Re: [boost] "Simple C++11 metaprogramming"

Subject: Re: [boost] "Simple C++11 metaprogramming"
From: pfultz2 (pfultz2_at_[hidden])
Date: 2015-05-31 14:05:31

> If one day we have static constexpr lambdas, wouldn't we be able to use
> some kind of lambda meta-functions

We don't need constexpr lambdas to achieve this. We can do higher-order type
computations using dependent typing. I show here how to implement boost
fusion
filter so it will take a lambda rather than a MPL placeholder expression:

http://pfultz2.com/blog/2015/01/24/dependent-typing/

Which is based on the ideas presented by Zach Laine and Matt Calabrese
several
years ago at boostcon. So in essence you can write this in C++14:

    auto numbers = simple_filter(some_fusion_sequence, [](auto x) 
    { 
        return is_integral<decltype(x)>() or
is_floating_point<decltype(x)>(); 
    });

There is no `constexpr` required by the user. It all happens through
dependent
typing. This is how Boost.Hana works as well. Furthermore, Boost.Hana is not
`constexpr`-metaprogramming(athough it makes use of `constexpr`), it is
actually
dependently-typed metaprogramming, which is much more powerful in current
C++.

> Is not that what the proposed Boost.Hana and Boost.Fit  do already using
> a specific trick? 

That trick, in general, is only used to achieve `constexpr` initialization
of
lambdas, which is necessary when declaring global lambdas. In can be used in
other ways to allow type deduction of lambdas in a `constexpr` context.
However,
in general, with dependently-typed metaprogramming this isn't necessary.

Paul

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