[antlr-interest] Is my brain crooked?

[Tim Gleason]

Thank you!   That's what I needed.  Makes the gammar unreadable, but ;)

--tim


On 6/16/07, Diehl, Matthew J <matthew.j.diehl at intel.com> wrote:
> I think I saw something like this in the book, but what you could try
> (if reordering your original grammar and using predicates doesn't work
> (times_expr times_op)=>), is:  (oh yeah, page 188 of the book)
>
> math_expr
>  : (times_expr -> times_expr)
>    (
>     times_op me2=math_expr -> ^(BIN_EXPR times_op $math_expr $me2)
>    )?
>  ;
>
> So, if there's just times_expr, then that's all that is returned (no
> tree), otherwise, a tree is created, and the previous parts of
> $math_expr (which is just times_expr) are added to the tree.  ...or
> something like that...
> If you don't already have the book, I highly recommend it.
>
> Matt
>
> -----Original Message-----
> From: antlr-interest-bounces at antlr.org
> [mailto:antlr-interest-bounces at antlr.org] On Behalf Of Tim Gleason
> Sent: Saturday, June 16, 2007 12:07 PM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] Is my brain crooked?
>
> I bought the ANTLR book with the idea that I would have some fun
> writing a few toy languages -- and maybe even learn a thing or two.
> But it seems /every/ grammar I try to write fails due to the LL(*)
> restriction.  My brain just seems unable to grok a grammar that ANTLR
> likes :(  I've written several recursive descent parsers in the past
> and never had these kinds of problems.
>
> Anyway, in re-writing my grammar over an over until I get it right, I
> seem to have something that sorta works, but now I can't figure out
> how to write the tree generator.
>
> A fragment of my original grammar looks like this:
>
> math_expr
>  :  times_expr                               -> times_expr
>  |  times_expr times_op math_expr -> ^(BIN_EXPR times_op times_expr
> math_expr)
>  ;
>
> times_op
>        :        ('*'|'/');
>
> That, of course, gave me errors.  I've rewritten the rule, like:
>
> math_expr
>  : times_expr ('*' times_expr)*
>  ;
>
> What I want is a tree if there is a '*', and no tree if not.  I know
> that I can add the '^' at the '*' -- but that will give me '*' at the
> root of my tree.  What I really want is a tree that looks like:
> (BIN_OP {operation} {left_expression} {right_expression})
>
> Is there an easy way to do this in my case?
>
> Thanks for your help!
> tim
>