[antlr-interest] always create an ast node for an optional token

[Felix Dorner]

Hey,

I have some problems with ambiguities of declarations/expressions. I 
therefore introduced an additional Token VAR='var' that's required prior 
to a declaration.
At some points however, I want to make that var token optional, because 
it's clear from the context that there is a variable declaration, like 
in a for statement:

Tokens{
VAR='var'
}

declaration: VAR ID -> ^(VAR ID)

forStatement : 'for' (VAR? ID 'in'...) -> ^(FOR ^(VAR ID ...))

You see, that I'd like to put a var node in the tree in all cases, even 
if it's ommited in the text in unambiguous situations. It seems like 
antlr doesn't like it this way, I get RewriteEmptyStreamExceptions in 
this case.

I resolved it by placing literal 'var's into the rules and make VAR an 
imaginary token, to decouple their meanings:

Tokens{
VAR;
}

declaration : 'var' ID -> ^(VAR ID)
for: 'for' '(' 'var'? ID ...)   -> ... ^(VAR ID)

 I guess that's an okay way to do it. Any other solutions?


Thanks
Felix