[GHC] #8348: Add Super-classes to libraries

[GHC]

#8348: Add Super-classes to libraries
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       Reporter:  wvv               |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  libraries/base    |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Haskell community has a long discussion how to implement a superclasses
 into Haskell.

 Now it is used `default` method. But it looks ugly!

 But all their abilities are already implemented!

 We need just 2 extensions: FlexibleInstances and UndecidableInstances

 {{{
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE UndecidableInstances #-}

 instance Monad m => Applicative m where
     pure  = return
     (<*>) = ap

 instance Monad m => Functor m where
     fmap = liftM

 instance Monad m => Bind m where
     (>>-) = flip (>>=)
     B.join = M.join
 }}}

 this code is valid!

 I've already defined 3 "superclassses" for Monad: Functor, Applicative and
 Bind!

 "superclass' instances" have unique quality from Programming Patterns and
 typeclasses ideology: do not inherit, extend!

 We could easily extend "superclasses" and make a lot of class'
 dependences.

 We don't need to insert inside the class some ugliness like

 {{{
   default return :: Applicative f => a -> f a
   return = pure
 }}}

 Next is much prettier!
 {{{
 instance Monoid m => Alternative m where
     (<|>) = mplus
     empty = mzero
 }}}

 We could even use it with `Generic` without any `default`:

 {{{
 class ToJSON a where
     toJSON   :: a -> Value

 instance (Generic a, GToJSON (Rep a)) => ToJSON a where
     toJSON = genericToJSON defaultOptions

 }}}

 So, I suggest to made true "Applicative and Functor are superclasses of
 Monad" and add all necessary superclass' instances to base libraries.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8348>
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