1 # ----------------------------------------------------------------------------- 2 # yacc_simple.py 3 # 4 # A simple, properly specifier grammar 5 # ----------------------------------------------------------------------------- 6 7 from .calclex import tokens 8 from ply import yacc 9 10 # Parsing rules 11 precedence = ( 12 ('left','PLUS','MINUS'), 13 ('left','TIMES','DIVIDE'), 14 ('right','UMINUS'), 15 ) 16 17 # dictionary of names 18 names = { } 19 20 def p_statement_assign(t): 21 'statement : NAME EQUALS expression' 22 names[t[1]] = t[3] 23 24 def p_statement_expr(t): 25 'statement : expression' 26 t[0] = t[1] 27 28 def p_expression_binop(t): 29 '''expression : expression PLUS expression 30 | expression MINUS expression 31 | expression TIMES expression 32 | expression DIVIDE expression''' 33 if t[2] == '+' : t[0] = t[1] + t[3] 34 elif t[2] == '-': t[0] = t[1] - t[3] 35 elif t[2] == '*': t[0] = t[1] * t[3] 36 elif t[2] == '/': t[0] = t[1] / t[3] 37 38 def p_expression_uminus(t): 39 'expression : MINUS expression %prec UMINUS' 40 t[0] = -t[2] 41 42 def p_expression_group(t): 43 'expression : LPAREN expression RPAREN' 44 t[0] = t[2] 45 46 def p_expression_number(t): 47 'expression : NUMBER' 48 t[0] = t[1] 49 50 def p_expression_name(t): 51 'expression : NAME' 52 try: 53 t[0] = names[t[1]] 54 except LookupError: 55 print("Undefined name '%s'" % t[1]) 56 t[0] = 0 57 58 def p_error(t): 59 print("Syntax error at '%s'" % t.value) 60 61 parser = yacc.yacc() 62 63 64 65 66 67
