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

21 Sep 2014

      Vicente J. Botet Escriba  writes:
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Le 17/09/14 23:30, Vicente J. Botet Escriba a écrit :
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Le 17/09/14 22:49, Louis Dionne a écrit :
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Vicente J. Botet Escriba  writes:
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In addition, I wonder if the category associated shouldn't depend on 
the type class we want to map.
TC::Instance>
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> {};
I think I don't understand what you are trying to achieve here. What exactly 
are CatA and T? What effect do you want to achieve in doing such a type class
instantiation (instantiation in Hana/Haskell terms).
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A deeper analysis of your code let me think that you have already 
something similar to what I was requesting.
[...]
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.
Given a data type D, it is possible to instantiate any unary type class TC 
from inside D's definition as follows:

    struct D {
        struct hana_TC {
            // type class definition
        };
    };

This is true for all unary type classes. However, since the instantiation of
the Record type class is very straightforward, a macro is provided to make it
even easier. Note that the ability to instantiate a type class from inside a
data type's definition is not documented right now.

Regards,
Louis