[antlr-interest] NoViableException with semantic predicates

[Bertalan Fodor]

Hello, I would need help in the following. I have grammar rules like this:

program: (definition)=>definition | expression;
expression: variable | { isMacro(input.LT(1))}? macro;
macro: keyword body;
definition: 'define' body;

Syntactically 'define' can be a keyword, and anything that can be a 
keyword can be a variable. That's why I'm using the semantic predicate.
I get NoViableAlt exception. If I run the debugger it doesn't go into 
the syntactic predicate checking, and the dfa.predict method of 
program() throws NoViableAltException. The semantic predicate is not 
called at all. It seems I'm misunderstanding something. Could you explain?

Thank you,

Bertalan Fodor