{-# language ImpredicativeTypes, UndecidableSuperClasses #-}

import Control.Monad.Reader.Class (MonadReader)
import Control.Monad.IO.Class (MonadIO)

newtype EffFun c m a = EffFun (c m => m a -> m a)

class (c1 m, c2 m) => Combine c1 c2 m
instance (c1 m, c2 m) => Combine c1 c2 m

f :: f (forall m a. EffFun (MonadReader ()) m a)
f = undefined

g :: f (forall m a. EffFun (MonadIO) m a)
g = undefined

h1, h2, h3 :: Applicative f => f (forall m a. EffFun (Combine (MonadReader ()) MonadIO) m a)
h1 = (\(EffFun f) (EffFun g) -> EffFun (f . g)) <$> f <*> g

h2 = specialisedCompose <$> f <*> g where

specialisedCompose ::
    (forall m a. EffFun (MonadReader ()) m a)
    -> (forall m a. EffFun (MonadIO) m a)
    -> forall m a. EffFun (Combine (MonadReader ()) MonadIO) m a
specialisedCompose (EffFun f) (EffFun g) = EffFun (f . g)

h3 = generalisedCompose <$> f <*> g where

generalisedCompose ::
    (forall m a. EffFun c1 m a)
    -> (forall m a. EffFun c2 m a)
    -> (forall m a. EffFun (Combine c1 c2) m a)
generalisedCompose f' g' =
  EffFun (case (f', g') of
            (EffFun f, EffFun g) -> f . g)

main = putStrLn "hello"