xref: /external/bison/doc/bison.info

This is ../../../doc/bison.info, produced by makeinfo version 4.13 from
../../../doc/bison.texi.

This manual (9 December 2012) is for GNU Bison (version 2.7), the GNU
parser generator.

   Copyright (C) 1988-1993, 1995, 1998-2012 Free Software Foundation,
Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover texts
     being "A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License."

     (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
     modify this GNU manual.  Buying copies from the FSF supports it in
     developing GNU and promoting software freedom."

INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* bison: (bison).       GNU parser generator (Yacc replacement).
END-INFO-DIR-ENTRY


File: bison.info,  Node: Top,  Next: Introduction,  Up: (dir)

Bison
*****

This manual (9 December 2012) is for GNU Bison (version 2.7), the GNU
parser generator.

   Copyright (C) 1988-1993, 1995, 1998-2012 Free Software Foundation,
Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover texts
     being "A GNU Manual," and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     "GNU Free Documentation License."

     (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
     modify this GNU manual.  Buying copies from the FSF supports it in
     developing GNU and promoting software freedom."

* Menu:

* Introduction::
* Conditions::
* Copying::             The GNU General Public License says
                          how you can copy and share Bison.

Tutorial sections:
* Concepts::            Basic concepts for understanding Bison.
* Examples::            Three simple explained examples of using Bison.

Reference sections:
* Grammar File::        Writing Bison declarations and rules.
* Interface::           C-language interface to the parser function `yyparse'.
* Algorithm::           How the Bison parser works at run-time.
* Error Recovery::      Writing rules for error recovery.
* Context Dependency::  What to do if your language syntax is too
                          messy for Bison to handle straightforwardly.
* Debugging::           Understanding or debugging Bison parsers.
* Invocation::          How to run Bison (to produce the parser implementation).
* Other Languages::     Creating C++ and Java parsers.
* FAQ::                 Frequently Asked Questions
* Table of Symbols::    All the keywords of the Bison language are explained.
* Glossary::            Basic concepts are explained.
* Copying This Manual:: License for copying this manual.
* Bibliography::        Publications cited in this manual.
* Index of Terms::      Cross-references to the text.

 --- The Detailed Node Listing ---

The Concepts of Bison

* Language and Grammar:: Languages and context-free grammars,
                           as mathematical ideas.
* Grammar in Bison::     How we represent grammars for Bison's sake.
* Semantic Values::      Each token or syntactic grouping can have
                           a semantic value (the value of an integer,
                           the name of an identifier, etc.).
* Semantic Actions::     Each rule can have an action containing C code.
* GLR Parsers::          Writing parsers for general context-free languages.
* Locations::            Overview of location tracking.
* Bison Parser::         What are Bison's input and output,
                           how is the output used?
* Stages::               Stages in writing and running Bison grammars.
* Grammar Layout::       Overall structure of a Bison grammar file.

Writing GLR Parsers

* Simple GLR Parsers::     Using GLR parsers on unambiguous grammars.
* Merging GLR Parses::     Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions::   Deferred semantic actions have special concerns.
* Compiler Requirements::  GLR parsers require a modern C compiler.

Examples

* RPN Calc::               Reverse polish notation calculator;
                             a first example with no operator precedence.
* Infix Calc::             Infix (algebraic) notation calculator.
                             Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @N and @$.
* Multi-function Calc::    Calculator with memory and trig functions.
                             It uses multiple data-types for semantic values.
* Exercises::              Ideas for improving the multi-function calculator.

Reverse Polish Notation Calculator

* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer::           The lexical analyzer.
* Rpcalc Main::            The controlling function.
* Rpcalc Error::           The error reporting function.
* Rpcalc Generate::        Running Bison on the grammar file.
* Rpcalc Compile::         Run the C compiler on the output code.

Grammar Rules for `rpcalc'

* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::

Location Tracking Calculator: `ltcalc'

* Ltcalc Declarations::    Bison and C declarations for ltcalc.
* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer::           The lexical analyzer.

Multi-Function Calculator: `mfcalc'

* Mfcalc Declarations::    Bison declarations for multi-function calculator.
* Mfcalc Rules::           Grammar rules for the calculator.
* Mfcalc Symbol Table::    Symbol table management subroutines.

Bison Grammar Files

* Grammar Outline::    Overall layout of the grammar file.
* Symbols::            Terminal and nonterminal symbols.
* Rules::              How to write grammar rules.
* Recursion::          Writing recursive rules.
* Semantics::          Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References::   Using named references in actions.
* Declarations::       All kinds of Bison declarations are described here.
* Multiple Parsers::   Putting more than one Bison parser in one program.

Outline of a Bison Grammar

* Prologue::              Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations::    Syntax and usage of the Bison declarations section.
* Grammar Rules::         Syntax and usage of the grammar rules section.
* Epilogue::              Syntax and usage of the epilogue.

Defining Language Semantics

* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Mid-Rule Actions::  Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.

Actions in Mid-Rule

* Using Mid-Rule Actions::       Putting an action in the middle of a rule.
* Mid-Rule Action Translation::  How mid-rule actions are actually processed.
* Mid-Rule Conflicts::           Mid-rule actions can cause conflicts.

Tracking Locations

* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.

Bison Declarations

* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Union Decl::        Declaring the set of all semantic value types.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Printer Decl::      Declaring how symbol values are displayed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Push Decl::         Requesting a push parser.
* Decl Summary::      Table of all Bison declarations.
* %define Summary::   Defining variables to adjust Bison's behavior.
* %code Summary::     Inserting code into the parser source.

Parser C-Language Interface

* Parser Function::         How to call `yyparse' and what it returns.
* Push Parser Function::    How to call `yypush_parse' and what it returns.
* Pull Parser Function::    How to call `yypull_parse' and what it returns.
* Parser Create Function::  How to call `yypstate_new' and what it returns.
* Parser Delete Function::  How to call `yypstate_delete' and what it returns.
* Lexical::                 You must supply a function `yylex'
                              which reads tokens.
* Error Reporting::         You must supply a function `yyerror'.
* Action Features::         Special features for use in actions.
* Internationalization::    How to let the parser speak in the user's
                              native language.

The Lexical Analyzer Function `yylex'

* Calling Convention::  How `yyparse' calls `yylex'.
* Token Values::        How `yylex' must return the semantic value
                          of the token it has read.
* Token Locations::     How `yylex' must return the text location
                          (line number, etc.) of the token, if the
                          actions want that.
* Pure Calling::        How the calling convention differs in a pure parser
                          (*note A Pure (Reentrant) Parser: Pure Decl.).

The Bison Parser Algorithm

* Lookahead::         Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR::         How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.

Operator Precedence

* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence in Bison grammars.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.
* Non Operators::     Using precedence for general conflicts.

Tuning LR

* LR Table Construction:: Choose a different construction algorithm.
* Default Reductions::    Disable default reductions.
* LAC::                   Correct lookahead sets in the parser states.
* Unreachable States::    Keep unreachable parser states for debugging.

Handling Context Dependencies

* Semantic Tokens::   Token parsing can depend on the semantic context.
* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
* Tie-in Recovery::   Lexical tie-ins have implications for how
                        error recovery rules must be written.

Debugging Your Parser

* Understanding::     Understanding the structure of your parser.
* Graphviz::          Getting a visual representation of the parser.
* Xml::               Getting a markup representation of the parser.
* Tracing::           Tracing the execution of your parser.

Tracing Your Parser

* Enabling Traces::             Activating run-time trace support
* Mfcalc Traces::               Extending `mfcalc' to support traces
* The YYPRINT Macro::           Obsolete interface for semantic value reports

Invoking Bison

* Bison Options::     All the options described in detail,
                        in alphabetical order by short options.
* Option Cross Key::  Alphabetical list of long options.
* Yacc Library::      Yacc-compatible `yylex' and `main'.

Parsers Written In Other Languages

* C++ Parsers::                 The interface to generate C++ parser classes
* Java Parsers::                The interface to generate Java parser classes

C++ Parsers

* C++ Bison Interface::         Asking for C++ parser generation
* C++ Semantic Values::         %union vs. C++
* C++ Location Values::         The position and location classes
* C++ Parser Interface::        Instantiating and running the parser
* C++ Scanner Interface::       Exchanges between yylex and parse
* A Complete C++ Example::      Demonstrating their use

C++ Location Values

* C++ position::                One point in the source file
* C++ location::                Two points in the source file
* User Defined Location Type::  Required interface for locations

A Complete C++ Example

* Calc++ --- C++ Calculator::   The specifications
* Calc++ Parsing Driver::       An active parsing context
* Calc++ Parser::               A parser class
* Calc++ Scanner::              A pure C++ Flex scanner
* Calc++ Top Level::            Conducting the band

Java Parsers

* Java Bison Interface::        Asking for Java parser generation
* Java Semantic Values::        %type and %token vs. Java
* Java Location Values::        The position and location classes
* Java Parser Interface::       Instantiating and running the parser
* Java Scanner Interface::      Specifying the scanner for the parser
* Java Action Features::        Special features for use in actions
* Java Differences::            Differences between C/C++ and Java Grammars
* Java Declarations Summary::   List of Bison declarations used with Java

Frequently Asked Questions

* Memory Exhausted::            Breaking the Stack Limits
* How Can I Reset the Parser::  `yyparse' Keeps some State
* Strings are Destroyed::       `yylval' Loses Track of Strings
* Implementing Gotos/Loops::    Control Flow in the Calculator
* Multiple start-symbols::      Factoring closely related grammars
* Secure?  Conform?::           Is Bison POSIX safe?
* I can't build Bison::         Troubleshooting
* Where can I find help?::      Troubleshouting
* Bug Reports::                 Troublereporting
* More Languages::              Parsers in C++, Java, and so on
* Beta Testing::                Experimenting development versions
* Mailing Lists::               Meeting other Bison users

Copying This Manual

* Copying This Manual::         License for copying this manual.


File: bison.info,  Node: Introduction,  Next: Conditions,  Prev: Top,  Up: Top

Introduction
************

"Bison" is a general-purpose parser generator that converts an
annotated context-free grammar into a deterministic LR or generalized
LR (GLR) parser employing LALR(1) parser tables.  As an experimental
feature, Bison can also generate IELR(1) or canonical LR(1) parser
tables.  Once you are proficient with Bison, you can use it to develop
a wide range of language parsers, from those used in simple desk
calculators to complex programming languages.

   Bison is upward compatible with Yacc: all properly-written Yacc
grammars ought to work with Bison with no change.  Anyone familiar with
Yacc should be able to use Bison with little trouble.  You need to be
fluent in C or C++ programming in order to use Bison or to understand
this manual.  Java is also supported as an experimental feature.

   We begin with tutorial chapters that explain the basic concepts of
using Bison and show three explained examples, each building on the
last.  If you don't know Bison or Yacc, start by reading these
chapters.  Reference chapters follow, which describe specific aspects
of Bison in detail.

   Bison was written originally by Robert Corbett.  Richard Stallman
made it Yacc-compatible.  Wilfred Hansen of Carnegie Mellon University
added multi-character string literals and other features.  Since then,
Bison has grown more robust and evolved many other new features thanks
to the hard work of a long list of volunteers.  For details, see the
`THANKS' and `ChangeLog' files included in the Bison distribution.

   This edition corresponds to version 2.7 of Bison.


File: bison.info,  Node: Conditions,  Next: Copying,  Prev: Introduction,  Up: Top

Conditions for Using Bison
**************************

The distribution terms for Bison-generated parsers permit using the
parsers in nonfree programs.  Before Bison version 2.2, these extra
permissions applied only when Bison was generating LALR(1) parsers in
C.  And before Bison version 1.24, Bison-generated parsers could be
used only in programs that were free software.

   The other GNU programming tools, such as the GNU C compiler, have
never had such a requirement.  They could always be used for nonfree
software.  The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.

   The main output of the Bison utility--the Bison parser implementation
file--contains a verbatim copy of a sizable piece of Bison, which is
the code for the parser's implementation.  (The actions from your
grammar are inserted into this implementation at one point, but most of
the rest of the implementation is not changed.)  When we applied the
GPL terms to the skeleton code for the parser's implementation, the
effect was to restrict the use of Bison output to free software.

   We didn't change the terms because of sympathy for people who want to
make software proprietary.  *Software should be free.*  But we
concluded that limiting Bison's use to free software was doing little to
encourage people to make other software free.  So we decided to make the
practical conditions for using Bison match the practical conditions for
using the other GNU tools.

   This exception applies when Bison is generating code for a parser.
You can tell whether the exception applies to a Bison output file by
inspecting the file for text beginning with "As a special
exception...".  The text spells out the exact terms of the exception.


File: bison.info,  Node: Copying,  Next: Concepts,  Prev: Conditions,  Up: Top

GNU GENERAL PUBLIC LICENSE
**************************

                        Version 3, 29 June 2007

     Copyright (C) 2007 Free Software Foundation, Inc. `http://fsf.org/'

     Everyone is permitted to copy and distribute verbatim copies of this
     license document, but changing it is not allowed.

Preamble
========

The GNU General Public License is a free, copyleft license for software
and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program--to make sure it remains
free software for all its users.  We, the Free Software Foundation, use
the GNU General Public License for most of our software; it applies
also to any other work released this way by its authors.  You can apply
it to your programs, too.

   When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.

   To protect your rights, we need to prevent others from denying you
these rights or asking you to surrender the rights.  Therefore, you
have certain responsibilities if you distribute copies of the software,
or if you modify it: responsibilities to respect the freedom of others.

   For example, if you distribute copies of such a program, whether
gratis or for a fee, you must pass on to the recipients the same
freedoms that you received.  You must make sure that they, too, receive
or can get the source code.  And you must show them these terms so they
know their rights.

   Developers that use the GNU GPL protect your rights with two steps:
(1) assert copyright on the software, and (2) offer you this License
giving you legal permission to copy, distribute and/or modify it.

   For the developers' and authors' protection, the GPL clearly explains
that there is no warranty for this free software.  For both users' and
authors' sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
authors of previous versions.

   Some devices are designed to deny users access to install or run
modified versions of the software inside them, although the
manufacturer can do so.  This is fundamentally incompatible with the
aim of protecting users' freedom to change the software.  The
systematic pattern of such abuse occurs in the area of products for
individuals to use, which is precisely where it is most unacceptable.
Therefore, we have designed this version of the GPL to prohibit the
practice for those products.  If such problems arise substantially in
other domains, we stand ready to extend this provision to those domains
in future versions of the GPL, as needed to protect the freedom of
users.

   Finally, every program is threatened constantly by software patents.
States should not allow patents to restrict development and use of
software on general-purpose computers, but in those that do, we wish to
avoid the special danger that patents applied to a free program could
make it effectively proprietary.  To prevent this, the GPL assures that
patents cannot be used to render the program non-free.

   The precise terms and conditions for copying, distribution and
modification follow.

TERMS AND CONDITIONS
====================

  0. Definitions.

     "This License" refers to version 3 of the GNU General Public
     License.

     "Copyright" also means copyright-like laws that apply to other
     kinds of works, such as semiconductor masks.

     "The Program" refers to any copyrightable work licensed under this
     License.  Each licensee is addressed as "you".  "Licensees" and
     "recipients" may be individuals or organizations.

     To "modify" a work means to copy from or adapt all or part of the
     work in a fashion requiring copyright permission, other than the
     making of an exact copy.  The resulting work is called a "modified
     version" of the earlier work or a work "based on" the earlier work.

     A "covered work" means either the unmodified Program or a work
     based on the Program.

     To "propagate" a work means to do anything with it that, without
     permission, would make you directly or secondarily liable for
     infringement under applicable copyright law, except executing it
     on a computer or modifying a private copy.  Propagation includes
     copying, distribution (with or without modification), making
     available to the public, and in some countries other activities as
     well.

     To "convey" a work means any kind of propagation that enables other
     parties to make or receive copies.  Mere interaction with a user
     through a computer network, with no transfer of a copy, is not
     conveying.

     An interactive user interface displays "Appropriate Legal Notices"
     to the extent that it includes a convenient and prominently visible
     feature that (1) displays an appropriate copyright notice, and (2)
     tells the user that there is no warranty for the work (except to
     the extent that warranties are provided), that licensees may
     convey the work under this License, and how to view a copy of this
     License.  If the interface presents a list of user commands or
     options, such as a menu, a prominent item in the list meets this
     criterion.

  1. Source Code.

     The "source code" for a work means the preferred form of the work
     for making modifications to it.  "Object code" means any
     non-source form of a work.

     A "Standard Interface" means an interface that either is an
     official standard defined by a recognized standards body, or, in
     the case of interfaces specified for a particular programming
     language, one that is widely used among developers working in that
     language.

     The "System Libraries" of an executable work include anything,
     other than the work as a whole, that (a) is included in the normal
     form of packaging a Major Component, but which is not part of that
     Major Component, and (b) serves only to enable use of the work
     with that Major Component, or to implement a Standard Interface
     for which an implementation is available to the public in source
     code form.  A "Major Component", in this context, means a major
     essential component (kernel, window system, and so on) of the
     specific operating system (if any) on which the executable work
     runs, or a compiler used to produce the work, or an object code
     interpreter used to run it.

     The "Corresponding Source" for a work in object code form means all
     the source code needed to generate, install, and (for an executable
     work) run the object code and to modify the work, including
     scripts to control those activities.  However, it does not include
     the work's System Libraries, or general-purpose tools or generally
     available free programs which are used unmodified in performing
     those activities but which are not part of the work.  For example,
     Corresponding Source includes interface definition files
     associated with source files for the work, and the source code for
     shared libraries and dynamically linked subprograms that the work
     is specifically designed to require, such as by intimate data
     communication or control flow between those subprograms and other
     parts of the work.

     The Corresponding Source need not include anything that users can
     regenerate automatically from other parts of the Corresponding
     Source.

     The Corresponding Source for a work in source code form is that
     same work.

  2. Basic Permissions.

     All rights granted under this License are granted for the term of
     copyright on the Program, and are irrevocable provided the stated
     conditions are met.  This License explicitly affirms your unlimited
     permission to run the unmodified Program.  The output from running
     a covered work is covered by this License only if the output,
     given its content, constitutes a covered work.  This License
     acknowledges your rights of fair use or other equivalent, as
     provided by copyright law.

     You may make, run and propagate covered works that you do not
     convey, without conditions so long as your license otherwise
     remains in force.  You may convey covered works to others for the
     sole purpose of having them make modifications exclusively for
     you, or provide you with facilities for running those works,
     provided that you comply with the terms of this License in
     conveying all material for which you do not control copyright.
     Those thus making or running the covered works for you must do so
     exclusively on your behalf, under your direction and control, on
     terms that prohibit them from making any copies of your
     copyrighted material outside their relationship with you.

     Conveying under any other circumstances is permitted solely under
     the conditions stated below.  Sublicensing is not allowed; section
     10 makes it unnecessary.

  3. Protecting Users' Legal Rights From Anti-Circumvention Law.

     No covered work shall be deemed part of an effective technological
     measure under any applicable law fulfilling obligations under
     article 11 of the WIPO copyright treaty adopted on 20 December
     1996, or similar laws prohibiting or restricting circumvention of
     such measures.

     When you convey a covered work, you waive any legal power to forbid
     circumvention of technological measures to the extent such
     circumvention is effected by exercising rights under this License
     with respect to the covered work, and you disclaim any intention
     to limit operation or modification of the work as a means of
     enforcing, against the work's users, your or third parties' legal
     rights to forbid circumvention of technological measures.

  4. Conveying Verbatim Copies.

     You may convey verbatim copies of the Program's source code as you
     receive it, in any medium, provided that you conspicuously and
     appropriately publish on each copy an appropriate copyright notice;
     keep intact all notices stating that this License and any
     non-permissive terms added in accord with section 7 apply to the
     code; keep intact all notices of the absence of any warranty; and
     give all recipients a copy of this License along with the Program.

     You may charge any price or no price for each copy that you convey,
     and you may offer support or warranty protection for a fee.

  5. Conveying Modified Source Versions.

     You may convey a work based on the Program, or the modifications to
     produce it from the Program, in the form of source code under the
     terms of section 4, provided that you also meet all of these
     conditions:

       a. The work must carry prominent notices stating that you
          modified it, and giving a relevant date.

       b. The work must carry prominent notices stating that it is
          released under this License and any conditions added under
          section 7.  This requirement modifies the requirement in
          section 4 to "keep intact all notices".

       c. You must license the entire work, as a whole, under this
          License to anyone who comes into possession of a copy.  This
          License will therefore apply, along with any applicable
          section 7 additional terms, to the whole of the work, and all
          its parts, regardless of how they are packaged.  This License
          gives no permission to license the work in any other way, but
          it does not invalidate such permission if you have separately
          received it.

       d. If the work has interactive user interfaces, each must display
          Appropriate Legal Notices; however, if the Program has
          interactive interfaces that do not display Appropriate Legal
          Notices, your work need not make them do so.

     A compilation of a covered work with other separate and independent
     works, which are not by their nature extensions of the covered
     work, and which are not combined with it such as to form a larger
     program, in or on a volume of a storage or distribution medium, is
     called an "aggregate" if the compilation and its resulting
     copyright are not used to limit the access or legal rights of the
     compilation's users beyond what the individual works permit.
     Inclusion of a covered work in an aggregate does not cause this
     License to apply to the other parts of the aggregate.

  6. Conveying Non-Source Forms.

     You may convey a covered work in object code form under the terms
     of sections 4 and 5, provided that you also convey the
     machine-readable Corresponding Source under the terms of this
     License, in one of these ways:

       a. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by the
          Corresponding Source fixed on a durable physical medium
          customarily used for software interchange.

       b. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by a
          written offer, valid for at least three years and valid for
          as long as you offer spare parts or customer support for that
          product model, to give anyone who possesses the object code
          either (1) a copy of the Corresponding Source for all the
          software in the product that is covered by this License, on a
          durable physical medium customarily used for software
          interchange, for a price no more than your reasonable cost of
          physically performing this conveying of source, or (2) access
          to copy the Corresponding Source from a network server at no
          charge.

       c. Convey individual copies of the object code with a copy of
          the written offer to provide the Corresponding Source.  This
          alternative is allowed only occasionally and noncommercially,
          and only if you received the object code with such an offer,
          in accord with subsection 6b.

       d. Convey the object code by offering access from a designated
          place (gratis or for a charge), and offer equivalent access
          to the Corresponding Source in the same way through the same
          place at no further charge.  You need not require recipients
          to copy the Corresponding Source along with the object code.
          If the place to copy the object code is a network server, the
          Corresponding Source may be on a different server (operated
          by you or a third party) that supports equivalent copying
          facilities, provided you maintain clear directions next to
          the object code saying where to find the Corresponding Source.
          Regardless of what server hosts the Corresponding Source, you
          remain obligated to ensure that it is available for as long
          as needed to satisfy these requirements.

       e. Convey the object code using peer-to-peer transmission,
          provided you inform other peers where the object code and
          Corresponding Source of the work are being offered to the
          general public at no charge under subsection 6d.


     A separable portion of the object code, whose source code is
     excluded from the Corresponding Source as a System Library, need
     not be included in conveying the object code work.

     A "User Product" is either (1) a "consumer product", which means
     any tangible personal property which is normally used for personal,
     family, or household purposes, or (2) anything designed or sold for
     incorporation into a dwelling.  In determining whether a product
     is a consumer product, doubtful cases shall be resolved in favor of
     coverage.  For a particular product received by a particular user,
     "normally used" refers to a typical or common use of that class of
     product, regardless of the status of the particular user or of the
     way in which the particular user actually uses, or expects or is
     expected to use, the product.  A product is a consumer product
     regardless of whether the product has substantial commercial,
     industrial or non-consumer uses, unless such uses represent the
     only significant mode of use of the product.

     "Installation Information" for a User Product means any methods,
     procedures, authorization keys, or other information required to
     install and execute modified versions of a covered work in that
     User Product from a modified version of its Corresponding Source.
     The information must suffice to ensure that the continued
     functioning of the modified object code is in no case prevented or
     interfered with solely because modification has been made.

     If you convey an object code work under this section in, or with,
     or specifically for use in, a User Product, and the conveying
     occurs as part of a transaction in which the right of possession
     and use of the User Product is transferred to the recipient in
     perpetuity or for a fixed term (regardless of how the transaction
     is characterized), the Corresponding Source conveyed under this
     section must be accompanied by the Installation Information.  But
     this requirement does not apply if neither you nor any third party
     retains the ability to install modified object code on the User
     Product (for example, the work has been installed in ROM).

     The requirement to provide Installation Information does not
     include a requirement to continue to provide support service,
     warranty, or updates for a work that has been modified or
     installed by the recipient, or for the User Product in which it
     has been modified or installed.  Access to a network may be denied
     when the modification itself materially and adversely affects the
     operation of the network or violates the rules and protocols for
     communication across the network.

     Corresponding Source conveyed, and Installation Information
     provided, in accord with this section must be in a format that is
     publicly documented (and with an implementation available to the
     public in source code form), and must require no special password
     or key for unpacking, reading or copying.

  7. Additional Terms.

     "Additional permissions" are terms that supplement the terms of
     this License by making exceptions from one or more of its
     conditions.  Additional permissions that are applicable to the
     entire Program shall be treated as though they were included in
     this License, to the extent that they are valid under applicable
     law.  If additional permissions apply only to part of the Program,
     that part may be used separately under those permissions, but the
     entire Program remains governed by this License without regard to
     the additional permissions.

     When you convey a copy of a covered work, you may at your option
     remove any additional permissions from that copy, or from any part
     of it.  (Additional permissions may be written to require their own
     removal in certain cases when you modify the work.)  You may place
     additional permissions on material, added by you to a covered work,
     for which you have or can give appropriate copyright permission.

     Notwithstanding any other provision of this License, for material
     you add to a covered work, you may (if authorized by the copyright
     holders of that material) supplement the terms of this License
     with terms:

       a. Disclaiming warranty or limiting liability differently from
          the terms of sections 15 and 16 of this License; or

       b. Requiring preservation of specified reasonable legal notices
          or author attributions in that material or in the Appropriate
          Legal Notices displayed by works containing it; or

       c. Prohibiting misrepresentation of the origin of that material,
          or requiring that modified versions of such material be
          marked in reasonable ways as different from the original
          version; or

       d. Limiting the use for publicity purposes of names of licensors
          or authors of the material; or

       e. Declining to grant rights under trademark law for use of some
          trade names, trademarks, or service marks; or

       f. Requiring indemnification of licensors and authors of that
          material by anyone who conveys the material (or modified
          versions of it) with contractual assumptions of liability to
          the recipient, for any liability that these contractual
          assumptions directly impose on those licensors and authors.

     All other non-permissive additional terms are considered "further
     restrictions" within the meaning of section 10.  If the Program as
     you received it, or any part of it, contains a notice stating that
     it is governed by this License along with a term that is a further
     restriction, you may remove that term.  If a license document
     contains a further restriction but permits relicensing or
     conveying under this License, you may add to a covered work
     material governed by the terms of that license document, provided
     that the further restriction does not survive such relicensing or
     conveying.

     If you add terms to a covered work in accord with this section, you
     must place, in the relevant source files, a statement of the
     additional terms that apply to those files, or a notice indicating
     where to find the applicable terms.

     Additional terms, permissive or non-permissive, may be stated in
     the form of a separately written license, or stated as exceptions;
     the above requirements apply either way.

  8. Termination.

     You may not propagate or modify a covered work except as expressly
     provided under this License.  Any attempt otherwise to propagate or
     modify it is void, and will automatically terminate your rights
     under this License (including any patent licenses granted under
     the third paragraph of section 11).

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly
     and finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from
     you under this License.  If your rights have been terminated and
     not permanently reinstated, you do not qualify to receive new
     licenses for the same material under section 10.

  9. Acceptance Not Required for Having Copies.

     You are not required to accept this License in order to receive or
     run a copy of the Program.  Ancillary propagation of a covered work
     occurring solely as a consequence of using peer-to-peer
     transmission to receive a copy likewise does not require
     acceptance.  However, nothing other than this License grants you
     permission to propagate or modify any covered work.  These actions
     infringe copyright if you do not accept this License.  Therefore,
     by modifying or propagating a covered work, you indicate your
     acceptance of this License to do so.

 10. Automatic Licensing of Downstream Recipients.

     Each time you convey a covered work, the recipient automatically
     receives a license from the original licensors, to run, modify and
     propagate that work, subject to this License.  You are not
     responsible for enforcing compliance by third parties with this
     License.

     An "entity transaction" is a transaction transferring control of an
     organization, or substantially all assets of one, or subdividing an
     organization, or merging organizations.  If propagation of a
     covered work results from an entity transaction, each party to that
     transaction who receives a copy of the work also receives whatever
     licenses to the work the party's predecessor in interest had or
     could give under the previous paragraph, plus a right to
     possession of the Corresponding Source of the work from the
     predecessor in interest, if the predecessor has it or can get it
     with reasonable efforts.

     You may not impose any further restrictions on the exercise of the
     rights granted or affirmed under this License.  For example, you
     may not impose a license fee, royalty, or other charge for
     exercise of rights granted under this License, and you may not
     initiate litigation (including a cross-claim or counterclaim in a
     lawsuit) alleging that any patent claim is infringed by making,
     using, selling, offering for sale, or importing the Program or any
     portion of it.

 11. Patents.

     A "contributor" is a copyright holder who authorizes use under this
     License of the Program or a work on which the Program is based.
     The work thus licensed is called the contributor's "contributor
     version".

     A contributor's "essential patent claims" are all patent claims
     owned or controlled by the contributor, whether already acquired or
     hereafter acquired, that would be infringed by some manner,
     permitted by this License, of making, using, or selling its
     contributor version, but do not include claims that would be
     infringed only as a consequence of further modification of the
     contributor version.  For purposes of this definition, "control"
     includes the right to grant patent sublicenses in a manner
     consistent with the requirements of this License.

     Each contributor grants you a non-exclusive, worldwide,
     royalty-free patent license under the contributor's essential
     patent claims, to make, use, sell, offer for sale, import and
     otherwise run, modify and propagate the contents of its
     contributor version.

     In the following three paragraphs, a "patent license" is any
     express agreement or commitment, however denominated, not to
     enforce a patent (such as an express permission to practice a
     patent or covenant not to sue for patent infringement).  To
     "grant" such a patent license to a party means to make such an
     agreement or commitment not to enforce a patent against the party.

