`newtype` IO

newtype_io.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Applicative

-- Don't export the 'MyIO' constructor
newtype MyIO a = MyIO { runMyIO :: IO a
                      } deriving (Functor, Applicative, Monad)


myGetLine :: MyIO String
myGetLine = MyIO getLine

myPutStrLn :: String -> MyIO ()
myPutStrLn = MyIO . putStrLn

main = runMyIO $ do
  a <- myGetLine
  myPutStrLn a