流式 IO

io-streams 是基於流的庫，專注於 Stream 抽象，但是用於 IO。它暴露了兩種型別：

• InputStream：只讀智慧手柄

• OutputStream：一個只寫的智慧控制代碼

我們可以用 makeInputStream::IO (Maybe a) -> IO (InputStream a) 建立一個流。使用 read::InputStream a -> IO (Maybe a) 從流中讀取 ，其中 Nothing 表示 EOF：

import Control.Monad (forever)
import qualified System.IO.Streams as S
import System.Random (randomRIO)

main::IO ()
main = do
  is <- S.makeInputStream $ randomInt  -- create an InputStream
  forever $ printStream =<< S.read is  -- forever read from that stream
  return ()

randomInt::IO (Maybe Int)
randomInt = do
  r <- randomRIO (1, 100)
  return $ Just r

printStream::Maybe Int -> IO ()
printStream Nothing  = print "Nada!"
printStream (Just a) = putStrLn $ show a