How to throw an exception in Antlr grammar

Question

I have this kind of a grammar,

locationPath returns [CustomParser xpathParser]  
    :^(LOCATION_PATH relativeLocationPath {**Want to throw a exception if this condition matches**})
    |^(LOCATION_PATH absoluteLocationPath {$xpathParser=$absoluteLocationPath.xpathParser;})
    ;

What is the way to do it? I tried with this one

locationPath returns [CustomParser xpathParser]  
    :^(LOCATION_PATH relativeLocationPath {throw new Exception})
    |^(LOCATION_PATH absoluteLocationPath {$xpathParser=$absoluteLocationPath.xpathParser;})

But with this one the generated code gives compile Error. Because that method loactionapth doesn't have throws clues at method signature.

Answer 1

Only one way to do this: throw an unchecked exception:

locationPath returns [CustomParser xpathParser]  
 : ^(LOCATION_PATH relativeLocationPath) {throw new RuntimeException("No way!");}
 | ^(LOCATION_PATH absoluteLocationPath {$xpathParser=$absoluteLocationPath.xpathParser;})
 ;

If the compiler still complains (I can't remember, and I'm not able to test right now), add an if(true) in front of it:

locationPath returns [CustomParser xpathParser]  
 : ^(LOCATION_PATH relativeLocationPath) {if(true) throw new RuntimeException("No way!");}
 | ^(LOCATION_PATH absoluteLocationPath {$xpathParser=$absoluteLocationPath.xpathParser;})
 ;