Reader Monad

data Reader e a = Reader (e -> a)

instance Functor (Reader e) where
  -- fmap :: (a -> b) -> Reader e a -> Reader e b
  fmap f (Reader g) = Reader (f.g)

instance Applicative (Reader e) where
-- pure :: a -> Reader e a
  pure x = Reader (\e -> x)

-- (<*>) :: Reader e (a -> b) -> Reader e a -> Reader e b
  rab <*> ra = Reader (\e -> let fab = runReader rab e
                                 fa  = runReader ra e
                             in  fab fa)

instance Monad (Reader e) where
  -- (>>=) :: Reader e a -> (a -> Reader e b) -> Reader e b
  ra >>= arb = 
    Reader (\e -> let a  = runReader ra e -- a
                      rb = arb a -- Reader e b
                  in  runReader rb e)


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

ask :: Reader e e
ask = (Reader id)

askfor f = fmap f ask

data Config = Conf { tweet_key  :: String
                   , api_secret :: String
                   }

postTweet :: String -> Reader Config String
postTweet t = do
  b <- valida t
  if b then return t
  else return "senha inválida"

valida :: String -> Reader Config Bool
valida x = do 
  key <- askfor tweet_key
  if key == "teste"
  then return True
  else return False

main = do let conf1  = Conf "teste" "senha"
              conf2  = Conf "erro" "senha"
              reader = postTweet "Meu tweet"
          putStrLn $ runReader reader conf1
          putStrLn $ runReader reader conf2