Pure IO

gistfile1.hs

import Control.Applicative (Applicative(..))
import Control.Monad (ap)

data MyIO a = Return a
            | Get (String -> MyIO a)
            | Put String (MyIO a)

instance Functor MyIO where
    fmap f (Return a) = Return (f a)
    fmap f (Get g)    = Get (fmap f . g)
    fmap f (Put a io) = Put a (fmap f io)
    
instance Applicative MyIO where
    pure = return
    (<*>) = ap

instance Monad MyIO where
    return = Return
    (Return a) >>= f = f a
    (Get f) >>= g = Get (\x -> f x >>= g)
    (Put s io) >>= f = Put s (io >>= f)

run :: (Read a, Show a) => MyIO a -> IO a
run (Return a) = return a
run (Get f)    = getLine >>= run . f
run (Put a io) = putStr a >> run io

get :: MyIO String
get = Get Return

put :: String -> MyIO ()
put s = Put s (Return ())

example :: MyIO ()
example = do
    put "A:\n"
    x <- get
    put $ "B: " ++ x ++ "\n"

main :: IO ()
main = run example