Re: [boost] [proto] grammar matching issue

From: Eric Niebler (eric_at_[hidden])
Date: 2007-06-14 12:09:37

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?

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com