[antlr-interest] Left factor? Syntactic predicates? Or another solution?

Naveen Chawla naveen.chwl at googlemail.com
Thu Oct 1 13:35:12 PDT 2009


2009/10/1 Gavin Lambert <antlr at mirality.co.nz>

> At 01:24 2/10/2009, Naveen Chawla wrote:
>> Thanks so much! Yes, I tried complement: object object? coincidentally
>> just before I read your message and it worked. Good news is that in the real
>> grammar they ARE actually equivalent. But is there no way of making
>> 'indirectObject' and 'object' appear as they are in the end structure if
>> there are 2 'object's in a row (they have slightly different meanings even
>> though syntactically the same)? Would a syntactic predicate solve this? For
>> example, complement: (indirectObject object)=>indirectObject object |
>> object; Is this the right one? (it doesn't seem to work on ANTLRWorks) Many
>> many thanks, N
> Which rules it passes through shouldn't have any bearing on the semantic
> meaning; the rules should just be concerned with matching logical units of
> tokens.
> You can assign different meanings to the objects via altering the output
> AST, eg:
> complement
>  : (o1=object -> ^(DIRECT $o1))
>    (o2=object -> ^(INDIRECT_DIRECT $o1 $o2))?
>  ;
> (There are lots of other possible tree layouts, of course.)
> If you're carrying out some action directly rather than outputting an AST,
> you can still use a similar technique (putting some of the code inside the
> optional block) to get different behaviour if the second object is actually
> present.
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