Function (Reader) Monad

fm.hs

foo :: Int -> Int
foo a = a + 1

bar :: Int -> Int
bar a = a - 1

foobar' :: Int -> [Int]
foobar' = sequence [foo, bar]

foobar :: Int -> Int
foobar = sum <$> sequence [foo, bar]

foo :: Int -> Int
foo a = a

:t fmap Just foo => fmap Just foo :: Int -> Maybe Int

foo :: Int -> Maybe Int
foo a = Just a

:t (fmap . fmap) Just foo => (fmap . fmap) Just foo :: Int -> Maybe (Maybe Int)