Haskell pre-monadic I/O

Question

I wonder how I/O were done in Haskell in the days when IO monad was still not invented. Anyone knows an example.

Edit: Can I/O be done without the IO Monad in modern

Answer 1

@sepp2k already clarified how this works, but i wanted to add a few words

I'm really sorry for this. I don't think it is possible to write this function without unsafePerformIO

Of course you can, you should almost never use unsafePerformIO http://chrisdone.com/posts/haskellers

I'm using slightly different Request type constructor, so that it does not take channel version (stdin / stdout like in @sepp2k's code). Here is my solution for this:

(Note: getFirstReq doesn't work on empty list, you would have to add a case for that, bu it should be trivial)

data Request = Readline
             | PutStrLn String

data Response = Success
              | Str String

type Dialog = [Response] -> [Request]


execRequest :: Request -> IO Response
execRequest Readline = getLine >>= \s -> return (Str s)
execRequest (PutStrLn s) = putStrLn s >> return Success


dialogToIOMonad :: Dialog -> IO ()
dialogToIOMonad dialog =
    let getFirstReq :: Dialog -> Request
        getFirstReq dialog = let (req:_) = dialog [] in req

        getTailReqs :: Dialog -> Response -> Dialog
        getTailReqs dialog resp =
            \resps -> let (_:reqs) = dialog (resp:resps) in reqs
    in do
        let req = getFirstReq dialog
        resp <- execRequest req
        dialogToIOMonad (getTailReqs dialog resp)