Re: [boost] why or_, and_ names? (was Re: Preliminary submission: Boost.Proto DSEL Framework

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-04-04 13:44:30

On 04/04/2007 01:14 PM, Eric Niebler wrote:
> Larry Evans wrote:
[snip]
>>Proto uses or_ and and_ for alternatives
>>and sequences. It seems that alt_ and seq_ would be closer
>>to the spirit names.   Any reason why those names were
>>preferred over alt_ and seq_?
> 
> 
> 
> Because for what they do, or_ and and_ are the right names. :-) You 
> realize that these do not create expression templates, right? 

Yes, but I guess I need to reread about expression templates.

> They are 
> part of proto's meta-grammar facility. If you want to know if an 
> expression type E matches grammar G1 or G2, you use:
> 
>    proto::matches<E, proto::or_<G1, G2> >
> 
> That's a compile-time boolean test. Proto::and_ behaves similarly. Like 
> mpl::or_ and mpl::and_ (and like || and &&) these do short-circuit 
> evaluation.
> 
> HTH,
> 

That sort of helps.  I guess I was just predisposed to thinking
of them as doing something similar to spirit's alternative<L,R> and
sequence<L,R> templates; however, I guess not.  Maybe my confusion is
somehow related to the possiblity of have placeholder's as arguments
to the templates.  I understand that this is related to proto's ability
to do transform of the template expressions, OTOH, spirit doesn't
transform templates, only runtime values (via the parse member function).

In short, I need to study proto some more :(