Hana: How do I create a tuple of types from a variant?

Question

If I have a variant, like so:

using my_variant = boost::variant<int, bool, std::string>;

Is there an easy way to extract the types the variant can contain into a Boost.Hana tuple so that the following holds:

using boost::hana::type;
static_assert(std::is_same<my_tuple, boost::hana::tuple<type<int>, type<bool>, type<std::string>>>{});

Answer 1

The following will work on develop (since e13d826):

#include <boost/hana.hpp>
#include <boost/hana/ext/boost/mpl.hpp>
#include <boost/variant.hpp>
#include <string>
namespace hana = boost::hana;


using my_variant = boost::variant<int, bool, std::string>;

constexpr auto my_tuple = hana::to<hana::tuple_tag>(my_variant::types{});

// Note:
// In general, don't use std::is_same to compare hana::tuple; use == in
// because it will also work if the tuple contains hana::basic_types.
static_assert(my_tuple == hana::tuple_t<int, bool, std::string>, "");

What e13d826 did was add support for mpl::list; only mpl::vector was supported before, and boost::variant<>::types is a mpl::list. This is why my answer took a while to come; I was implementing that :-).

Edit

I did not explain why I'm using constexpr auto my_tuple = ... instead of using my_tuple = decltype(...). Well, the reason is simply because knowing the type of my_tuple is not really useful, since you can't rely on it. Indeed, if you look at the documentation of hana::type, it's written that you can't rely on hana::type<T> being anything specific. There are good reasons for this, but from a usability point of view it means that you can't rely on the type of hana::tuple<hana::type<...>, ...> being anything specific either. When doing type-level computations, prefer value-level encoding (thus auto my_tuple = ...) to type-level encoding. This is the specificity of Hana over MPL & friends, and you should try to stick with it as much as possible, or you'll find Hana to be really clunky (because not written with that in mind).

Answer 2

This doesn't use any hana features, but should work.

First, a transcribe type function that takes a template, and an instance of a different template, and transcribes the types in the second into the first:

template<template<class...>class To, class From>
struct transcribe;
template<template<class...>class To, class From>
using transcribe_t=typename transcribe<To,From>::type;

template<template<class...>class Z, template<class...>class Src, class...Ts>
struct transcribe<Z, Src<Ts...>> {
  using type=Z<Ts...>;
};

Now, a template that takes types, and returns a hana tuple of hana types:

template<class...Ts>
using tuple_of_types = boost::hana::tuple<boost::hana::type<Ts>...>;

And we are done:

template<class Src>
using get_types_from = transcribe_t< tuple_of_types, Src >;

using result = get_types_from< my_variant >;

get_types_from is because a type function that extracts the template arguments of an arbitrary template seems useful.