[Haskell-cafe] Why the reluctance to introduce the Functor requirement on Monad?

[Maciej Marcin Piechotka]

On Mon, 2011-07-25 at 00:11 -0400, August Sodora wrote:
> Out of (perhaps naive) curiosity, what difficulties does allowing such
> overriding introduce? Wouldn't the module system prevent the ambiguity
> of which implementation to use?
> 
> August Sodora
> augsod at gmail.com
> (201) 280-8138
> 

class A a where
  a :: a

class A a => B b where
  b :: b
  
  a = b

class A a => C c where
  c :: c

  a = c

data BC = B | C deriving Show

instance B BC where
  b = B

instance C BC where
  c = C

show (a :: BC) == ???

Regards
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