Re: [boost] [hana]recursive data type?

25 Jun 2015

      Larry Evans  writes:
...
On 06/24/2015 05:25 AM, Larry Evans wrote:
...
How would one define a recursive data type in hana
something described by the type equation:
hana_list<T> = hana::nothing | std::pair
where hana_list<T> is a list of T's terminated by nothing?
I think this:
https://gist.github.com/cppljevans/f270db4ddeaf8bdd5733
would be a start.
Sorry for the late reply; you had the time to do it yourself!
Indeed, what you wrote would be a good start. Then, you might
want to make your List a model of some concepts, like Iterable:

    namespace boost { namespace hana {
        // This would require `_const` to inherit publicly from
        // `_pair`.
        template <>
        struct head_impl<List> {
            template 
            static constexpr auto apply(_pair const& xs) {
                return hana::first(xs);
            }
        };

        template <>
        struct tail_impl<List> {
            template 
            static constexpr auto apply(_pair const& xs) {
                return hana::second(xs);
            }
        };

        template <>
        struct is_empty_impl<List> {
            template <typename Xs>
            static constexpr auto apply(Xs const& xs) {
                return hana::bool_<!Xs::is_cons>;
            }
        };
    }}

However, note that a recursive implementation of a list might not lead to
the best compile-time performance.

Regards,
Louis

Re: [boost] [hana]recursive data type?

Louis Dionne