Add Super-classes to libraries

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.

Trac metadata

Trac field	Value
Version	7.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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