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

I'd prefer an example that works with modern GHC.

For GHC 8.6.5:

import Control.Concurrent.Chan(newChan, getChanContents, writeChan) 
import Control.Monad((<=<))

type Dialogue = [Response] -> [Request]
data Request  = Getq | Putq Char
data Response = Getp Char | Putp

runDialogue :: Dialogue -> IO ()
runDialogue d =
  do ch <- newChan
     l <- getChanContents ch
     mapM_ (writeChan ch <=< respond) (d l)

respond :: Request -> IO Response
respond Getq     = fmap Getp getChar
respond (Putq c) = putChar c >> return Putp

where the type declarations are from page 14 of How to Declare an Imperative by Philip Wadler. Test programs are left as an exercise for curious readers :-)

If anyone is wondering:

 -- from ghc-8.6.5/libraries/base/Control/Concurrent/Chan.hs, lines 132-139
getChanContents :: Chan a -> IO [a]
getChanContents ch
  = unsafeInterleaveIO (do
        x  <- readChan ch
        xs <- getChanContents ch
        return (x:xs)
    )

yes - unsafeInterleaveIO does make an appearance.