Re: [boost] [proto] grammar matching issue

From: Maurizio Vitale (maurizio.vitale_at_[hidden])
Date: 2007-06-14 15:42:08

On Jun 14, 2007, at 12:57 PM, Eric Niebler wrote:

>
> Maurizio Vitale wrote:
>> 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.
>
> I'll give the issue some thought. One possibility is shoehorn into  
> proto
> the concept of a nullary expression, which is what terminals should
> rightly be. Of course, what is there already works, so IMO there are
> bigger fish to fry first.
>

I think "usability" considerations play a big role in a library  
acceptance. In this case, for instance,
if one's code is correct everything works fine: just have terminals  
first and the unary_expr
pattern last.

But when writing the code in the first instance, the patterns for  
terminals may be wrong . It is
not so easy (at least for me) to find what's going wrong in a complex  
mesh of transforms when
some other pattern matches unexpectedly. You just get the wrong  
result. With some luck this
results in a type error somewhere else, but otherwise finding the  
problem in absence
of a meta debugger is, mmm, challenging.

Btw, for now I've wrapped my unary_expr pattern in the following way:

        proto::and_<
                proto::not_<proto::terminal<proto::_> >,
                proto::unary_expr<proto::_, proto::_>
        >

so it is not terrible, but it might bite others in future.

Best regards,

        Maurizio