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List:       haskell-cafe
Subject:    Re: [Haskell-cafe] Inheritance constraints
From:       Gábor_Lehel <illissius () gmail ! com>
Date:       2012-06-07 16:55:11
Message-ID: CAPNUp09R6kPF+fViijNg1aHe7JrWZ+NPFk8_o5exEEDRn9XmDA () mail ! gmail ! com
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On Thu, Jun 7, 2012 at 1:42 PM, Miguel Mitrofanov <miguelimo38@yandex.ru> wrote:
> Hi cafe!
>
> The ConstraintKinds extension makes it possible to create classes of classes, like this:
>
> class F c where cfmap :: c f => (a -> b) -> f a -> f b
> instance F Functor where cfmap = fmap
> instance F Monad where cfmap = liftM
>
> So, basically we are saying that "c" is a class that has some special properties.
>
> Is it possible to express the property "c is a subclass of a specific class d"? Like
> class F c where Functor f => c f

If I'm understanding you correctly, this is similar to something I
recently filed as a feature request:

http://hackage.haskell.org/trac/ghc/ticket/5927

In the meantime it's possible to emulate it to a limited extent with
the Sub type from the constraints package:

http://hackage.haskell.org/packages/archive/constraints/0.3/doc/html/Data-Constraint.html#g:3

Out of curiosity, do you have any specific use case in mind?

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