io-stream sample 2

io-stream sample 2.hs

{-# LANGUAGE LambdaCase #-}

module SampleStream where

import Control.Monad.IO.Class (liftIO)
import System.IO.Streams                as S

main :: IO ()
main = do
  i <- S.fromList [1,2,3,9,4]       >>= \i'  ->
       fromGenerator (doNothing i') >>= \i'' ->
       fromGenerator (modify i'')
  o <- writeConsole
  connect i o

writeConsole :: (Show a) => IO (OutputStream a)
writeConsole = S.makeOutputStream $ \case
  Just x  -> print x
  Nothing -> return ()

doNothing :: InputStream Int -> Generator Int ()
doNothing is = do
  ma <- liftIO $ S.read is
  case ma of
    Nothing -> return ()  -- stop
    Just a  -> do         -- as it is.
      yield a
      doNothing is

modify :: InputStream Int -> Generator Int ()
modify is = do
  ma <- liftIO $ S.read is
  case ma of
    Nothing -> return ()  -- stop
    Just 9  -> return ()  -- stop
    Just 1  -> do         -- duplicate
      yield 1
      yield 1
      modify is
    Just a  -> do         -- as it is.
      yield a
      modify is