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

[GHC]

#8021: Multiple constraint classes - the alternative to superclass
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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:                  
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Comment(by wvv):

 And you could allow to write classes as these:
 {{{
 class interface Monoid where  where    -- Constructor (!) must be empty
    mempty  :: *            -- kinds only
    mappend :: * -> * -> *  -- kinds only

 class Mononoid a where ...

 class Mononoid m where ...
 }}}

 Inside class interface constructor must be empty, because:

 {{{
 () => Monoid a      :: *
 Monad m => Monoid m :: * -> *
 Arrow c => Monoid c :: * -> * -> *
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8021#comment:2>
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