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

Subject: Re: [boost] [hana]Either generalization & missing variant
From: Michael Caisse (mcaisse-lists_at_[hidden])
Date: 2015-06-08 16:57:35

On 06/08/2015 01:55 PM, Louis Dionne wrote:
> Michael Caisse <mcaisse-lists <at> ciere.com> writes:
>
>>
>> [...]
>>
>> Converting a type list to a variant is a common thing I need to do.
>> Creating yet-one-more-variant type is probably not in Hana's best
>> interest. Variant is a priority for C++17 and there are already a
>> variety of other implementations.
>>
>> If you provide an adapter for boost::variant (possibly as a util so it
>> isn't a dependency of the core library) then that may satisfy the
>> concerns. Optionally, getting an MPL style typelist from Hana would also
>> solve these types of interfacing issues.
>
> If all you need is to make a variant type (boost::variant<...>) from a
> sequence of types (make_tuple(type<T>...) in Hana), the following will do:
>
>      auto types = make_tuple(...); // a tuple of Type objects in Hana
>      using Variant = decltype(unpack(types, template_<boost::variant>))::type;
>      // Variant is now your boost::variant<...>
>
> Basically, we're considering boost::variant as a function on types
> (via template_), and then we're applying that function to the contents
> of the tuple (via unpack). Finally, we take everything back to the type
> level by using decltype(...)::type.
>
> Regards,
> Louis
>
>

Thank you for the example Louis.

-- 
Michael Caisse
ciere consulting
ciere.com