SpecializedIO.hs

{-# LANGUAGE FlexibleContexts #-}
module SpecializedIO where

import Control.Monad.Free

data OnlyPrintLine a = OnlyPrintLine String a

instance Functor OnlyPrintLine where
  fmap f (OnlyPrintLine s a) = OnlyPrintLine s (f a)

type OnlyPrintLineF = Free OnlyPrintLine

-- | OnlyPrintLineF a ~> IO a
interpret :: OnlyPrintLineF a -> IO a
interpret (Free (OnlyPrintLine s a)) = putStrLn s >> interpret a
interpret (Pure a) = return a

-- | Pretty interface
pp :: MonadFree OnlyPrintLine m => String -> m ()
pp s = liftF (OnlyPrintLine s ())

main :: OnlyPrintLineF ()
main = do
  pp "hello world!"
  pp "I can't do anything except println now!"