whileを使ったループ

while.hs

import Data.IORef

doWhile :: Monad m => m Bool -> m a -> m a
doWhile test body = do
  x <- body
  testResult <- test
  if testResult
    then doWhile test body
    else return x

while :: Monad m => m Bool -> m a -> m (Maybe a)
while test body = do
  testResult <- test
  if testResult
    then doWhile test body >>= \x -> return (Just x)
    else return Nothing

example :: IO ()
example = do
  i <- newIORef 0
  putStrLn $ "Can you guess the number I have?"

  while ((/= 42) `fmap` readIORef i) $ do
    a <- getLine
    putStrLn $ "You said:" ++ a
    writeIORef i $ read a
    putStrLn "That's right!"

  return ()

example2 :: IO ()
example2 = do

  flag <- newIORef True

  putStrLn $ "Can you guess the number I have?"

  while (readIORef flag) $ do
    a <- getLine
    putStrLn $ "You said:" ++ a
    let i = read a in
      writeIORef flag (i /= 42)
    putStrLn "That's right!"

  return ()