     If you convey a covered work, knowingly relying on a patent
     license, and the Corresponding Source of the work is not available
     for anyone to copy, free of charge and under the terms of this
     License, through a publicly available network server or other
     readily accessible means, then you must either (1) cause the
     Corresponding Source to be so available, or (2) arrange to deprive
     yourself of the benefit of the patent license for this particular
     work, or (3) arrange, in a manner consistent with the requirements
     of this License, to extend the patent license to downstream
     recipients.  "Knowingly relying" means you have actual knowledge
     that, but for the patent license, your conveying the covered work
     in a country, or your recipient's use of the covered work in a
     country, would infringe one or more identifiable patents in that
     country that you have reason to believe are valid.

     If, pursuant to or in connection with a single transaction or
     arrangement, you convey, or propagate by procuring conveyance of, a
     covered work, and grant a patent license to some of the parties
     receiving the covered work authorizing them to use, propagate,
     modify or convey a specific copy of the covered work, then the
     patent license you grant is automatically extended to all
     recipients of the covered work and works based on it.

     A patent license is "discriminatory" if it does not include within
     the scope of its coverage, prohibits the exercise of, or is
     conditioned on the non-exercise of one or more of the rights that
     are specifically granted under this License.  You may not convey a
     covered work if you are a party to an arrangement with a third
     party that is in the business of distributing software, under
     which you make payment to the third party based on the extent of
     your activity of conveying the work, and under which the third
     party grants, to any of the parties who would receive the covered
     work from you, a discriminatory patent license (a) in connection
     with copies of the covered work conveyed by you (or copies made
     from those copies), or (b) primarily for and in connection with
     specific products or compilations that contain the covered work,
     unless you entered into that arrangement, or that patent license
     was granted, prior to 28 March 2007.

     Nothing in this License shall be construed as excluding or limiting
     any implied license or other defenses to infringement that may
     otherwise be available to you under applicable patent law.

 12. No Surrender of Others' Freedom.

     If conditions are imposed on you (whether by court order,
     agreement or otherwise) that contradict the conditions of this
     License, they do not excuse you from the conditions of this
     License.  If you cannot convey a covered work so as to satisfy
     simultaneously your obligations under this License and any other
     pertinent obligations, then as a consequence you may not convey it
     at all.  For example, if you agree to terms that obligate you to
     collect a royalty for further conveying from those to whom you
     convey the Program, the only way you could satisfy both those
     terms and this License would be to refrain entirely from conveying
     the Program.

 13. Use with the GNU Affero General Public License.

     Notwithstanding any other provision of this License, you have
     permission to link or combine any covered work with a work licensed
     under version 3 of the GNU Affero General Public License into a
     single combined work, and to convey the resulting work.  The terms
     of this License will continue to apply to the part which is the
     covered work, but the special requirements of the GNU Affero
     General Public License, section 13, concerning interaction through
     a network will apply to the combination as such.

 14. Revised Versions of this License.

     The Free Software Foundation may publish revised and/or new
     versions of the GNU General Public License from time to time.
     Such new versions will be similar in spirit to the present
     version, but may differ in detail to address new problems or
     concerns.

     Each version is given a distinguishing version number.  If the
     Program specifies that a certain numbered version of the GNU
     General Public License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that numbered version or of any later version published by the
     Free Software Foundation.  If the Program does not specify a
     version number of the GNU General Public License, you may choose
     any version ever published by the Free Software Foundation.

     If the Program specifies that a proxy can decide which future
     versions of the GNU General Public License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Program.

     Later license versions may give you additional or different
     permissions.  However, no additional obligations are imposed on any
     author or copyright holder as a result of your choosing to follow a
     later version.

 15. Disclaimer of Warranty.

     THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
     APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE
     COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS"
     WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED,
     INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
     MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE
     RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.
     SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
     NECESSARY SERVICING, REPAIR OR CORRECTION.

 16. Limitation of Liability.

     IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
     WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES
     AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU
     FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR
     CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE
     THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA
     BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
     PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
     PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF
     THE POSSIBILITY OF SUCH DAMAGES.

 17. Interpretation of Sections 15 and 16.

     If the disclaimer of warranty and limitation of liability provided
     above cannot be given local legal effect according to their terms,
     reviewing courts shall apply local law that most closely
     approximates an absolute waiver of all civil liability in
     connection with the Program, unless a warranty or assumption of
     liability accompanies a copy of the Program in return for a fee.


END OF TERMS AND CONDITIONS
===========================

How to Apply These Terms to Your New Programs
=============================================

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

   To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least the
"copyright" line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
     Copyright (C) YEAR NAME OF AUTHOR

     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation, either version 3 of the License, or (at
     your option) any later version.

     This program is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with this program.  If not, see `http://www.gnu.org/licenses/'.

   Also add information on how to contact you by electronic and paper
mail.

   If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

     PROGRAM Copyright (C) YEAR NAME OF AUTHOR
     This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type `show c' for details.

   The hypothetical commands `show w' and `show c' should show the
appropriate parts of the General Public License.  Of course, your
program's commands might be different; for a GUI interface, you would
use an "about box".

   You should also get your employer (if you work as a programmer) or
school, if any, to sign a "copyright disclaimer" for the program, if
necessary.  For more information on this, and how to apply and follow
the GNU GPL, see `http://www.gnu.org/licenses/'.

   The GNU General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use the
GNU Lesser General Public License instead of this License.  But first,
please read `http://www.gnu.org/philosophy/why-not-lgpl.html'.


File: bison.info,  Node: Concepts,  Next: Examples,  Prev: Copying,  Up: Top

1 The Concepts of Bison
***********************

This chapter introduces many of the basic concepts without which the
details of Bison will not make sense.  If you do not already know how to
use Bison or Yacc, we suggest you start by reading this chapter
carefully.

* Menu:

* Language and Grammar:: Languages and context-free grammars,
                           as mathematical ideas.
* Grammar in Bison::     How we represent grammars for Bison's sake.
* Semantic Values::      Each token or syntactic grouping can have
                           a semantic value (the value of an integer,
                           the name of an identifier, etc.).
* Semantic Actions::     Each rule can have an action containing C code.
* GLR Parsers::          Writing parsers for general context-free languages.
* Locations::            Overview of location tracking.
* Bison Parser::         What are Bison's input and output,
                           how is the output used?
* Stages::               Stages in writing and running Bison grammars.
* Grammar Layout::       Overall structure of a Bison grammar file.


File: bison.info,  Node: Language and Grammar,  Next: Grammar in Bison,  Up: Concepts

1.1 Languages and Context-Free Grammars
=======================================

In order for Bison to parse a language, it must be described by a
"context-free grammar".  This means that you specify one or more
"syntactic groupings" and give rules for constructing them from their
parts.  For example, in the C language, one kind of grouping is called
an `expression'.  One rule for making an expression might be, "An
expression can be made of a minus sign and another expression".
Another would be, "An expression can be an integer".  As you can see,
rules are often recursive, but there must be at least one rule which
leads out of the recursion.

   The most common formal system for presenting such rules for humans
to read is "Backus-Naur Form" or "BNF", which was developed in order to
specify the language Algol 60.  Any grammar expressed in BNF is a
context-free grammar.  The input to Bison is essentially
machine-readable BNF.

   There are various important subclasses of context-free grammars.
Although it can handle almost all context-free grammars, Bison is
optimized for what are called LR(1) grammars.  In brief, in these
grammars, it must be possible to tell how to parse any portion of an
input string with just a single token of lookahead.  For historical
reasons, Bison by default is limited by the additional restrictions of
LALR(1), which is hard to explain simply.  *Note Mysterious
Conflicts::, for more information on this.  As an experimental feature,
you can escape these additional restrictions by requesting IELR(1) or
canonical LR(1) parser tables.  *Note LR Table Construction::, to learn
how.

   Parsers for LR(1) grammars are "deterministic", meaning roughly that
the next grammar rule to apply at any point in the input is uniquely
determined by the preceding input and a fixed, finite portion (called a
"lookahead") of the remaining input.  A context-free grammar can be
"ambiguous", meaning that there are multiple ways to apply the grammar
rules to get the same inputs.  Even unambiguous grammars can be
"nondeterministic", meaning that no fixed lookahead always suffices to
determine the next grammar rule to apply.  With the proper
declarations, Bison is also able to parse these more general
context-free grammars, using a technique known as GLR parsing (for
Generalized LR).  Bison's GLR parsers are able to handle any
context-free grammar for which the number of possible parses of any
given string is finite.

   In the formal grammatical rules for a language, each kind of
syntactic unit or grouping is named by a "symbol".  Those which are
built by grouping smaller constructs according to grammatical rules are
called "nonterminal symbols"; those which can't be subdivided are called
"terminal symbols" or "token types".  We call a piece of input
corresponding to a single terminal symbol a "token", and a piece
corresponding to a single nonterminal symbol a "grouping".

   We can use the C language as an example of what symbols, terminal and
nonterminal, mean.  The tokens of C are identifiers, constants (numeric
and string), and the various keywords, arithmetic operators and
punctuation marks.  So the terminal symbols of a grammar for C include
`identifier', `number', `string', plus one symbol for each keyword,
operator or punctuation mark: `if', `return', `const', `static', `int',
`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)

   Here is a simple C function subdivided into tokens:

     int             /* keyword `int' */
     square (int x)  /* identifier, open-paren, keyword `int',
                        identifier, close-paren */
     {               /* open-brace */
       return x * x; /* keyword `return', identifier, asterisk,
                        identifier, semicolon */
     }               /* close-brace */

   The syntactic groupings of C include the expression, the statement,
the declaration, and the function definition.  These are represented in
the grammar of C by nonterminal symbols `expression', `statement',
`declaration' and `function definition'.  The full grammar uses dozens
of additional language constructs, each with its own nonterminal
symbol, in order to express the meanings of these four.  The example
above is a function definition; it contains one declaration, and one
statement.  In the statement, each `x' is an expression and so is `x *
x'.

   Each nonterminal symbol must have grammatical rules showing how it
is made out of simpler constructs.  For example, one kind of C
statement is the `return' statement; this would be described with a
grammar rule which reads informally as follows:

     A `statement' can be made of a `return' keyword, an `expression'
     and a `semicolon'.

There would be many other rules for `statement', one for each kind of
statement in C.

   One nonterminal symbol must be distinguished as the special one which
defines a complete utterance in the language.  It is called the "start
symbol".  In a compiler, this means a complete input program.  In the C
language, the nonterminal symbol `sequence of definitions and
declarations' plays this role.

   For example, `1 + 2' is a valid C expression--a valid part of a C
program--but it is not valid as an _entire_ C program.  In the
context-free grammar of C, this follows from the fact that `expression'
is not the start symbol.

   The Bison parser reads a sequence of tokens as its input, and groups
the tokens using the grammar rules.  If the input is valid, the end
result is that the entire token sequence reduces to a single grouping
whose symbol is the grammar's start symbol.  If we use a grammar for C,
the entire input must be a `sequence of definitions and declarations'.
If not, the parser reports a syntax error.


File: bison.info,  Node: Grammar in Bison,  Next: Semantic Values,  Prev: Language and Grammar,  Up: Concepts

1.2 From Formal Rules to Bison Input
====================================

A formal grammar is a mathematical construct.  To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a "Bison grammar" file.  *Note Bison Grammar Files: Grammar File.

   A nonterminal symbol in the formal grammar is represented in Bison
input as an identifier, like an identifier in C.  By convention, it
should be in lower case, such as `expr', `stmt' or `declaration'.

   The Bison representation for a terminal symbol is also called a
"token type".  Token types as well can be represented as C-like
identifiers.  By convention, these identifiers should be upper case to
distinguish them from nonterminals: for example, `INTEGER',
`IDENTIFIER', `IF' or `RETURN'.  A terminal symbol that stands for a
particular keyword in the language should be named after that keyword
converted to upper case.  The terminal symbol `error' is reserved for
error recovery.  *Note Symbols::.

   A terminal symbol can also be represented as a character literal,
just like a C character constant.  You should do this whenever a token
is just a single character (parenthesis, plus-sign, etc.): use that
same character in a literal as the terminal symbol for that token.

   A third way to represent a terminal symbol is with a C string
constant containing several characters.  *Note Symbols::, for more
information.

   The grammar rules also have an expression in Bison syntax.  For
example, here is the Bison rule for a C `return' statement.  The
semicolon in quotes is a literal character token, representing part of
the C syntax for the statement; the naked semicolon, and the colon, are
Bison punctuation used in every rule.

     stmt: RETURN expr ';' ;

*Note Syntax of Grammar Rules: Rules.


File: bison.info,  Node: Semantic Values,  Next: Semantic Actions,  Prev: Grammar in Bison,  Up: Concepts

1.3 Semantic Values
===================

A formal grammar selects tokens only by their classifications: for
example, if a rule mentions the terminal symbol `integer constant', it
means that _any_ integer constant is grammatically valid in that
position.  The precise value of the constant is irrelevant to how to
parse the input: if `x+4' is grammatical then `x+1' or `x+3989' is
equally grammatical.

   But the precise value is very important for what the input means
once it is parsed.  A compiler is useless if it fails to distinguish
between 4, 1 and 3989 as constants in the program!  Therefore, each
token in a Bison grammar has both a token type and a "semantic value".
*Note Defining Language Semantics: Semantics, for details.

   The token type is a terminal symbol defined in the grammar, such as
`INTEGER', `IDENTIFIER' or `',''.  It tells everything you need to know
to decide where the token may validly appear and how to group it with
other tokens.  The grammar rules know nothing about tokens except their
types.

   The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier.  (A token such as `','' which is just punctuation doesn't
need to have any semantic value.)

   For example, an input token might be classified as token type
`INTEGER' and have the semantic value 4.  Another input token might
have the same token type `INTEGER' but value 3989.  When a grammar rule
says that `INTEGER' is allowed, either of these tokens is acceptable
because each is an `INTEGER'.  When the parser accepts the token, it
keeps track of the token's semantic value.

   Each grouping can also have a semantic value as well as its
nonterminal symbol.  For example, in a calculator, an expression
typically has a semantic value that is a number.  In a compiler for a
programming language, an expression typically has a semantic value that
is a tree structure describing the meaning of the expression.


File: bison.info,  Node: Semantic Actions,  Next: GLR Parsers,  Prev: Semantic Values,  Up: Concepts

1.4 Semantic Actions
====================

In order to be useful, a program must do more than parse input; it must
also produce some output based on the input.  In a Bison grammar, a
grammar rule can have an "action" made up of C statements.  Each time
the parser recognizes a match for that rule, the action is executed.
*Note Actions::.

   Most of the time, the purpose of an action is to compute the
semantic value of the whole construct from the semantic values of its
parts.  For example, suppose we have a rule which says an expression
can be the sum of two expressions.  When the parser recognizes such a
sum, each of the subexpressions has a semantic value which describes
how it was built up.  The action for this rule should create a similar
sort of value for the newly recognized larger expression.

   For example, here is a rule that says an expression can be the sum of
two subexpressions:

     expr: expr '+' expr   { $$ = $1 + $3; } ;

The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.


File: bison.info,  Node: GLR Parsers,  Next: Locations,  Prev: Semantic Actions,  Up: Concepts

1.5 Writing GLR Parsers
=======================

In some grammars, Bison's deterministic LR(1) parsing algorithm cannot
decide whether to apply a certain grammar rule at a given point.  That
is, it may not be able to decide (on the basis of the input read so
far) which of two possible reductions (applications of a grammar rule)
applies, or whether to apply a reduction or read more of the input and
apply a reduction later in the input.  These are known respectively as
"reduce/reduce" conflicts (*note Reduce/Reduce::), and "shift/reduce"
conflicts (*note Shift/Reduce::).

   To use a grammar that is not easily modified to be LR(1), a more
general parsing algorithm is sometimes necessary.  If you include
`%glr-parser' among the Bison declarations in your file (*note Grammar
Outline::), the result is a Generalized LR (GLR) parser.  These parsers
handle Bison grammars that contain no unresolved conflicts (i.e., after
applying precedence declarations) identically to deterministic parsers.
However, when faced with unresolved shift/reduce and reduce/reduce
conflicts, GLR parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities.  Each of
the resulting parsers can again split, so that at any given time, there
can be any number of possible parses being explored.  The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next.  Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.

   During the time that there are multiple parsers, semantic actions are
recorded, but not performed.  When a parser disappears, its recorded
semantic actions disappear as well, and are never performed.  When a
reduction makes two parsers identical, causing them to merge, Bison
records both sets of semantic actions.  Whenever the last two parsers
merge, reverting to the single-parser case, Bison resolves all the
outstanding actions either by precedences given to the grammar rules
involved, or by performing both actions, and then calling a designated
user-defined function on the resulting values to produce an arbitrary
merged result.

* Menu:

* Simple GLR Parsers::     Using GLR parsers on unambiguous grammars.
* Merging GLR Parses::     Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions::   Deferred semantic actions have special concerns.
* Compiler Requirements::  GLR parsers require a modern C compiler.


File: bison.info,  Node: Simple GLR Parsers,  Next: Merging GLR Parses,  Up: GLR Parsers

1.5.1 Using GLR on Unambiguous Grammars
---------------------------------------

In the simplest cases, you can use the GLR algorithm to parse grammars
that are unambiguous but fail to be LR(1).  Such grammars typically
require more than one symbol of lookahead.

   Consider a problem that arises in the declaration of enumerated and
subrange types in the programming language Pascal.  Here are some
examples:

     type subrange = lo .. hi;
     type enum = (a, b, c);

The original language standard allows only numeric literals and
constant identifiers for the subrange bounds (`lo' and `hi'), but
Extended Pascal (ISO/IEC 10206) and many other Pascal implementations
allow arbitrary expressions there.  This gives rise to the following
situation, containing a superfluous pair of parentheses:

     type subrange = (a) .. b;

Compare this to the following declaration of an enumerated type with
only one value:

     type enum = (a);

(These declarations are contrived, but they are syntactically valid,
and more-complicated cases can come up in practical programs.)

   These two declarations look identical until the `..' token.  With
normal LR(1) one-token lookahead it is not possible to decide between
the two forms when the identifier `a' is parsed.  It is, however,
desirable for a parser to decide this, since in the latter case `a'
must become a new identifier to represent the enumeration value, while
in the former case `a' must be evaluated with its current meaning,
which may be a constant or even a function call.

   You could parse `(a)' as an "unspecified identifier in parentheses",
to be resolved later, but this typically requires substantial
contortions in both semantic actions and large parts of the grammar,
where the parentheses are nested in the recursive rules for expressions.

   You might think of using the lexer to distinguish between the two
forms by returning different tokens for currently defined and undefined
identifiers.  But if these declarations occur in a local scope, and `a'
is defined in an outer scope, then both forms are possible--either
locally redefining `a', or using the value of `a' from the outer scope.
So this approach cannot work.

   A simple solution to this problem is to declare the parser to use
the GLR algorithm.  When the GLR parser reaches the critical state, it
merely splits into two branches and pursues both syntax rules
simultaneously.  Sooner or later, one of them runs into a parsing
error.  If there is a `..' token before the next `;', the rule for
enumerated types fails since it cannot accept `..' anywhere; otherwise,
the subrange type rule fails since it requires a `..' token.  So one of
the branches fails silently, and the other one continues normally,
performing all the intermediate actions that were postponed during the
split.

   If the input is syntactically incorrect, both branches fail and the
parser reports a syntax error as usual.

   The effect of all this is that the parser seems to "guess" the
correct branch to take, or in other words, it seems to use more
lookahead than the underlying LR(1) algorithm actually allows for.  In
this example, LR(2) would suffice, but also some cases that are not
LR(k) for any k can be handled this way.

   In general, a GLR parser can take quadratic or cubic worst-case time,
and the current Bison parser even takes exponential time and space for
some grammars.  In practice, this rarely happens, and for many grammars
it is possible to prove that it cannot happen.  The present example
contains only one conflict between two rules, and the type-declaration
context containing the conflict cannot be nested.  So the number of
branches that can exist at any time is limited by the constant 2, and
the parsing time is still linear.

   Here is a Bison grammar corresponding to the example above.  It
parses a vastly simplified form of Pascal type declarations.

     %token TYPE DOTDOT ID

     %left '+' '-'
     %left '*' '/'

     %%

     type_decl: TYPE ID '=' type ';' ;

     type:
       '(' id_list ')'
     | expr DOTDOT expr
     ;

     id_list:
       ID
     | id_list ',' ID
     ;

     expr:
       '(' expr ')'
     | expr '+' expr
     | expr '-' expr
     | expr '*' expr
     | expr '/' expr
     | ID
     ;

   When used as a normal LR(1) grammar, Bison correctly complains about
one reduce/reduce conflict.  In the conflicting situation the parser
chooses one of the alternatives, arbitrarily the one declared first.
Therefore the following correct input is not recognized:

     type t = (a) .. b;

   The parser can be turned into a GLR parser, while also telling Bison
to be silent about the one known reduce/reduce conflict, by adding
these two declarations to the Bison grammar file (before the first
`%%'):

     %glr-parser
     %expect-rr 1

No change in the grammar itself is required.  Now the parser recognizes
all valid declarations, according to the limited syntax above,
transparently.  In fact, the user does not even notice when the parser
splits.

   So here we have a case where we can use the benefits of GLR, almost
without disadvantages.  Even in simple cases like this, however, there
are at least two potential problems to beware.  First, always analyze
the conflicts reported by Bison to make sure that GLR splitting is only
done where it is intended.  A GLR parser splitting inadvertently may
cause problems less obvious than an LR parser statically choosing the
wrong alternative in a conflict.  Second, consider interactions with
the lexer (*note Semantic Tokens::) with great care.  Since a split
parser consumes tokens without performing any actions during the split,
the lexer cannot obtain information via parser actions.  Some cases of
lexer interactions can be eliminated by using GLR to shift the
complications from the lexer to the parser.  You must check the
remaining cases for correctness.

   In our example, it would be safe for the lexer to return tokens
based on their current meanings in some symbol table, because no new
symbols are defined in the middle of a type declaration.  Though it is
possible for a parser to define the enumeration constants as they are
parsed, before the type declaration is completed, it actually makes no
difference since they cannot be used within the same enumerated type
declaration.


File: bison.info,  Node: Merging GLR Parses,  Next: GLR Semantic Actions,  Prev: Simple GLR Parsers,  Up: GLR Parsers

1.5.2 Using GLR to Resolve Ambiguities
--------------------------------------

Let's consider an example, vastly simplified from a C++ grammar.

     %{
       #include <stdio.h>
       #define YYSTYPE char const *
       int yylex (void);
       void yyerror (char const *);
     %}

     %token TYPENAME ID

     %right '='
     %left '+'

     %glr-parser

     %%

     prog:
       /* Nothing.  */
     | prog stmt   { printf ("\n"); }
     ;

     stmt:
       expr ';'  %dprec 1
     | decl      %dprec 2
     ;

     expr:
       ID               { printf ("%s ", $$); }
     | TYPENAME '(' expr ')'
                        { printf ("%s <cast> ", $1); }
     | expr '+' expr    { printf ("+ "); }
     | expr '=' expr    { printf ("= "); }
     ;

     decl:
       TYPENAME declarator ';'
                        { printf ("%s <declare> ", $1); }
     | TYPENAME declarator '=' expr ';'
                        { printf ("%s <init-declare> ", $1); }
     ;

     declarator:
       ID               { printf ("\"%s\" ", $1); }
     | '(' declarator ')'
     ;

This models a problematic part of the C++ grammar--the ambiguity between
certain declarations and statements.  For example,

     T (x) = y+z;

parses as either an `expr' or a `stmt' (assuming that `T' is recognized
as a `TYPENAME' and `x' as an `ID').  Bison detects this as a
reduce/reduce conflict between the rules `expr : ID' and `declarator :
ID', which it cannot resolve at the time it encounters `x' in the
example above.  Since this is a GLR parser, it therefore splits the
problem into two parses, one for each choice of resolving the
reduce/reduce conflict.  Unlike the example from the previous section
(*note Simple GLR Parsers::), however, neither of these parses "dies,"
because the grammar as it stands is ambiguous.  One of the parsers
eventually reduces `stmt : expr ';'' and the other reduces `stmt :
decl', after which both parsers are in an identical state: they've seen
`prog stmt' and have the same unprocessed input remaining.  We say that
these parses have "merged."

   At this point, the GLR parser requires a specification in the
grammar of how to choose between the competing parses.  In the example
above, the two `%dprec' declarations specify that Bison is to give
precedence to the parse that interprets the example as a `decl', which
implies that `x' is a declarator.  The parser therefore prints

     "x" y z + T <init-declare>

   The `%dprec' declarations only come into play when more than one
parse survives.  Consider a different input string for this parser:

     T (x) + y;

This is another example of using GLR to parse an unambiguous construct,
as shown in the previous section (*note Simple GLR Parsers::).  Here,
there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters `x', it does not have
enough information to resolve the reduce/reduce conflict (again,
between `x' as an `expr' or a `declarator').  In this case, no
precedence declaration is used.  Again, the parser splits into two, one
assuming that `x' is an `expr', and the other assuming `x' is a
`declarator'.  The second of these parsers then vanishes when it sees
`+', and the parser prints

     x T <cast> y +

   Suppose that instead of resolving the ambiguity, you wanted to see
all the possibilities.  For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other.  To do so, you could change the declaration of `stmt' as follows:

     stmt:
       expr ';'  %merge <stmtMerge>
     | decl      %merge <stmtMerge>
     ;

and define the `stmtMerge' function as:

     static YYSTYPE
     stmtMerge (YYSTYPE x0, YYSTYPE x1)
     {
       printf ("<OR> ");
       return "";
     }

with an accompanying forward declaration in the C declarations at the
beginning of the file:

     %{
       #define YYSTYPE char const *
       static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
     %}

With these declarations, the resulting parser parses the first example
as both an `expr' and a `decl', and prints

     "x" y z + T <init-declare> x T <cast> y z + = <OR>

   Bison requires that all of the productions that participate in any
particular merge have identical `%merge' clauses.  Otherwise, the
ambiguity would be unresolvable, and the parser will report an error
during any parse that results in the offending merge.


File: bison.info,  Node: GLR Semantic Actions,  Next: Compiler Requirements,  Prev: Merging GLR Parses,  Up: GLR Parsers

1.5.3 GLR Semantic Actions
--------------------------

By definition, a deferred semantic action is not performed at the same
time as the associated reduction.  This raises caveats for several
Bison features you might use in a semantic action in a GLR parser.

   In any semantic action, you can examine `yychar' to determine the
type of the lookahead token present at the time of the associated
reduction.  After checking that `yychar' is not set to `YYEMPTY' or
`YYEOF', you can then examine `yylval' and `yylloc' to determine the
lookahead token's semantic value and location, if any.  In a
nondeferred semantic action, you can also modify any of these variables
to influence syntax analysis.  *Note Lookahead Tokens: Lookahead.

   In a deferred semantic action, it's too late to influence syntax
analysis.  In this case, `yychar', `yylval', and `yylloc' are set to
shallow copies of the values they had at the time of the associated
reduction.  For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to
change with future versions of Bison.  For example, if a semantic
action might be deferred, you should never write it to invoke
`yyclearin' (*note Action Features::) or to attempt to free memory
referenced by `yylval'.

   Another Bison feature requiring special consideration is `YYERROR'
(*note Action Features::), which you can invoke in a semantic action to
initiate error recovery.  During deterministic GLR operation, the
effect of `YYERROR' is the same as its effect in a deterministic parser.
In a deferred semantic action, its effect is undefined.

   Also, see *note Default Action for Locations: Location Default
Action, which describes a special usage of `YYLLOC_DEFAULT' in GLR
parsers.


File: bison.info,  Node: Compiler Requirements,  Prev: GLR Semantic Actions,  Up: GLR Parsers

1.5.4 Considerations when Compiling GLR Parsers
-----------------------------------------------

The GLR parsers require a compiler for ISO C89 or later.  In addition,
they use the `inline' keyword, which is not C89, but is C99 and is a
common extension in pre-C99 compilers.  It is up to the user of these
parsers to handle portability issues.  For instance, if using Autoconf
and the Autoconf macro `AC_C_INLINE', a mere

     %{
       #include <config.h>
     %}

will suffice.  Otherwise, we suggest

     %{
       #if (__STDC_VERSION__ < 199901 && ! defined __GNUC__ \
            && ! defined inline)
       # define inline
       #endif
     %}


File: bison.info,  Node: Locations,  Next: Bison Parser,  Prev: GLR Parsers,  Up: Concepts

1.6 Locations
=============

Many applications, like interpreters or compilers, have to produce
verbose and useful error messages.  To achieve this, one must be able
to keep track of the "textual location", or "location", of each
syntactic construct.  Bison provides a mechanism for handling these
locations.

   Each token has a semantic value.  In a similar fashion, each token
has an associated location, but the type of locations is the same for
all tokens and groupings.  Moreover, the output parser is equipped with
a default data structure for storing locations (*note Tracking
Locations::, for more details).

   Like semantic values, locations can be reached in actions using a
dedicated set of constructs.  In the example above, the location of the
whole grouping is `@$', while the locations of the subexpressions are
`@1' and `@3'.

   When a rule is matched, a default action is used to compute the
semantic value of its left hand side (*note Actions::).  In the same
way, another default action is used for locations.  However, the action
for locations is general enough for most cases, meaning there is
usually no need to describe for each rule how `@$' should be formed.
When building a new location for a given grouping, the default behavior
of the output parser is to take the beginning of the first symbol, and
the end of the last symbol.


File: bison.info,  Node: Bison Parser,  Next: Stages,  Prev: Locations,  Up: Concepts

1.7 Bison Output: the Parser Implementation File
================================================

When you run Bison, you give it a Bison grammar file as input.  The
most important output is a C source file that implements a parser for
the language described by the grammar.  This parser is called a "Bison
parser", and this file is called a "Bison parser implementation file".
Keep in mind that the Bison utility and the Bison parser are two
distinct programs: the Bison utility is a program whose output is the
Bison parser implementation file that becomes part of your program.

   The job of the Bison parser is to group tokens into groupings
according to the grammar rules--for example, to build identifiers and
operators into expressions.  As it does this, it runs the actions for
the grammar rules it uses.

   The tokens come from a function called the "lexical analyzer" that
you must supply in some fashion (such as by writing it in C).  The Bison
parser calls the lexical analyzer each time it wants a new token.  It
doesn't know what is "inside" the tokens (though their semantic values
may reflect this).  Typically the lexical analyzer makes the tokens by
parsing characters of text, but Bison does not depend on this.  *Note
The Lexical Analyzer Function `yylex': Lexical.

   The Bison parser implementation file is C code which defines a
function named `yyparse' which implements that grammar.  This function
does not make a complete C program: you must supply some additional
functions.  One is the lexical analyzer.  Another is an error-reporting
function which the parser calls to report an error.  In addition, a
complete C program must start with a function called `main'; you have
to provide this, and arrange for it to call `yyparse' or the parser
will never run.  *Note Parser C-Language Interface: Interface.

