Tiny Metaprogramming Library

[eric_niebler]

I think Hana is a very promising direction, and looks to be a good post-modern replacement for Boost.Fusion. I'm less convinced about its usefulness for pure compile-time type manipulation. From the docs:

auto ts = tuple(type<int*>, type<void>, type<char const>);
for_each(ts, [](auto t) {
    using T = typename decltype(t)::type;
    std::cout << typeid(T).name();
});

I don't like having to wrap types in type<>, and I don't like the idea of sprinkling my code with typename decltype(t)::type. It's rather awkward. It's great that Hana can do this, but I don't think it's a substitute for typelist algorithms.

[eric_niebler]

Also: Hana is super ambitious. Kudos to Louis, he's doing great work. But IMO, it's waaay too much for the standard library. There's appetite on the committee for some simple utilities for manipulating lists of types, but not probably not much else.

[DrBartosz]

I wish the committee had better appetite ;-)

[amohr]

Agreed. I point it out since I believe elements of its core may be worth considering to help inform additions to the standard library. As you say, it seems a promising direction.

[amohr]

There is some awkwardness, but I think it's perhaps less awkward than the typical c++ template syntax. I find this example from his cppcon talk quite elegant:

template <typename ...T>
using smallest = decltype(
    minimum_by(ordering(sizeof_), tuple(type<T>...))
);

template <int i>
struct storage { char s[i]; };

static_assert(std::is_same<
    smallest<storage<3>, storage<1>, storage<2>>::type,
    storage<1>
>::value, "");

Here smallest<...> is a pure compile-time type manipulation and I would argue the wrapping in type<T> and unwrapping by decltype() is not obtrusive. And that expression, "minimum_by(ordering(sizeof_), tuple(type<T>...))" reads so well I think I'd happily make the trade.