Re: [boost] [proto] weird transform behaviour

From: Eric Niebler (eric_at_[hidden])
Date: 2007-05-10 14:15:54

Maurizio Vitale wrote:
> I've hit something that I find weird in the behaviour of transforms. I don't know what is the intention.
> 
> Basically, what seem to be happening is that if a transform doesn't cover all the cases but a pattern
> matches at some external level, then that is taken as a match.
> For instance, given the following transform:
> 
>     struct width_transform : proto::or_ <
>       terminal_width<proto::terminal< number<_> > >,
>       binary_width<proto::binary_expr<proto::tag::plus, width_transform, width_transform> >
>     > {};
> 
> and evaluating the transform for proto::binary_expr<proto::tag::minus, something, something>, I get binary_width "called".
> If I add a pattern for proto::binary_expr<_, width_transform, width_transform> then that pattern is used.
> 
> I would expect a failure at compile time, but in any case adding a wider pattern after a narrower pattern should never change
> whether the former matches or not.
> 
> Unless I'm making something stupid, in which case just let me know, I'm used to it.

Yes, the assumption is that you will only be applying a transform to 
expressions that match the grammar. You can enforce this via:

   // make sure it matches
   BOOST_MPL_ASSERT((matches<Expr, width_transform>));
   // apply the transform
   width_transform::call(expr, state, visitor);

It would be nice if it failed loudly, instead of silently doing the 
wrong thing, though. I'll looking into changing this.

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