We already could write and compile next code:

{-# LANGUAGE PolyKinds, FlexibleContexts, TypeFamilies #-}

{-# LANGUAGE  FlexibleInstances, UndecidableInstances #-}

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


instance Monoid [b] where
 type Pr [b] = [b]
mempty = []
mappend = (++)
 
instance (Monad m, MonadPlus m) => Monoid ( m :: * -> * ) where 
type Pr m = m Int               -- Unfortunally, we can't write now type Pr m = forall b. m b

                                         --Illegal polymorphic or qualified type: forall b. m b
mempty = mzero
mappend = mplus
 

This code is already valid.

But we can't use any written instance at all, the ambiguity problem.

29 / 06 / 2013  17:39:25 user Ben Franksen [via Haskell] ([hidden email]) wrote:

This is a kind error. The m in Monoid has kind *, while the one in Monad has  
kind * -> *.

I suggest you (try to) rewrite your proposal to eliminate these kind errors
before any further elaboration.

Cheers
--
Ben Franksen
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