antlr4compiler-theoryambiguous-grammardangling-else

How does the latest ANTLR4 resolve the "dangling else" ambiguity?

I am using antlr 'org.antlr:antlr4:4.9.2' and come across the "dangling else" ambiguity problem; see the following grammar IfStat.g4.

// file: IfStat.g4
grammar IfStat;

stat : 'if' expr 'then' stat
     | 'if' expr 'then' stat 'else' stat
     | expr
     ;

expr : ID ;

ID : LETTER (LETTER | [0-9])* ;
fragment LETTER : [a-zA-Z] ;

WS  : [ \t\n\r]+ -> skip ;

I tested this grammar against the input "if a then if b then c else d". It is parsed as `"if a then (if b then c else d)" as expected. How does ANTLR4 resolve this ambiguity?

Solution

  • ANTLR will choose the first possible (successful) path it is able to make.

    You can enable ANTLR to report such ambiguities in your grammar. Check this Q&A for that: Ambiguity in grammar not reported by ANTLR