   Aside from the token type names and the symbols in the actions you
write, all symbols defined in the Bison parser implementation file
itself begin with `yy' or `YY'.  This includes interface functions such
as the lexical analyzer function `yylex', the error reporting function
`yyerror' and the parser function `yyparse' itself.  This also includes
numerous identifiers used for internal purposes.  Therefore, you should
avoid using C identifiers starting with `yy' or `YY' in the Bison
grammar file except for the ones defined in this manual.  Also, you
should avoid using the C identifiers `malloc' and `free' for anything
other than their usual meanings.

   In some cases the Bison parser implementation file includes system
headers, and in those cases your code should respect the identifiers
reserved by those headers.  On some non-GNU hosts, `<alloca.h>',
`<malloc.h>', `<stddef.h>', and `<stdlib.h>' are included as needed to
declare memory allocators and related types.  `<libintl.h>' is included
if message translation is in use (*note Internationalization::).  Other
system headers may be included if you define `YYDEBUG' to a nonzero
value (*note Tracing Your Parser: Tracing.).


File: bison.info,  Node: Stages,  Next: Grammar Layout,  Prev: Bison Parser,  Up: Concepts

1.8 Stages in Using Bison
=========================

The actual language-design process using Bison, from grammar
specification to a working compiler or interpreter, has these parts:

  1. Formally specify the grammar in a form recognized by Bison (*note
     Bison Grammar Files: Grammar File.).  For each grammatical rule in
     the language, describe the action that is to be taken when an
     instance of that rule is recognized.  The action is described by a
     sequence of C statements.

  2. Write a lexical analyzer to process input and pass tokens to the
     parser.  The lexical analyzer may be written by hand in C (*note
     The Lexical Analyzer Function `yylex': Lexical.).  It could also
     be produced using Lex, but the use of Lex is not discussed in this
     manual.

  3. Write a controlling function that calls the Bison-produced parser.

  4. Write error-reporting routines.

   To turn this source code as written into a runnable program, you
must follow these steps:

  1. Run Bison on the grammar to produce the parser.

  2. Compile the code output by Bison, as well as any other source
     files.

  3. Link the object files to produce the finished product.


File: bison.info,  Node: Grammar Layout,  Prev: Stages,  Up: Concepts

1.9 The Overall Layout of a Bison Grammar
=========================================

The input file for the Bison utility is a "Bison grammar file".  The
general form of a Bison grammar file is as follows:

     %{
     PROLOGUE
     %}

     BISON DECLARATIONS

     %%
     GRAMMAR RULES
     %%
     EPILOGUE

The `%%', `%{' and `%}' are punctuation that appears in every Bison
grammar file to separate the sections.

   The prologue may define types and variables used in the actions.
You can also use preprocessor commands to define macros used there, and
use `#include' to include header files that do any of these things.
You need to declare the lexical analyzer `yylex' and the error printer
`yyerror' here, along with any other global identifiers used by the
actions in the grammar rules.

   The Bison declarations declare the names of the terminal and
nonterminal symbols, and may also describe operator precedence and the
data types of semantic values of various symbols.

   The grammar rules define how to construct each nonterminal symbol
from its parts.

   The epilogue can contain any code you want to use.  Often the
definitions of functions declared in the prologue go here.  In a simple
program, all the rest of the program can go here.


File: bison.info,  Node: Examples,  Next: Grammar File,  Prev: Concepts,  Up: Top

2 Examples
**********

Now we show and explain several sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
calculator -- later extended to track "locations" -- and a
multi-function calculator.  All produce usable, though limited,
interactive desk-top calculators.

   These examples are simple, but Bison grammars for real programming
languages are written the same way.  You can copy these examples into a
source file to try them.

* Menu:

* RPN Calc::               Reverse polish notation calculator;
                             a first example with no operator precedence.
* Infix Calc::             Infix (algebraic) notation calculator.
                             Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @N and @$.
* Multi-function Calc::    Calculator with memory and trig functions.
                             It uses multiple data-types for semantic values.
* Exercises::              Ideas for improving the multi-function calculator.


File: bison.info,  Node: RPN Calc,  Next: Infix Calc,  Up: Examples

2.1 Reverse Polish Notation Calculator
======================================

The first example is that of a simple double-precision "reverse polish
notation" calculator (a calculator using postfix operators).  This
example provides a good starting point, since operator precedence is
not an issue.  The second example will illustrate how operator
precedence is handled.

   The source code for this calculator is named `rpcalc.y'.  The `.y'
extension is a convention used for Bison grammar files.

* Menu:

* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer::           The lexical analyzer.
* Rpcalc Main::            The controlling function.
* Rpcalc Error::           The error reporting function.
* Rpcalc Generate::        Running Bison on the grammar file.
* Rpcalc Compile::         Run the C compiler on the output code.


File: bison.info,  Node: Rpcalc Declarations,  Next: Rpcalc Rules,  Up: RPN Calc

2.1.1 Declarations for `rpcalc'
-------------------------------

Here are the C and Bison declarations for the reverse polish notation
calculator.  As in C, comments are placed between `/*...*/'.

     /* Reverse polish notation calculator.  */

     %{
       #define YYSTYPE double
       #include <math.h>
       int yylex (void);
       void yyerror (char const *);
     %}

     %token NUM

     %% /* Grammar rules and actions follow.  */

   The declarations section (*note The prologue: Prologue.) contains two
preprocessor directives and two forward declarations.

   The `#define' directive defines the macro `YYSTYPE', thus specifying
the C data type for semantic values of both tokens and groupings (*note
Data Types of Semantic Values: Value Type.).  The Bison parser will use
whatever type `YYSTYPE' is defined as; if you don't define it, `int' is
the default.  Because we specify `double', each token and each
expression has an associated value, which is a floating point number.

   The `#include' directive is used to declare the exponentiation
function `pow'.

   The forward declarations for `yylex' and `yyerror' are needed
because the C language requires that functions be declared before they
are used.  These functions will be defined in the epilogue, but the
parser calls them so they must be declared in the prologue.

   The second section, Bison declarations, provides information to Bison
about the token types (*note The Bison Declarations Section: Bison
Declarations.).  Each terminal symbol that is not a single-character
literal must be declared here.  (Single-character literals normally
don't need to be declared.)  In this example, all the arithmetic
operators are designated by single-character literals, so the only
terminal symbol that needs to be declared is `NUM', the token type for
numeric constants.


File: bison.info,  Node: Rpcalc Rules,  Next: Rpcalc Lexer,  Prev: Rpcalc Declarations,  Up: RPN Calc

2.1.2 Grammar Rules for `rpcalc'
--------------------------------

Here are the grammar rules for the reverse polish notation calculator.

     input:
       /* empty */
     | input line
     ;

     line:
       '\n'
     | exp '\n'      { printf ("%.10g\n", $1); }
     ;

     exp:
       NUM           { $$ = $1;           }
     | exp exp '+'   { $$ = $1 + $2;      }
     | exp exp '-'   { $$ = $1 - $2;      }
     | exp exp '*'   { $$ = $1 * $2;      }
     | exp exp '/'   { $$ = $1 / $2;      }
     | exp exp '^'   { $$ = pow ($1, $2); }  /* Exponentiation */
     | exp 'n'       { $$ = -$1;          }  /* Unary minus    */
     ;
     %%

   The groupings of the rpcalc "language" defined here are the
expression (given the name `exp'), the line of input (`line'), and the
complete input transcript (`input').  Each of these nonterminal symbols
has several alternate rules, joined by the vertical bar `|' which is
read as "or".  The following sections explain what these rules mean.

   The semantics of the language is determined by the actions taken
when a grouping is recognized.  The actions are the C code that appears
inside braces.  *Note Actions::.

   You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules.  In each action, the
pseudo-variable `$$' stands for the semantic value for the grouping
that the rule is going to construct.  Assigning a value to `$$' is the
main job of most actions.  The semantic values of the components of the
rule are referred to as `$1', `$2', and so on.

* Menu:

* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::


File: bison.info,  Node: Rpcalc Input,  Next: Rpcalc Line,  Up: Rpcalc Rules

2.1.2.1 Explanation of `input'
..............................

Consider the definition of `input':

     input:
       /* empty */
     | input line
     ;

   This definition reads as follows: "A complete input is either an
empty string, or a complete input followed by an input line".  Notice
that "complete input" is defined in terms of itself.  This definition
is said to be "left recursive" since `input' appears always as the
leftmost symbol in the sequence.  *Note Recursive Rules: Recursion.

   The first alternative is empty because there are no symbols between
the colon and the first `|'; this means that `input' can match an empty
string of input (no tokens).  We write the rules this way because it is
legitimate to type `Ctrl-d' right after you start the calculator.  It's
conventional to put an empty alternative first and write the comment
`/* empty */' in it.

   The second alternate rule (`input line') handles all nontrivial
input.  It means, "After reading any number of lines, read one more
line if possible."  The left recursion makes this rule into a loop.
Since the first alternative matches empty input, the loop can be
executed zero or more times.

   The parser function `yyparse' continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.


File: bison.info,  Node: Rpcalc Line,  Next: Rpcalc Expr,  Prev: Rpcalc Input,  Up: Rpcalc Rules

2.1.2.2 Explanation of `line'
.............................

Now consider the definition of `line':

     line:
       '\n'
     | exp '\n'  { printf ("%.10g\n", $1); }
     ;

   The first alternative is a token which is a newline character; this
means that rpcalc accepts a blank line (and ignores it, since there is
no action).  The second alternative is an expression followed by a
newline.  This is the alternative that makes rpcalc useful.  The
semantic value of the `exp' grouping is the value of `$1' because the
`exp' in question is the first symbol in the alternative.  The action
prints this value, which is the result of the computation the user
asked for.

   This action is unusual because it does not assign a value to `$$'.
As a consequence, the semantic value associated with the `line' is
uninitialized (its value will be unpredictable).  This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed
the value of the user's input line, that value is no longer needed.


File: bison.info,  Node: Rpcalc Expr,  Prev: Rpcalc Line,  Up: Rpcalc Rules

2.1.2.3 Explanation of `expr'
.............................

The `exp' grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just
numbers.  The second handles an addition-expression, which looks like
two expressions followed by a plus-sign.  The third handles
subtraction, and so on.

     exp:
       NUM
     | exp exp '+'     { $$ = $1 + $2;    }
     | exp exp '-'     { $$ = $1 - $2;    }
     ...
     ;

   We have used `|' to join all the rules for `exp', but we could
equally well have written them separately:

     exp: NUM ;
     exp: exp exp '+'     { $$ = $1 + $2; };
     exp: exp exp '-'     { $$ = $1 - $2; };
     ...

   Most of the rules have actions that compute the value of the
expression in terms of the value of its parts.  For example, in the
rule for addition, `$1' refers to the first component `exp' and `$2'
refers to the second one.  The third component, `'+'', has no meaningful
associated semantic value, but if it had one you could refer to it as
`$3'.  When `yyparse' recognizes a sum expression using this rule, the
sum of the two subexpressions' values is produced as the value of the
entire expression.  *Note Actions::.

   You don't have to give an action for every rule.  When a rule has no
action, Bison by default copies the value of `$1' into `$$'.  This is
what happens in the first rule (the one that uses `NUM').

   The formatting shown here is the recommended convention, but Bison
does not require it.  You can add or change white space as much as you
wish.  For example, this:

     exp: NUM | exp exp '+' {$$ = $1 + $2; } | ... ;

means the same thing as this:

     exp:
       NUM
     | exp exp '+'    { $$ = $1 + $2; }
     | ...
     ;

The latter, however, is much more readable.


File: bison.info,  Node: Rpcalc Lexer,  Next: Rpcalc Main,  Prev: Rpcalc Rules,  Up: RPN Calc

2.1.3 The `rpcalc' Lexical Analyzer
-----------------------------------

The lexical analyzer's job is low-level parsing: converting characters
or sequences of characters into tokens.  The Bison parser gets its
tokens by calling the lexical analyzer.  *Note The Lexical Analyzer
Function `yylex': Lexical.

   Only a simple lexical analyzer is needed for the RPN calculator.
This lexical analyzer skips blanks and tabs, then reads in numbers as
`double' and returns them as `NUM' tokens.  Any other character that
isn't part of a number is a separate token.  Note that the token-code
for such a single-character token is the character itself.

   The return value of the lexical analyzer function is a numeric code
which represents a token type.  The same text used in Bison rules to
stand for this token type is also a C expression for the numeric code
for the type.  This works in two ways.  If the token type is a
character literal, then its numeric code is that of the character; you
can use the same character literal in the lexical analyzer to express
the number.  If the token type is an identifier, that identifier is
defined by Bison as a C macro whose definition is the appropriate
number.  In this example, therefore, `NUM' becomes a macro for `yylex'
to use.

   The semantic value of the token (if it has one) is stored into the
global variable `yylval', which is where the Bison parser will look for
it.  (The C data type of `yylval' is `YYSTYPE', which was defined at
the beginning of the grammar; *note Declarations for `rpcalc': Rpcalc
Declarations.)

   A token type code of zero is returned if the end-of-input is
encountered.  (Bison recognizes any nonpositive value as indicating
end-of-input.)

   Here is the code for the lexical analyzer:

     /* The lexical analyzer returns a double floating point
        number on the stack and the token NUM, or the numeric code
        of the character read if not a number.  It skips all blanks
        and tabs, and returns 0 for end-of-input.  */

     #include <ctype.h>

     int
     yylex (void)
     {
       int c;

       /* Skip white space.  */
       while ((c = getchar ()) == ' ' || c == '\t')
         continue;
       /* Process numbers.  */
       if (c == '.' || isdigit (c))
         {
           ungetc (c, stdin);
           scanf ("%lf", &yylval);
           return NUM;
         }
       /* Return end-of-input.  */
       if (c == EOF)
         return 0;
       /* Return a single char.  */
       return c;
     }


File: bison.info,  Node: Rpcalc Main,  Next: Rpcalc Error,  Prev: Rpcalc Lexer,  Up: RPN Calc

2.1.4 The Controlling Function
------------------------------

In keeping with the spirit of this example, the controlling function is
kept to the bare minimum.  The only requirement is that it call
`yyparse' to start the process of parsing.

     int
     main (void)
     {
       return yyparse ();
     }


File: bison.info,  Node: Rpcalc Error,  Next: Rpcalc Generate,  Prev: Rpcalc Main,  Up: RPN Calc

2.1.5 The Error Reporting Routine
---------------------------------

When `yyparse' detects a syntax error, it calls the error reporting
function `yyerror' to print an error message (usually but not always
`"syntax error"').  It is up to the programmer to supply `yyerror'
(*note Parser C-Language Interface: Interface.), so here is the
definition we will use:

     #include <stdio.h>

     /* Called by yyparse on error.  */
     void
     yyerror (char const *s)
     {
       fprintf (stderr, "%s\n", s);
     }

   After `yyerror' returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(*note Error Recovery::).  Otherwise, `yyparse' returns nonzero.  We
have not written any error rules in this example, so any invalid input
will cause the calculator program to exit.  This is not clean behavior
for a real calculator, but it is adequate for the first example.


File: bison.info,  Node: Rpcalc Generate,  Next: Rpcalc Compile,  Prev: Rpcalc Error,  Up: RPN Calc

2.1.6 Running Bison to Make the Parser
--------------------------------------

Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files.  For such a
simple example, the easiest thing is to put everything in one file, the
grammar file.  The definitions of `yylex', `yyerror' and `main' go at
the end, in the epilogue of the grammar file (*note The Overall Layout
of a Bison Grammar: Grammar Layout.).

   For a large project, you would probably have several source files,
and use `make' to arrange to recompile them.

   With all the source in the grammar file, you use the following
command to convert it into a parser implementation file:

     bison FILE.y

In this example, the grammar file is called `rpcalc.y' (for "Reverse
Polish CALCulator").  Bison produces a parser implementation file named
`FILE.tab.c', removing the `.y' from the grammar file name.  The parser
implementation file contains the source code for `yyparse'.  The
additional functions in the grammar file (`yylex', `yyerror' and
`main') are copied verbatim to the parser implementation file.


File: bison.info,  Node: Rpcalc Compile,  Prev: Rpcalc Generate,  Up: RPN Calc

2.1.7 Compiling the Parser Implementation File
----------------------------------------------

Here is how to compile and run the parser implementation file:

     # List files in current directory.
     $ ls
     rpcalc.tab.c  rpcalc.y

     # Compile the Bison parser.
     # `-lm' tells compiler to search math library for `pow'.
     $ cc -lm -o rpcalc rpcalc.tab.c

     # List files again.
     $ ls
     rpcalc  rpcalc.tab.c  rpcalc.y

   The file `rpcalc' now contains the executable code.  Here is an
example session using `rpcalc'.

     $ rpcalc
     4 9 +
     13
     3 7 + 3 4 5 *+-
     -13
     3 7 + 3 4 5 * + - n              Note the unary minus, `n'
     13
     5 6 / 4 n +
     -3.166666667
     3 4 ^                            Exponentiation
     81
     ^D                               End-of-file indicator
     $


File: bison.info,  Node: Infix Calc,  Next: Simple Error Recovery,  Prev: RPN Calc,  Up: Examples

2.2 Infix Notation Calculator: `calc'
=====================================

We now modify rpcalc to handle infix operators instead of postfix.
Infix notation involves the concept of operator precedence and the need
for parentheses nested to arbitrary depth.  Here is the Bison code for
`calc.y', an infix desk-top calculator.

     /* Infix notation calculator.  */

     %{
       #define YYSTYPE double
       #include <math.h>
       #include <stdio.h>
       int yylex (void);
       void yyerror (char const *);
     %}

     /* Bison declarations.  */
     %token NUM
     %left '-' '+'
     %left '*' '/'
     %left NEG     /* negation--unary minus */
     %right '^'    /* exponentiation */

     %% /* The grammar follows.  */
     input:
       /* empty */
     | input line
     ;

     line:
       '\n'
     | exp '\n'  { printf ("\t%.10g\n", $1); }
     ;

     exp:
       NUM                { $$ = $1;           }
     | exp '+' exp        { $$ = $1 + $3;      }
     | exp '-' exp        { $$ = $1 - $3;      }
     | exp '*' exp        { $$ = $1 * $3;      }
     | exp '/' exp        { $$ = $1 / $3;      }
     | '-' exp  %prec NEG { $$ = -$2;          }
     | exp '^' exp        { $$ = pow ($1, $3); }
     | '(' exp ')'        { $$ = $2;           }
     ;
     %%

The functions `yylex', `yyerror' and `main' can be the same as before.

   There are two important new features shown in this code.

   In the second section (Bison declarations), `%left' declares token
types and says they are left-associative operators.  The declarations
`%left' and `%right' (right associativity) take the place of `%token'
which is used to declare a token type name without associativity.
(These tokens are single-character literals, which ordinarily don't
need to be declared.  We declare them here to specify the
associativity.)

   Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence.  Hence, exponentiation
has the highest precedence, unary minus (`NEG') is next, followed by
`*' and `/', and so on.  *Note Operator Precedence: Precedence.

   The other important new feature is the `%prec' in the grammar
section for the unary minus operator.  The `%prec' simply instructs
Bison that the rule `| '-' exp' has the same precedence as `NEG'--in
this case the next-to-highest.  *Note Context-Dependent Precedence:
Contextual Precedence.

   Here is a sample run of `calc.y':

     $ calc
     4 + 4.5 - (34/(8*3+-3))
     6.880952381
     -56 + 2
     -54
     3 ^ 2
     9


File: bison.info,  Node: Simple Error Recovery,  Next: Location Tracking Calc,  Prev: Infix Calc,  Up: Examples

2.3 Simple Error Recovery
=========================

Up to this point, this manual has not addressed the issue of "error
recovery"--how to continue parsing after the parser detects a syntax
error.  All we have handled is error reporting with `yyerror'.  Recall
that by default `yyparse' returns after calling `yyerror'.  This means
that an erroneous input line causes the calculator program to exit.
Now we show how to rectify this deficiency.

   The Bison language itself includes the reserved word `error', which
may be included in the grammar rules.  In the example below it has been
added to one of the alternatives for `line':

     line:
       '\n'
     | exp '\n'   { printf ("\t%.10g\n", $1); }
     | error '\n' { yyerrok;                  }
     ;

   This addition to the grammar allows for simple error recovery in the
event of a syntax error.  If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for `line', and
parsing will continue.  (The `yyerror' function is still called upon to
print its message as well.)  The action executes the statement
`yyerrok', a macro defined automatically by Bison; its meaning is that
error recovery is complete (*note Error Recovery::).  Note the
difference between `yyerrok' and `yyerror'; neither one is a misprint.

   This form of error recovery deals with syntax errors.  There are
other kinds of errors; for example, division by zero, which raises an
exception signal that is normally fatal.  A real calculator program
must handle this signal and use `longjmp' to return to `main' and
resume parsing input lines; it would also have to discard the rest of
the current line of input.  We won't discuss this issue further because
it is not specific to Bison programs.


File: bison.info,  Node: Location Tracking Calc,  Next: Multi-function Calc,  Prev: Simple Error Recovery,  Up: Examples

2.4 Location Tracking Calculator: `ltcalc'
==========================================

This example extends the infix notation calculator with location
tracking.  This feature will be used to improve the error messages.  For
the sake of clarity, this example is a simple integer calculator, since
most of the work needed to use locations will be done in the lexical
analyzer.

* Menu:

* Ltcalc Declarations::    Bison and C declarations for ltcalc.
* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer::           The lexical analyzer.


File: bison.info,  Node: Ltcalc Declarations,  Next: Ltcalc Rules,  Up: Location Tracking Calc

2.4.1 Declarations for `ltcalc'
-------------------------------

The C and Bison declarations for the location tracking calculator are
the same as the declarations for the infix notation calculator.

     /* Location tracking calculator.  */

     %{
       #define YYSTYPE int
       #include <math.h>
       int yylex (void);
       void yyerror (char const *);
     %}

     /* Bison declarations.  */
     %token NUM

     %left '-' '+'
     %left '*' '/'
     %left NEG
     %right '^'

     %% /* The grammar follows.  */

Note there are no declarations specific to locations.  Defining a data
type for storing locations is not needed: we will use the type provided
by default (*note Data Types of Locations: Location Type.), which is a
four member structure with the following integer fields: `first_line',
`first_column', `last_line' and `last_column'.  By conventions, and in
accordance with the GNU Coding Standards and common practice, the line
and column count both start at 1.


File: bison.info,  Node: Ltcalc Rules,  Next: Ltcalc Lexer,  Prev: Ltcalc Declarations,  Up: Location Tracking Calc

2.4.2 Grammar Rules for `ltcalc'
--------------------------------

Whether handling locations or not has no effect on the syntax of your
language.  Therefore, grammar rules for this example will be very close
to those of the previous example: we will only modify them to benefit
from the new information.

   Here, we will use locations to report divisions by zero, and locate
the wrong expressions or subexpressions.

     input:
       /* empty */
     | input line
     ;

     line:
       '\n'
     | exp '\n' { printf ("%d\n", $1); }
     ;

     exp:
       NUM           { $$ = $1; }
     | exp '+' exp   { $$ = $1 + $3; }
     | exp '-' exp   { $$ = $1 - $3; }
     | exp '*' exp   { $$ = $1 * $3; }
     | exp '/' exp
         {
           if ($3)
             $$ = $1 / $3;
           else
             {
               $$ = 1;
               fprintf (stderr, "%d.%d-%d.%d: division by zero",
                        @3.first_line, @3.first_column,
                        @3.last_line, @3.last_column);
             }
         }
     | '-' exp %prec NEG     { $$ = -$2; }
     | exp '^' exp           { $$ = pow ($1, $3); }
     | '(' exp ')'           { $$ = $2; }

   This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables `@N' for rule components, and the
pseudo-variable `@$' for groupings.

   We don't need to assign a value to `@$': the output parser does it
automatically.  By default, before executing the C code of each action,
`@$' is set to range from the beginning of `@1' to the end of `@N', for
a rule with N components.  This behavior can be redefined (*note
Default Action for Locations: Location Default Action.), and for very
specific rules, `@$' can be computed by hand.


File: bison.info,  Node: Ltcalc Lexer,  Prev: Ltcalc Rules,  Up: Location Tracking Calc

2.4.3 The `ltcalc' Lexical Analyzer.
------------------------------------

Until now, we relied on Bison's defaults to enable location tracking.
The next step is to rewrite the lexical analyzer, and make it able to
feed the parser with the token locations, as it already does for
semantic values.

   To this end, we must take into account every single character of the
input text, to avoid the computed locations of being fuzzy or wrong:

     int
     yylex (void)
     {
       int c;

       /* Skip white space.  */
       while ((c = getchar ()) == ' ' || c == '\t')
         ++yylloc.last_column;

       /* Step.  */
       yylloc.first_line = yylloc.last_line;
       yylloc.first_column = yylloc.last_column;

       /* Process numbers.  */
       if (isdigit (c))
         {
           yylval = c - '0';
           ++yylloc.last_column;
           while (isdigit (c = getchar ()))
             {
               ++yylloc.last_column;
               yylval = yylval * 10 + c - '0';
             }
           ungetc (c, stdin);
           return NUM;
         }

       /* Return end-of-input.  */
       if (c == EOF)
         return 0;

       /* Return a single char, and update location.  */
       if (c == '\n')
         {
           ++yylloc.last_line;
           yylloc.last_column = 0;
         }
       else
         ++yylloc.last_column;
       return c;
     }

   Basically, the lexical analyzer performs the same processing as
before: it skips blanks and tabs, and reads numbers or single-character
tokens.  In addition, it updates `yylloc', the global variable (of type
`YYLTYPE') containing the token's location.

   Now, each time this function returns a token, the parser has its
number as well as its semantic value, and its location in the text.
The last needed change is to initialize `yylloc', for example in the
controlling function:

     int
     main (void)
     {
       yylloc.first_line = yylloc.last_line = 1;
       yylloc.first_column = yylloc.last_column = 0;
       return yyparse ();
     }

   Remember that computing locations is not a matter of syntax.  Every
character must be associated to a location update, whether it is in
valid input, in comments, in literal strings, and so on.


File: bison.info,  Node: Multi-function Calc,  Next: Exercises,  Prev: Location Tracking Calc,  Up: Examples

2.5 Multi-Function Calculator: `mfcalc'
=======================================

Now that the basics of Bison have been discussed, it is time to move on
to a more advanced problem.  The above calculators provided only five
functions, `+', `-', `*', `/' and `^'.  It would be nice to have a
calculator that provides other mathematical functions such as `sin',
`cos', etc.

   It is easy to add new operators to the infix calculator as long as
they are only single-character literals.  The lexical analyzer `yylex'
passes back all nonnumeric characters as tokens, so new grammar rules
suffice for adding a new operator.  But we want something more
flexible: built-in functions whose syntax has this form:

     FUNCTION_NAME (ARGUMENT)

At the same time, we will add memory to the calculator, by allowing you
to create named variables, store values in them, and use them later.
Here is a sample session with the multi-function calculator:

     $ mfcalc
     pi = 3.141592653589
     3.1415926536
     sin(pi)
     0.0000000000
     alpha = beta1 = 2.3
     2.3000000000
     alpha
     2.3000000000
     ln(alpha)
     0.8329091229
     exp(ln(beta1))
     2.3000000000
     $

   Note that multiple assignment and nested function calls are
permitted.

* Menu:

* Mfcalc Declarations::    Bison declarations for multi-function calculator.
* Mfcalc Rules::           Grammar rules for the calculator.
* Mfcalc Symbol Table::    Symbol table management subroutines.


File: bison.info,  Node: Mfcalc Declarations,  Next: Mfcalc Rules,  Up: Multi-function Calc

2.5.1 Declarations for `mfcalc'
-------------------------------

Here are the C and Bison declarations for the multi-function calculator.

     %{
       #include <math.h>  /* For math functions, cos(), sin(), etc.  */
       #include "calc.h"  /* Contains definition of `symrec'.  */
       int yylex (void);
       void yyerror (char const *);
     %}

     %union {
       double    val;   /* For returning numbers.  */
       symrec  *tptr;   /* For returning symbol-table pointers.  */
     }
     %token <val>  NUM        /* Simple double precision number.  */
     %token <tptr> VAR FNCT   /* Variable and function.  */
     %type  <val>  exp

     %right '='
     %left '-' '+'
     %left '*' '/'
     %left NEG     /* negation--unary minus */
     %right '^'    /* exponentiation */

   The above grammar introduces only two new features of the Bison
language.  These features allow semantic values to have various data
types (*note More Than One Value Type: Multiple Types.).

   The `%union' declaration specifies the entire list of possible types;
this is instead of defining `YYSTYPE'.  The allowable types are now
double-floats (for `exp' and `NUM') and pointers to entries in the
symbol table.  *Note The Collection of Value Types: Union Decl.

   Since values can now have various types, it is necessary to
associate a type with each grammar symbol whose semantic value is used.
These symbols are `NUM', `VAR', `FNCT', and `exp'.  Their declarations
are augmented with information about their data type (placed between
angle brackets).

   The Bison construct `%type' is used for declaring nonterminal
symbols, just as `%token' is used for declaring token types.  We have
not used `%type' before because nonterminal symbols are normally
declared implicitly by the rules that define them.  But `exp' must be
declared explicitly so we can specify its value type.  *Note
Nonterminal Symbols: Type Decl.


File: bison.info,  Node: Mfcalc Rules,  Next: Mfcalc Symbol Table,  Prev: Mfcalc Declarations,  Up: Multi-function Calc

2.5.2 Grammar Rules for `mfcalc'
--------------------------------

Here are the grammar rules for the multi-function calculator.  Most of
them are copied directly from `calc'; three rules, those which mention
`VAR' or `FNCT', are new.

     %% /* The grammar follows.  */
     input:
       /* empty */
     | input line
     ;

     line:
       '\n'
     | exp '\n'   { printf ("%.10g\n", $1); }
     | error '\n' { yyerrok;                }
     ;

     exp:
       NUM                { $$ = $1;                         }
     | VAR                { $$ = $1->value.var;              }
     | VAR '=' exp        { $$ = $3; $1->value.var = $3;     }
     | FNCT '(' exp ')'   { $$ = (*($1->value.fnctptr))($3); }
     | exp '+' exp        { $$ = $1 + $3;                    }
     | exp '-' exp        { $$ = $1 - $3;                    }
     | exp '*' exp        { $$ = $1 * $3;                    }
     | exp '/' exp        { $$ = $1 / $3;                    }
     | '-' exp  %prec NEG { $$ = -$2;                        }
     | exp '^' exp        { $$ = pow ($1, $3);               }
     | '(' exp ')'        { $$ = $2;                         }
     ;
     /* End of grammar.  */
     %%


File: bison.info,  Node: Mfcalc Symbol Table,  Prev: Mfcalc Rules,  Up: Multi-function Calc

2.5.3 The `mfcalc' Symbol Table
-------------------------------

The multi-function calculator requires a symbol table to keep track of
the names and meanings of variables and functions.  This doesn't affect
the grammar rules (except for the actions) or the Bison declarations,
but it requires some additional C functions for support.

