Monad instance of Fetch from ICFP2014 Haxl paper

Fetch.hs

instance Monad Fetch where
  return a = Fetch $ \ref -> return (Done a)

  Fetch m >>= k = Fetch $ \ref -> do
    r <- m ref
    case r of
      Done a       -> unFetch (k a) ref
      Blocked br c -> return (Blocked br (c >>= k))
      Throw e      -> return (Throw e)

GenHaxl.hs

instance Monad (GenHaxl u) where
  return a = GenHaxl $ \_env _ref -> return (Done a)
  GenHaxl m >>= k = GenHaxl $ \env ref -> do
    e <- m env ref
    case e of
      Done a       -> unHaxl (k a) env ref
      Throw e      -> return (Throw e)
      Blocked cont -> return (Blocked (cont :>>= k))
  fail msg = GenHaxl $ \_env _ref ->
    return $ Throw $ toException $ MonadFail $ Text.pack msg