readerMonad.hs

--the reader monad is supposed to model computations that have access to some
--read-only environment. In imperative code such environment is often
--implemented as a global object. In functional languages we need to pass it
--as an argument to every function that might potentially need access to it. 
--The reader monad hides this process
newtype Reader e a = Reader (e -> a)

reader :: (e -> a) -> Reader e a
reader f = Reader f

runReader :: Reader e a -> e -> a
runReader (Reader f) x = f x

ask :: Reader e e
ask = reader (\e -> e)

instance Monad (Reader e) where
    return x = reader (\_ -> x)
    rd >>= k = reader $ \env ->
        let x = runReader rd env
            act' = k x
        in runReader act' env

type Env = Reader String
-- curried version of
-- type Env a = Reader String a

test :: Env Int
test = do
    s <- ask
    return $ read s + 1

main = print $ runReader test "13"