gistfile1.hs

{-# LANGUAGE DeriveFunctor #-}
import Control.Monad.Free

--data Free f a = Pure a | Free (f (Free f a))
--instance Functor f => Monad (Free f) where
--    return = Pure
--    (Pure a) >>= f = f a
--    (Free fr) >>= f = Free (fmap (>>= f) fr)
--
--liftF :: Functor f => f a -> Free f a
--liftF =  Free . fmap Pure
--
data Terminal a =
    WriteLine String a
    | ReadLine (String -> a)
    deriving (Functor)

writeLine :: String -> Free Terminal ()
writeLine s = liftF $ WriteLine s ()
readLine :: Free Terminal String
readLine = liftF $ ReadLine id

helloWorld :: Free Terminal ()
helloWorld = do
    writeLine "Hi, what's your name?"
    name <- readLine
    writeLine $ "Hello " ++ name

interpretIO :: Free Terminal a -> IO a
interpretIO (Free (WriteLine s a)) = putStrLn s >> interpretIO a
interpretIO (Free (ReadLine f)) = getLine >>= interpretIO . f
interpretIO (Pure a) = return a