   The symbol table itself consists of a linked list of records.  Its
definition, which is kept in the header `calc.h', is as follows.  It
provides for either functions or variables to be placed in the table.

     /* Function type.  */
     typedef double (*func_t) (double);

     /* Data type for links in the chain of symbols.  */
     struct symrec
     {
       char *name;  /* name of symbol */
       int type;    /* type of symbol: either VAR or FNCT */
       union
       {
         double var;      /* value of a VAR */
         func_t fnctptr;  /* value of a FNCT */
       } value;
       struct symrec *next;  /* link field */
     };

     typedef struct symrec symrec;

     /* The symbol table: a chain of `struct symrec'.  */
     extern symrec *sym_table;

     symrec *putsym (char const *, int);
     symrec *getsym (char const *);

   The new version of `main' includes a call to `init_table', a
function that initializes the symbol table.  Here it is, and
`init_table' as well:

     #include <stdio.h>

     /* Called by yyparse on error.  */
     void
     yyerror (char const *s)
     {
       fprintf (stderr, "%s\n", s);
     }

     struct init
     {
       char const *fname;
       double (*fnct) (double);
     };

     struct init const arith_fncts[] =
     {
       "sin",  sin,
       "cos",  cos,
       "atan", atan,
       "ln",   log,
       "exp",  exp,
       "sqrt", sqrt,
       0, 0
     };

     /* The symbol table: a chain of `struct symrec'.  */
     symrec *sym_table;

     /* Put arithmetic functions in table.  */
     void
     init_table (void)
     {
       int i;
       for (i = 0; arith_fncts[i].fname != 0; i++)
         {
           symrec *ptr = putsym (arith_fncts[i].fname, FNCT);
           ptr->value.fnctptr = arith_fncts[i].fnct;
         }
     }

     int
     main (void)
     {
       init_table ();
       return yyparse ();
     }

   By simply editing the initialization list and adding the necessary
include files, you can add additional functions to the calculator.

   Two important functions allow look-up and installation of symbols in
the symbol table.  The function `putsym' is passed a name and the type
(`VAR' or `FNCT') of the object to be installed.  The object is linked
to the front of the list, and a pointer to the object is returned.  The
function `getsym' is passed the name of the symbol to look up.  If
found, a pointer to that symbol is returned; otherwise zero is returned.

     #include <stdlib.h> /* malloc. */
     #include <string.h> /* strlen. */

     symrec *
     putsym (char const *sym_name, int sym_type)
     {
       symrec *ptr = (symrec *) malloc (sizeof (symrec));
       ptr->name = (char *) malloc (strlen (sym_name) + 1);
       strcpy (ptr->name,sym_name);
       ptr->type = sym_type;
       ptr->value.var = 0; /* Set value to 0 even if fctn.  */
       ptr->next = (struct symrec *)sym_table;
       sym_table = ptr;
       return ptr;
     }

     symrec *
     getsym (char const *sym_name)
     {
       symrec *ptr;
       for (ptr = sym_table; ptr != (symrec *) 0;
            ptr = (symrec *)ptr->next)
         if (strcmp (ptr->name,sym_name) == 0)
           return ptr;
       return 0;
     }

   The function `yylex' must now recognize variables, numeric values,
and the single-character arithmetic operators.  Strings of alphanumeric
characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.

   The string is passed to `getsym' for look up in the symbol table.  If
the name appears in the table, a pointer to its location and its type
(`VAR' or `FNCT') is returned to `yyparse'.  If it is not already in
the table, then it is installed as a `VAR' using `putsym'.  Again, a
pointer and its type (which must be `VAR') is returned to `yyparse'.

   No change is needed in the handling of numeric values and arithmetic
operators in `yylex'.

     #include <ctype.h>

     int
     yylex (void)
     {
       int c;

       /* Ignore white space, get first nonwhite character.  */
       while ((c = getchar ()) == ' ' || c == '\t')
         continue;

       if (c == EOF)
         return 0;

       /* Char starts a number => parse the number.         */
       if (c == '.' || isdigit (c))
         {
           ungetc (c, stdin);
           scanf ("%lf", &yylval.val);
           return NUM;
         }

       /* Char starts an identifier => read the name.       */
       if (isalpha (c))
         {
           /* Initially make the buffer long enough
              for a 40-character symbol name.  */
           static size_t length = 40;
           static char *symbuf = 0;
           symrec *s;
           int i;

           if (!symbuf)
             symbuf = (char *) malloc (length + 1);

           i = 0;
           do
             {
               /* If buffer is full, make it bigger.        */
               if (i == length)
                 {
                   length *= 2;
                   symbuf = (char *) realloc (symbuf, length + 1);
                 }
               /* Add this character to the buffer.         */
               symbuf[i++] = c;
               /* Get another character.                    */
               c = getchar ();
             }
           while (isalnum (c));

           ungetc (c, stdin);
           symbuf[i] = '\0';

           s = getsym (symbuf);
           if (s == 0)
             s = putsym (symbuf, VAR);
           yylval.tptr = s;
           return s->type;
         }

       /* Any other character is a token by itself.        */
       return c;
     }

   The error reporting function is unchanged, and the new version of
`main' includes a call to `init_table' and sets the `yydebug' on user
demand (*Note Tracing Your Parser: Tracing, for details):

     /* Called by yyparse on error.  */
     void
     yyerror (char const *s)
     {
       fprintf (stderr, "%s\n", s);
     }

     int
     main (int argc, char const* argv[])
     {
       int i;
       /* Enable parse traces on option -p.  */
       for (i = 1; i < argc; ++i)
         if (!strcmp(argv[i], "-p"))
           yydebug = 1;
       init_table ();
       return yyparse ();
     }

   This program is both powerful and flexible.  You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as `pi' or `e' as well.


File: bison.info,  Node: Exercises,  Prev: Multi-function Calc,  Up: Examples

2.6 Exercises
=============

  1. Add some new functions from `math.h' to the initialization list.

  2. Add another array that contains constants and their values.  Then
     modify `init_table' to add these constants to the symbol table.
     It will be easiest to give the constants type `VAR'.

  3. Make the program report an error if the user refers to an
     uninitialized variable in any way except to store a value in it.


File: bison.info,  Node: Grammar File,  Next: Interface,  Prev: Examples,  Up: Top

3 Bison Grammar Files
*********************

Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.

   The Bison grammar file conventionally has a name ending in `.y'.
*Note Invoking Bison: Invocation.

* Menu:

* Grammar Outline::    Overall layout of the grammar file.
* Symbols::            Terminal and nonterminal symbols.
* Rules::              How to write grammar rules.
* Recursion::          Writing recursive rules.
* Semantics::          Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References::   Using named references in actions.
* Declarations::       All kinds of Bison declarations are described here.
* Multiple Parsers::   Putting more than one Bison parser in one program.


File: bison.info,  Node: Grammar Outline,  Next: Symbols,  Up: Grammar File

3.1 Outline of a Bison Grammar
==============================

A Bison grammar file has four main sections, shown here with the
appropriate delimiters:

     %{
       PROLOGUE
     %}

     BISON DECLARATIONS

     %%
     GRAMMAR RULES
     %%

     EPILOGUE

   Comments enclosed in `/* ... */' may appear in any of the sections.
As a GNU extension, `//' introduces a comment that continues until end
of line.

* Menu:

* Prologue::              Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations::    Syntax and usage of the Bison declarations section.
* Grammar Rules::         Syntax and usage of the grammar rules section.
* Epilogue::              Syntax and usage of the epilogue.


File: bison.info,  Node: Prologue,  Next: Prologue Alternatives,  Up: Grammar Outline

3.1.1 The prologue
------------------

The PROLOGUE section contains macro definitions and declarations of
functions and variables that are used in the actions in the grammar
rules.  These are copied to the beginning of the parser implementation
file so that they precede the definition of `yyparse'.  You can use
`#include' to get the declarations from a header file.  If you don't
need any C declarations, you may omit the `%{' and `%}' delimiters that
bracket this section.

   The PROLOGUE section is terminated by the first occurrence of `%}'
that is outside a comment, a string literal, or a character constant.

   You may have more than one PROLOGUE section, intermixed with the
BISON DECLARATIONS.  This allows you to have C and Bison declarations
that refer to each other.  For example, the `%union' declaration may
use types defined in a header file, and you may wish to prototype
functions that take arguments of type `YYSTYPE'.  This can be done with
two PROLOGUE blocks, one before and one after the `%union' declaration.

     %{
       #define _GNU_SOURCE
       #include <stdio.h>
       #include "ptypes.h"
     %}

     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %{
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
     %}

     ...

   When in doubt, it is usually safer to put prologue code before all
Bison declarations, rather than after.  For example, any definitions of
feature test macros like `_GNU_SOURCE' or `_POSIX_C_SOURCE' should
appear before all Bison declarations, as feature test macros can affect
the behavior of Bison-generated `#include' directives.


File: bison.info,  Node: Prologue Alternatives,  Next: Bison Declarations,  Prev: Prologue,  Up: Grammar Outline

3.1.2 Prologue Alternatives
---------------------------

The functionality of PROLOGUE sections can often be subtle and
inflexible.  As an alternative, Bison provides a `%code' directive with
an explicit qualifier field, which identifies the purpose of the code
and thus the location(s) where Bison should generate it.  For C/C++,
the qualifier can be omitted for the default location, or it can be one
of `requires', `provides', `top'.  *Note %code Summary::.

   Look again at the example of the previous section:

     %{
       #define _GNU_SOURCE
       #include <stdio.h>
       #include "ptypes.h"
     %}

     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %{
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
     %}

     ...

Notice that there are two PROLOGUE sections here, but there's a subtle
distinction between their functionality.  For example, if you decide to
override Bison's default definition for `YYLTYPE', in which PROLOGUE
section should you write your new definition?  You should write it in
the first since Bison will insert that code into the parser
implementation file _before_ the default `YYLTYPE' definition.  In
which PROLOGUE section should you prototype an internal function,
`trace_token', that accepts `YYLTYPE' and `yytokentype' as arguments?
You should prototype it in the second since Bison will insert that code
_after_ the `YYLTYPE' and `yytokentype' definitions.

   This distinction in functionality between the two PROLOGUE sections
is established by the appearance of the `%union' between them.  This
behavior raises a few questions.  First, why should the position of a
`%union' affect definitions related to `YYLTYPE' and `yytokentype'?
Second, what if there is no `%union'?  In that case, the second kind of
PROLOGUE section is not available.  This behavior is not intuitive.

   To avoid this subtle `%union' dependency, rewrite the example using a
`%code top' and an unqualified `%code'.  Let's go ahead and add the new
`YYLTYPE' definition and the `trace_token' prototype at the same time:

     %code top {
       #define _GNU_SOURCE
       #include <stdio.h>

       /* WARNING: The following code really belongs
        * in a `%code requires'; see below.  */

       #include "ptypes.h"
       #define YYLTYPE YYLTYPE
       typedef struct YYLTYPE
       {
         int first_line;
         int first_column;
         int last_line;
         int last_column;
         char *filename;
       } YYLTYPE;
     }

     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %code {
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
       static void trace_token (enum yytokentype token, YYLTYPE loc);
     }

     ...

In this way, `%code top' and the unqualified `%code' achieve the same
functionality as the two kinds of PROLOGUE sections, but it's always
explicit which kind you intend.  Moreover, both kinds are always
available even in the absence of `%union'.

   The `%code top' block above logically contains two parts.  The first
two lines before the warning need to appear near the top of the parser
implementation file.  The first line after the warning is required by
`YYSTYPE' and thus also needs to appear in the parser implementation
file.  However, if you've instructed Bison to generate a parser header
file (*note %defines: Decl Summary.), you probably want that line to
appear before the `YYSTYPE' definition in that header file as well.
The `YYLTYPE' definition should also appear in the parser header file
to override the default `YYLTYPE' definition there.

   In other words, in the `%code top' block above, all but the first two
lines are dependency code required by the `YYSTYPE' and `YYLTYPE'
definitions.  Thus, they belong in one or more `%code requires':

     %code top {
       #define _GNU_SOURCE
       #include <stdio.h>
     }

     %code requires {
       #include "ptypes.h"
     }
     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %code requires {
       #define YYLTYPE YYLTYPE
       typedef struct YYLTYPE
       {
         int first_line;
         int first_column;
         int last_line;
         int last_column;
         char *filename;
       } YYLTYPE;
     }

     %code {
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
       static void trace_token (enum yytokentype token, YYLTYPE loc);
     }

     ...

Now Bison will insert `#include "ptypes.h"' and the new `YYLTYPE'
definition before the Bison-generated `YYSTYPE' and `YYLTYPE'
definitions in both the parser implementation file and the parser
header file.  (By the same reasoning, `%code requires' would also be
the appropriate place to write your own definition for `YYSTYPE'.)

   When you are writing dependency code for `YYSTYPE' and `YYLTYPE',
you should prefer `%code requires' over `%code top' regardless of
whether you instruct Bison to generate a parser header file.  When you
are writing code that you need Bison to insert only into the parser
implementation file and that has no special need to appear at the top
of that file, you should prefer the unqualified `%code' over `%code
top'.  These practices will make the purpose of each block of your code
explicit to Bison and to other developers reading your grammar file.
Following these practices, we expect the unqualified `%code' and `%code
requires' to be the most important of the four PROLOGUE alternatives.

   At some point while developing your parser, you might decide to
provide `trace_token' to modules that are external to your parser.
Thus, you might wish for Bison to insert the prototype into both the
parser header file and the parser implementation file.  Since this
function is not a dependency required by `YYSTYPE' or `YYLTYPE', it
doesn't make sense to move its prototype to a `%code requires'.  More
importantly, since it depends upon `YYLTYPE' and `yytokentype', `%code
requires' is not sufficient.  Instead, move its prototype from the
unqualified `%code' to a `%code provides':

     %code top {
       #define _GNU_SOURCE
       #include <stdio.h>
     }

     %code requires {
       #include "ptypes.h"
     }
     %union {
       long int n;
       tree t;  /* `tree' is defined in `ptypes.h'. */
     }

     %code requires {
       #define YYLTYPE YYLTYPE
       typedef struct YYLTYPE
       {
         int first_line;
         int first_column;
         int last_line;
         int last_column;
         char *filename;
       } YYLTYPE;
     }

     %code provides {
       void trace_token (enum yytokentype token, YYLTYPE loc);
     }

     %code {
       static void print_token_value (FILE *, int, YYSTYPE);
       #define YYPRINT(F, N, L) print_token_value (F, N, L)
     }

     ...

Bison will insert the `trace_token' prototype into both the parser
header file and the parser implementation file after the definitions
for `yytokentype', `YYLTYPE', and `YYSTYPE'.

   The above examples are careful to write directives in an order that
reflects the layout of the generated parser implementation and header
files: `%code top', `%code requires', `%code provides', and then
`%code'.  While your grammar files may generally be easier to read if
you also follow this order, Bison does not require it.  Instead, Bison
lets you choose an organization that makes sense to you.

   You may declare any of these directives multiple times in the
grammar file.  In that case, Bison concatenates the contained code in
declaration order.  This is the only way in which the position of one
of these directives within the grammar file affects its functionality.

   The result of the previous two properties is greater flexibility in
how you may organize your grammar file.  For example, you may organize
semantic-type-related directives by semantic type:

     %code requires { #include "type1.h" }
     %union { type1 field1; }
     %destructor { type1_free ($$); } <field1>
     %printer { type1_print (yyoutput, $$); } <field1>

     %code requires { #include "type2.h" }
     %union { type2 field2; }
     %destructor { type2_free ($$); } <field2>
     %printer { type2_print (yyoutput, $$); } <field2>

You could even place each of the above directive groups in the rules
section of the grammar file next to the set of rules that uses the
associated semantic type.  (In the rules section, you must terminate
each of those directives with a semicolon.)  And you don't have to
worry that some directive (like a `%union') in the definitions section
is going to adversely affect their functionality in some
counter-intuitive manner just because it comes first.  Such an
organization is not possible using PROLOGUE sections.

   This section has been concerned with explaining the advantages of
the four PROLOGUE alternatives over the original Yacc PROLOGUE.
However, in most cases when using these directives, you shouldn't need
to think about all the low-level ordering issues discussed here.
Instead, you should simply use these directives to label each block of
your code according to its purpose and let Bison handle the ordering.
`%code' is the most generic label.  Move code to `%code requires',
`%code provides', or `%code top' as needed.


File: bison.info,  Node: Bison Declarations,  Next: Grammar Rules,  Prev: Prologue Alternatives,  Up: Grammar Outline

3.1.3 The Bison Declarations Section
------------------------------------

The BISON DECLARATIONS section contains declarations that define
terminal and nonterminal symbols, specify precedence, and so on.  In
some simple grammars you may not need any declarations.  *Note Bison
Declarations: Declarations.


File: bison.info,  Node: Grammar Rules,  Next: Epilogue,  Prev: Bison Declarations,  Up: Grammar Outline

3.1.4 The Grammar Rules Section
-------------------------------

The "grammar rules" section contains one or more Bison grammar rules,
and nothing else.  *Note Syntax of Grammar Rules: Rules.

   There must always be at least one grammar rule, and the first `%%'
(which precedes the grammar rules) may never be omitted even if it is
the first thing in the file.


File: bison.info,  Node: Epilogue,  Prev: Grammar Rules,  Up: Grammar Outline

3.1.5 The epilogue
------------------

The EPILOGUE is copied verbatim to the end of the parser implementation
file, just as the PROLOGUE is copied to the beginning.  This is the
most convenient place to put anything that you want to have in the
parser implementation file but which need not come before the
definition of `yyparse'.  For example, the definitions of `yylex' and
`yyerror' often go here.  Because C requires functions to be declared
before being used, you often need to declare functions like `yylex' and
`yyerror' in the Prologue, even if you define them in the Epilogue.
*Note Parser C-Language Interface: Interface.

   If the last section is empty, you may omit the `%%' that separates it
from the grammar rules.

   The Bison parser itself contains many macros and identifiers whose
names start with `yy' or `YY', so it is a good idea to avoid using any
such names (except those documented in this manual) in the epilogue of
the grammar file.


File: bison.info,  Node: Symbols,  Next: Rules,  Prev: Grammar Outline,  Up: Grammar File

3.2 Symbols, Terminal and Nonterminal
=====================================

"Symbols" in Bison grammars represent the grammatical classifications
of the language.

   A "terminal symbol" (also known as a "token type") represents a
class of syntactically equivalent tokens.  You use the symbol in grammar
rules to mean that a token in that class is allowed.  The symbol is
represented in the Bison parser by a numeric code, and the `yylex'
function returns a token type code to indicate what kind of token has
been read.  You don't need to know what the code value is; you can use
the symbol to stand for it.

   A "nonterminal symbol" stands for a class of syntactically
equivalent groupings.  The symbol name is used in writing grammar rules.
By convention, it should be all lower case.

   Symbol names can contain letters, underscores, periods, and
non-initial digits and dashes.  Dashes in symbol names are a GNU
extension, incompatible with POSIX Yacc.  Periods and dashes make
symbol names less convenient to use with named references, which
require brackets around such names (*note Named References::).
Terminal symbols that contain periods or dashes make little sense:
since they are not valid symbols (in most programming languages) they
are not exported as token names.

   There are three ways of writing terminal symbols in the grammar:

   * A "named token type" is written with an identifier, like an
     identifier in C.  By convention, it should be all upper case.  Each
     such name must be defined with a Bison declaration such as
     `%token'.  *Note Token Type Names: Token Decl.

   * A "character token type" (or "literal character token") is written
     in the grammar using the same syntax used in C for character
     constants; for example, `'+'' is a character token type.  A
     character token type doesn't need to be declared unless you need to
     specify its semantic value data type (*note Data Types of Semantic
     Values: Value Type.), associativity, or precedence (*note Operator
     Precedence: Precedence.).

     By convention, a character token type is used only to represent a
     token that consists of that particular character.  Thus, the token
     type `'+'' is used to represent the character `+' as a token.
     Nothing enforces this convention, but if you depart from it, your
     program will confuse other readers.

     All the usual escape sequences used in character literals in C can
     be used in Bison as well, but you must not use the null character
     as a character literal because its numeric code, zero, signifies
     end-of-input (*note Calling Convention for `yylex': Calling
     Convention.).  Also, unlike standard C, trigraphs have no special
     meaning in Bison character literals, nor is backslash-newline
     allowed.

   * A "literal string token" is written like a C string constant; for
     example, `"<="' is a literal string token.  A literal string token
     doesn't need to be declared unless you need to specify its semantic
     value data type (*note Value Type::), associativity, or precedence
     (*note Precedence::).

     You can associate the literal string token with a symbolic name as
     an alias, using the `%token' declaration (*note Token
     Declarations: Token Decl.).  If you don't do that, the lexical
     analyzer has to retrieve the token number for the literal string
     token from the `yytname' table (*note Calling Convention::).

     *Warning*: literal string tokens do not work in Yacc.

     By convention, a literal string token is used only to represent a
     token that consists of that particular string.  Thus, you should
     use the token type `"<="' to represent the string `<=' as a token.
     Bison does not enforce this convention, but if you depart from it,
     people who read your program will be confused.

     All the escape sequences used in string literals in C can be used
     in Bison as well, except that you must not use a null character
     within a string literal.  Also, unlike Standard C, trigraphs have
     no special meaning in Bison string literals, nor is
     backslash-newline allowed.  A literal string token must contain
     two or more characters; for a token containing just one character,
     use a character token (see above).

   How you choose to write a terminal symbol has no effect on its
grammatical meaning.  That depends only on where it appears in rules and
on when the parser function returns that symbol.

   The value returned by `yylex' is always one of the terminal symbols,
except that a zero or negative value signifies end-of-input.  Whichever
way you write the token type in the grammar rules, you write it the
same way in the definition of `yylex'.  The numeric code for a
character token type is simply the positive numeric code of the
character, so `yylex' can use the identical value to generate the
requisite code, though you may need to convert it to `unsigned char' to
avoid sign-extension on hosts where `char' is signed.  Each named token
type becomes a C macro in the parser implementation file, so `yylex'
can use the name to stand for the code.  (This is why periods don't
make sense in terminal symbols.)  *Note Calling Convention for `yylex':
Calling Convention.

   If `yylex' is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there.  Use the `-d'
option when you run Bison, so that it will write these macro definitions
into a separate header file `NAME.tab.h' which you can include in the
other source files that need it.  *Note Invoking Bison: Invocation.

   If you want to write a grammar that is portable to any Standard C
host, you must use only nonnull character tokens taken from the basic
execution character set of Standard C.  This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the
characters in the following C-language string:

     "\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_{|}~"

   The `yylex' function and Bison must use a consistent character set
and encoding for character tokens.  For example, if you run Bison in an
ASCII environment, but then compile and run the resulting program in an
environment that uses an incompatible character set like EBCDIC, the
resulting program may not work because the tables generated by Bison
will assume ASCII numeric values for character tokens.  It is standard
practice for software distributions to contain C source files that were
generated by Bison in an ASCII environment, so installers on platforms
that are incompatible with ASCII must rebuild those files before
compiling them.

   The symbol `error' is a terminal symbol reserved for error recovery
(*note Error Recovery::); you shouldn't use it for any other purpose.
In particular, `yylex' should never return this value.  The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a `%token' declaration.


File: bison.info,  Node: Rules,  Next: Recursion,  Prev: Symbols,  Up: Grammar File

3.3 Syntax of Grammar Rules
===========================

A Bison grammar rule has the following general form:

     RESULT: COMPONENTS...;

where RESULT is the nonterminal symbol that this rule describes, and
COMPONENTS are various terminal and nonterminal symbols that are put
together by this rule (*note Symbols::).

   For example,

     exp: exp '+' exp;

says that two groupings of type `exp', with a `+' token in between, can
be combined into a larger grouping of type `exp'.

   White space in rules is significant only to separate symbols.  You
can add extra white space as you wish.

   Scattered among the components can be ACTIONS that determine the
semantics of the rule.  An action looks like this:

     {C STATEMENTS}

This is an example of "braced code", that is, C code surrounded by
braces, much like a compound statement in C.  Braced code can contain
any sequence of C tokens, so long as its braces are balanced.  Bison
does not check the braced code for correctness directly; it merely
copies the code to the parser implementation file, where the C compiler
can check it.

   Within braced code, the balanced-brace count is not affected by
braces within comments, string literals, or character constants, but it
is affected by the C digraphs `<%' and `%>' that represent braces.  At
the top level braced code must be terminated by `}' and not by a
digraph.  Bison does not look for trigraphs, so if braced code uses
trigraphs you should ensure that they do not affect the nesting of
braces or the boundaries of comments, string literals, or character
constants.

   Usually there is only one action and it follows the components.
*Note Actions::.

   Multiple rules for the same RESULT can be written separately or can
be joined with the vertical-bar character `|' as follows:

     RESULT:
       RULE1-COMPONENTS...
     | RULE2-COMPONENTS...
     ...
     ;

They are still considered distinct rules even when joined in this way.

   If COMPONENTS in a rule is empty, it means that RESULT can match the
empty string.  For example, here is how to define a comma-separated
sequence of zero or more `exp' groupings:

     expseq:
       /* empty */
     | expseq1
     ;

     expseq1:
       exp
     | expseq1 ',' exp
     ;

It is customary to write a comment `/* empty */' in each rule with no
components.


File: bison.info,  Node: Recursion,  Next: Semantics,  Prev: Rules,  Up: Grammar File

3.4 Recursive Rules
===================

A rule is called "recursive" when its RESULT nonterminal appears also
on its right hand side.  Nearly all Bison grammars need to use
recursion, because that is the only way to define a sequence of any
number of a particular thing.  Consider this recursive definition of a
comma-separated sequence of one or more expressions:

     expseq1:
       exp
     | expseq1 ',' exp
     ;

Since the recursive use of `expseq1' is the leftmost symbol in the
right hand side, we call this "left recursion".  By contrast, here the
same construct is defined using "right recursion":

     expseq1:
       exp
     | exp ',' expseq1
     ;

Any kind of sequence can be defined using either left recursion or right
recursion, but you should always use left recursion, because it can
parse a sequence of any number of elements with bounded stack space.
Right recursion uses up space on the Bison stack in proportion to the
number of elements in the sequence, because all the elements must be
shifted onto the stack before the rule can be applied even once.  *Note
The Bison Parser Algorithm: Algorithm, for further explanation of this.

   "Indirect" or "mutual" recursion occurs when the result of the rule
does not appear directly on its right hand side, but does appear in
rules for other nonterminals which do appear on its right hand side.

   For example:

     expr:
       primary
     | primary '+' primary
     ;

     primary:
       constant
     | '(' expr ')'
     ;

defines two mutually-recursive nonterminals, since each refers to the
other.


File: bison.info,  Node: Semantics,  Next: Tracking Locations,  Prev: Recursion,  Up: Grammar File

3.5 Defining Language Semantics
===============================

The grammar rules for a language determine only the syntax.  The
semantics are determined by the semantic values associated with various
tokens and groupings, and by the actions taken when various groupings
are recognized.

   For example, the calculator calculates properly because the value
associated with each expression is the proper number; it adds properly
because the action for the grouping `X + Y' is to add the numbers
associated with X and Y.

* Menu:

* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Mid-Rule Actions::  Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.


File: bison.info,  Node: Value Type,  Next: Multiple Types,  Up: Semantics

3.5.1 Data Types of Semantic Values
-----------------------------------

In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs.  This was true in the
RPN and infix calculator examples (*note Reverse Polish Notation
Calculator: RPN Calc.).

   Bison normally uses the type `int' for semantic values if your
program uses the same data type for all language constructs.  To
specify some other type, define `YYSTYPE' as a macro, like this:

     #define YYSTYPE double

`YYSTYPE''s replacement list should be a type name that does not
contain parentheses or square brackets.  This macro definition must go
in the prologue of the grammar file (*note Outline of a Bison Grammar:
Grammar Outline.).


File: bison.info,  Node: Multiple Types,  Next: Actions,  Prev: Value Type,  Up: Semantics

3.5.2 More Than One Value Type
------------------------------

In most programs, you will need different data types for different kinds
of tokens and groupings.  For example, a numeric constant may need type
`int' or `long int', while a string constant needs type `char *', and
an identifier might need a pointer to an entry in the symbol table.

   To use more than one data type for semantic values in one parser,
Bison requires you to do two things:

   * Specify the entire collection of possible data types, either by
     using the `%union' Bison declaration (*note The Collection of
     Value Types: Union Decl.), or by using a `typedef' or a `#define'
     to define `YYSTYPE' to be a union type whose member names are the
     type tags.

   * Choose one of those types for each symbol (terminal or
     nonterminal) for which semantic values are used.  This is done for
     tokens with the `%token' Bison declaration (*note Token Type
     Names: Token Decl.)  and for groupings with the `%type' Bison
     declaration (*note Nonterminal Symbols: Type Decl.).


File: bison.info,  Node: Actions,  Next: Action Types,  Prev: Multiple Types,  Up: Semantics

3.5.3 Actions
-------------

An action accompanies a syntactic rule and contains C code to be
executed each time an instance of that rule is recognized.  The task of
most actions is to compute a semantic value for the grouping built by
the rule from the semantic values associated with tokens or smaller
groupings.

   An action consists of braced code containing C statements, and can be
placed at any position in the rule; it is executed at that position.
Most rules have just one action at the end of the rule, following all
the components.  Actions in the middle of a rule are tricky and used
only for special purposes (*note Actions in Mid-Rule: Mid-Rule
Actions.).

   The C code in an action can refer to the semantic values of the
components matched by the rule with the construct `$N', which stands
for the value of the Nth component.  The semantic value for the
grouping being constructed is `$$'.  In addition, the semantic values
of symbols can be accessed with the named references construct `$NAME'
or `$[NAME]'.  Bison translates both of these constructs into
expressions of the appropriate type when it copies the actions into the
parser implementation file.  `$$' (or `$NAME', when it stands for the
current grouping) is translated to a modifiable lvalue, so it can be
assigned to.

