Re: [Boost-users] [Boost.Hana] how to merge two or more maps

Subject: Re: [Boost-users] [Boost.Hana] how to merge two or more maps
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2016-08-24 21:06:46

Hana does not provide a way to merge two maps out of the box, although that
would be very useful. I added a feature request at [1]. For now, you can use
`fold_left` with `insert` to insert each element of the 

    #include <boost/hana.hpp>
    #include <string>
    namespace hana = boost::hana;
    using namespace hana::literals;

    constexpr auto m1 = hana::make_map(
        hana::make_pair("key1"_s, hana::type_c<std::string>),
        hana::make_pair("key2"_s, hana::type_c<std::string>)
    );

    constexpr auto m2 = hana::make_map(
        hana::make_pair("key3"_s, hana::type_c<std::string>),
        hana::make_pair("key4"_s, hana::type_c<std::string>),
        hana::make_pair("key5"_s, hana::type_c<std::string>)
    );

    constexpr auto result = hana::fold_left(m1, m2, hana::insert);
    constexpr auto expected = hana::make_map(
        hana::make_pair("key1"_s, hana::type_c<std::string>),
        hana::make_pair("key2"_s, hana::type_c<std::string>),
        hana::make_pair("key3"_s, hana::type_c<std::string>),
        hana::make_pair("key4"_s, hana::type_c<std::string>),
        hana::make_pair("key5"_s, hana::type_c<std::string>)
    );

    static_assert(result == expected, "");

    int main() { }

Also, while posting on this list is OK, please note that the preferred way
to
ask Hana-related questions is on StackOverflow with the [boost-hana] tag, as
documented at [2]. This allows everybody to benefit from questions and
answers,
and I am registered to the tag so there's no chance that I miss your
question
as on this list.

Regards,
Louis

[1]: https://github.com/boostorg/hana/issues/299
[2]: http://boostorg.github.io/hana/#tutorial-support

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