Question

I do not see how to handle the error of a ANTLR grammar:

****************error message*********
Decision can match input such as "{'+', '-'} IDENT" using multiple alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input
 |---> add: mult (('+'|'-') mult)*;
***************************************

This is mostly copied from the example of Scott, but I do not see why his works well but mine got stuck and how to get out of it?

—————following Sample.g—————-

grammar Sample;

options { 
  language = Java;
}


 program
      :  'program' IDENT '='
          (constant| variable)*
          'begin'
          (statement)*
          'end' IDENT '.'
      ; 

constant:
       'constant' IDENT ':' type ':=' expression ';' 
      ;

type: 'integer';

variable: 'var' IDENT (',' IDENT)* ':' type  ';';


statement: 'var' IDENT ':=' INTEGER ';' ;

//expression

term: IDENT |'(' expression ')'|INTEGER;
negation: 'not'* term;
unary: ('+'|'-')* negation;
mult: unary (('*'|'/'|'mod') unary)*;

[XXX Errorfor the following line]

add: mult (('+'|'-') mult)*;
relation: add (('='|'/='|'<'|'<=') add)*;
expression: (relation ('and'|'or') relation)*;


END : 'end';

CONSTANT : 'constant';

INTEGER: '0'| (('1'..'9') ('0'..'9')*);   

IDENT: ('a'..'z'|'A'..'Z')('a'..'z'|'A'..'Z'|'0'..'9')*;

WS: ('\n'|' '|'\t'|'r'|'\f')+ {$channel=HIDDEN;};

Answer 1

You’ve misplaced a parenthesis in:

expression: (relation ('and'|'or') relation)*;

making your grammar ambiguous: the parser cannot decide when it sees a - IDENT if it should be a part of an add– or unary-rule.

For example, your rule expression now matches this:

relation ('and'|'or') relation relation ('and'|'or') relation

i.e., two relation rules directly placed after each other. If the parser now stumbles upon input like this:

- A - B

the parser “sees” two possibilities to parse this input:

1 (unary expression & unary expression)

2 (unary & add expression)

It should be:

expression: relation (('and'|'or') relation)*;

instead, so the there can never be 2 successive expressions (and no ambiguity!).