   Here is a typical example:

     exp:
     ...
     | exp '+' exp     { $$ = $1 + $3; }

   Or, in terms of named references:

     exp[result]:
     ...
     | exp[left] '+' exp[right]  { $result = $left + $right; }

This rule constructs an `exp' from two smaller `exp' groupings
connected by a plus-sign token.  In the action, `$1' and `$3' (`$left'
and `$right') refer to the semantic values of the two component `exp'
groupings, which are the first and third symbols on the right hand side
of the rule.  The sum is stored into `$$' (`$result') so that it
becomes the semantic value of the addition-expression just recognized
by the rule.  If there were a useful semantic value associated with the
`+' token, it could be referred to as `$2'.

   *Note Named References::, for more information about using the named
references construct.

   Note that the vertical-bar character `|' is really a rule separator,
and actions are attached to a single rule.  This is a difference with
tools like Flex, for which `|' stands for either "or", or "the same
action as that of the next rule".  In the following example, the action
is triggered only when `b' is found:

     a-or-b: 'a'|'b'   { a_or_b_found = 1; };

   If you don't specify an action for a rule, Bison supplies a default:
`$$ = $1'.  Thus, the value of the first symbol in the rule becomes the
value of the whole rule.  Of course, the default action is valid only
if the two data types match.  There is no meaningful default action for
an empty rule; every empty rule must have an explicit action unless the
rule's value does not matter.

   `$N' with N zero or negative is allowed for reference to tokens and
groupings on the stack _before_ those that match the current rule.
This is a very risky practice, and to use it reliably you must be
certain of the context in which the rule is applied.  Here is a case in
which you can use this reliably:

     foo:
       expr bar '+' expr  { ... }
     | expr bar '-' expr  { ... }
     ;

     bar:
       /* empty */    { previous_expr = $0; }
     ;

   As long as `bar' is used only in the fashion shown here, `$0' always
refers to the `expr' which precedes `bar' in the definition of `foo'.

   It is also possible to access the semantic value of the lookahead
token, if any, from a semantic action.  This semantic value is stored
in `yylval'.  *Note Special Features for Use in Actions: Action
Features.


File: bison.info,  Node: Action Types,  Next: Mid-Rule Actions,  Prev: Actions,  Up: Semantics

3.5.4 Data Types of Values in Actions
-------------------------------------

If you have chosen a single data type for semantic values, the `$$' and
`$N' constructs always have that data type.

   If you have used `%union' to specify a variety of data types, then
you must declare a choice among these types for each terminal or
nonterminal symbol that can have a semantic value.  Then each time you
use `$$' or `$N', its data type is determined by which symbol it refers
to in the rule.  In this example,

     exp:
       ...
     | exp '+' exp    { $$ = $1 + $3; }

`$1' and `$3' refer to instances of `exp', so they all have the data
type declared for the nonterminal symbol `exp'.  If `$2' were used, it
would have the data type declared for the terminal symbol `'+'',
whatever that might be.

   Alternatively, you can specify the data type when you refer to the
value, by inserting `<TYPE>' after the `$' at the beginning of the
reference.  For example, if you have defined types as shown here:

     %union {
       int itype;
       double dtype;
     }

then you can write `$<itype>1' to refer to the first subunit of the
rule as an integer, or `$<dtype>1' to refer to it as a double.


File: bison.info,  Node: Mid-Rule Actions,  Prev: Action Types,  Up: Semantics

3.5.5 Actions in Mid-Rule
-------------------------

Occasionally it is useful to put an action in the middle of a rule.
These actions are written just like usual end-of-rule actions, but they
are executed before the parser even recognizes the following components.

* Menu:

* Using Mid-Rule Actions::       Putting an action in the middle of a rule.
* Mid-Rule Action Translation::  How mid-rule actions are actually processed.
* Mid-Rule Conflicts::           Mid-rule actions can cause conflicts.


File: bison.info,  Node: Using Mid-Rule Actions,  Next: Mid-Rule Action Translation,  Up: Mid-Rule Actions

3.5.5.1 Using Mid-Rule Actions
..............................

A mid-rule action may refer to the components preceding it using `$N',
but it may not refer to subsequent components because it is run before
they are parsed.

   The mid-rule action itself counts as one of the components of the
rule.  This makes a difference when there is another action later in
the same rule (and usually there is another at the end): you have to
count the actions along with the symbols when working out which number
N to use in `$N'.

   The mid-rule action can also have a semantic value.  The action can
set its value with an assignment to `$$', and actions later in the rule
can refer to the value using `$N'.  Since there is no symbol to name
the action, there is no way to declare a data type for the value in
advance, so you must use the `$<...>N' construct to specify a data type
each time you refer to this value.

   There is no way to set the value of the entire rule with a mid-rule
action, because assignments to `$$' do not have that effect.  The only
way to set the value for the entire rule is with an ordinary action at
the end of the rule.

   Here is an example from a hypothetical compiler, handling a `let'
statement that looks like `let (VARIABLE) STATEMENT' and serves to
create a variable named VARIABLE temporarily for the duration of
STATEMENT.  To parse this construct, we must put VARIABLE into the
symbol table while STATEMENT is parsed, then remove it afterward.  Here
is how it is done:

     stmt:
       "let" '(' var ')'
         {
           $<context>$ = push_context ();
           declare_variable ($3);
         }
       stmt
         {
           $$ = $6;
           pop_context ($<context>5);
         }

As soon as `let (VARIABLE)' has been recognized, the first action is
run.  It saves a copy of the current semantic context (the list of
accessible variables) as its semantic value, using alternative
`context' in the data-type union.  Then it calls `declare_variable' to
add the new variable to that list.  Once the first action is finished,
the embedded statement `stmt' can be parsed.

   Note that the mid-rule action is component number 5, so the `stmt' is
component number 6.  Named references can be used to improve the
readability and maintainability (*note Named References::):

     stmt:
       "let" '(' var ')'
         {
           $<context>let = push_context ();
           declare_variable ($3);
         }[let]
       stmt
         {
           $$ = $6;
           pop_context ($<context>let);
         }

   After the embedded statement is parsed, its semantic value becomes
the value of the entire `let'-statement.  Then the semantic value from
the earlier action is used to restore the prior list of variables.  This
removes the temporary `let'-variable from the list so that it won't
appear to exist while the rest of the program is parsed.

   In the above example, if the parser initiates error recovery (*note
Error Recovery::) while parsing the tokens in the embedded statement
`stmt', it might discard the previous semantic context `$<context>5'
without restoring it.  Thus, `$<context>5' needs a destructor (*note
Freeing Discarded Symbols: Destructor Decl.).  However, Bison currently
provides no means to declare a destructor specific to a particular
mid-rule action's semantic value.

   One solution is to bury the mid-rule action inside a nonterminal
symbol and to declare a destructor for that symbol:

     %type <context> let
     %destructor { pop_context ($$); } let

     %%

     stmt:
       let stmt
         {
           $$ = $2;
           pop_context ($let);
         };

     let:
       "let" '(' var ')'
         {
           $let = push_context ();
           declare_variable ($3);
         };

Note that the action is now at the end of its rule.  Any mid-rule
action can be converted to an end-of-rule action in this way, and this
is what Bison actually does to implement mid-rule actions.


File: bison.info,  Node: Mid-Rule Action Translation,  Next: Mid-Rule Conflicts,  Prev: Using Mid-Rule Actions,  Up: Mid-Rule Actions

3.5.5.2 Mid-Rule Action Translation
...................................

As hinted earlier, mid-rule actions are actually transformed into
regular rules and actions.  The various reports generated by Bison
(textual, graphical, etc., see *note Understanding Your Parser:
Understanding.)  reveal this translation, best explained by means of an
example.  The following rule:

     exp: { a(); } "b" { c(); } { d(); } "e" { f(); };

is translated into:

     $@1: /* empty */ { a(); };
     $@2: /* empty */ { c(); };
     $@3: /* empty */ { d(); };
     exp: $@1 "b" $@2 $@3 "e" { f(); };

with new nonterminal symbols `$@N', where N is a number.

   A mid-rule action is expected to generate a value if it uses `$$', or
the (final) action uses `$N' where N denote the mid-rule action.  In
that case its nonterminal is rather named `@N':

     exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };

is translated into

     @1: /* empty */ { a(); };
     @2: /* empty */ { $$ = c(); };
     $@3: /* empty */ { d(); };
     exp: @1 "b" @2 $@3 "e" { f = $1; }

   There are probably two errors in the above example: the first
mid-rule action does not generate a value (it does not use `$$'
although the final action uses it), and the value of the second one is
not used (the final action does not use `$3').  Bison reports these
errors when the `midrule-value' warnings are enabled (*note Invoking
Bison: Invocation.):

     $ bison -fcaret -Wmidrule-value mid.y
     mid.y:2.6-13: warning: unset value: $$
      exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
           ^^^^^^^^
     mid.y:2.19-31: warning: unused value: $3
      exp: { a(); } "b" { $$ = c(); } { d(); } "e" { f = $1; };
                        ^^^^^^^^^^^^^


File: bison.info,  Node: Mid-Rule Conflicts,  Prev: Mid-Rule Action Translation,  Up: Mid-Rule Actions

3.5.5.3 Conflicts due to Mid-Rule Actions
.........................................

Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute
the action.  For example, the following two rules, without mid-rule
actions, can coexist in a working parser because the parser can shift
the open-brace token and look at what follows before deciding whether
there is a declaration or not:

     compound:
       '{' declarations statements '}'
     | '{' statements '}'
     ;

But when we add a mid-rule action as follows, the rules become
nonfunctional:

     compound:
       { prepare_for_local_variables (); }
          '{' declarations statements '}'
     |    '{' statements '}'
     ;

Now the parser is forced to decide whether to run the mid-rule action
when it has read no farther than the open-brace.  In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly.  (The open-brace token is what is called
the "lookahead" token at this time, since the parser is still deciding
what to do about it.  *Note Lookahead Tokens: Lookahead.)

   You might think that you could correct the problem by putting
identical actions into the two rules, like this:

     compound:
       { prepare_for_local_variables (); }
         '{' declarations statements '}'
     | { prepare_for_local_variables (); }
         '{' statements '}'
     ;

But this does not help, because Bison does not realize that the two
actions are identical.  (Bison never tries to understand the C code in
an action.)

   If the grammar is such that a declaration can be distinguished from a
statement by the first token (which is true in C), then one solution
which does work is to put the action after the open-brace, like this:

     compound:
       '{' { prepare_for_local_variables (); }
         declarations statements '}'
     | '{' statements '}'
     ;

Now the first token of the following declaration or statement, which
would in any case tell Bison which rule to use, can still do so.

   Another solution is to bury the action inside a nonterminal symbol
which serves as a subroutine:

     subroutine:
       /* empty */  { prepare_for_local_variables (); }
     ;

     compound:
       subroutine '{' declarations statements '}'
     | subroutine '{' statements '}'
     ;

Now Bison can execute the action in the rule for `subroutine' without
deciding which rule for `compound' it will eventually use.


File: bison.info,  Node: Tracking Locations,  Next: Named References,  Prev: Semantics,  Up: Grammar File

3.6 Tracking Locations
======================

Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional
information, especially symbol locations.

   The way locations are handled is defined by providing a data type,
and actions to take when rules are matched.

* Menu:

* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.


File: bison.info,  Node: Location Type,  Next: Actions and Locations,  Up: Tracking Locations

3.6.1 Data Type of Locations
----------------------------

Defining a data type for locations is much simpler than for semantic
values, since all tokens and groupings always use the same type.

   You can specify the type of locations by defining a macro called
`YYLTYPE', just as you can specify the semantic value type by defining
a `YYSTYPE' macro (*note Value Type::).  When `YYLTYPE' is not defined,
Bison uses a default structure type with four members:

     typedef struct YYLTYPE
     {
       int first_line;
       int first_column;
       int last_line;
       int last_column;
     } YYLTYPE;

   When `YYLTYPE' is not defined, at the beginning of the parsing, Bison
initializes all these fields to 1 for `yylloc'.  To initialize `yylloc'
with a custom location type (or to chose a different initialization),
use the `%initial-action' directive.  *Note Performing Actions before
Parsing: Initial Action Decl.


File: bison.info,  Node: Actions and Locations,  Next: Location Default Action,  Prev: Location Type,  Up: Tracking Locations

3.6.2 Actions and Locations
---------------------------

Actions are not only useful for defining language semantics, but also
for describing the behavior of the output parser with locations.

   The most obvious way for building locations of syntactic groupings
is very similar to the way semantic values are computed.  In a given
rule, several constructs can be used to access the locations of the
elements being matched.  The location of the Nth component of the right
hand side is `@N', while the location of the left hand side grouping is
`@$'.

   In addition, the named references construct `@NAME' and `@[NAME]'
may also be used to address the symbol locations.  *Note Named
References::, for more information about using the named references
construct.

   Here is a basic example using the default data type for locations:

     exp:
       ...
     | exp '/' exp
         {
           @$.first_column = @1.first_column;
           @$.first_line = @1.first_line;
           @$.last_column = @3.last_column;
           @$.last_line = @3.last_line;
           if ($3)
             $$ = $1 / $3;
           else
             {
               $$ = 1;
               fprintf (stderr,
                        "Division by zero, l%d,c%d-l%d,c%d",
                        @3.first_line, @3.first_column,
                        @3.last_line, @3.last_column);
             }
         }

   As for semantic values, there is a default action for locations that
is run each time a rule is matched.  It sets the beginning of `@$' to
the beginning of the first symbol, and the end of `@$' to the end of the
last symbol.

   With this default action, the location tracking can be fully
automatic.  The example above simply rewrites this way:

     exp:
       ...
     | exp '/' exp
         {
           if ($3)
             $$ = $1 / $3;
           else
             {
               $$ = 1;
               fprintf (stderr,
                        "Division by zero, l%d,c%d-l%d,c%d",
                        @3.first_line, @3.first_column,
                        @3.last_line, @3.last_column);
             }
         }

   It is also possible to access the location of the lookahead token,
if any, from a semantic action.  This location is stored in `yylloc'.
*Note Special Features for Use in Actions: Action Features.


File: bison.info,  Node: Location Default Action,  Prev: Actions and Locations,  Up: Tracking Locations

3.6.3 Default Action for Locations
----------------------------------

Actually, actions are not the best place to compute locations.  Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each rule.
The `YYLLOC_DEFAULT' macro is invoked each time a rule is matched,
before the associated action is run.  It is also invoked while
processing a syntax error, to compute the error's location.  Before
reporting an unresolvable syntactic ambiguity, a GLR parser invokes
`YYLLOC_DEFAULT' recursively to compute the location of that ambiguity.

   Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.

   The `YYLLOC_DEFAULT' macro takes three parameters.  The first one is
the location of the grouping (the result of the computation).  When a
rule is matched, the second parameter identifies locations of all right
hand side elements of the rule being matched, and the third parameter
is the size of the rule's right hand side.  When a GLR parser reports
an ambiguity, which of multiple candidate right hand sides it passes to
`YYLLOC_DEFAULT' is undefined.  When processing a syntax error, the
second parameter identifies locations of the symbols that were
discarded during error processing, and the third parameter is the
number of discarded symbols.

   By default, `YYLLOC_DEFAULT' is defined this way:

     # define YYLLOC_DEFAULT(Cur, Rhs, N)                      \
     do                                                        \
       if (N)                                                  \
         {                                                     \
           (Cur).first_line   = YYRHSLOC(Rhs, 1).first_line;   \
           (Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \
           (Cur).last_line    = YYRHSLOC(Rhs, N).last_line;    \
           (Cur).last_column  = YYRHSLOC(Rhs, N).last_column;  \
         }                                                     \
       else                                                    \
         {                                                     \
           (Cur).first_line   = (Cur).last_line   =            \
             YYRHSLOC(Rhs, 0).last_line;                       \
           (Cur).first_column = (Cur).last_column =            \
             YYRHSLOC(Rhs, 0).last_column;                     \
         }                                                     \
     while (0)

where `YYRHSLOC (rhs, k)' is the location of the Kth symbol in RHS when
K is positive, and the location of the symbol just before the reduction
when K and N are both zero.

   When defining `YYLLOC_DEFAULT', you should consider that:

   * All arguments are free of side-effects.  However, only the first
     one (the result) should be modified by `YYLLOC_DEFAULT'.

   * For consistency with semantic actions, valid indexes within the
     right hand side range from 1 to N.  When N is zero, only 0 is a
     valid index, and it refers to the symbol just before the reduction.
     During error processing N is always positive.

   * Your macro should parenthesize its arguments, if need be, since the
     actual arguments may not be surrounded by parentheses.  Also, your
     macro should expand to something that can be used as a single
     statement when it is followed by a semicolon.


File: bison.info,  Node: Named References,  Next: Declarations,  Prev: Tracking Locations,  Up: Grammar File

3.7 Named References
====================

As described in the preceding sections, the traditional way to refer to
any semantic value or location is a "positional reference", which takes
the form `$N', `$$', `@N', and `@$'.  However, such a reference is not
very descriptive.  Moreover, if you later decide to insert or remove
symbols in the right-hand side of a grammar rule, the need to renumber
such references can be tedious and error-prone.

   To avoid these issues, you can also refer to a semantic value or
location using a "named reference".  First of all, original symbol
names may be used as named references.  For example:

     invocation: op '(' args ')'
       { $invocation = new_invocation ($op, $args, @invocation); }

Positional and named references can be mixed arbitrarily.  For example:

     invocation: op '(' args ')'
       { $$ = new_invocation ($op, $args, @$); }

However, sometimes regular symbol names are not sufficient due to
ambiguities:

     exp: exp '/' exp
       { $exp = $exp / $exp; } // $exp is ambiguous.

     exp: exp '/' exp
       { $$ = $1 / $exp; } // One usage is ambiguous.

     exp: exp '/' exp
       { $$ = $1 / $3; } // No error.

When ambiguity occurs, explicitly declared names may be used for values
and locations.  Explicit names are declared as a bracketed name after a
symbol appearance in rule definitions.  For example:
     exp[result]: exp[left] '/' exp[right]
       { $result = $left / $right; }

In order to access a semantic value generated by a mid-rule action, an
explicit name may also be declared by putting a bracketed name after the
closing brace of the mid-rule action code:
     exp[res]: exp[x] '+' {$left = $x;}[left] exp[right]
       { $res = $left + $right; }

In references, in order to specify names containing dots and dashes, an
explicit bracketed syntax `$[name]' and `@[name]' must be used:
     if-stmt: "if" '(' expr ')' "then" then.stmt ';'
       { $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); }

   It often happens that named references are followed by a dot, dash
or other C punctuation marks and operators.  By default, Bison will read
`$name.suffix' as a reference to symbol value `$name' followed by
`.suffix', i.e., an access to the `suffix' field of the semantic value.
In order to force Bison to recognize `name.suffix' in its entirety as
the name of a semantic value, the bracketed syntax `$[name.suffix]'
must be used.

   The named references feature is experimental.  More user feedback
will help to stabilize it.


File: bison.info,  Node: Declarations,  Next: Multiple Parsers,  Prev: Named References,  Up: Grammar File

3.8 Bison Declarations
======================

The "Bison declarations" section of a Bison grammar defines the symbols
used in formulating the grammar and the data types of semantic values.
*Note Symbols::.

   All token type names (but not single-character literal tokens such as
`'+'' and `'*'') must be declared.  Nonterminal symbols must be
declared if you need to specify which data type to use for the semantic
value (*note More Than One Value Type: Multiple Types.).

   The first rule in the grammar file also specifies the start symbol,
by default.  If you want some other symbol to be the start symbol, you
must declare it explicitly (*note Languages and Context-Free Grammars:
Language and Grammar.).

* Menu:

* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Union Decl::        Declaring the set of all semantic value types.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Printer Decl::      Declaring how symbol values are displayed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Push Decl::         Requesting a push parser.
* Decl Summary::      Table of all Bison declarations.
* %define Summary::   Defining variables to adjust Bison's behavior.
* %code Summary::     Inserting code into the parser source.


File: bison.info,  Node: Require Decl,  Next: Token Decl,  Up: Declarations

3.8.1 Require a Version of Bison
--------------------------------

You may require the minimum version of Bison to process the grammar.  If
the requirement is not met, `bison' exits with an error (exit status
63).

     %require "VERSION"


File: bison.info,  Node: Token Decl,  Next: Precedence Decl,  Prev: Require Decl,  Up: Declarations

3.8.2 Token Type Names
----------------------

The basic way to declare a token type name (terminal symbol) is as
follows:

     %token NAME

   Bison will convert this into a `#define' directive in the parser, so
that the function `yylex' (if it is in this file) can use the name NAME
to stand for this token type's code.

   Alternatively, you can use `%left', `%right', or `%nonassoc' instead
of `%token', if you wish to specify associativity and precedence.
*Note Operator Precedence: Precedence Decl.

   You can explicitly specify the numeric code for a token type by
appending a nonnegative decimal or hexadecimal integer value in the
field immediately following the token name:

     %token NUM 300
     %token XNUM 0x12d // a GNU extension

It is generally best, however, to let Bison choose the numeric codes for
all token types.  Bison will automatically select codes that don't
conflict with each other or with normal characters.

   In the event that the stack type is a union, you must augment the
`%token' or other token declaration to include the data type
alternative delimited by angle-brackets (*note More Than One Value
Type: Multiple Types.).

   For example:

     %union {              /* define stack type */
       double val;
       symrec *tptr;
     }
     %token <val> NUM      /* define token NUM and its type */

   You can associate a literal string token with a token type name by
writing the literal string at the end of a `%token' declaration which
declares the name.  For example:

     %token arrow "=>"

For example, a grammar for the C language might specify these names with
equivalent literal string tokens:

     %token  <operator>  OR      "||"
     %token  <operator>  LE 134  "<="
     %left  OR  "<="

Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules.  The
`yylex' function can use the token name or the literal string to obtain
the token type code number (*note Calling Convention::).  Syntax error
messages passed to `yyerror' from the parser will reference the literal
string instead of the token name.

   The token numbered as 0 corresponds to end of file; the following
line allows for nicer error messages referring to "end of file" instead
of "$end":

     %token END 0 "end of file"


File: bison.info,  Node: Precedence Decl,  Next: Union Decl,  Prev: Token Decl,  Up: Declarations

3.8.3 Operator Precedence
-------------------------

Use the `%left', `%right' or `%nonassoc' declaration to declare a token
and specify its precedence and associativity, all at once.  These are
called "precedence declarations".  *Note Operator Precedence:
Precedence, for general information on operator precedence.

   The syntax of a precedence declaration is nearly the same as that of
`%token': either

     %left SYMBOLS...

or

     %left <TYPE> SYMBOLS...

   And indeed any of these declarations serves the purposes of `%token'.
But in addition, they specify the associativity and relative precedence
for all the SYMBOLS:

   * The associativity of an operator OP determines how repeated uses
     of the operator nest: whether `X OP Y OP Z' is parsed by grouping
     X with Y first or by grouping Y with Z first.  `%left' specifies
     left-associativity (grouping X with Y first) and `%right'
     specifies right-associativity (grouping Y with Z first).
     `%nonassoc' specifies no associativity, which means that `X OP Y
     OP Z' is considered a syntax error.

   * The precedence of an operator determines how it nests with other
     operators.  All the tokens declared in a single precedence
     declaration have equal precedence and nest together according to
     their associativity.  When two tokens declared in different
     precedence declarations associate, the one declared later has the
     higher precedence and is grouped first.

   For backward compatibility, there is a confusing difference between
the argument lists of `%token' and precedence declarations.  Only a
`%token' can associate a literal string with a token type name.  A
precedence declaration always interprets a literal string as a
reference to a separate token.  For example:

     %left  OR "<="         // Does not declare an alias.
     %left  OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".


File: bison.info,  Node: Union Decl,  Next: Type Decl,  Prev: Precedence Decl,  Up: Declarations

3.8.4 The Collection of Value Types
-----------------------------------

The `%union' declaration specifies the entire collection of possible
data types for semantic values.  The keyword `%union' is followed by
braced code containing the same thing that goes inside a `union' in C.

   For example:

     %union {
       double val;
       symrec *tptr;
     }

This says that the two alternative types are `double' and `symrec *'.
They are given names `val' and `tptr'; these names are used in the
`%token' and `%type' declarations to pick one of the types for a
terminal or nonterminal symbol (*note Nonterminal Symbols: Type Decl.).

   As an extension to POSIX, a tag is allowed after the `union'.  For
example:

     %union value {
       double val;
       symrec *tptr;
     }

specifies the union tag `value', so the corresponding C type is `union
value'.  If you do not specify a tag, it defaults to `YYSTYPE'.

   As another extension to POSIX, you may specify multiple `%union'
declarations; their contents are concatenated.  However, only the first
`%union' declaration can specify a tag.

   Note that, unlike making a `union' declaration in C, you need not
write a semicolon after the closing brace.

   Instead of `%union', you can define and use your own union type
`YYSTYPE' if your grammar contains at least one `<TYPE>' tag.  For
example, you can put the following into a header file `parser.h':

     union YYSTYPE {
       double val;
       symrec *tptr;
     };
     typedef union YYSTYPE YYSTYPE;

and then your grammar can use the following instead of `%union':

     %{
     #include "parser.h"
     %}
     %type <val> expr
     %token <tptr> ID


File: bison.info,  Node: Type Decl,  Next: Initial Action Decl,  Prev: Union Decl,  Up: Declarations

3.8.5 Nonterminal Symbols
-------------------------

When you use `%union' to specify multiple value types, you must declare
the value type of each nonterminal symbol for which values are used.
This is done with a `%type' declaration, like this:

     %type <TYPE> NONTERMINAL...

Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the
name given in the `%union' to the alternative that you want (*note The
Collection of Value Types: Union Decl.).  You can give any number of
nonterminal symbols in the same `%type' declaration, if they have the
same value type.  Use spaces to separate the symbol names.

   You can also declare the value type of a terminal symbol.  To do
this, use the same `<TYPE>' construction in a declaration for the
terminal symbol.  All kinds of token declarations allow `<TYPE>'.


File: bison.info,  Node: Initial Action Decl,  Next: Destructor Decl,  Prev: Type Decl,  Up: Declarations

3.8.6 Performing Actions before Parsing
---------------------------------------

Sometimes your parser needs to perform some initializations before
parsing.  The `%initial-action' directive allows for such arbitrary
code.

 -- Directive: %initial-action { CODE }
     Declare that the braced CODE must be invoked before parsing each
     time `yyparse' is called.  The CODE may use `$$' (or `$<TAG>$')
     and `@$' -- initial value and location of the lookahead -- and the
     `%parse-param'.

   For instance, if your locations use a file name, you may use

     %parse-param { char const *file_name };
     %initial-action
     {
       @$.initialize (file_name);
     };


File: bison.info,  Node: Destructor Decl,  Next: Printer Decl,  Prev: Initial Action Decl,  Up: Declarations

3.8.7 Freeing Discarded Symbols
-------------------------------

During error recovery (*note Error Recovery::), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet.  If the parser runs out of memory,
or if it returns via `YYABORT' or `YYACCEPT', all the symbols on the
stack must be discarded.  Even if the parser succeeds, it must discard
the start symbol.

   When discarded symbols convey heap based information, this memory is
lost.  While this behavior can be tolerable for batch parsers, such as
in traditional compilers, it is unacceptable for programs like shells or
protocol implementations that may parse and execute indefinitely.

   The `%destructor' directive defines code that is called when a
symbol is automatically discarded.

 -- Directive: %destructor { CODE } SYMBOLS
     Invoke the braced CODE whenever the parser discards one of the
     SYMBOLS.  Within CODE, `$$' (or `$<TAG>$') designates the semantic
     value associated with the discarded symbol, and `@$' designates
     its location.  The additional parser parameters are also available
     (*note The Parser Function `yyparse': Parser Function.).

     When a symbol is listed among SYMBOLS, its `%destructor' is called
     a per-symbol `%destructor'.  You may also define a per-type
     `%destructor' by listing a semantic type tag among SYMBOLS.  In
     that case, the parser will invoke this CODE whenever it discards
     any grammar symbol that has that semantic type tag unless that
     symbol has its own per-symbol `%destructor'.

     Finally, you can define two different kinds of default
     `%destructor's.  (These default forms are experimental.  More user
     feedback will help to determine whether they should become
     permanent features.)  You can place each of `<*>' and `<>' in the
     SYMBOLS list of exactly one `%destructor' declaration in your
     grammar file.  The parser will invoke the CODE associated with one
     of these whenever it discards any user-defined grammar symbol that
     has no per-symbol and no per-type `%destructor'.  The parser uses
     the CODE for `<*>' in the case of such a grammar symbol for which
     you have formally declared a semantic type tag (`%type' counts as
     such a declaration, but `$<tag>$' does not).  The parser uses the
     CODE for `<>' in the case of such a grammar symbol that has no
     declared semantic type tag.

For example:

     %union { char *string; }
     %token <string> STRING1
     %token <string> STRING2
     %type  <string> string1
     %type  <string> string2
     %union { char character; }
     %token <character> CHR
     %type  <character> chr
     %token TAGLESS

     %destructor { } <character>
     %destructor { free ($$); } <*>
     %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
     %destructor { printf ("Discarding tagless symbol.\n"); } <>

guarantees that, when the parser discards any user-defined symbol that
has a semantic type tag other than `<character>', it passes its
semantic value to `free' by default.  However, when the parser discards
a `STRING1' or a `string1', it also prints its line number to `stdout'.
It performs only the second `%destructor' in this case, so it invokes
`free' only once.  Finally, the parser merely prints a message whenever
it discards any symbol, such as `TAGLESS', that has no semantic type
tag.

   A Bison-generated parser invokes the default `%destructor's only for
user-defined as opposed to Bison-defined symbols.  For example, the
parser will not invoke either kind of default `%destructor' for the
special Bison-defined symbols `$accept', `$undefined', or `$end' (*note
Bison Symbols: Table of Symbols.), none of which you can reference in
your grammar.  It also will not invoke either for the `error' token
(*note error: Table of Symbols.), which is always defined by Bison
regardless of whether you reference it in your grammar.  However, it
may invoke one of them for the end token (token 0) if you redefine it
from `$end' to, for example, `END':

     %token END 0

   Finally, Bison will never invoke a `%destructor' for an unreferenced
mid-rule semantic value (*note Actions in Mid-Rule: Mid-Rule Actions.).
That is, Bison does not consider a mid-rule to have a semantic value if
you do not reference `$$' in the mid-rule's action or `$N' (where N is
the right-hand side symbol position of the mid-rule) in any later
action in that rule.  However, if you do reference either, the
Bison-generated parser will invoke the `<>' `%destructor' whenever it
discards the mid-rule symbol.


