Question

特别是，我需要能够将CGI monad与IO monad结合起来，但是如何将IO monad与Maybe monad相结合的示例可能会更好......

Answer 1

我假设你想使用Maybe monad提前终止（比如C中的break或return）。

在这种情况下，您应该使用MaybeT包中的MaybeT（cabal install MaybeT）。

main = do
  runMaybeT . forever $ do
    liftIO $ putStrLn "I won't stop until you type pretty please"
    line <- liftIO getLine
    when ("pretty please" == line) mzero
  return ()

MaybeT是monad的monad变换器版本。

Monad变形金刚“向其他monad添加功能”。

Answer 2

您并未确切地说明要如何合并IO和Maybe，但我认为您有许多函数会返回IO (Maybe a)，您希望轻松合并它们。基本上，您希望将IO (Maybe a)视为一个单独的类型，并使用它自己的Monad实例：

newtype IOMaybe a = IOM (IO (Maybe a))

-- "unpack" a value of the new type
runIOMaybe :: IOMaybe a -> IO (Maybe a)
runIOMaybe (IOM a) = a

instance Monad IOMaybe where
   -- bind operator
   (IOM ioa) >>= f = IOM $ do
      a <- ioa
      case a of
         Nothing -> return Nothing
         Just v  -> runIOMaybe (f v)

   -- return
   return a = IOM (return (Just a))

-- maybe also some convenience functions
returnIO :: IO a -> IOMaybe a
returnIO ioa = IOM $ do
   v <- ioa
   return (Just v)

returnMaybe :: Maybe a -> IOMaybe a
returnMaybe ma = IOM (return ma)

使用此功能，您可以使用do - 符号来合并返回IO (Maybe a)，IO a或Maybe a的函数：

f1 :: Int -> IO (Maybe Int)
f1 0 = return Nothing
f1 a = return (Just a)

main = runIOMaybe $ do
   returnIO $ putStrLn "Hello"
   a <- returnMaybe $ Just 2
   IOM $ f1 a
   return ()

一般来说，组合和修改像这样的monad的东西叫做monad transformer，而GHC附带的package包括monad变换器用于常见情况。如果此monad变换器库中有适合您的场景的内容取决于您希望如何组合Maybe和IO。

Answer 3

你想在什么意义上结合monad？

f :: Int -> IO (Maybe Int)
f x = do
    putStrLn "Hello world!"
    return $ if x == 0 then Nothing else Just x

可以评估为：

[1 of 1] Compiling Main             ( maybe-io.hs, interpreted )
Ok, modules loaded: Main.
*Main> f 0
Hello world!
Nothing
*Main> f 3
Hello world!
Just 3

如何在Haskell中组合monad？

3 个答案: