[antlr-interest] error in tree grammar syntax...

[Gavin Lambert]

At 10:51 30/12/2008, pady wrote:
 >I was under the assumption that the paranthesis is what makes 
the
 >parser choose different paths...so to differentiate input like
[...]
 >About the "optional statements", this is where my understanding 
on
 >rewrite rules is poor...i dont understand how to differentiate
 >the 2 optional statements using more structure in the ast...ie
 >the syntax to use is what i am not clear on.

statement
   : assignment
   | ^('if' boolExpression statement*
       ^('else' (statement statement?)? )? )
   ;

This will permit zero, one, or two statements in the "else" 
section; if only one statement is supplied then it will 
unambiguously be recognised as the first of the two.  I also 
removed the redundant assignment from the "if" section.

I'm still not sure why you'd want to define the else block like 
that, though; generally the if and else blocks should be symmetric 
-- they should both either only accept one statement (which might 
be a composite block) or any number of statements (depending on 
how you choose to write the parser rule and how you want to 
structure the AST).  It's a little strange to make one side open 
and the other restricted like that.