Convert an `IO String` into an infinite string.

infinite-string-stream.hs

-- Run this example with runhaskell:
--
-- $ runhaskell infinite-string-stream.hs
--
-- Type in some text and press enter.
-- Repeat as long as you like.
-- At the end press CTRL-d to quit.
--
-- Using unsafeInterleaveIO the text you put in will appear immediately after
-- each new line. Without it, the output will only be printed after you quit
-- with CTRL-d.

import System.IO.Unsafe
import System.IO

withStringStream :: IO (Maybe String) -> (String -> IO a) -> IO a
withStringStream m f = f =<< go
  where
    go = do
      x <- m
      case x of
        Just s -> (s ++) <$> unsafeInterleaveIO go
        Nothing -> return ""

input :: IO (Maybe String)
input = do
  b <- isEOF
  if b
    then return Nothing
    else Just <$> getLine

main :: IO ()
main = do
  hSetEcho stdin False
  hSetBuffering stdin  LineBuffering
  hSetBuffering stdout NoBuffering
  withStringStream input print