App.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}

module App where

class Apply f g ret | f g -> ret where
  app :: f -> g -> ret

instance Applicative f => Apply (f (a -> b)) (f a) (f b) where
  fs `app` xs = fs <*> xs

instance (Applicative f, Applicative g) => Apply (f (g (a -> b))) (f (g a)) (f (g b)) where
  gfs `app` gxs = pure app <*> gfs <*> gxs

funcs :: [Int -> Int]
funcs = [id]

args :: [Int]
args = [3]

test1 :: [Int]
test1 = funcs `app` args

test2 :: [[Int]]
test2 = [funcs] `app` [args]

main :: IO ()
main = print test1 >> print test2