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  1. {-# language DeriveFunctor #-}
  2.  
  3. import Control.Monad
  4.  
  5. data IOpure a 
  6.   = Return a
  7.   | Output String (IOpure a)
  8.   | Input (String -> IOpure a)
  9.   deriving Functor
  10.  
  11. instance Applicative IOpure where
  12.    pure = Return
  13.    (<*>) = ap
  14.  
  15. instance Monad IOpure where
  16.    Return x >>= f = f x
  17.    Output s io >>= f = Output s (io >>= f)
  18.    Input k >>= f = Input (\s -> k s >>= f)
  19.  
  20. class Monad m => Interaction m where
  21.   getInput :: m String
  22.   produceOutput :: String -> m ()
  23.  
  24. instance Interaction IOpure where
  25.   getInput = Input Return
  26.   produceOutput x = Output x (Return ())
  27.  
  28. echoingUser :: IOpure a -> a
  29. echoingUser = go "no output"
  30.    where
  31.    go _ (Return x)    = x
  32.    go _ (Output o io) = go o io
  33.    go o (Input k)     = go o (k o)
  34.  
  35. interactiveProgram :: Interaction m => m (String, String)
  36. interactiveProgram = do
  37.   produceOutput "Jeff"
  38.   name <- getInput
  39.   produceOutput "Bob"
  40.   name2 <- getInput
  41.   return (name, name2)
  42.  
  43. main :: IO ()
  44. main = print (echoingUser interactiveProgram)
  45.  
Success #stdin #stdout 0s 4492KB
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Standard input is empty
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("Jeff","Bob")
https://ideone.com/ImMQCv
language:
Haskell (ghc 8.4.4)
created:
5 years ago
visibility:
public

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