Re: [GHC] #8021: Multiple constraint classes - the alternative to superclass

6 Jul 2013

      #8021: Multiple constraint classes - the alternative to superclass
-----------------------------+----------------------------------------------
Reporter:  wvv               |          Owner:                  
    Type:  feature request   |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.3             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------

Comment(by wvv):

 We already could define

 {{{
 {-# LANGUAGE PolyKinds, FlexibleContexts, TypeFamilies #-}
 {-# LANGUAGE FlexibleInstances, UndecidableInstances, RankNTypes #-}
 module Monoids where

 import Control.Monad
 import Control.Arrow

 class Monoid (a :: k) where
         type PrE a :: *
         mempty :: PrE a
         mappend :: PrE a -> PrE a -> PrE a

 instance Monoid [b] where
         type PrE [b] = [b]
         mempty = []
         mappend = (++)

 instance (Monad m, MonadPlus m) => Monoid ( m b ) where
         type PrE (m b) = m b
         mempty = mzero
         mappend = mplus

 instance (Arrow a, ArrowPlus a, ArrowZero a) => Monoid ( a b c ) where
         type PrE (a b c) = a b c
         mempty = zeroArrow
         mappend = (<+>)
 }}}

 And this program is valid.
 We can't use it at all, but as we see, the GHC is already has most
 features to implement -XClassInterfaces extension.

 I suggest to rewrite this program:

 {{{
 {-# LANGUAGE PolyKinds, FlexibleContexts, TypeFamilies #-}
 {-# LANGUAGE FlexibleInstances, UndecidableInstances, RankNTypes #-}
 module Monoids where

 import Control.Monad
 import Control.Arrow

 class interface Monoid (a :: k) where
         type PrE a :: *
         mempty :: PrE a
         mappend :: PrE a -> PrE a -> PrE a

 class Monoid [b] where
         type PrE [b] = [b]
         mempty = []
         mappend = (++)

 class (Monad m, MonadPlus m) => Monoid (forall b. m b ) where
         type PrE (m b) = m b
         mempty = mzero
         mappend = mplus

 class (Arrow a, ArrowPlus a, ArrowZero a) => Monoid (forall b c. a b c)
 where
         type PrE (a b c) = a b c
         mempty = zeroArrow
         mappend = (<+>)
 }}}

 that's all!

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/8021#comment:5
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