Generated internal parser contains java syntax error

[SimonCockx]

I'm seeing a syntax error in the generated InternalMyDslParser.java file:

        // InternalMyDsl.g:139:7: ( ( ( ',' ( ( ruleSwitchCaseOrDefault ) ) ) )=> ( ',' ( ( ruleSwitchCaseOrDefault ) ) ) )*
        loop:
        do {
            int alt=;  // SEE THIS LINE
            switch (alt) {
        	case 1 :
        	    ...
        	    break;
        	default :
        	    break loop;
            }
        } while (true);

Minimal repro grammar file:

grammar org.xtext.example.mydsl.MyDsl with org.eclipse.xtext.common.Terminals

generate myDsl "http://www.xtext.org/example/mydsl/MyDsl"

Expression:
	expression=SwitchOperation;

SwitchOperation returns Expression:
	Integer
	(
		=>({SwitchOperation.argument=current} operator='switch' =>(cases+=SwitchCaseOrDefault =>(',' cases+=SwitchCaseOrDefault)*)?)
	)*
;

SwitchCaseOrDefault:
	'default' expression=Expression
 	| guard=Integer 'then' expression=Expression
;

Integer returns Expression:
	value=INT
;

Is there any workaround for this?

[szarnekow]

Can you try to remove the nested predicates)

[szarnekow]

Looks like they should be replaced by first-set predicates in all three cases. Use -> instead of the fat arrow

[SimonCockx]

I'll try out your suggestion, thanks.

I wonder if this works for a more complex grammar as well though. I'll try it out on my actual grammar (see this line here: https://github.com/REGnosys/rune-dsl/blob/switch-fix/rosetta-lang/src/main/java/com/regnosys/rosetta/Rosetta.xtext#L676), and see if I need to "enhance" the example a bit to make more sense. I'll come back.

[SimonCockx]

@szarnekow This did indeed fixes the issue. I did a review of my expression parser rules, and figured I could use a first token predicate -> most of the time, except in a few places. Thanks for that.

Should I close the issue, or keep it open, since the actual generation issue is not fixed?