[antlr-interest] a grammar problem

[John B. Brodie]

Greetings!
On Sun, 2009-12-13 at 14:20 +0100, Hans-Martin Adorf wrote:
> here is an excerpt on a SchemeNumber grammar which is part of a Scheme
> grammar that I am toying with.
> 
> grammar SchemeNumber;
> tokens {
>     HASH = '#' ;
> }
> prefix2        : RADIX2 EXACTNESS?
>         | EXACTNESS? RADIX2
>         ;       
> RADIX2        : HASH ('b'|'B');
> EXACTNESS    : HASH ('i'|'I'|'e'|'E');
> 
> The problem is in rule 'prefix2' since token RADIX2 can be matched in
> two different ways. Scheme requires that I am able to state the
> following prefixes:
> 
> "#b", "#e#b" and "#b#e". 
> 
> The radix description is mandatory whereas the exactness is optional,
> and can come before ore after the radix description. 
> 
> Any idea how to change the grammar above so that ANTLR no longer
> complains and disables alternatives?

perhaps (untested)....

prefix2 : RADIX2 EXACTNESS? | EXACTNESS RADIX2 ;


(as an aside, are you sure you want to do this in the parser? does your
lexer ignore whitespace? if so, does scheme permit whitespace between
the radix and exactness tokens? because your lexer would accept that.)