Effect of MonoLocalBinds

monolocalbinds.hs

{-# LANGUAGE ExplicitForAll #-}

-- With MonomorphismRestriction (which is ON by default,)
-- any constrained type variable (e.g. `m` in `Monad m`)
-- are not generalized
{-# LANGUAGE NoMonomorphismRestriction #-}

-- MonoLocalBinds prevents generalization of non-top-level binding
{-# LANGUAGE MonoLocalBinds #-}
-- Compare for
-- {-# LANGUAGE NoMonoLocalBinds #-}
module Main where

main :: IO ()
main = putStrLn (result 0)

--------

class Monad m => MonadXXX m where
    xxx :: Int -> m Int

instance MonadXXX Maybe where
    xxx = Just

instance MonadXXX IO where
    xxx a = putStrLn "xxx" >> pure a

genericAction :: forall m. MonadXXX m => Int -> Int -> m Int
genericAction x y = xxx (x + y)

foo1 :: Maybe Int -> String
foo1 = show

foo2 :: m Int -> String
foo2 _ = "foo2"

result :: Int -> String
result n = (_foo1 action) ++ (foo2 action)
  where
    action = genericAction n n