Larry Evans <cppljevans <at> suddenlink.net> 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<T,hana_list<T> >

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<Head, Tail>` to inherit publicly from // `_pair<Head, Tail>`. template <> struct head_impl<List> { template <typename Head, typename Tail> static constexpr auto apply(_pair<Head, Tail> const& xs) { return hana::first(xs); } }; template <> struct tail_impl<List> { template <typename Head, typename Tail> static constexpr auto apply(_pair<Head, Tail> 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