Re: [boost] [boost:proto] adding new tags

From: Eric Niebler (eric_at_[hidden])
Date: 2007-05-09 12:13:10

Maurizio Vitale wrote:
> Eric Niebler <eric_at_[hidden]> writes:
> 
>> Tough questions. You've touched on a weakness of proto. I hope to make 
>> improvements in this area after BoostCon ...
> 
> I think proto is a very nice addition to boost, and I'm sure with a few touches to make it 
> easier to use for applications different from what it has been designed for it can get
> rapidly to the point it can be submitted formally.

Fair. This particular area certainly needs improvement. But are you also 
referring to your desire for something like proto::eval() to work even 
when you don't have an expression object to evaluate? I don't consider 
that a common use case.

> Do you have a timeframe in mind for this?

Sometime after BoostCon. That's all I can say right now.

>> An improvement to the return type would be:
>>
>> proto::expr<
>>      my_plus
>>    , proto::args2<
>>          typename proto::as_arg<Left>::type
>>        , typename proto::as_arg<Right>::type
>>      >
>>  > const
> 
> I have quickly tried this and it doesn't seem to work: as it is gcc complains about
> as_arg not defining a type in boost::proto (which surely looks like it is from the headers).
> But indeed as_arg is the right thing here, so I'll try some more.

Sorry, try: typename proto::result_of::as_arg<Left>::type

>>>   - a simple example for this would be very nice for the documentation.
>> Agreed.
> 
> Please include how to make those new constructs displayable. You need something like:
> 
>       struct my_plus {};
>       inline char const *proto_tag_name(my_plus) { return "my_plus"; }
> 
> It is important that proto_tag_name is defined in the same namespace as my_plus, otherwise it won't be found
> (Koenig lookup).

Sure.

> In my case my_plus is outside proto::tag and things seem fine with a mix of tags inside and outside
> the proto::tag namespace, but it would be nice to have this confirmed and spelled out in the docs.

Yes, the tag type doesn't need to be in the proto namespace.

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