rank2.hs

{-# LANGUAGE Rank2Types #-}

import Data.Function
import Control.Monad

data M m a = M a

instance Monad (M m) where
  return = M
  (M a) >>= f = f a

runZ :: (forall m . M m a) -> a
runZ (M z) = z

data T a b = T {unT :: b}

foo :: Int -> M z (T z Int)
foo i = M (T i)

bar :: T z Int -> T z Int -> M z Int
bar = (return .). (+) `on` unT

-- λ> runZ $ join $ liftM2 (bar) (foo 1) (foo 2)
-- 3