io-m.hs

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}

module Sample(perform, runAsIO, runAsSt) where

import Text.Printf (printf)
import qualified Control.Monad.Trans.State as S

class Monad m => ConsoleR m where
  writeLine :: String -> m ()
  readLine  :: m String

perform :: ConsoleR m => m ()
perform = do writeLine "What is your name?"
             name <- readLine
             writeLine (printf "Hello %s" name)

instance ConsoleR IO where
  writeLine = putStrLn
  readLine  = getLine

instance ConsoleR (S.State String) where
  writeLine l = S.modify (++ "WL:" ++ l)
  readLine    = S.state (\s -> ("some_name", s ++ "RL:some_name"))

runAsIO :: IO ()
runAsIO = perform :: IO ()

runAsSt :: S.State String ()
runAsSt = perform :: S.State String ()