   "Discarded symbols" are the following:

   * stacked symbols popped during the first phase of error recovery,

   * incoming terminals during the second phase of error recovery,

   * the current lookahead and the entire stack (except the current
     right-hand side symbols) when the parser returns immediately, and

   * the current lookahead and the entire stack (including the current
     right-hand side symbols) when the C++ parser (`lalr1.cc') catches
     an exception in `parse',

   * the start symbol, when the parser succeeds.

   The parser can "return immediately" because of an explicit call to
`YYABORT' or `YYACCEPT', or failed error recovery, or memory exhaustion.

   Right-hand side symbols of a rule that explicitly triggers a syntax
error via `YYERROR' are not discarded automatically.  As a rule of
thumb, destructors are invoked only when user actions cannot manage the
memory.


File: bison.info,  Node: Printer Decl,  Next: Expect Decl,  Prev: Destructor Decl,  Up: Declarations

3.8.8 Printing Semantic Values
------------------------------

When run-time traces are enabled (*note Tracing Your Parser: Tracing.),
the parser reports its actions, such as reductions.  When a symbol
involved in an action is reported, only its kind is displayed, as the
parser cannot know how semantic values should be formatted.

   The `%printer' directive defines code that is called when a symbol is
reported.  Its syntax is the same as `%destructor' (*note Freeing
Discarded Symbols: Destructor Decl.).

 -- Directive: %printer { CODE } SYMBOLS
     Invoke the braced CODE whenever the parser displays one of the
     SYMBOLS.  Within CODE, `yyoutput' denotes the output stream (a
     `FILE*' in C, and an `std::ostream&' in C++), `$$' (or `$<TAG>$')
     designates the semantic value associated with the symbol, and `@$'
     its location.  The additional parser parameters are also available
     (*note The Parser Function `yyparse': Parser Function.).

     The SYMBOLS are defined as for `%destructor' (*note Freeing
     Discarded Symbols: Destructor Decl.): they can be per-type (e.g.,
     `<ival>'), per-symbol (e.g., `exp', `NUM', `"float"'), typed
     per-default (i.e., `<*>', or untyped per-default (i.e., `<>').

For example:

     %union { char *string; }
     %token <string> STRING1
     %token <string> STRING2
     %type  <string> string1
     %type  <string> string2
     %union { char character; }
     %token <character> CHR
     %type  <character> chr
     %token TAGLESS

     %printer { fprintf (yyoutput, "'%c'", $$); } <character>
     %printer { fprintf (yyoutput, "&%p", $$); } <*>
     %printer { fprintf (yyoutput, "\"%s\"", $$); } STRING1 string1
     %printer { fprintf (yyoutput, "<>"); } <>

guarantees that, when the parser print any symbol that has a semantic
type tag other than `<character>', it display the address of the
semantic value by default.  However, when the parser displays a
`STRING1' or a `string1', it formats it as a string in double quotes.
It performs only the second `%printer' in this case, so it prints only
once.  Finally, the parser print `<>' for any symbol, such as `TAGLESS',
that has no semantic type tag.  See also


File: bison.info,  Node: Expect Decl,  Next: Start Decl,  Prev: Printer Decl,  Up: Declarations

3.8.9 Suppressing Conflict Warnings
-----------------------------------

Bison normally warns if there are any conflicts in the grammar (*note
Shift/Reduce Conflicts: Shift/Reduce.), but most real grammars have
harmless shift/reduce conflicts which are resolved in a predictable way
and would be difficult to eliminate.  It is desirable to suppress the
warning about these conflicts unless the number of conflicts changes.
You can do this with the `%expect' declaration.

   The declaration looks like this:

     %expect N

   Here N is a decimal integer.  The declaration says there should be N
shift/reduce conflicts and no reduce/reduce conflicts.  Bison reports
an error if the number of shift/reduce conflicts differs from N, or if
there are any reduce/reduce conflicts.

   For deterministic parsers, reduce/reduce conflicts are more serious,
and should be eliminated entirely.  Bison will always report
reduce/reduce conflicts for these parsers.  With GLR parsers, however,
both kinds of conflicts are routine; otherwise, there would be no need
to use GLR parsing.  Therefore, it is also possible to specify an
expected number of reduce/reduce conflicts in GLR parsers, using the
declaration:

     %expect-rr N

   In general, using `%expect' involves these steps:

   * Compile your grammar without `%expect'.  Use the `-v' option to
     get a verbose list of where the conflicts occur.  Bison will also
     print the number of conflicts.

   * Check each of the conflicts to make sure that Bison's default
     resolution is what you really want.  If not, rewrite the grammar
     and go back to the beginning.

   * Add an `%expect' declaration, copying the number N from the number
     which Bison printed.  With GLR parsers, add an `%expect-rr'
     declaration as well.

   Now Bison will report an error if you introduce an unexpected
conflict, but will keep silent otherwise.


File: bison.info,  Node: Start Decl,  Next: Pure Decl,  Prev: Expect Decl,  Up: Declarations

3.8.10 The Start-Symbol
-----------------------

Bison assumes by default that the start symbol for the grammar is the
first nonterminal specified in the grammar specification section.  The
programmer may override this restriction with the `%start' declaration
as follows:

     %start SYMBOL


File: bison.info,  Node: Pure Decl,  Next: Push Decl,  Prev: Start Decl,  Up: Declarations

3.8.11 A Pure (Reentrant) Parser
--------------------------------

A "reentrant" program is one which does not alter in the course of
execution; in other words, it consists entirely of "pure" (read-only)
code.  Reentrancy is important whenever asynchronous execution is
possible; for example, a nonreentrant program may not be safe to call
from a signal handler.  In systems with multiple threads of control, a
nonreentrant program must be called only within interlocks.

   Normally, Bison generates a parser which is not reentrant.  This is
suitable for most uses, and it permits compatibility with Yacc.  (The
standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with `yylex',
including `yylval' and `yylloc'.)

   Alternatively, you can generate a pure, reentrant parser.  The Bison
declaration `%define api.pure' says that you want the parser to be
reentrant.  It looks like this:

     %define api.pure full

   The result is that the communication variables `yylval' and `yylloc'
become local variables in `yyparse', and a different calling convention
is used for the lexical analyzer function `yylex'.  *Note Calling
Conventions for Pure Parsers: Pure Calling, for the details of this.
The variable `yynerrs' becomes local in `yyparse' in pull mode but it
becomes a member of yypstate in push mode.  (*note The Error Reporting
Function `yyerror': Error Reporting.).  The convention for calling
`yyparse' itself is unchanged.

   Whether the parser is pure has nothing to do with the grammar rules.
You can generate either a pure parser or a nonreentrant parser from any
valid grammar.


File: bison.info,  Node: Push Decl,  Next: Decl Summary,  Prev: Pure Decl,  Up: Declarations

3.8.12 A Push Parser
--------------------

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   A pull parser is called once and it takes control until all its input
is completely parsed.  A push parser, on the other hand, is called each
time a new token is made available.

   A push parser is typically useful when the parser is part of a main
event loop in the client's application.  This is typically a
requirement of a GUI, when the main event loop needs to be triggered
within a certain time period.

   Normally, Bison generates a pull parser.  The following Bison
declaration says that you want the parser to be a push parser (*note
api.push-pull: %define Summary.):

     %define api.push-pull push

   In almost all cases, you want to ensure that your push parser is also
a pure parser (*note A Pure (Reentrant) Parser: Pure Decl.).  The only
time you should create an impure push parser is to have backwards
compatibility with the impure Yacc pull mode interface.  Unless you know
what you are doing, your declarations should look like this:

     %define api.pure full
     %define api.push-pull push

   There is a major notable functional difference between the pure push
parser and the impure push parser.  It is acceptable for a pure push
parser to have many parser instances, of the same type of parser, in
memory at the same time.  An impure push parser should only use one
parser at a time.

   When a push parser is selected, Bison will generate some new symbols
in the generated parser.  `yypstate' is a structure that the generated
parser uses to store the parser's state.  `yypstate_new' is the
function that will create a new parser instance.  `yypstate_delete'
will free the resources associated with the corresponding parser
instance.  Finally, `yypush_parse' is the function that should be
called whenever a token is available to provide the parser.  A trivial
example of using a pure push parser would look like this:

     int status;
     yypstate *ps = yypstate_new ();
     do {
       status = yypush_parse (ps, yylex (), NULL);
     } while (status == YYPUSH_MORE);
     yypstate_delete (ps);

   If the user decided to use an impure push parser, a few things about
the generated parser will change.  The `yychar' variable becomes a
global variable instead of a variable in the `yypush_parse' function.
For this reason, the signature of the `yypush_parse' function is
changed to remove the token as a parameter.  A nonreentrant push parser
example would thus look like this:

     extern int yychar;
     int status;
     yypstate *ps = yypstate_new ();
     do {
       yychar = yylex ();
       status = yypush_parse (ps);
     } while (status == YYPUSH_MORE);
     yypstate_delete (ps);

   That's it. Notice the next token is put into the global variable
`yychar' for use by the next invocation of the `yypush_parse' function.

   Bison also supports both the push parser interface along with the
pull parser interface in the same generated parser.  In order to get
this functionality, you should replace the `%define api.push-pull push'
declaration with the `%define api.push-pull both' declaration.  Doing
this will create all of the symbols mentioned earlier along with the
two extra symbols, `yyparse' and `yypull_parse'.  `yyparse' can be used
exactly as it normally would be used.  However, the user should note
that it is implemented in the generated parser by calling
`yypull_parse'.  This makes the `yyparse' function that is generated
with the `%define api.push-pull both' declaration slower than the normal
`yyparse' function.  If the user calls the `yypull_parse' function it
will parse the rest of the input stream.  It is possible to
`yypush_parse' tokens to select a subgrammar and then `yypull_parse'
the rest of the input stream.  If you would like to switch back and
forth between between parsing styles, you would have to write your own
`yypull_parse' function that knows when to quit looking for input.  An
example of using the `yypull_parse' function would look like this:

     yypstate *ps = yypstate_new ();
     yypull_parse (ps); /* Will call the lexer */
     yypstate_delete (ps);

   Adding the `%define api.pure full' declaration does exactly the same
thing to the generated parser with `%define api.push-pull both' as it
did for `%define api.push-pull push'.


File: bison.info,  Node: Decl Summary,  Next: %define Summary,  Prev: Push Decl,  Up: Declarations

3.8.13 Bison Declaration Summary
--------------------------------

Here is a summary of the declarations used to define a grammar:

 -- Directive: %union
     Declare the collection of data types that semantic values may have
     (*note The Collection of Value Types: Union Decl.).

 -- Directive: %token
     Declare a terminal symbol (token type name) with no precedence or
     associativity specified (*note Token Type Names: Token Decl.).

 -- Directive: %right
     Declare a terminal symbol (token type name) that is
     right-associative (*note Operator Precedence: Precedence Decl.).

 -- Directive: %left
     Declare a terminal symbol (token type name) that is
     left-associative (*note Operator Precedence: Precedence Decl.).

 -- Directive: %nonassoc
     Declare a terminal symbol (token type name) that is nonassociative
     (*note Operator Precedence: Precedence Decl.).  Using it in a way
     that would be associative is a syntax error.

 -- Directive: %type
     Declare the type of semantic values for a nonterminal symbol
     (*note Nonterminal Symbols: Type Decl.).

 -- Directive: %start
     Specify the grammar's start symbol (*note The Start-Symbol: Start
     Decl.).

 -- Directive: %expect
     Declare the expected number of shift-reduce conflicts (*note
     Suppressing Conflict Warnings: Expect Decl.).


In order to change the behavior of `bison', use the following
directives:

 -- Directive: %code {CODE}
 -- Directive: %code QUALIFIER {CODE}
     Insert CODE verbatim into the output parser source at the default
     location or at the location specified by QUALIFIER.  *Note %code
     Summary::.

 -- Directive: %debug
     In the parser implementation file, define the macro `YYDEBUG' (or
     `PREFIXDEBUG' with `%define api.prefix PREFIX', see *note Multiple
     Parsers in the Same Program: Multiple Parsers.) to 1 if it is not
     already defined, so that the debugging facilities are compiled.
     *Note Tracing Your Parser: Tracing.

 -- Directive: %define VARIABLE
 -- Directive: %define VARIABLE VALUE
 -- Directive: %define VARIABLE "VALUE"
     Define a variable to adjust Bison's behavior.  *Note %define
     Summary::.

 -- Directive: %defines
     Write a parser header file containing macro definitions for the
     token type names defined in the grammar as well as a few other
     declarations.  If the parser implementation file is named `NAME.c'
     then the parser header file is named `NAME.h'.

     For C parsers, the parser header file declares `YYSTYPE' unless
     `YYSTYPE' is already defined as a macro or you have used a
     `<TYPE>' tag without using `%union'.  Therefore, if you are using
     a `%union' (*note More Than One Value Type: Multiple Types.) with
     components that require other definitions, or if you have defined
     a `YYSTYPE' macro or type definition (*note Data Types of Semantic
     Values: Value Type.), you need to arrange for these definitions to
     be propagated to all modules, e.g., by putting them in a
     prerequisite header that is included both by your parser and by any
     other module that needs `YYSTYPE'.

     Unless your parser is pure, the parser header file declares
     `yylval' as an external variable.  *Note A Pure (Reentrant)
     Parser: Pure Decl.

     If you have also used locations, the parser header file declares
     `YYLTYPE' and `yylloc' using a protocol similar to that of the
     `YYSTYPE' macro and `yylval'.  *Note Tracking Locations::.

     This parser header file is normally essential if you wish to put
     the definition of `yylex' in a separate source file, because
     `yylex' typically needs to be able to refer to the above-mentioned
     declarations and to the token type codes.  *Note Semantic Values
     of Tokens: Token Values.

     If you have declared `%code requires' or `%code provides', the
     output header also contains their code.  *Note %code Summary::.

     The generated header is protected against multiple inclusions with
     a C preprocessor guard: `YY_PREFIX_FILE_INCLUDED', where PREFIX
     and FILE are the prefix (*note Multiple Parsers in the Same
     Program: Multiple Parsers.) and generated file name turned
     uppercase, with each series of non alphanumerical characters
     converted to a single underscore.

     For instance with `%define api.prefix "calc"' and `%defines
     "lib/parse.h"', the header will be guarded as follows.
          #ifndef YY_CALC_LIB_PARSE_H_INCLUDED
          # define YY_CALC_LIB_PARSE_H_INCLUDED
          ...
          #endif /* ! YY_CALC_LIB_PARSE_H_INCLUDED */

 -- Directive: %defines DEFINES-FILE
     Same as above, but save in the file DEFINES-FILE.

 -- Directive: %destructor
     Specify how the parser should reclaim the memory associated to
     discarded symbols.  *Note Freeing Discarded Symbols: Destructor
     Decl.

 -- Directive: %file-prefix "PREFIX"
     Specify a prefix to use for all Bison output file names.  The names
     are chosen as if the grammar file were named `PREFIX.y'.

 -- Directive: %language "LANGUAGE"
     Specify the programming language for the generated parser.
     Currently supported languages include C, C++, and Java.  LANGUAGE
     is case-insensitive.


 -- Directive: %locations
     Generate the code processing the locations (*note Special Features
     for Use in Actions: Action Features.).  This mode is enabled as
     soon as the grammar uses the special `@N' tokens, but if your
     grammar does not use it, using `%locations' allows for more
     accurate syntax error messages.

 -- Directive: %no-lines
     Don't generate any `#line' preprocessor commands in the parser
     implementation file.  Ordinarily Bison writes these commands in the
     parser implementation file so that the C compiler and debuggers
     will associate errors and object code with your source file (the
     grammar file).  This directive causes them to associate errors
     with the parser implementation file, treating it as an independent
     source file in its own right.

 -- Directive: %output "FILE"
     Specify FILE for the parser implementation file.

 -- Directive: %pure-parser
     Deprecated version of `%define api.pure' (*note api.pure: %define
     Summary.), for which Bison is more careful to warn about
     unreasonable usage.

 -- Directive: %require "VERSION"
     Require version VERSION or higher of Bison.  *Note Require a
     Version of Bison: Require Decl.

 -- Directive: %skeleton "FILE"
     Specify the skeleton to use.

     If FILE does not contain a `/', FILE is the name of a skeleton
     file in the Bison installation directory.  If it does, FILE is an
     absolute file name or a file name relative to the directory of the
     grammar file.  This is similar to how most shells resolve commands.

 -- Directive: %token-table
     Generate an array of token names in the parser implementation file.
     The name of the array is `yytname'; `yytname[I]' is the name of
     the token whose internal Bison token code number is I.  The first
     three elements of `yytname' correspond to the predefined tokens
     `"$end"', `"error"', and `"$undefined"'; after these come the
     symbols defined in the grammar file.

     The name in the table includes all the characters needed to
     represent the token in Bison.  For single-character literals and
     literal strings, this includes the surrounding quoting characters
     and any escape sequences.  For example, the Bison single-character
     literal `'+'' corresponds to a three-character name, represented
     in C as `"'+'"'; and the Bison two-character literal string `"\\/"'
     corresponds to a five-character name, represented in C as
     `"\"\\\\/\""'.

     When you specify `%token-table', Bison also generates macro
     definitions for macros `YYNTOKENS', `YYNNTS', and `YYNRULES', and
     `YYNSTATES':

    `YYNTOKENS'
          The highest token number, plus one.

    `YYNNTS'
          The number of nonterminal symbols.

    `YYNRULES'
          The number of grammar rules,

    `YYNSTATES'
          The number of parser states (*note Parser States::).

 -- Directive: %verbose
     Write an extra output file containing verbose descriptions of the
     parser states and what is done for each type of lookahead token in
     that state.  *Note Understanding Your Parser: Understanding, for
     more information.

 -- Directive: %yacc
     Pretend the option `--yacc' was given, i.e., imitate Yacc,
     including its naming conventions.  *Note Bison Options::, for more.


File: bison.info,  Node: %define Summary,  Next: %code Summary,  Prev: Decl Summary,  Up: Declarations

3.8.14 %define Summary
----------------------

There are many features of Bison's behavior that can be controlled by
assigning the feature a single value.  For historical reasons, some
such features are assigned values by dedicated directives, such as
`%start', which assigns the start symbol.  However, newer such features
are associated with variables, which are assigned by the `%define'
directive:

 -- Directive: %define VARIABLE
 -- Directive: %define VARIABLE VALUE
 -- Directive: %define VARIABLE "VALUE"
     Define VARIABLE to VALUE.

     VALUE must be placed in quotation marks if it contains any
     character other than a letter, underscore, period, or non-initial
     dash or digit.  Omitting `"VALUE"' entirely is always equivalent
     to specifying `""'.

     It is an error if a VARIABLE is defined by `%define' multiple
     times, but see *note -D NAME[=VALUE]: Bison Options.

   The rest of this section summarizes variables and values that
`%define' accepts.

   Some VARIABLEs take Boolean values.  In this case, Bison will
complain if the variable definition does not meet one of the following
four conditions:

  1. `VALUE' is `true'

  2. `VALUE' is omitted (or `""' is specified).  This is equivalent to
     `true'.

  3. `VALUE' is `false'.

  4. VARIABLE is never defined.  In this case, Bison selects a default
     value.

   What VARIABLEs are accepted, as well as their meanings and default
values, depend on the selected target language and/or the parser
skeleton (*note %language: Decl Summary, *note %skeleton: Decl
Summary.).  Unaccepted VARIABLEs produce an error.  Some of the
accepted VARIABLEs are:

   * `api.location.type' 

        * Language(s): C++, Java

        * Purpose: Define the location type.  *Note User Defined
          Location Type::.

        * Accepted Values: String

        * Default Value: none

        * History: introduced in Bison 2.7

   * `api.prefix' 

        * Language(s): All

        * Purpose: Rename exported symbols.  *Note Multiple Parsers in
          the Same Program: Multiple Parsers.

        * Accepted Values: String

        * Default Value: `yy'

        * History: introduced in Bison 2.6

   * `api.pure' 

        * Language(s): C

        * Purpose: Request a pure (reentrant) parser program.  *Note A
          Pure (Reentrant) Parser: Pure Decl.

        * Accepted Values: `true', `false', `full'

          The value may be omitted: this is equivalent to specifying
          `true', as is the case for Boolean values.

          When `%define api.pure full' is used, the parser is made
          reentrant. This changes the signature for `yylex' (*note Pure
          Calling::), and also that of `yyerror' when the tracking of
          locations has been activated, as shown below.

          The `true' value is very similar to the `full' value, the only
          difference is in the signature of `yyerror' on Yacc parsers
          without `%parse-param', for historical reasons.

          I.e., if `%locations %define api.pure' is passed then the
          prototypes for `yyerror' are:

               void yyerror (char const *msg);                 // Yacc parsers.
               void yyerror (YYLTYPE *locp, char const *msg);  // GLR parsers.

          But if `%locations %define api.pure %parse-param {int
          *nastiness}' is used, then both parsers have the same
          signature:

               void yyerror (YYLTYPE *llocp, int *nastiness, char const *msg);

          (*note The Error Reporting Function `yyerror': Error
          Reporting.)

        * Default Value: `false'

        * History: the `full' value was introduced in Bison 2.7

   * `api.push-pull' 

        * Language(s): C (deterministic parsers only)

        * Purpose: Request a pull parser, a push parser, or both.
          *Note A Push Parser: Push Decl.  (The current push parsing
          interface is experimental and may evolve.  More user feedback
          will help to stabilize it.)

        * Accepted Values: `pull', `push', `both'

        * Default Value: `pull'

   * `lr.default-reductions' 

        * Language(s): all

        * Purpose: Specify the kind of states that are permitted to
          contain default reductions.  *Note Default Reductions::.
          (The ability to specify where default reductions should be
          used is experimental.  More user feedback will help to
          stabilize it.)

        * Accepted Values: `most', `consistent', `accepting'

        * Default Value:
             * `accepting' if `lr.type' is `canonical-lr'.

             * `most' otherwise.

   * `lr.keep-unreachable-states' 

        * Language(s): all

        * Purpose: Request that Bison allow unreachable parser states to
          remain in the parser tables.  *Note Unreachable States::.

        * Accepted Values: Boolean

        * Default Value: `false'

   * `lr.type' 

        * Language(s): all

        * Purpose: Specify the type of parser tables within the LR(1)
          family.  *Note LR Table Construction::.  (This feature is
          experimental.  More user feedback will help to stabilize it.)

        * Accepted Values: `lalr', `ielr', `canonical-lr'

        * Default Value: `lalr'

   * `namespace' 

        * Languages(s): C++

        * Purpose: Specify the namespace for the parser class.  For
          example, if you specify:

               %define namespace "foo::bar"

          Bison uses `foo::bar' verbatim in references such as:

               foo::bar::parser::semantic_type

          However, to open a namespace, Bison removes any leading `::'
          and then splits on any remaining occurrences:

               namespace foo { namespace bar {
                 class position;
                 class location;
               } }

        * Accepted Values: Any absolute or relative C++ namespace
          reference without a trailing `"::"'.  For example, `"foo"' or
          `"::foo::bar"'.

        * Default Value: The value specified by `%name-prefix', which
          defaults to `yy'.  This usage of `%name-prefix' is for
          backward compatibility and can be confusing since
          `%name-prefix' also specifies the textual prefix for the
          lexical analyzer function.  Thus, if you specify
          `%name-prefix', it is best to also specify `%define
          namespace' so that `%name-prefix' _only_ affects the lexical
          analyzer function.  For example, if you specify:

               %define namespace "foo"
               %name-prefix "bar::"

          The parser namespace is `foo' and `yylex' is referenced as
          `bar::lex'.

   * `parse.lac' 

        * Languages(s): C (deterministic parsers only)

        * Purpose: Enable LAC (lookahead correction) to improve syntax
          error handling.  *Note LAC::.

        * Accepted Values: `none', `full'

        * Default Value: `none'


File: bison.info,  Node: %code Summary,  Prev: %define Summary,  Up: Declarations

3.8.15 %code Summary
--------------------

The `%code' directive inserts code verbatim into the output parser
source at any of a predefined set of locations.  It thus serves as a
flexible and user-friendly alternative to the traditional Yacc
prologue, `%{CODE%}'.  This section summarizes the functionality of
`%code' for the various target languages supported by Bison.  For a
detailed discussion of how to use `%code' in place of `%{CODE%}' for
C/C++ and why it is advantageous to do so, *note Prologue
Alternatives::.

 -- Directive: %code {CODE}
     This is the unqualified form of the `%code' directive.  It inserts
     CODE verbatim at a language-dependent default location in the
     parser implementation.

     For C/C++, the default location is the parser implementation file
     after the usual contents of the parser header file.  Thus, the
     unqualified form replaces `%{CODE%}' for most purposes.

     For Java, the default location is inside the parser class.

 -- Directive: %code QUALIFIER {CODE}
     This is the qualified form of the `%code' directive.  QUALIFIER
     identifies the purpose of CODE and thus the location(s) where
     Bison should insert it.  That is, if you need to specify
     location-sensitive CODE that does not belong at the default
     location selected by the unqualified `%code' form, use this form
     instead.

   For any particular qualifier or for the unqualified form, if there
are multiple occurrences of the `%code' directive, Bison concatenates
the specified code in the order in which it appears in the grammar file.

   Not all qualifiers are accepted for all target languages.  Unaccepted
qualifiers produce an error.  Some of the accepted qualifiers are:

   * requires 

        * Language(s): C, C++

        * Purpose: This is the best place to write dependency code
          required for `YYSTYPE' and `YYLTYPE'.  In other words, it's
          the best place to define types referenced in `%union'
          directives, and it's the best place to override Bison's
          default `YYSTYPE' and `YYLTYPE' definitions.

        * Location(s): The parser header file and the parser
          implementation file before the Bison-generated `YYSTYPE' and
          `YYLTYPE' definitions.

   * provides 

        * Language(s): C, C++

        * Purpose: This is the best place to write additional
          definitions and declarations that should be provided to other
          modules.

        * Location(s): The parser header file and the parser
          implementation file after the Bison-generated `YYSTYPE',
          `YYLTYPE', and token definitions.

   * top 

        * Language(s): C, C++

        * Purpose: The unqualified `%code' or `%code requires' should
          usually be more appropriate than `%code top'.  However,
          occasionally it is necessary to insert code much nearer the
          top of the parser implementation file.  For example:

               %code top {
                 #define _GNU_SOURCE
                 #include <stdio.h>
               }

        * Location(s): Near the top of the parser implementation file.

   * imports 

        * Language(s): Java

        * Purpose: This is the best place to write Java import
          directives.

        * Location(s): The parser Java file after any Java package
          directive and before any class definitions.

   Though we say the insertion locations are language-dependent, they
are technically skeleton-dependent.  Writers of non-standard skeletons
however should choose their locations consistently with the behavior of
the standard Bison skeletons.


File: bison.info,  Node: Multiple Parsers,  Prev: Declarations,  Up: Grammar File

3.9 Multiple Parsers in the Same Program
========================================

Most programs that use Bison parse only one language and therefore
contain only one Bison parser.  But what if you want to parse more than
one language with the same program?  Then you need to avoid name
conflicts between different definitions of functions and variables such
as `yyparse', `yylval'.  To use different parsers from the same
compilation unit, you also need to avoid conflicts on types and macros
(e.g., `YYSTYPE') exported in the generated header.

   The easy way to do this is to define the `%define' variable
`api.prefix'.  With different `api.prefix's it is guaranteed that
headers do not conflict when included together, and that compiled
objects can be linked together too.  Specifying `%define api.prefix
PREFIX' (or passing the option `-Dapi.prefix=PREFIX', see *note
Invoking Bison: Invocation.) renames the interface functions and
variables of the Bison parser to start with PREFIX instead of `yy', and
all the macros to start by PREFIX (i.e., PREFIX upper-cased) instead of
`YY'.

   The renamed symbols include `yyparse', `yylex', `yyerror',
`yynerrs', `yylval', `yylloc', `yychar' and `yydebug'.  If you use a
push parser, `yypush_parse', `yypull_parse', `yypstate', `yypstate_new'
and `yypstate_delete' will also be renamed.  The renamed macros include
`YYSTYPE', `YYLTYPE', and `YYDEBUG', which is treated specifically --
more about this below.

   For example, if you use `%define api.prefix c', the names become
`cparse', `clex', ..., `CSTYPE', `CLTYPE', and so on.

   The `%define' variable `api.prefix' works in two different ways.  In
the implementation file, it works by adding macro definitions to the
beginning of the parser implementation file, defining `yyparse' as
`PREFIXparse', and so on:

     #define YYSTYPE CTYPE
     #define yyparse cparse
     #define yylval  clval
     ...
     YYSTYPE yylval;
     int yyparse (void);

   This effectively substitutes one name for the other in the entire
parser implementation file, thus the "original" names (`yylex',
`YYSTYPE', ...) are also usable in the parser implementation file.

   However, in the parser header file, the symbols are defined renamed,
for instance:

     extern CSTYPE clval;
     int cparse (void);

   The macro `YYDEBUG' is commonly used to enable the tracing support in
parsers.  To comply with this tradition, when `api.prefix' is used,
`YYDEBUG' (not renamed) is used as a default value:

     /* Enabling traces.  */
     #ifndef CDEBUG
     # if defined YYDEBUG
     #  if YYDEBUG
     #   define CDEBUG 1
     #  else
     #   define CDEBUG 0
     #  endif
     # else
     #  define CDEBUG 0
     # endif
     #endif
     #if CDEBUG
     extern int cdebug;
     #endif



   Prior to Bison 2.6, a feature similar to `api.prefix' was provided by
the obsolete directive `%name-prefix' (*note Bison Symbols: Table of
Symbols.) and the option `--name-prefix' (*note Bison Options::).


File: bison.info,  Node: Interface,  Next: Algorithm,  Prev: Grammar File,  Up: Top

4 Parser C-Language Interface
*****************************

The Bison parser is actually a C function named `yyparse'.  Here we
describe the interface conventions of `yyparse' and the other functions
that it needs to use.

   Keep in mind that the parser uses many C identifiers starting with
`yy' and `YY' for internal purposes.  If you use such an identifier
(aside from those in this manual) in an action or in epilogue in the
grammar file, you are likely to run into trouble.

* Menu:

* Parser Function::         How to call `yyparse' and what it returns.
* Push Parser Function::    How to call `yypush_parse' and what it returns.
* Pull Parser Function::    How to call `yypull_parse' and what it returns.
* Parser Create Function::  How to call `yypstate_new' and what it returns.
* Parser Delete Function::  How to call `yypstate_delete' and what it returns.
* Lexical::                 You must supply a function `yylex'
                              which reads tokens.
* Error Reporting::         You must supply a function `yyerror'.
* Action Features::         Special features for use in actions.
* Internationalization::    How to let the parser speak in the user's
                              native language.


