Re: [boost] [proto] design of expr<>

From: Larry Evans (cppljevans_at_[hidden])
Date: 2008-03-21 16:24:02

On 03/20/08 20:17, Eric Niebler wrote:
> Larry Evans wrote:
>> However, I'd imagine some people would want
>> this restricted to just two elements, which could be
>> done with something like:
> <snip>
>> And now, if size<Args>::value is
>> restricted to the Arity in
>> tag_arity<shift_right,Arity> then it
>> would be restricted to 2.
> 
> Users may very well want to restrict tag::plus to arity 2 *in their 
> domain*, but they shouldn't be foisting that decision on other consumers 
> of Proto. 

Good point. I agree. then just define:

template<class Tag,int Arity,class Domain=default_domain>
tag_arity{};

which has no restrictions on arity, but it allow the user to
specify restrictions by specializaing on Domain.

> The best way to ensure that expressions have the arity you 
> expect is to encode that in the grammar for your domain, and check it 
> with proto::matches.
> 

Like in the attached?