Re: [boost] Boost.Hana - Type Classes, data types and categories (tags).

Subject: Re: [boost] Boost.Hana - Type Classes, data types and categories (tags).
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2014-09-21 08:28:45

Le 17/09/14 23:30, Vicente J. Botet Escriba a écrit :
> Le 17/09/14 22:49, Louis Dionne a écrit :
>> Vicente J. Botet Escriba <vicente.botet <at> wanadoo.fr> writes:
>
>>> In addition, I wonder if the category associated shouldn't depend on 
>>> the
>>> type class we want to map.
>>>
>>> TC::Instance<category_t<T, TC>>
>> I'm not sure why that would be useful. Do you have a use case in mind?
>
> After more thoughts the user can always specialize TC::Instance<T> 
> inheriting from TC::Instance<CatA>.
>
> struct TC::Instance<T> : TC::Instance<CatA> {};
>
A deeper analysis of your code let me think that you have already 
something similar to what I was requesting.

     #define 
BOOST_HANA_TYPECLASS(NAME)                                      \
         /** @cond 
*/                                                        \
         template <typename T, typename = 
void>                              \
         struct 
instance                                                     \
             : instance<T, 
::boost::hana::when<true>>                        \
         { 
};                                                                \
\
         template <typename 
T>                                               \
         struct instance<T, decltype((void)(typename T::hana_ ## 
NAME*)0)>   \
             : T::hana_ ## 
NAME                                              \
         { 
};                                                                \

The previous specialization let the data type class define directly how 
is it could be seen as an instance of a given type class NAME.
I see however that this has been used only fro the Record type class.

boost/hana/record/macros.hpp:    struct hana_Record : 
::boost::hana::Record::mcd {                       \

So here you have the use_case you were requesting.

Best,.
Vicente