Re: [boost] [hana]Either generalization & missing variant

Subject: Re: [boost] [hana]Either generalization & missing variant
From: Larry Evans (cppljevans_at_[hidden])
Date: 2015-06-08 12:53:17

On 06/08/2015 10:12 AM, Larry Evans wrote:
[snip]

> wrappers.  Why not provide wrapper's tagged by some
> enumerable?  For example, something like this:
> 
>       template
>       < typename X
>       , typename Enumerable=unsigned
>       , Enumerable tag=Tag()
>       >
>     struct _tagged //instead of _left and _right
>     : operators::adl
>     {
>         X value;
>     .
>     .
>     .
>         template <typename... F>
>         constexpr decltype(auto) go(F...&& f) const&
>         {  return (hana::arg<unsigned(tag)>(f...))(value);
>         }
>     .
>     .
>     .
>     };
> 
> This would be more general than the binary Either.  In
> addition, _tagged could be used elsewhere.  For example, it
> could be used in a multiple inheritance implementation of
> tuple.

Which I see hana actually does here:

https://github.com/ldionne/hana/blob/master/include/boost/hana/detail/closure.hpp#L26

but instead of _tagged<X,n>, element<n,X> is used where element is at:

https://github.com/ldionne/hana/blob/master/include/boost/hana/detail/closure.hpp#L20

[snip]