Re: [boost] [proto]purpose of binary_expr<T,L,R>::type ?

From: Eric Niebler (eric_at_[hidden])
Date: 2008-03-03 01:21:02

Larry Evans wrote:
> 
> If the template arguments are grammars then shift_right<L,R>::type is
> not an expression type.

Correct, it's a grammar type.

> AFAICT, it's of no use.

That's not true. The fact that shift_right<L,R>::type creates 
expressions when L and R are expressions, and grammars when L and R are 
grammars, is an important symmetry that is leveraged extensively within 
Proto. And what would the alternative be? To have two classes, 
expression::shift_right<> that generates expression types, and 
grammar::shift_right<> that creates grammar rules? This would surely be 
worse for users. Why have two when one works just as well? Or are you 
suggesting something else?

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