1 // Copyright 2011 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 /* 6 Package template implements data-driven templates for generating textual output. 7 8 To generate HTML output, see package html/template, which has the same interface 9 as this package but automatically secures HTML output against certain attacks. 10 11 Templates are executed by applying them to a data structure. Annotations in the 12 template refer to elements of the data structure (typically a field of a struct 13 or a key in a map) to control execution and derive values to be displayed. 14 Execution of the template walks the structure and sets the cursor, represented 15 by a period '.' and called "dot", to the value at the current location in the 16 structure as execution proceeds. 17 18 The input text for a template is UTF-8-encoded text in any format. 19 "Actions"--data evaluations or control structures--are delimited by 20 "{{" and "}}"; all text outside actions is copied to the output unchanged. 21 Except for raw strings, actions may not span newlines, although comments can. 22 23 Once parsed, a template may be executed safely in parallel. 24 25 Here is a trivial example that prints "17 items are made of wool". 26 27 type Inventory struct { 28 Material string 29 Count uint 30 } 31 sweaters := Inventory{"wool", 17} 32 tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}") 33 if err != nil { panic(err) } 34 err = tmpl.Execute(os.Stdout, sweaters) 35 if err != nil { panic(err) } 36 37 More intricate examples appear below. 38 39 Actions 40 41 Here is the list of actions. "Arguments" and "pipelines" are evaluations of 42 data, defined in detail below. 43 44 */ 45 // {{/* a comment */}} 46 // A comment; discarded. May contain newlines. 47 // Comments do not nest and must start and end at the 48 // delimiters, as shown here. 49 /* 50 51 {{pipeline}} 52 The default textual representation of the value of the pipeline 53 is copied to the output. 54 55 {{if pipeline}} T1 {{end}} 56 If the value of the pipeline is empty, no output is generated; 57 otherwise, T1 is executed. The empty values are false, 0, any 58 nil pointer or interface value, and any array, slice, map, or 59 string of length zero. 60 Dot is unaffected. 61 62 {{if pipeline}} T1 {{else}} T0 {{end}} 63 If the value of the pipeline is empty, T0 is executed; 64 otherwise, T1 is executed. Dot is unaffected. 65 66 {{if pipeline}} T1 {{else if pipeline}} T0 {{end}} 67 To simplify the appearance of if-else chains, the else action 68 of an if may include another if directly; the effect is exactly 69 the same as writing 70 {{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}} 71 72 {{range pipeline}} T1 {{end}} 73 The value of the pipeline must be an array, slice, map, or channel. 74 If the value of the pipeline has length zero, nothing is output; 75 otherwise, dot is set to the successive elements of the array, 76 slice, or map and T1 is executed. If the value is a map and the 77 keys are of basic type with a defined order ("comparable"), the 78 elements will be visited in sorted key order. 79 80 {{range pipeline}} T1 {{else}} T0 {{end}} 81 The value of the pipeline must be an array, slice, map, or channel. 82 If the value of the pipeline has length zero, dot is unaffected and 83 T0 is executed; otherwise, dot is set to the successive elements 84 of the array, slice, or map and T1 is executed. 85 86 {{template "name"}} 87 The template with the specified name is executed with nil data. 88 89 {{template "name" pipeline}} 90 The template with the specified name is executed with dot set 91 to the value of the pipeline. 92 93 {{with pipeline}} T1 {{end}} 94 If the value of the pipeline is empty, no output is generated; 95 otherwise, dot is set to the value of the pipeline and T1 is 96 executed. 97 98 {{with pipeline}} T1 {{else}} T0 {{end}} 99 If the value of the pipeline is empty, dot is unaffected and T0 100 is executed; otherwise, dot is set to the value of the pipeline 101 and T1 is executed. 102 103 Arguments 104 105 An argument is a simple value, denoted by one of the following. 106 107 - A boolean, string, character, integer, floating-point, imaginary 108 or complex constant in Go syntax. These behave like Go's untyped 109 constants. 110 - The keyword nil, representing an untyped Go nil. 111 - The character '.' (period): 112 . 113 The result is the value of dot. 114 - A variable name, which is a (possibly empty) alphanumeric string 115 preceded by a dollar sign, such as 116 $piOver2 117 or 118 $ 119 The result is the value of the variable. 120 Variables are described below. 121 - The name of a field of the data, which must be a struct, preceded 122 by a period, such as 123 .Field 124 The result is the value of the field. Field invocations may be 125 chained: 126 .Field1.Field2 127 Fields can also be evaluated on variables, including chaining: 128 $x.Field1.Field2 129 - The name of a key of the data, which must be a map, preceded 130 by a period, such as 131 .Key 132 The result is the map element value indexed by the key. 133 Key invocations may be chained and combined with fields to any 134 depth: 135 .Field1.Key1.Field2.Key2 136 Although the key must be an alphanumeric identifier, unlike with 137 field names they do not need to start with an upper case letter. 