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Subject: Re: [boost] "Simple C++11 metaprogramming"
From: Eric Niebler (eniebler_at_[hidden])
Date: 2015-08-19 19:31:05

On 8/14/2015 6:05 AM, Bruno Dutra wrote:
> After maturing my understanding of metaprogramming a little further, I
> come to the conclusion eager metafunction evaluation using template
> aliases and lazy metafunction evaluation through explicit access of a
> nested ::type are two sides of the same coin and choosing one over the
> other is mostly just a matter of taste.
> That said, I'd like to share two reasons why I prefer the lazy version:
> 1. Most tools for metaprogramming in the standard library are lazy, so
> following the principle of least surprise, lazy it shall remain

I think that's primarily, or at least in part, because alias templates
didn't exist before C++11. Just because we did things that way in the
past doesn't necessarily mean it's the best way now.

> 2. Lazy evaluation of metafunctions allow for expressive construction
> of "lambda expressions" as formalized by MPL

True, but in my experience, lambdas are not often needed.

> I concur with Eric that functional composition is a killer feature and
> I strongly believe it should constitute the very core of any
> metaprogramming library. I just go a step further and greatly simplify
> things by getting rid of "metafunction classes" altogether. I've
> managed to transfer the entire burden of abstraction to an analogous
> of MPL's apply, which expects a "lambda expression" and a set of
> arguments and, through a handful of partial specializations, directly
> evaluates it. Atop apply<>, bind<> becomes a one-liner, as do
> everything else just as gracefully. Oh and this way one also avoids
> dealing with core issue #1430, since MPL's quote<> is no more.

Although interesting from a design perspective, I suspect that if you
benchmark you'll find this approach is too heavy. Compile-time lambdas
are expensive. Turning *every* metafunction evaluation into a lambda
evaluation is going to kill compile-times.

Apologies if I've misunderstood.

> If one looks closer, by doing the actual recursive metafunction
> evaluation in a SFINAE context behind the scenes, apply<> becomes a
> monadic bind of metafunctions, which, from this perspective, are
> themselves nothing more than optionals. Such concepts bring great
> expressiveness to a functional world.

You refer to your eval template that returns just<Ret> or nothing? It's
interesting, but essentially the same as turning evaluation failures
into a SFINAE-able condition. It has the same pros and cons, too. If you
get back a "nothing" from a complicated computation, you're left
wondering why. I don't have a good solution to that yet.

> I've been exploring this idea with my library Metal [1] (yes the name
> kindda sucks, I know, but it is what it is) which is an evolution of
> my earlier attempt of reimplementing MPL. Not that gave up on the
> earlier idea, it just has grown far beyond that.
> [1]:


Eric Niebler

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