[Haskell-cafe] Custom monad using ST

[Yves Parès]

Hello,

I am trying to make a monad that uses ST internally.
But even when reducing this to the simplest case I'm still cramped by the
's' phantom type :

{-# LANGUAGE Rank2Types #-}

newtype MyST a = MyST (forall s. ST s a)
-- ^ I cannot use " deriving (Monad) " through GeneralizedNewtypeDeriving

runMyST (MyST m) = runST m
-- ^ works thanks to declaration of 's' at rank 2 in the definition of MyST
--   It refuses to compile if MyST is declared as such:
--   data MyST s a = MyST (ST s a)

instance Monad MyST where
  return = MyST . return           -- and this does not compile
  (MyST m) >>= f = MyST $ do
    x <- m
    case f x of
      (MyST m) -> m


If you try it, GHC will complain:
Simple.hs:13:20:
    Couldn't match expected type `forall s. ST s a'
                with actual type `ST s a'
    Expected type: a -> forall s1. ST s1 a
      Actual type: a -> ST s a
    In the second argument of `(.)', namely
      `(return :: a -> (forall s. ST s a))'
    In the expression: MyST . (return :: a -> (forall s. ST s a))
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