〈Haskell Tutorial（21）來寫些迴圈吧！〉的解答

IO.hs

while :: Bool -> IO () -> IO ()
while cond value = do
   if cond then value
           else return ()

forever' :: IO a -> IO ()
forever' value = do
    while True $ do
        value
        forever' value

sequence' :: [IO a] -> IO [a]
sequence' [] = return []
sequence' (x:xs) = do
    value <- x
    values <- sequence' xs
    return (value : values)

mapM' :: (a1 -> IO a) -> [a1] -> IO [a]
mapM' f xs = sequence' $ map f xs

mapM_' :: (a1 -> IO a) -> [a1] -> IO ()
mapM_' f xs = do
    mapM' f xs
    return ()

forM' :: [a1] -> (a1 -> IO a) -> IO [a]
forM' xs f = sequence' $ map f xs