Reader.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
    
newtype Reader r a = Reader { runReader :: r -> a }
    deriving (Functor, Applicative, Monad)

ask :: Reader a a
ask = Reader id

succ :: Reader Int Int
succ = do
    x <- ask
    pure (x + 1)
    
-- The same thing without Reader:

ask' :: a -> a
ask' = id

succ' :: Int -> Int
succ' = do
    x <- ask'
    pure (x + 1)