[antlr-interest] Why doesn't this work?

[Gavin Lambert]

At 03:12 9/04/2009, Indhu Bharathi wrote:
>INT_FLOAT_PATTERN
>     :    (NUMBER DOT NUMBER LETTER ) => NUMBER DOT NUMBER LETTER
>         { $type=PATTERN; }
>
>     |    ( NUMBER DOT NUMBER ) =>  NUMBER DOT NUMBER
>         { $type=FLOAT_LIT; }
>
>     |    (NUMBER) => NUMBER
>         { $type=INT_LIT; }
>
>     ;

I'm not sure why that isn't working, but you might want to 
left-factor it a bit more than you have:

INT_FLOAT_PATTERN /* or you could just make this INT_LIT */
   : NUMBER { $type = INT_LIT; }
     ( (DOT NUMBER) => DOT NUMBER { $type = FLOAT_LIT; }
       (LETTER { $type = PATTERN; })?
     )?
   ;

This should be more efficient (because it will only do the 
lookahead-backtracking once), and hopefully it'll actually work 
(though I haven't tested it).