Re: [boost] [proto] grammar matching issue

From: Maurizio Vitale (maurizio.vitale_at_[hidden])
Date: 2007-06-14 12:21:19

On Jun 14, 2007, at 12:09 PM, Eric Niebler wrote:

> Maurizio Vitale wrote:
>> This one took me a long time to understand, and now that I think I  
>> understand it, I'm pretty sure I don't like
>>
>> Apparently (at least in the situation shown below, with user  
>> defined domain and expressions) unary_expr matches
>> terminal< >. This was surprising to me because I was working under  
>> the assumption that unary_expr matched only
>> bona-fide unary C++ operators.
>
>
> Your understanding of the issue is correct. The implementation is dead
> simple, and the behavior you're seeing naturally falls out of the
> decision to make terminals behave like degenerate unary expressions.
> They are unary expressions because they store one datum inside, just
> like a "real" unary expression does.
>
> I agree that letting terminals match unary_expr<_,_> is a little
> surprising, though. I can easily patch up proto::matches<> such that
> tag::terminal does not match _, so that a terminal will not match
> unary_expr<_,_>. It *will* still match unary_expr<tag::terminal,_>
> though. Would that be ok?
>

Would be helpful (and would solve my specific case completely), but
if you could find a way to completely hide the fact that a terminal is
a unary_expr from the user I think it would be much cleaner for
a boost::proto release.

Regards,

        Maurizio