[boost] [proto] weird transform behaviour

From: Maurizio Vitale (mav_at_[hidden])
Date: 2007-05-10 13:46:19

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.

Best regards,

     Maurizio