138 Keys can also be evaluated on variables, including chaining: 139 $x.key1.key2 140 - The name of a niladic method of the data, preceded by a period, 141 such as 142 .Method 143 The result is the value of invoking the method with dot as the 144 receiver, dot.Method(). Such a method must have one return value (of 145 any type) or two return values, the second of which is an error. 146 If it has two and the returned error is non-nil, execution terminates 147 and an error is returned to the caller as the value of Execute. 148 Method invocations may be chained and combined with fields and keys 149 to any depth: 150 .Field1.Key1.Method1.Field2.Key2.Method2 151 Methods can also be evaluated on variables, including chaining: 152 $x.Method1.Field 153 - The name of a niladic function, such as 154 fun 155 The result is the value of invoking the function, fun(). The return 156 types and values behave as in methods. Functions and function 157 names are described below. 158 - A parenthesized instance of one the above, for grouping. The result 159 may be accessed by a field or map key invocation. 160 print (.F1 arg1) (.F2 arg2) 161 (.StructValuedMethod "arg").Field 162 163 Arguments may evaluate to any type; if they are pointers the implementation 164 automatically indirects to the base type when required. 165 If an evaluation yields a function value, such as a function-valued 166 field of a struct, the function is not invoked automatically, but it 167 can be used as a truth value for an if action and the like. To invoke 168 it, use the call function, defined below. 169 170 A pipeline is a possibly chained sequence of "commands". A command is a simple 171 value (argument) or a function or method call, possibly with multiple arguments: 172 173 Argument 174 The result is the value of evaluating the argument. 175 .Method [Argument...] 176 The method can be alone or the last element of a chain but, 177 unlike methods in the middle of a chain, it can take arguments. 178 The result is the value of calling the method with the 179 arguments: 180 dot.Method(Argument1, etc.) 181 functionName [Argument...] 182 The result is the value of calling the function associated 183 with the name: 184 function(Argument1, etc.) 185 Functions and function names are described below. 186 187 Pipelines 188 189 A pipeline may be "chained" by separating a sequence of commands with pipeline 190 characters '|'. In a chained pipeline, the result of the each command is 191 passed as the last argument of the following command. The output of the final 192 command in the pipeline is the value of the pipeline. 193 194 The output of a command will be either one value or two values, the second of 195 which has type error. If that second value is present and evaluates to 196 non-nil, execution terminates and the error is returned to the caller of 197 Execute. 198 199 Variables 200 201 A pipeline inside an action may initialize a variable to capture the result. 202 The initialization has syntax 203 204 $variable := pipeline 205 206 where $variable is the name of the variable. An action that declares a 207 variable produces no output. 208 209 If a "range" action initializes a variable, the variable is set to the 210 successive elements of the iteration. Also, a "range" may declare two 211 variables, separated by a comma: 212 213 range $index, $element := pipeline 214 215 in which case $index and $element are set to the successive values of the 216 array/slice index or map key and element, respectively. Note that if there is 217 only one variable, it is assigned the element; this is opposite to the 218 convention in Go range clauses. 219 220 A variable's scope extends to the "end" action of the control structure ("if", 221 "with", or "range") in which it is declared, or to the end of the template if 222 there is no such control structure. A template invocation does not inherit 223 variables from the point of its invocation. 224 225 When execution begins, $ is set to the data argument passed to Execute, that is, 226 to the starting value of dot. 227 228 Examples 229 230 Here are some example one-line templates demonstrating pipelines and variables. 231 All produce the quoted word "output": 232 233 {{"\"output\""}} 234 A string constant. 235 {{`"output"`}} 236 A raw string constant. 237 {{printf "%q" "output"}} 238 A function call. 239 {{"output" | printf "%q"}} 240 A function call whose final argument comes from the previous 241 command. 242 {{printf "%q" (print "out" "put")}} 243 A parenthesized argument. 244 {{"put" | printf "%s%s" "out" | printf "%q"}} 245 A more elaborate call. 246 {{"output" | printf "%s" | printf "%q"}} 247 A longer chain. 248 {{with "output"}}{{printf "%q" .}}{{end}} 249 A with action using dot. 250 {{with $x := "output" | printf "%q"}}{{$x}}{{end}} 251 A with action that creates and uses a variable. 252 {{with $x := "output"}}{{printf "%q" $x}}{{end}} 253 A with action that uses the variable in another action. 254 {{with $x := "output"}}{{$x | printf "%q"}}{{end}} 255 The same, but pipelined. 256 257 Functions 258 259 During execution functions are found in two function maps: first in the 260 template, then in the global function map. By default, no functions are defined 261 in the template but the Funcs method can be used to add them. 262 263 Predefined global functions are named as follows. 264 265 and 266 Returns the boolean AND of its arguments by returning the 267 first empty argument or the last argument, that is, 268 "and x y" behaves as "if x then y else x". All the 269 arguments are evaluated. 