So the next step is to trace through the parser and ensure that the language it recognises is exactly the language you want to recognize.īut that's not possible without seeing the entire grammar, or at least a much larger extract. Or, at least, the input does not conform to the grammar. Most of the time, when p_error is called, the input contains a syntax error. In other words, there is only one lexical anslyser, and as far as you have tested, it's working just fine. That could mean the token has not been correctly recognised, but we know that's not the case here the lexical analyser identifies the token, and ply.yacc uses ply.lex to split the input into tokens. Ply's parser calls p_error when it is unable to find an appropriate action for the lookahead token. The error message is, at best, misleading. With all due respect, this is not a reliable diagnostic tool. Your evidence for the claim that Ply's parser component ( ply.yacc) is not correctly recognising = as the ASSIGN token is a message which you yourself produce, in the p_error method. I've tried moving the token regex definitions thinking maybe there was some prioritization happening between EQUALS AND ASSIGN to no avail. Print ('Line %s, illegal token %s' % (p.lineno, p.value)) However when parsing as follows (excluding unrelated definitions) def p_assign(p):Īssign : ID array_assign ASSIGN expression SEMICOLON Using this simple technique we are able toĭisambiguate our grammar.I'm using ply and defined my tokens as reserved = I get the expected tokens LexToken(MAIN,'main',1,0) Consequently multiplication andĭivision have higher precedence than addition and subtraction. The lastĭefinition listed has the highest precedence. We may specify %left, for left-associative or %right for right associative. This is followed byĭefinitions for the arithmetic operators. The tokens for INTEGER and VARIABLE are utilized by yacc to create #defines in y.tab.h for use in lex.

Consequently multiply has a higherįollows. When we reduce rule r2, we emit the multiply instruction. Until a single nonterminal, the start symbol, remains in the stack. Step 2 applies rule r3 to the stack to change x to E. Yacc defines each of these as a small integer using a preprocessor define. The tokens in our grammar are the parts of speech: NOUN, PRONOUN, VERB, ADVERB, ADJECTIVE, PREPOSITION, and CONJUNCTION.

This process continues until we have shifted all input to the stackĪnd only the starting nonterminal remains on the stack. We solve this problem by letting yacc define the token codes. The matched tokensĪre known as a handle and we are reducing the handle to the lhs of the Stack, and the lhs of the production is pushed on the stack. In other words the matched tokens of the rhs are popped off the

Production we replace the matched tokens on the stack with the lhs of the

When the top of the stack matches the rhs of a The stack while remaining input is to the right of the dot.

Online lex and yacc compiler error