Constrained monads with multi-parameter typeclasses

CMonad.hs

{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-}

import Data.Set (Set)
import qualified Data.Set as Set

class Unit m a where
  unit :: a -> m a

class (Unit m a, Unit m b) => Bind m a b where
  bind :: m a -> (a -> m b) -> m b

instance Unit [] a where
  unit = (:[])

instance Bind [] a b where
  bind m k = concatMap k m

instance Unit Set a where
  unit = Set.singleton

instance Ord b => Bind Set a b where
  bind m k = Set.fold (Set.union . k) Set.empty m

{-
[1, 2, 3] `bind` \a ->
  [1, 2, 3] `bind` \b ->
    unit (a * b)
==
[1, 2, 3, 2, 4, 6, 3, 6, 9]

Set.fromList [1, 2, 3] `bind` \a ->
  Set.fromList [1, 2, 3] `bind` \b ->
    unit (a * b)
==
Set.fromList [1, 2, 3, 4, 6, 9]
-}