File: bison.info,  Node: Parser Function,  Next: Push Parser Function,  Up: Interface

4.1 The Parser Function `yyparse'
=================================

You call the function `yyparse' to cause parsing to occur.  This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error.  You can also
write an action which directs `yyparse' to return immediately without
reading further.

 -- Function: int yyparse (void)
     The value returned by `yyparse' is 0 if parsing was successful
     (return is due to end-of-input).

     The value is 1 if parsing failed because of invalid input, i.e.,
     input that contains a syntax error or that causes `YYABORT' to be
     invoked.

     The value is 2 if parsing failed due to memory exhaustion.

   In an action, you can cause immediate return from `yyparse' by using
these macros:

 -- Macro: YYACCEPT
     Return immediately with value 0 (to report success).

 -- Macro: YYABORT
     Return immediately with value 1 (to report failure).

   If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way.  To do so, use the
declaration `%parse-param':

 -- Directive: %parse-param {ARGUMENT-DECLARATION}
     Declare that an argument declared by the braced-code
     ARGUMENT-DECLARATION is an additional `yyparse' argument.  The
     ARGUMENT-DECLARATION is used when declaring functions or
     prototypes.  The last identifier in ARGUMENT-DECLARATION must be
     the argument name.

   Here's an example.  Write this in the parser:

     %parse-param {int *nastiness}
     %parse-param {int *randomness}

Then call the parser like this:

     {
       int nastiness, randomness;
       ...  /* Store proper data in `nastiness' and `randomness'.  */
       value = yyparse (&nastiness, &randomness);
       ...
     }

In the grammar actions, use expressions like this to refer to the data:

     exp: ...    { ...; *randomness += 1; ... }

Using the following:
     %parse-param {int *randomness}

   Results in these signatures:
     void yyerror (int *randomness, const char *msg);
     int  yyparse (int *randomness);

Or, if both `%define api.pure full' (or just `%define api.pure') and
`%locations' are used:

     void yyerror (YYLTYPE *llocp, int *randomness, const char *msg);
     int  yyparse (int *randomness);


File: bison.info,  Node: Push Parser Function,  Next: Pull Parser Function,  Prev: Parser Function,  Up: Interface

4.2 The Push Parser Function `yypush_parse'
===========================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypush_parse' to parse a single token.  This
function is available if either the `%define api.push-pull push' or
`%define api.push-pull both' declaration is used.  *Note A Push Parser:
Push Decl.

 -- Function: int yypush_parse (yypstate *yyps)
     The value returned by `yypush_parse' is the same as for yyparse
     with the following exception: it returns `YYPUSH_MORE' if more
     input is required to finish parsing the grammar.


File: bison.info,  Node: Pull Parser Function,  Next: Parser Create Function,  Prev: Push Parser Function,  Up: Interface

4.3 The Pull Parser Function `yypull_parse'
===========================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypull_parse' to parse the rest of the input
stream.  This function is available if the `%define api.push-pull both'
declaration is used.  *Note A Push Parser: Push Decl.

 -- Function: int yypull_parse (yypstate *yyps)
     The value returned by `yypull_parse' is the same as for `yyparse'.


File: bison.info,  Node: Parser Create Function,  Next: Parser Delete Function,  Prev: Pull Parser Function,  Up: Interface

4.4 The Parser Create Function `yystate_new'
============================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypstate_new' to create a new parser instance.
This function is available if either the `%define api.push-pull push' or
`%define api.push-pull both' declaration is used.  *Note A Push Parser:
Push Decl.

 -- Function: yypstate* yypstate_new (void)
     The function will return a valid parser instance if there was
     memory available or 0 if no memory was available.  In impure mode,
     it will also return 0 if a parser instance is currently allocated.


File: bison.info,  Node: Parser Delete Function,  Next: Lexical,  Prev: Parser Create Function,  Up: Interface

4.5 The Parser Delete Function `yystate_delete'
===============================================

(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)

   You call the function `yypstate_delete' to delete a parser instance.
function is available if either the `%define api.push-pull push' or
`%define api.push-pull both' declaration is used.  *Note A Push Parser:
Push Decl.

 -- Function: void yypstate_delete (yypstate *yyps)
     This function will reclaim the memory associated with a parser
     instance.  After this call, you should no longer attempt to use
     the parser instance.


File: bison.info,  Node: Lexical,  Next: Error Reporting,  Prev: Parser Delete Function,  Up: Interface

4.6 The Lexical Analyzer Function `yylex'
=========================================

The "lexical analyzer" function, `yylex', recognizes tokens from the
input stream and returns them to the parser.  Bison does not create
this function automatically; you must write it so that `yyparse' can
call it.  The function is sometimes referred to as a lexical scanner.

   In simple programs, `yylex' is often defined at the end of the Bison
grammar file.  If `yylex' is defined in a separate source file, you
need to arrange for the token-type macro definitions to be available
there.  To do this, use the `-d' option when you run Bison, so that it
will write these macro definitions into the separate parser header
file, `NAME.tab.h', which you can include in the other source files
that need it.  *Note Invoking Bison: Invocation.

* Menu:

* Calling Convention::  How `yyparse' calls `yylex'.
* Token Values::        How `yylex' must return the semantic value
                          of the token it has read.
* Token Locations::     How `yylex' must return the text location
                          (line number, etc.) of the token, if the
                          actions want that.
* Pure Calling::        How the calling convention differs in a pure parser
                          (*note A Pure (Reentrant) Parser: Pure Decl.).


File: bison.info,  Node: Calling Convention,  Next: Token Values,  Up: Lexical

4.6.1 Calling Convention for `yylex'
------------------------------------

The value that `yylex' returns must be the positive numeric code for
the type of token it has just found; a zero or negative value signifies
end-of-input.

   When a token is referred to in the grammar rules by a name, that name
in the parser implementation file becomes a C macro whose definition is
the proper numeric code for that token type.  So `yylex' can use the
name to indicate that type.  *Note Symbols::.

   When a token is referred to in the grammar rules by a character
literal, the numeric code for that character is also the code for the
token type.  So `yylex' can simply return that character code, possibly
converted to `unsigned char' to avoid sign-extension.  The null
character must not be used this way, because its code is zero and that
signifies end-of-input.

   Here is an example showing these things:

     int
     yylex (void)
     {
       ...
       if (c == EOF)    /* Detect end-of-input.  */
         return 0;
       ...
       if (c == '+' || c == '-')
         return c;      /* Assume token type for `+' is '+'.  */
       ...
       return INT;      /* Return the type of the token.  */
       ...
     }

This interface has been designed so that the output from the `lex'
utility can be used without change as the definition of `yylex'.

   If the grammar uses literal string tokens, there are two ways that
`yylex' can determine the token type codes for them:

   * If the grammar defines symbolic token names as aliases for the
     literal string tokens, `yylex' can use these symbolic names like
     all others.  In this case, the use of the literal string tokens in
     the grammar file has no effect on `yylex'.

   * `yylex' can find the multicharacter token in the `yytname' table.
     The index of the token in the table is the token type's code.  The
     name of a multicharacter token is recorded in `yytname' with a
     double-quote, the token's characters, and another double-quote.
     The token's characters are escaped as necessary to be suitable as
     input to Bison.

     Here's code for looking up a multicharacter token in `yytname',
     assuming that the characters of the token are stored in
     `token_buffer', and assuming that the token does not contain any
     characters like `"' that require escaping.

          for (i = 0; i < YYNTOKENS; i++)
            {
              if (yytname[i] != 0
                  && yytname[i][0] == '"'
                  && ! strncmp (yytname[i] + 1, token_buffer,
                                strlen (token_buffer))
                  && yytname[i][strlen (token_buffer) + 1] == '"'
                  && yytname[i][strlen (token_buffer) + 2] == 0)
                break;
            }

     The `yytname' table is generated only if you use the
     `%token-table' declaration.  *Note Decl Summary::.


File: bison.info,  Node: Token Values,  Next: Token Locations,  Prev: Calling Convention,  Up: Lexical

4.6.2 Semantic Values of Tokens
-------------------------------

In an ordinary (nonreentrant) parser, the semantic value of the token
must be stored into the global variable `yylval'.  When you are using
just one data type for semantic values, `yylval' has that type.  Thus,
if the type is `int' (the default), you might write this in `yylex':

       ...
       yylval = value;  /* Put value onto Bison stack.  */
       return INT;      /* Return the type of the token.  */
       ...

   When you are using multiple data types, `yylval''s type is a union
made from the `%union' declaration (*note The Collection of Value
Types: Union Decl.).  So when you store a token's value, you must use
the proper member of the union.  If the `%union' declaration looks like
this:

     %union {
       int intval;
       double val;
       symrec *tptr;
     }

then the code in `yylex' might look like this:

       ...
       yylval.intval = value; /* Put value onto Bison stack.  */
       return INT;            /* Return the type of the token.  */
       ...


File: bison.info,  Node: Token Locations,  Next: Pure Calling,  Prev: Token Values,  Up: Lexical

4.6.3 Textual Locations of Tokens
---------------------------------

If you are using the `@N'-feature (*note Tracking Locations::) in
actions to keep track of the textual locations of tokens and groupings,
then you must provide this information in `yylex'.  The function
`yyparse' expects to find the textual location of a token just parsed
in the global variable `yylloc'.  So `yylex' must store the proper data
in that variable.

   By default, the value of `yylloc' is a structure and you need only
initialize the members that are going to be used by the actions.  The
four members are called `first_line', `first_column', `last_line' and
`last_column'.  Note that the use of this feature makes the parser
noticeably slower.

   The data type of `yylloc' has the name `YYLTYPE'.


File: bison.info,  Node: Pure Calling,  Prev: Token Locations,  Up: Lexical

4.6.4 Calling Conventions for Pure Parsers
------------------------------------------

When you use the Bison declaration `%define api.pure full' to request a
pure, reentrant parser, the global communication variables `yylval' and
`yylloc' cannot be used.  (*Note A Pure (Reentrant) Parser: Pure Decl.)
In such parsers the two global variables are replaced by pointers
passed as arguments to `yylex'.  You must declare them as shown here,
and pass the information back by storing it through those pointers.

     int
     yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
     {
       ...
       *lvalp = value;  /* Put value onto Bison stack.  */
       return INT;      /* Return the type of the token.  */
       ...
     }

   If the grammar file does not use the `@' constructs to refer to
textual locations, then the type `YYLTYPE' will not be defined.  In
this case, omit the second argument; `yylex' will be called with only
one argument.

   If you wish to pass the additional parameter data to `yylex', use
`%lex-param' just like `%parse-param' (*note Parser Function::).

 -- Directive: lex-param {ARGUMENT-DECLARATION}
     Declare that the braced-code ARGUMENT-DECLARATION is an additional
     `yylex' argument declaration.

For instance:

     %lex-param   {int *nastiness}

results in the following signature:

     int yylex (int *nastiness);

If `%define api.pure full' (or just `%define api.pure') is added:

     int yylex (YYSTYPE *lvalp, int *nastiness);


File: bison.info,  Node: Error Reporting,  Next: Action Features,  Prev: Lexical,  Up: Interface

4.7 The Error Reporting Function `yyerror'
==========================================

The Bison parser detects a "syntax error" or "parse error" whenever it
reads a token which cannot satisfy any syntax rule.  An action in the
grammar can also explicitly proclaim an error, using the macro
`YYERROR' (*note Special Features for Use in Actions: Action Features.).

   The Bison parser expects to report the error by calling an error
reporting function named `yyerror', which you must supply.  It is
called by `yyparse' whenever a syntax error is found, and it receives
one argument.  For a syntax error, the string is normally
`"syntax error"'.

   If you invoke the directive `%error-verbose' in the Bison
declarations section (*note The Bison Declarations Section: Bison
Declarations.), then Bison provides a more verbose and specific error
message string instead of just plain `"syntax error"'.  However, that
message sometimes contains incorrect information if LAC is not enabled
(*note LAC::).

   The parser can detect one other kind of error: memory exhaustion.
This can happen when the input contains constructions that are very
deeply nested.  It isn't likely you will encounter this, since the Bison
parser normally extends its stack automatically up to a very large
limit.  But if memory is exhausted, `yyparse' calls `yyerror' in the
usual fashion, except that the argument string is `"memory exhausted"'.

   In some cases diagnostics like `"syntax error"' are translated
automatically from English to some other language before they are
passed to `yyerror'.  *Note Internationalization::.

   The following definition suffices in simple programs:

     void
     yyerror (char const *s)
     {
       fprintf (stderr, "%s\n", s);
     }

   After `yyerror' returns to `yyparse', the latter will attempt error
recovery if you have written suitable error recovery grammar rules
(*note Error Recovery::).  If recovery is impossible, `yyparse' will
immediately return 1.

   Obviously, in location tracking pure parsers, `yyerror' should have
an access to the current location. With `%define api.pure', this is
indeed the case for the GLR parsers, but not for the Yacc parser, for
historical reasons, and this is the why `%define api.pure full' should
be prefered over `%define api.pure'.

   When `%locations %define api.pure full' is used, `yyerror' has the
following signature:

     void yyerror (YYLTYPE *locp, char const *msg);

The prototypes are only indications of how the code produced by Bison
uses `yyerror'.  Bison-generated code always ignores the returned
value, so `yyerror' can return any type, including `void'.  Also,
`yyerror' can be a variadic function; that is why the message is always
passed last.

   Traditionally `yyerror' returns an `int' that is always ignored, but
this is purely for historical reasons, and `void' is preferable since
it more accurately describes the return type for `yyerror'.

   The variable `yynerrs' contains the number of syntax errors reported
so far.  Normally this variable is global; but if you request a pure
parser (*note A Pure (Reentrant) Parser: Pure Decl.)  then it is a
local variable which only the actions can access.


File: bison.info,  Node: Action Features,  Next: Internationalization,  Prev: Error Reporting,  Up: Interface

4.8 Special Features for Use in Actions
=======================================

Here is a table of Bison constructs, variables and macros that are
useful in actions.

 -- Variable: $$
     Acts like a variable that contains the semantic value for the
     grouping made by the current rule.  *Note Actions::.

 -- Variable: $N
     Acts like a variable that contains the semantic value for the Nth
     component of the current rule.  *Note Actions::.

 -- Variable: $<TYPEALT>$
     Like `$$' but specifies alternative TYPEALT in the union specified
     by the `%union' declaration.  *Note Data Types of Values in
     Actions: Action Types.

 -- Variable: $<TYPEALT>N
     Like `$N' but specifies alternative TYPEALT in the union specified
     by the `%union' declaration.  *Note Data Types of Values in
     Actions: Action Types.

 -- Macro: YYABORT `;'
     Return immediately from `yyparse', indicating failure.  *Note The
     Parser Function `yyparse': Parser Function.

 -- Macro: YYACCEPT `;'
     Return immediately from `yyparse', indicating success.  *Note The
     Parser Function `yyparse': Parser Function.

 -- Macro: YYBACKUP (TOKEN, VALUE)`;'
     Unshift a token.  This macro is allowed only for rules that reduce
     a single value, and only when there is no lookahead token.  It is
     also disallowed in GLR parsers.  It installs a lookahead token
     with token type TOKEN and semantic value VALUE; then it discards
     the value that was going to be reduced by this rule.

     If the macro is used when it is not valid, such as when there is a
     lookahead token already, then it reports a syntax error with a
     message `cannot back up' and performs ordinary error recovery.

     In either case, the rest of the action is not executed.

 -- Macro: YYEMPTY
     Value stored in `yychar' when there is no lookahead token.

 -- Macro: YYEOF
     Value stored in `yychar' when the lookahead is the end of the input
     stream.

 -- Macro: YYERROR `;'
     Cause an immediate syntax error.  This statement initiates error
     recovery just as if the parser itself had detected an error;
     however, it does not call `yyerror', and does not print any
     message.  If you want to print an error message, call `yyerror'
     explicitly before the `YYERROR;' statement.  *Note Error
     Recovery::.

 -- Macro: YYRECOVERING
     The expression `YYRECOVERING ()' yields 1 when the parser is
     recovering from a syntax error, and 0 otherwise.  *Note Error
     Recovery::.

 -- Variable: yychar
     Variable containing either the lookahead token, or `YYEOF' when the
     lookahead is the end of the input stream, or `YYEMPTY' when no
     lookahead has been performed so the next token is not yet known.
     Do not modify `yychar' in a deferred semantic action (*note GLR
     Semantic Actions::).  *Note Lookahead Tokens: Lookahead.

 -- Macro: yyclearin `;'
     Discard the current lookahead token.  This is useful primarily in
     error rules.  Do not invoke `yyclearin' in a deferred semantic
     action (*note GLR Semantic Actions::).  *Note Error Recovery::.

 -- Macro: yyerrok `;'
     Resume generating error messages immediately for subsequent syntax
     errors.  This is useful primarily in error rules.  *Note Error
     Recovery::.

 -- Variable: yylloc
     Variable containing the lookahead token location when `yychar' is
     not set to `YYEMPTY' or `YYEOF'.  Do not modify `yylloc' in a
     deferred semantic action (*note GLR Semantic Actions::).  *Note
     Actions and Locations: Actions and Locations.

 -- Variable: yylval
     Variable containing the lookahead token semantic value when
     `yychar' is not set to `YYEMPTY' or `YYEOF'.  Do not modify
     `yylval' in a deferred semantic action (*note GLR Semantic
     Actions::).  *Note Actions: Actions.

 -- Value: @$
     Acts like a structure variable containing information on the
     textual location of the grouping made by the current rule.  *Note
     Tracking Locations::.


 -- Value: @N
     Acts like a structure variable containing information on the
     textual location of the Nth component of the current rule.  *Note
     Tracking Locations::.


File: bison.info,  Node: Internationalization,  Prev: Action Features,  Up: Interface

4.9 Parser Internationalization
===============================

A Bison-generated parser can print diagnostics, including error and
tracing messages.  By default, they appear in English.  However, Bison
also supports outputting diagnostics in the user's native language.  To
make this work, the user should set the usual environment variables.
*Note The User's View: (gettext)Users.  For example, the shell command
`export LC_ALL=fr_CA.UTF-8' might set the user's locale to French
Canadian using the UTF-8 encoding.  The exact set of available locales
depends on the user's installation.

   The maintainer of a package that uses a Bison-generated parser
enables the internationalization of the parser's output through the
following steps.  Here we assume a package that uses GNU Autoconf and
GNU Automake.

  1. Into the directory containing the GNU Autoconf macros used by the
     package --often called `m4'-- copy the `bison-i18n.m4' file
     installed by Bison under `share/aclocal/bison-i18n.m4' in Bison's
     installation directory.  For example:

          cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4

  2. In the top-level `configure.ac', after the `AM_GNU_GETTEXT'
     invocation, add an invocation of `BISON_I18N'.  This macro is
     defined in the file `bison-i18n.m4' that you copied earlier.  It
     causes `configure' to find the value of the `BISON_LOCALEDIR'
     variable, and it defines the source-language symbol `YYENABLE_NLS'
     to enable translations in the Bison-generated parser.

  3. In the `main' function of your program, designate the directory
     containing Bison's runtime message catalog, through a call to
     `bindtextdomain' with domain name `bison-runtime'.  For example:

          bindtextdomain ("bison-runtime", BISON_LOCALEDIR);

     Typically this appears after any other call `bindtextdomain
     (PACKAGE, LOCALEDIR)' that your package already has.  Here we rely
     on `BISON_LOCALEDIR' to be defined as a string through the
     `Makefile'.

  4. In the `Makefile.am' that controls the compilation of the `main'
     function, make `BISON_LOCALEDIR' available as a C preprocessor
     macro, either in `DEFS' or in `AM_CPPFLAGS'.  For example:

          DEFS = @DEFS@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'

     or:

          AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'

  5. Finally, invoke the command `autoreconf' to generate the build
     infrastructure.


File: bison.info,  Node: Algorithm,  Next: Error Recovery,  Prev: Interface,  Up: Top

5 The Bison Parser Algorithm
****************************

As Bison reads tokens, it pushes them onto a stack along with their
semantic values.  The stack is called the "parser stack".  Pushing a
token is traditionally called "shifting".

   For example, suppose the infix calculator has read `1 + 5 *', with a
`3' to come.  The stack will have four elements, one for each token
that was shifted.

   But the stack does not always have an element for each token read.
When the last N tokens and groupings shifted match the components of a
grammar rule, they can be combined according to that rule.  This is
called "reduction".  Those tokens and groupings are replaced on the
stack by a single grouping whose symbol is the result (left hand side)
of that rule.  Running the rule's action is part of the process of
reduction, because this is what computes the semantic value of the
resulting grouping.

   For example, if the infix calculator's parser stack contains this:

     1 + 5 * 3

and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:

     expr: expr '*' expr;

Then the stack contains just these three elements:

     1 + 15

At this point, another reduction can be made, resulting in the single
value 16.  Then the newline token can be shifted.

   The parser tries, by shifts and reductions, to reduce the entire
input down to a single grouping whose symbol is the grammar's
start-symbol (*note Languages and Context-Free Grammars: Language and
Grammar.).

   This kind of parser is known in the literature as a bottom-up parser.

* Menu:

* Lookahead::         Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR::         How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.


File: bison.info,  Node: Lookahead,  Next: Shift/Reduce,  Up: Algorithm

5.1 Lookahead Tokens
====================

The Bison parser does _not_ always reduce immediately as soon as the
last N tokens and groupings match a rule.  This is because such a
simple strategy is inadequate to handle most languages.  Instead, when a
reduction is possible, the parser sometimes "looks ahead" at the next
token in order to decide what to do.

   When a token is read, it is not immediately shifted; first it
becomes the "lookahead token", which is not on the stack.  Now the
parser can perform one or more reductions of tokens and groupings on
the stack, while the lookahead token remains off to the side.  When no
more reductions should take place, the lookahead token is shifted onto
the stack.  This does not mean that all possible reductions have been
done; depending on the token type of the lookahead token, some rules
may choose to delay their application.

   Here is a simple case where lookahead is needed.  These three rules
define expressions which contain binary addition operators and postfix
unary factorial operators (`!'), and allow parentheses for grouping.

     expr:
       term '+' expr
     | term
     ;

     term:
       '(' expr ')'
     | term '!'
     | "number"
     ;

   Suppose that the tokens `1 + 2' have been read and shifted; what
should be done?  If the following token is `)', then the first three
tokens must be reduced to form an `expr'.  This is the only valid
course, because shifting the `)' would produce a sequence of symbols
`term ')'', and no rule allows this.

   If the following token is `!', then it must be shifted immediately so
that `2 !' can be reduced to make a `term'.  If instead the parser were
to reduce before shifting, `1 + 2' would become an `expr'.  It would
then be impossible to shift the `!' because doing so would produce on
the stack the sequence of symbols `expr '!''.  No rule allows that
sequence.

   The lookahead token is stored in the variable `yychar'.  Its
semantic value and location, if any, are stored in the variables
`yylval' and `yylloc'.  *Note Special Features for Use in Actions:
Action Features.


File: bison.info,  Node: Shift/Reduce,  Next: Precedence,  Prev: Lookahead,  Up: Algorithm

5.2 Shift/Reduce Conflicts
==========================

Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:

     if_stmt:
       "if" expr "then" stmt
     | "if" expr "then" stmt "else" stmt
     ;

Here `"if"', `"then"' and `"else"' are terminal symbols for specific
keyword tokens.

   When the `"else"' token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule.  But it is also legitimate to shift the
`"else"', because that would lead to eventual reduction by the second
rule.

   This situation, where either a shift or a reduction would be valid,
is called a "shift/reduce conflict".  Bison is designed to resolve
these conflicts by choosing to shift, unless otherwise directed by
operator precedence declarations.  To see the reason for this, let's
contrast it with the other alternative.

   Since the parser prefers to shift the `"else"', the result is to
attach the else-clause to the innermost if-statement, making these two
inputs equivalent:

     if x then if y then win; else lose;

     if x then do; if y then win; else lose; end;

   But if the parser chose to reduce when possible rather than shift,
the result would be to attach the else-clause to the outermost
if-statement, making these two inputs equivalent:

     if x then if y then win; else lose;

     if x then do; if y then win; end; else lose;

   The conflict exists because the grammar as written is ambiguous:
either parsing of the simple nested if-statement is legitimate.  The
established convention is that these ambiguities are resolved by
attaching the else-clause to the innermost if-statement; this is what
Bison accomplishes by choosing to shift rather than reduce.  (It would
ideally be cleaner to write an unambiguous grammar, but that is very
hard to do in this case.)  This particular ambiguity was first
encountered in the specifications of Algol 60 and is called the
"dangling `else'" ambiguity.

   To avoid warnings from Bison about predictable, legitimate
shift/reduce conflicts, you can use the `%expect N' declaration.  There
will be no warning as long as the number of shift/reduce conflicts is
exactly N, and Bison will report an error if there is a different
number.  *Note Suppressing Conflict Warnings: Expect Decl.  However, we
don't recommend the use of `%expect' (except `%expect 0'!), as an equal
number of conflicts does not mean that they are the _same_.  When
possible, you should rather use precedence directives to _fix_ the
conflicts explicitly (*note Using Precedence For Non Operators: Non
Operators.).

   The definition of `if_stmt' above is solely to blame for the
conflict, but the conflict does not actually appear without additional
rules.  Here is a complete Bison grammar file that actually manifests
the conflict:

     %%
     stmt:
       expr
     | if_stmt
     ;

     if_stmt:
       "if" expr "then" stmt
     | "if" expr "then" stmt "else" stmt
     ;

     expr:
       "identifier"
     ;


File: bison.info,  Node: Precedence,  Next: Contextual Precedence,  Prev: Shift/Reduce,  Up: Algorithm

5.3 Operator Precedence
=======================

Another situation where shift/reduce conflicts appear is in arithmetic
expressions.  Here shifting is not always the preferred resolution; the
Bison declarations for operator precedence allow you to specify when to
shift and when to reduce.

* Menu:

* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence in Bison grammars.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.
* Non Operators::     Using precedence for general conflicts.


File: bison.info,  Node: Why Precedence,  Next: Using Precedence,  Up: Precedence

5.3.1 When Precedence is Needed
-------------------------------

Consider the following ambiguous grammar fragment (ambiguous because the
input `1 - 2 * 3' can be parsed in two different ways):

     expr:
       expr '-' expr
     | expr '*' expr
     | expr '<' expr
     | '(' expr ')'
     ...
     ;

Suppose the parser has seen the tokens `1', `-' and `2'; should it
reduce them via the rule for the subtraction operator?  It depends on
the next token.  Of course, if the next token is `)', we must reduce;
shifting is invalid because no single rule can reduce the token
sequence `- 2 )' or anything starting with that.  But if the next token
is `*' or `<', we have a choice: either shifting or reduction would
allow the parse to complete, but with different results.

   To decide which one Bison should do, we must consider the results.
If the next operator token OP is shifted, then it must be reduced first
in order to permit another opportunity to reduce the difference.  The
result is (in effect) `1 - (2 OP 3)'.  On the other hand, if the
subtraction is reduced before shifting OP, the result is
`(1 - 2) OP 3'.  Clearly, then, the choice of shift or reduce should
depend on the relative precedence of the operators `-' and OP: `*'
should be shifted first, but not `<'.

   What about input such as `1 - 2 - 5'; should this be `(1 - 2) - 5'
or should it be `1 - (2 - 5)'?  For most operators we prefer the
former, which is called "left association".  The latter alternative,
"right association", is desirable for assignment operators.  The choice
of left or right association is a matter of whether the parser chooses
to shift or reduce when the stack contains `1 - 2' and the lookahead
token is `-': shifting makes right-associativity.


File: bison.info,  Node: Using Precedence,  Next: Precedence Examples,  Prev: Why Precedence,  Up: Precedence

5.3.2 Specifying Operator Precedence
------------------------------------

Bison allows you to specify these choices with the operator precedence
declarations `%left' and `%right'.  Each such declaration contains a
list of tokens, which are operators whose precedence and associativity
is being declared.  The `%left' declaration makes all those operators
left-associative and the `%right' declaration makes them
right-associative.  A third alternative is `%nonassoc', which declares
that it is a syntax error to find the same operator twice "in a row".

   The relative precedence of different operators is controlled by the
order in which they are declared.  The first `%left' or `%right'
declaration in the file declares the operators whose precedence is
lowest, the next such declaration declares the operators whose
precedence is a little higher, and so on.


File: bison.info,  Node: Precedence Examples,  Next: How Precedence,  Prev: Using Precedence,  Up: Precedence

5.3.3 Precedence Examples
-------------------------

In our example, we would want the following declarations:

     %left '<'
     %left '-'
     %left '*'

   In a more complete example, which supports other operators as well,
we would declare them in groups of equal precedence.  For example,
`'+'' is declared with `'-'':

     %left '<' '>' '=' "!=" "<=" ">="
     %left '+' '-'
     %left '*' '/'


File: bison.info,  Node: How Precedence,  Next: Non Operators,  Prev: Precedence Examples,  Up: Precedence

5.3.4 How Precedence Works
--------------------------

The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared.  The second effect is to assign
precedence levels to certain rules: each rule gets its precedence from
the last terminal symbol mentioned in the components.  (You can also
specify explicitly the precedence of a rule.  *Note Context-Dependent
Precedence: Contextual Precedence.)

   Finally, the resolution of conflicts works by comparing the
precedence of the rule being considered with that of the lookahead
token.  If the token's precedence is higher, the choice is to shift.
If the rule's precedence is higher, the choice is to reduce.  If they
have equal precedence, the choice is made based on the associativity of
that precedence level.  The verbose output file made by `-v' (*note
Invoking Bison: Invocation.) says how each conflict was resolved.

   Not all rules and not all tokens have precedence.  If either the
rule or the lookahead token has no precedence, then the default is to
shift.


File: bison.info,  Node: Non Operators,  Prev: How Precedence,  Up: Precedence

5.3.5 Using Precedence For Non Operators
----------------------------------------

Using properly precedence and associativity directives can help fixing
shift/reduce conflicts that do not involve arithmetics-like operators.
For instance, the "dangling `else'" problem (*note Shift/Reduce
Conflicts: Shift/Reduce.) can be solved elegantly in two different ways.

