{-# language DeriveFunctor #-}

import Control.Monad

data IOpure a 
  = Return a
  | Output String (IOpure a)
  | Input (String -> IOpure a)
  deriving Functor

instance Applicative IOpure where
   pure = Return
   (<*>) = ap

instance Monad IOpure where
   Return x >>= f = f x
   Output s io >>= f = Output s (io >>= f)
   Input k >>= f = Input (\s -> k s >>= f)
   
class Monad m => Interaction m where
  getInput :: m String
  produceOutput :: String -> m ()

instance Interaction IOpure where
  getInput = Input Return
  produceOutput x = Output x (Return ())

echoingUser :: IOpure a -> a
echoingUser = go "no output"
   where
   go _ (Return x)    = x
   go _ (Output o io) = go o io
   go o (Input k)     = go o (k o)

interactiveProgram :: Interaction m => m (String, String)
interactiveProgram = do
  produceOutput "Jeff"
  name <- getInput
  produceOutput "Bob"
  name2 <- getInput
  return (name, name2)

main :: IO ()
main = print (echoingUser interactiveProgram)