Interfaces - the Golden Path of Haskell?

[Wvv]

We already could define

{-# LANGUAGE PolyKinds, FlexibleContexts, TypeFamilies, 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 write this program like this:

{-# LANGUAGE ClassInterfaces, PolyKinds, FlexibleContexts, TypeFamilies,
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!



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