   In the present case, the conflict is between the token `"else"'
willing to be shifted, and the rule `if_stmt: "if" expr "then" stmt',
asking for reduction.  By default, the precedence of a rule is that of
its last token, here `"then"', so the conflict will be solved
appropriately by giving `"else"' a precedence higher than that of
`"then"', for instance as follows:

     %nonassoc "then"
     %nonassoc "else"

   Alternatively, you may give both tokens the same precedence, in
which case associativity is used to solve the conflict.  To preserve
the shift action, use right associativity:

     %right "then" "else"

   Neither solution is perfect however.  Since Bison does not provide,
so far, support for "scoped" precedence, both force you to declare the
precedence of these keywords with respect to the other operators your
grammar.  Therefore, instead of being warned about new conflicts you
would be unaware of (e.g., a shift/reduce conflict due to `if test then
1 else 2 + 3' being ambiguous: `if test then 1 else (2 + 3)' or `(if
test then 1 else 2) + 3'?), the conflict will be already "fixed".


File: bison.info,  Node: Contextual Precedence,  Next: Parser States,  Prev: Precedence,  Up: Algorithm

5.4 Context-Dependent Precedence
================================

Often the precedence of an operator depends on the context.  This sounds
outlandish at first, but it is really very common.  For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary
operator.

   The Bison precedence declarations, `%left', `%right' and
`%nonassoc', can only be used once for a given token; so a token has
only one precedence declared in this way.  For context-dependent
precedence, you need to use an additional mechanism: the `%prec'
modifier for rules.

   The `%prec' modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that
rule.  It's not necessary for that symbol to appear otherwise in the
rule.  The modifier's syntax is:

     %prec TERMINAL-SYMBOL

and it is written after the components of the rule.  Its effect is to
assign the rule the precedence of TERMINAL-SYMBOL, overriding the
precedence that would be deduced for it in the ordinary way.  The
altered rule precedence then affects how conflicts involving that rule
are resolved (*note Operator Precedence: Precedence.).

   Here is how `%prec' solves the problem of unary minus.  First,
declare a precedence for a fictitious terminal symbol named `UMINUS'.
There are no tokens of this type, but the symbol serves to stand for its
precedence:

     ...
     %left '+' '-'
     %left '*'
     %left UMINUS

   Now the precedence of `UMINUS' can be used in specific rules:

     exp:
       ...
     | exp '-' exp
       ...
     | '-' exp %prec UMINUS


File: bison.info,  Node: Parser States,  Next: Reduce/Reduce,  Prev: Contextual Precedence,  Up: Algorithm

5.5 Parser States
=================

The function `yyparse' is implemented using a finite-state machine.
The values pushed on the parser stack are not simply token type codes;
they represent the entire sequence of terminal and nonterminal symbols
at or near the top of the stack.  The current state collects all the
information about previous input which is relevant to deciding what to
do next.

   Each time a lookahead token is read, the current parser state
together with the type of lookahead token are looked up in a table.
This table entry can say, "Shift the lookahead token."  In this case,
it also specifies the new parser state, which is pushed onto the top of
the parser stack.  Or it can say, "Reduce using rule number N."  This
means that a certain number of tokens or groupings are taken off the
top of the stack, and replaced by one grouping.  In other words, that
number of states are popped from the stack, and one new state is pushed.

   There is one other alternative: the table can say that the lookahead
token is erroneous in the current state.  This causes error processing
to begin (*note Error Recovery::).


File: bison.info,  Node: Reduce/Reduce,  Next: Mysterious Conflicts,  Prev: Parser States,  Up: Algorithm

5.6 Reduce/Reduce Conflicts
===========================

A reduce/reduce conflict occurs if there are two or more rules that
apply to the same sequence of input.  This usually indicates a serious
error in the grammar.

   For example, here is an erroneous attempt to define a sequence of
zero or more `word' groupings.

     sequence:
       /* empty */    { printf ("empty sequence\n"); }
     | maybeword
     | sequence word  { printf ("added word %s\n", $2); }
     ;

     maybeword:
       /* empty */   { printf ("empty maybeword\n"); }
     | word          { printf ("single word %s\n", $1); }
     ;

The error is an ambiguity: there is more than one way to parse a single
`word' into a `sequence'.  It could be reduced to a `maybeword' and
then into a `sequence' via the second rule.  Alternatively,
nothing-at-all could be reduced into a `sequence' via the first rule,
and this could be combined with the `word' using the third rule for
`sequence'.

   There is also more than one way to reduce nothing-at-all into a
`sequence'.  This can be done directly via the first rule, or
indirectly via `maybeword' and then the second rule.

   You might think that this is a distinction without a difference,
because it does not change whether any particular input is valid or
not.  But it does affect which actions are run.  One parsing order runs
the second rule's action; the other runs the first rule's action and
the third rule's action.  In this example, the output of the program
changes.

   Bison resolves a reduce/reduce conflict by choosing to use the rule
that appears first in the grammar, but it is very risky to rely on
this.  Every reduce/reduce conflict must be studied and usually
eliminated.  Here is the proper way to define `sequence':

     sequence:
       /* empty */    { printf ("empty sequence\n"); }
     | sequence word  { printf ("added word %s\n", $2); }
     ;

   Here is another common error that yields a reduce/reduce conflict:

     sequence:
       /* empty */
     | sequence words
     | sequence redirects
     ;

     words:
       /* empty */
     | words word
     ;

     redirects:
       /* empty */
     | redirects redirect
     ;

The intention here is to define a sequence which can contain either
`word' or `redirect' groupings.  The individual definitions of
`sequence', `words' and `redirects' are error-free, but the three
together make a subtle ambiguity: even an empty input can be parsed in
infinitely many ways!

   Consider: nothing-at-all could be a `words'.  Or it could be two
`words' in a row, or three, or any number.  It could equally well be a
`redirects', or two, or any number.  Or it could be a `words' followed
by three `redirects' and another `words'.  And so on.

   Here are two ways to correct these rules.  First, to make it a
single level of sequence:

     sequence:
       /* empty */
     | sequence word
     | sequence redirect
     ;

   Second, to prevent either a `words' or a `redirects' from being
empty:

     sequence:
       /* empty */
     | sequence words
     | sequence redirects
     ;

     words:
       word
     | words word
     ;

     redirects:
       redirect
     | redirects redirect
     ;

   Yet this proposal introduces another kind of ambiguity!  The input
`word word' can be parsed as a single `words' composed of two `word's,
or as two one-`word' `words' (and likewise for `redirect'/`redirects').
However this ambiguity is now a shift/reduce conflict, and therefore it
can now be addressed with precedence directives.

   To simplify the matter, we will proceed with `word' and `redirect'
being tokens: `"word"' and `"redirect"'.

   To prefer the longest `words', the conflict between the token
`"word"' and the rule `sequence: sequence words' must be resolved as a
shift.  To this end, we use the same techniques as exposed above, see
*note Using Precedence For Non Operators: Non Operators.  One solution
relies on precedences: use `%prec' to give a lower precedence to the
rule:

     %nonassoc "word"
     %nonassoc "sequence"
     %%
     sequence:
       /* empty */
     | sequence word      %prec "sequence"
     | sequence redirect  %prec "sequence"
     ;

     words:
       word
     | words "word"
     ;

   Another solution relies on associativity: provide both the token and
the rule with the same precedence, but make them right-associative:

     %right "word" "redirect"
     %%
     sequence:
       /* empty */
     | sequence word      %prec "word"
     | sequence redirect  %prec "redirect"
     ;


File: bison.info,  Node: Mysterious Conflicts,  Next: Tuning LR,  Prev: Reduce/Reduce,  Up: Algorithm

5.7 Mysterious Conflicts
========================

Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:

     %%
     def: param_spec return_spec ',';
     param_spec:
       type
     | name_list ':' type
     ;
     return_spec:
       type
     | name ':' type
     ;
     type: "id";
     name: "id";
     name_list:
       name
     | name ',' name_list
     ;

   It would seem that this grammar can be parsed with only a single
token of lookahead: when a `param_spec' is being read, an `"id"' is a
`name' if a comma or colon follows, or a `type' if another `"id"'
follows.  In other words, this grammar is LR(1).

   However, for historical reasons, Bison cannot by default handle all
LR(1) grammars.  In this grammar, two contexts, that after an `"id"' at
the beginning of a `param_spec' and likewise at the beginning of a
`return_spec', are similar enough that Bison assumes they are the same.
They appear similar because the same set of rules would be active--the
rule for reducing to a `name' and that for reducing to a `type'.  Bison
is unable to determine at that stage of processing that the rules would
require different lookahead tokens in the two contexts, so it makes a
single parser state for them both.  Combining the two contexts causes a
conflict later.  In parser terminology, this occurrence means that the
grammar is not LALR(1).

   For many practical grammars (specifically those that fall into the
non-LR(1) class), the limitations of LALR(1) result in difficulties
beyond just mysterious reduce/reduce conflicts.  The best way to fix
all these problems is to select a different parser table construction
algorithm.  Either IELR(1) or canonical LR(1) would suffice, but the
former is more efficient and easier to debug during development.  *Note
LR Table Construction::, for details.  (Bison's IELR(1) and canonical
LR(1) implementations are experimental.  More user feedback will help
to stabilize them.)

   If you instead wish to work around LALR(1)'s limitations, you can
often fix a mysterious conflict by identifying the two parser states
that are being confused, and adding something to make them look
distinct.  In the above example, adding one rule to `return_spec' as
follows makes the problem go away:

     ...
     return_spec:
       type
     | name ':' type
     | "id" "bogus"       /* This rule is never used.  */
     ;

   This corrects the problem because it introduces the possibility of an
additional active rule in the context after the `"id"' at the beginning
of `return_spec'.  This rule is not active in the corresponding context
in a `param_spec', so the two contexts receive distinct parser states.
As long as the token `"bogus"' is never generated by `yylex', the added
rule cannot alter the way actual input is parsed.

   In this particular example, there is another way to solve the
problem: rewrite the rule for `return_spec' to use `"id"' directly
instead of via `name'.  This also causes the two confusing contexts to
have different sets of active rules, because the one for `return_spec'
activates the altered rule for `return_spec' rather than the one for
`name'.

     param_spec:
       type
     | name_list ':' type
     ;
     return_spec:
       type
     | "id" ':' type
     ;

   For a more detailed exposition of LALR(1) parsers and parser
generators, *note DeRemer 1982: Bibliography.


File: bison.info,  Node: Tuning LR,  Next: Generalized LR Parsing,  Prev: Mysterious Conflicts,  Up: Algorithm

5.8 Tuning LR
=============

The default behavior of Bison's LR-based parsers is chosen mostly for
historical reasons, but that behavior is often not robust.  For
example, in the previous section, we discussed the mysterious conflicts
that can be produced by LALR(1), Bison's default parser table
construction algorithm.  Another example is Bison's `%error-verbose'
directive, which instructs the generated parser to produce verbose
syntax error messages, which can sometimes contain incorrect
information.

   In this section, we explore several modern features of Bison that
allow you to tune fundamental aspects of the generated LR-based
parsers.  Some of these features easily eliminate shortcomings like
those mentioned above.  Others can be helpful purely for understanding
your parser.

   Most of the features discussed in this section are still
experimental.  More user feedback will help to stabilize them.

* Menu:

* LR Table Construction:: Choose a different construction algorithm.
* Default Reductions::    Disable default reductions.
* LAC::                   Correct lookahead sets in the parser states.
* Unreachable States::    Keep unreachable parser states for debugging.


File: bison.info,  Node: LR Table Construction,  Next: Default Reductions,  Up: Tuning LR

5.8.1 LR Table Construction
---------------------------

For historical reasons, Bison constructs LALR(1) parser tables by
default.  However, LALR does not possess the full language-recognition
power of LR.  As a result, the behavior of parsers employing LALR
parser tables is often mysterious.  We presented a simple example of
this effect in *note Mysterious Conflicts::.

   As we also demonstrated in that example, the traditional approach to
eliminating such mysterious behavior is to restructure the grammar.
Unfortunately, doing so correctly is often difficult.  Moreover, merely
discovering that LALR causes mysterious behavior in your parser can be
difficult as well.

   Fortunately, Bison provides an easy way to eliminate the possibility
of such mysterious behavior altogether.  You simply need to activate a
more powerful parser table construction algorithm by using the `%define
lr.type' directive.

 -- Directive: %define lr.type TYPE
     Specify the type of parser tables within the LR(1) family.  The
     accepted values for TYPE are:

        * `lalr' (default)

        * `ielr'

        * `canonical-lr'

     (This feature is experimental. More user feedback will help to
     stabilize it.)

   For example, to activate IELR, you might add the following directive
to you grammar file:

     %define lr.type ielr

For the example in *note Mysterious Conflicts::, the mysterious
conflict is then eliminated, so there is no need to invest time in
comprehending the conflict or restructuring the grammar to fix it.  If,
during future development, the grammar evolves such that all mysterious
behavior would have disappeared using just LALR, you need not fear that
continuing to use IELR will result in unnecessarily large parser tables.
That is, IELR generates LALR tables when LALR (using a deterministic
parsing algorithm) is sufficient to support the full
language-recognition power of LR.  Thus, by enabling IELR at the start
of grammar development, you can safely and completely eliminate the
need to consider LALR's shortcomings.

   While IELR is almost always preferable, there are circumstances
where LALR or the canonical LR parser tables described by Knuth (*note
Knuth 1965: Bibliography.) can be useful.  Here we summarize the
relative advantages of each parser table construction algorithm within
Bison:

   * LALR

     There are at least two scenarios where LALR can be worthwhile:

        * GLR without static conflict resolution.

          When employing GLR parsers (*note GLR Parsers::), if you do
          not resolve any conflicts statically (for example, with
          `%left' or `%prec'), then the parser explores all potential
          parses of any given input.  In this case, the choice of
          parser table construction algorithm is guaranteed not to alter
          the language accepted by the parser.  LALR parser tables are
          the smallest parser tables Bison can currently construct, so
          they may then be preferable.  Nevertheless, once you begin to
          resolve conflicts statically, GLR behaves more like a
          deterministic parser in the syntactic contexts where those
          conflicts appear, and so either IELR or canonical LR can then
          be helpful to avoid LALR's mysterious behavior.

        * Malformed grammars.

          Occasionally during development, an especially malformed
          grammar with a major recurring flaw may severely impede the
          IELR or canonical LR parser table construction algorithm.
          LALR can be a quick way to construct parser tables in order
          to investigate such problems while ignoring the more subtle
          differences from IELR and canonical LR.

   * IELR

     IELR (Inadequacy Elimination LR) is a minimal LR algorithm.  That
     is, given any grammar (LR or non-LR), parsers using IELR or
     canonical LR parser tables always accept exactly the same set of
     sentences.  However, like LALR, IELR merges parser states during
     parser table construction so that the number of parser states is
     often an order of magnitude less than for canonical LR.  More
     importantly, because canonical LR's extra parser states may contain
     duplicate conflicts in the case of non-LR grammars, the number of
     conflicts for IELR is often an order of magnitude less as well.
     This effect can significantly reduce the complexity of developing
     a grammar.

   * Canonical LR

     While inefficient, canonical LR parser tables can be an
     interesting means to explore a grammar because they possess a
     property that IELR and LALR tables do not.  That is, if
     `%nonassoc' is not used and default reductions are left disabled
     (*note Default Reductions::), then, for every left context of
     every canonical LR state, the set of tokens accepted by that state
     is guaranteed to be the exact set of tokens that is syntactically
     acceptable in that left context.  It might then seem that an
     advantage of canonical LR parsers in production is that, under the
     above constraints, they are guaranteed to detect a syntax error as
     soon as possible without performing any unnecessary reductions.
     However, IELR parsers that use LAC are also able to achieve this
     behavior without sacrificing `%nonassoc' or default reductions.
     For details and a few caveats of LAC, *note LAC::.

   For a more detailed exposition of the mysterious behavior in LALR
parsers and the benefits of IELR, *note Denny 2008 March: Bibliography,
and *note Denny 2010 November: Bibliography.


File: bison.info,  Node: Default Reductions,  Next: LAC,  Prev: LR Table Construction,  Up: Tuning LR

5.8.2 Default Reductions
------------------------

After parser table construction, Bison identifies the reduction with the
largest lookahead set in each parser state.  To reduce the size of the
parser state, traditional Bison behavior is to remove that lookahead
set and to assign that reduction to be the default parser action.  Such
a reduction is known as a "default reduction".

   Default reductions affect more than the size of the parser tables.
They also affect the behavior of the parser:

   * Delayed `yylex' invocations.

     A "consistent state" is a state that has only one possible parser
     action.  If that action is a reduction and is encoded as a default
     reduction, then that consistent state is called a "defaulted
     state".  Upon reaching a defaulted state, a Bison-generated parser
     does not bother to invoke `yylex' to fetch the next token before
     performing the reduction.  In other words, whether default
     reductions are enabled in consistent states determines how soon a
     Bison-generated parser invokes `yylex' for a token: immediately
     when it _reaches_ that token in the input or when it eventually
     _needs_ that token as a lookahead to determine the next parser
     action.  Traditionally, default reductions are enabled, and so the
     parser exhibits the latter behavior.

     The presence of defaulted states is an important consideration when
     designing `yylex' and the grammar file.  That is, if the behavior
     of `yylex' can influence or be influenced by the semantic actions
     associated with the reductions in defaulted states, then the delay
     of the next `yylex' invocation until after those reductions is
     significant.  For example, the semantic actions might pop a scope
     stack that `yylex' uses to determine what token to return.  Thus,
     the delay might be necessary to ensure that `yylex' does not look
     up the next token in a scope that should already be considered
     closed.

   * Delayed syntax error detection.

     When the parser fetches a new token by invoking `yylex', it checks
     whether there is an action for that token in the current parser
     state.  The parser detects a syntax error if and only if either
     (1) there is no action for that token or (2) the action for that
     token is the error action (due to the use of `%nonassoc').
     However, if there is a default reduction in that state (which
     might or might not be a defaulted state), then it is impossible
     for condition 1 to exist.  That is, all tokens have an action.
     Thus, the parser sometimes fails to detect the syntax error until
     it reaches a later state.

     While default reductions never cause the parser to accept
     syntactically incorrect sentences, the delay of syntax error
     detection can have unexpected effects on the behavior of the
     parser.  However, the delay can be caused anyway by parser state
     merging and the use of `%nonassoc', and it can be fixed by another
     Bison feature, LAC.  We discuss the effects of delayed syntax
     error detection and LAC more in the next section (*note LAC::).

   For canonical LR, the only default reduction that Bison enables by
default is the accept action, which appears only in the accepting
state, which has no other action and is thus a defaulted state.
However, the default accept action does not delay any `yylex'
invocation or syntax error detection because the accept action ends the
parse.

   For LALR and IELR, Bison enables default reductions in nearly all
states by default.  There are only two exceptions.  First, states that
have a shift action on the `error' token do not have default reductions
because delayed syntax error detection could then prevent the `error'
token from ever being shifted in that state.  However, parser state
merging can cause the same effect anyway, and LAC fixes it in both
cases, so future versions of Bison might drop this exception when LAC
is activated.  Second, GLR parsers do not record the default reduction
as the action on a lookahead token for which there is a conflict.  The
correct action in this case is to split the parse instead.

   To adjust which states have default reductions enabled, use the
`%define lr.default-reductions' directive.

 -- Directive: %define lr.default-reductions WHERE
     Specify the kind of states that are permitted to contain default
     reductions.  The accepted values of WHERE are:
        * `most' (default for LALR and IELR)

        * `consistent'

        * `accepting' (default for canonical LR)

     (The ability to specify where default reductions are permitted is
     experimental.  More user feedback will help to stabilize it.)


File: bison.info,  Node: LAC,  Next: Unreachable States,  Prev: Default Reductions,  Up: Tuning LR

5.8.3 LAC
---------

Canonical LR, IELR, and LALR can suffer from a couple of problems upon
encountering a syntax error.  First, the parser might perform additional
parser stack reductions before discovering the syntax error.  Such
reductions can perform user semantic actions that are unexpected because
they are based on an invalid token, and they cause error recovery to
begin in a different syntactic context than the one in which the
invalid token was encountered.  Second, when verbose error messages are
enabled (*note Error Reporting::), the expected token list in the
syntax error message can both contain invalid tokens and omit valid
tokens.

   The culprits for the above problems are `%nonassoc', default
reductions in inconsistent states (*note Default Reductions::), and
parser state merging.  Because IELR and LALR merge parser states, they
suffer the most.  Canonical LR can suffer only if `%nonassoc' is used
or if default reductions are enabled for inconsistent states.

   LAC (Lookahead Correction) is a new mechanism within the parsing
algorithm that solves these problems for canonical LR, IELR, and LALR
without sacrificing `%nonassoc', default reductions, or state merging.
You can enable LAC with the `%define parse.lac' directive.

 -- Directive: %define parse.lac VALUE
     Enable LAC to improve syntax error handling.
        * `none' (default)

        * `full'
     (This feature is experimental.  More user feedback will help to
     stabilize it.  Moreover, it is currently only available for
     deterministic parsers in C.)

   Conceptually, the LAC mechanism is straight-forward.  Whenever the
parser fetches a new token from the scanner so that it can determine
the next parser action, it immediately suspends normal parsing and
performs an exploratory parse using a temporary copy of the normal
parser state stack.  During this exploratory parse, the parser does not
perform user semantic actions.  If the exploratory parse reaches a
shift action, normal parsing then resumes on the normal parser stacks.
If the exploratory parse reaches an error instead, the parser reports a
syntax error.  If verbose syntax error messages are enabled, the parser
must then discover the list of expected tokens, so it performs a
separate exploratory parse for each token in the grammar.

   There is one subtlety about the use of LAC.  That is, when in a
consistent parser state with a default reduction, the parser will not
attempt to fetch a token from the scanner because no lookahead is
needed to determine the next parser action.  Thus, whether default
reductions are enabled in consistent states (*note Default
Reductions::) affects how soon the parser detects a syntax error:
immediately when it _reaches_ an erroneous token or when it eventually
_needs_ that token as a lookahead to determine the next parser action.
The latter behavior is probably more intuitive, so Bison currently
provides no way to achieve the former behavior while default reductions
are enabled in consistent states.

   Thus, when LAC is in use, for some fixed decision of whether to
enable default reductions in consistent states, canonical LR and IELR
behave almost exactly the same for both syntactically acceptable and
syntactically unacceptable input.  While LALR still does not support
the full language-recognition power of canonical LR and IELR, LAC at
least enables LALR's syntax error handling to correctly reflect LALR's
language-recognition power.

   There are a few caveats to consider when using LAC:

   * Infinite parsing loops.

     IELR plus LAC does have one shortcoming relative to canonical LR.
     Some parsers generated by Bison can loop infinitely.  LAC does not
     fix infinite parsing loops that occur between encountering a
     syntax error and detecting it, but enabling canonical LR or
     disabling default reductions sometimes does.

   * Verbose error message limitations.

     Because of internationalization considerations, Bison-generated
     parsers limit the size of the expected token list they are willing
     to report in a verbose syntax error message.  If the number of
     expected tokens exceeds that limit, the list is simply dropped
     from the message.  Enabling LAC can increase the size of the list
     and thus cause the parser to drop it.  Of course, dropping the
     list is better than reporting an incorrect list.

   * Performance.

     Because LAC requires many parse actions to be performed twice, it
     can have a performance penalty.  However, not all parse actions
     must be performed twice.  Specifically, during a series of default
     reductions in consistent states and shift actions, the parser
     never has to initiate an exploratory parse.  Moreover, the most
     time-consuming tasks in a parse are often the file I/O, the
     lexical analysis performed by the scanner, and the user's semantic
     actions, but none of these are performed during the exploratory
     parse.  Finally, the base of the temporary stack used during an
     exploratory parse is a pointer into the normal parser state stack
     so that the stack is never physically copied.  In our experience,
     the performance penalty of LAC has proved insignificant for
     practical grammars.

   While the LAC algorithm shares techniques that have been recognized
in the parser community for years, for the publication that introduces
LAC, *note Denny 2010 May: Bibliography.


File: bison.info,  Node: Unreachable States,  Prev: LAC,  Up: Tuning LR

5.8.4 Unreachable States
------------------------

If there exists no sequence of transitions from the parser's start
state to some state S, then Bison considers S to be an "unreachable
state".  A state can become unreachable during conflict resolution if
Bison disables a shift action leading to it from a predecessor state.

   By default, Bison removes unreachable states from the parser after
conflict resolution because they are useless in the generated parser.
However, keeping unreachable states is sometimes useful when trying to
understand the relationship between the parser and the grammar.

 -- Directive: %define lr.keep-unreachable-states VALUE
     Request that Bison allow unreachable states to remain in the
     parser tables.  VALUE must be a Boolean.  The default is `false'.

   There are a few caveats to consider:

   * Missing or extraneous warnings.

     Unreachable states may contain conflicts and may use rules not
     used in any other state.  Thus, keeping unreachable states may
     induce warnings that are irrelevant to your parser's behavior, and
     it may eliminate warnings that are relevant.  Of course, the
     change in warnings may actually be relevant to a parser table
     analysis that wants to keep unreachable states, so this behavior
     will likely remain in future Bison releases.

   * Other useless states.

     While Bison is able to remove unreachable states, it is not
     guaranteed to remove other kinds of useless states.  Specifically,
     when Bison disables reduce actions during conflict resolution,
     some goto actions may become useless, and thus some additional
     states may become useless.  If Bison were to compute which goto
     actions were useless and then disable those actions, it could
     identify such states as unreachable and then remove those states.
     However, Bison does not compute which goto actions are useless.


File: bison.info,  Node: Generalized LR Parsing,  Next: Memory Management,  Prev: Tuning LR,  Up: Algorithm

5.9 Generalized LR (GLR) Parsing
================================

Bison produces _deterministic_ parsers that choose uniquely when to
reduce and which reduction to apply based on a summary of the preceding
input and on one extra token of lookahead.  As a result, normal Bison
handles a proper subset of the family of context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (*note Mysterious Conflicts::), there
are languages where Bison's default choice of how to summarize the
input seen so far loses necessary information.

   When you use the `%glr-parser' declaration in your grammar file,
Bison generates a parser that uses a different algorithm, called
Generalized LR (or GLR).  A Bison GLR parser uses the same basic
algorithm for parsing as an ordinary Bison parser, but behaves
differently in cases where there is a shift-reduce conflict that has not
been resolved by precedence rules (*note Precedence::) or a
reduce-reduce conflict.  When a GLR parser encounters such a situation,
it effectively _splits_ into a several parsers, one for each possible
shift or reduction.  These parsers then proceed as usual, consuming
tokens in lock-step.  Some of the stacks may encounter other conflicts
and split further, with the result that instead of a sequence of states,
a Bison GLR parsing stack is what is in effect a tree of states.

   In effect, each stack represents a guess as to what the proper parse
is.  Additional input may indicate that a guess was wrong, in which case
the appropriate stack silently disappears.  Otherwise, the semantics
actions generated in each stack are saved, rather than being executed
immediately.  When a stack disappears, its saved semantic actions never
get executed.  When a reduction causes two stacks to become equivalent,
their sets of semantic actions are both saved with the state that
results from the reduction.  We say that two stacks are equivalent when
they both represent the same sequence of states, and each pair of
corresponding states represents a grammar symbol that produces the same
segment of the input token stream.

   Whenever the parser makes a transition from having multiple states
to having one, it reverts to the normal deterministic parsing
algorithm, after resolving and executing the saved-up actions.  At this
transition, some of the states on the stack will have semantic values
that are sets (actually multisets) of possible actions.  The parser
tries to pick one of the actions by first finding one whose rule has
the highest dynamic precedence, as set by the `%dprec' declaration.
Otherwise, if the alternative actions are not ordered by precedence,
but there the same merging function is declared for both rules by the
`%merge' declaration, Bison resolves and evaluates both and then calls
the merge function on the result.  Otherwise, it reports an ambiguity.

   It is possible to use a data structure for the GLR parsing tree that
permits the processing of any LR(1) grammar in linear time (in the size
of the input), any unambiguous (not necessarily LR(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time.  However, Bison currently
uses a simpler data structure that requires time proportional to the
length of the input times the maximum number of stacks required for any
prefix of the input.  Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process.  Such badly
behaving examples, however, are not generally of practical interest.
Usually, nondeterminism in a grammar is local--the parser is "in doubt"
only for a few tokens at a time.  Therefore, the current data structure
should generally be adequate.  On LR(1) portions of a grammar, in
particular, it is only slightly slower than with the deterministic
LR(1) Bison parser.

   For a more detailed exposition of GLR parsers, *note Scott 2000:
Bibliography.


File: bison.info,  Node: Memory Management,  Prev: Generalized LR Parsing,  Up: Algorithm

5.10 Memory Management, and How to Avoid Memory Exhaustion
==========================================================

The Bison parser stack can run out of memory if too many tokens are
shifted and not reduced.  When this happens, the parser function
`yyparse' calls `yyerror' and then returns 2.

   Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
recursion, see *note Recursive Rules: Recursion.

   By defining the macro `YYMAXDEPTH', you can control how deep the
parser stack can become before memory is exhausted.  Define the macro
with a value that is an integer.  This value is the maximum number of
tokens that can be shifted (and not reduced) before overflow.

   The stack space allowed is not necessarily allocated.  If you
specify a large value for `YYMAXDEPTH', the parser normally allocates a
small stack at first, and then makes it bigger by stages as needed.
This increasing allocation happens automatically and silently.
Therefore, you do not need to make `YYMAXDEPTH' painfully small merely
to save space for ordinary inputs that do not need much stack.

   However, do not allow `YYMAXDEPTH' to be a value so large that
arithmetic overflow could occur when calculating the size of the stack
space.  Also, do not allow `YYMAXDEPTH' to be less than `YYINITDEPTH'.

   The default value of `YYMAXDEPTH', if you do not define it, is 10000.

   You can control how much stack is allocated initially by defining the
macro `YYINITDEPTH' to a positive integer.  For the deterministic
parser in C, this value must be a compile-time constant unless you are
assuming C99 or some other target language or compiler that allows
variable-length arrays.  The default is 200.

   Do not allow `YYINITDEPTH' to be greater than `YYMAXDEPTH'.

   Because of semantic differences between C and C++, the deterministic
parsers in C produced by Bison cannot grow when compiled by C++
compilers.  In this precise case (compiling a C parser as C++) you are
suggested to grow `YYINITDEPTH'.  The Bison maintainers hope to fix
this deficiency in a future release.


File: bison.info,  Node: Error Recovery,  Next: Context Dependency,  Prev: Algorithm,  Up: Top

6 Error Recovery
****************

It is not usually acceptable to have a program terminate on a syntax
error.  For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should
accept another expression.

   In a simple interactive command parser where each input is one line,
it may be sufficient to allow `yyparse' to return 1 on error and have
the caller ignore the rest of the input line when that happens (and