{-# LANGUAGE MultiParamTypeClasses,
             UndecidableInstances,
             FlexibleInstances,
             NoImplicitPrelude #-}

-- http://www.randomhacks.net/articles/2007/03/15/data-set-monad-haskell-macros
-- http://groups.google.com/group/fa.haskell/browse_thread/thread/407a828c2729fdd5/c15ef7d3f7c258c2
import Prelude hiding (return, fail, (>>=))
import qualified Prelude
import qualified Data.Set as S

class Monad1 m a where
  return :: a -> m a
  fail :: String -> m a

class (Monad1 m a, Monad1 m b) =>
      Monad2 m a b where
  (>>=) :: m a -> (a -> m b) -> m b

instance (Ord a) =>
         Monad1 S.Set a where
  return = S.singleton
  fail _ = S.empty

instance (Ord a, Ord b) =>
         Monad2 S.Set a b where
  m >>= f = (setJoin . S.map f) m
    where setJoin = S.unions . S.toList

setExample = do
  x <- S.fromList [1,2,4]
  y <- S.fromList [1,2,4]
  return (x*y)

main = putStrLn $ show setExample