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Subject: Re: [boost] "Simple C++11 metaprogramming"
From: Bruno Dutra (brunocodutra_at_[hidden])
Date: 2015-06-02 11:59:43

2015-06-02 11:54 GMT-03:00, Peter Dimov <lists_at_[hidden]>:
> [...]
> I do agree that for branching one does need laziness. So if you want, f.ex.
> template<class Def, template<class...> class F, class... T>
> using eval_or_default = /**/
> // if is_evaluable<F, T...> returns F<T...> else returns Def
> then I agree that you have to have a lazy if.
> My basic question is "do you need laziness for something else apart from
> lazy if?"
> The lazy version is not hard to recover, but I'm still wondering whether
> it's even necessary outside of if.
> template<template<class...> class F, class... T> struct mp_defer
> {
> using type = F<T...>;
> };

mp_defer is a reasonable way of providing a lazy interface if it is
only meant to be used in conjunction with your proposed
eval_or_default. Apart from branching, laziness is also required for
lambdas as presented by MPL, but if you are to argue that those are
not necessary, than perhaps mp_defer might indeed be all you need. In
any case I'd rather stick to the typename $<>::type vs $_t<> duality
to mimic the standard library, but that of course is just a matter of
personal taste.

At any rate I think I agree with Louis that if one is restricted to
such very simple cases, than perhaps <type_traits> is all one needs.
That is, in my opinion any metaprogramming library should just go
ahead and provide the more complex lambda facilities if it is to prove
itself really useful for the end user.

Bruno Dutra

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