270 call 271 Returns the result of calling the first argument, which 272 must be a function, with the remaining arguments as parameters. 273 Thus "call .X.Y 1 2" is, in Go notation, dot.X.Y(1, 2) where 274 Y is a func-valued field, map entry, or the like. 275 The first argument must be the result of an evaluation 276 that yields a value of function type (as distinct from 277 a predefined function such as print). The function must 278 return either one or two result values, the second of which 279 is of type error. If the arguments don't match the function 280 or the returned error value is non-nil, execution stops. 281 html 282 Returns the escaped HTML equivalent of the textual 283 representation of its arguments. 284 index 285 Returns the result of indexing its first argument by the 286 following arguments. Thus "index x 1 2 3" is, in Go syntax, 287 x[1][2][3]. Each indexed item must be a map, slice, or array. 288 js 289 Returns the escaped JavaScript equivalent of the textual 290 representation of its arguments. 291 len 292 Returns the integer length of its argument. 293 not 294 Returns the boolean negation of its single argument. 295 or 296 Returns the boolean OR of its arguments by returning the 297 first non-empty argument or the last argument, that is, 298 "or x y" behaves as "if x then x else y". All the 299 arguments are evaluated. 300 print 301 An alias for fmt.Sprint 302 printf 303 An alias for fmt.Sprintf 304 println 305 An alias for fmt.Sprintln 306 urlquery 307 Returns the escaped value of the textual representation of 308 its arguments in a form suitable for embedding in a URL query. 309 310 The boolean functions take any zero value to be false and a non-zero 311 value to be true. 312 313 There is also a set of binary comparison operators defined as 314 functions: 315 316 eq 317 Returns the boolean truth of arg1 == arg2 318 ne 319 Returns the boolean truth of arg1 != arg2 320 lt 321 Returns the boolean truth of arg1 < arg2 322 le 323 Returns the boolean truth of arg1 <= arg2 324 gt 325 Returns the boolean truth of arg1 > arg2 326 ge 327 Returns the boolean truth of arg1 >= arg2 328 329 For simpler multi-way equality tests, eq (only) accepts two or more 330 arguments and compares the second and subsequent to the first, 331 returning in effect 332 333 arg1==arg2 || arg1==arg3 || arg1==arg4 ... 334 335 (Unlike with || in Go, however, eq is a function call and all the 336 arguments will be evaluated.) 337 338 The comparison functions work on basic types only (or named basic 339 types, such as "type Celsius float32"). They implement the Go rules 340 for comparison of values, except that size and exact type are 341 ignored, so any integer value, signed or unsigned, may be compared 342 with any other integer value. (The arithmetic value is compared, 343 not the bit pattern, so all negative integers are less than all 344 unsigned integers.) However, as usual, one may not compare an int 345 with a float32 and so on. 346 347 Associated templates 348 349 Each template is named by a string specified when it is created. Also, each 350 template is associated with zero or more other templates that it may invoke by 351 name; such associations are transitive and form a name space of templates. 352 353 A template may use a template invocation to instantiate another associated 354 template; see the explanation of the "template" action above. The name must be 355 that of a template associated with the template that contains the invocation. 356 357 Nested template definitions 358 359 When parsing a template, another template may be defined and associated with the 360 template being parsed. Template definitions must appear at the top level of the 361 template, much like global variables in a Go program. 362 363 The syntax of such definitions is to surround each template declaration with a 364 "define" and "end" action. 365 366 The define action names the template being created by providing a string 367 constant. Here is a simple example: 368 369 `{{define "T1"}}ONE{{end}} 370 {{define "T2"}}TWO{{end}} 371 {{define "T3"}}{{template "T1"}} {{template "T2"}}{{end}} 372 {{template "T3"}}` 373 374 This defines two templates, T1 and T2, and a third T3 that invokes the other two 375 when it is executed. Finally it invokes T3. If executed this template will 376 produce the text 377 378 ONE TWO 379 380 By construction, a template may reside in only one association. If it's 381 necessary to have a template addressable from multiple associations, the 382 template definition must be parsed multiple times to create distinct *Template 383 values, or must be copied with the Clone or AddParseTree method. 384 385 Parse may be called multiple times to assemble the various associated templates; 386 see the ParseFiles and ParseGlob functions and methods for simple ways to parse 387 related templates stored in files. 388 389 A template may be executed directly or through ExecuteTemplate, which executes 390 an associated template identified by name. To invoke our example above, we 391 might write, 392 393 err := tmpl.Execute(os.Stdout, "no data needed") 394 if err != nil { 395 log.Fatalf("execution failed: %s", err) 396 } 397 398 or to invoke a particular template explicitly by name, 399 400 err := tmpl.ExecuteTemplate(os.Stdout, "T2", "no data needed") 401 if err != nil { 402 log.Fatalf("execution failed: %s", err) 403 } 404 405 */ 406 package template 407
