haskell - 如何在另一个 monad 中使用 Maybe monad？

Question

我有这个代码(在 happstack 内，但可能只是 IO monad):

accountHandler conn = do
  sessionId <- optional $ readCookieValue "sessionId"

  case sessionId of
    Nothing -> seeOther ("/" :: String) $ toResponse ()
    Just s  -> do
      result <- loggedInUserId conn s

      case result of
        Just userId -> seeOther ("/account/" ++ unUserId userId) $ toResponse ()
        Nothing -> seeOther ("/" :: String) $ toResponse ()

我想删除嵌套的 case 语句并编写如下内容:

accountHandler conn = do

  let action = do
                sessionId <- optional $ readCookieValue "sessionId"
                userId    <- loggedInUserId conn sessionId

                return $ seeOther ("/account/" ++ userId)

  maybe (seeOther ("/" :: String)) id action $ toResponse ()

...但 userId 最终成为一种Maybe String 类型，而不仅仅是String。如何使用 Maybe monad 评估嵌套的 do block ？ (我也接受删除嵌套案例的不同重构。)

更新:下面是同一问题的通用版本，尽管是人为的:

module Main where

getAnswer expected = do
  l <- getLine

  if l == expected
    then return $ Just l
    else return $ Nothing

main = do
  a <- getAnswer "a"

  case a of
    Nothing -> putStrLn "nope"
    Just x -> do
      b <- getAnswer x

      case b of
        Nothing -> putStrLn "nope"
        Just _ -> putStrLn "correct!"

Answer 1

好的，通过您的通用示例，我可以使用 Control¸Monad.Transformers 做一些事情。这允许您创建一堆 monad。您可以在这里查看:http://hackage.haskell.org/package/transformers-0.3.0.0/docs/Control-Monad-Trans-Maybe.html您可以将 MaybeT 应用于 IO (Maybe a) 类型的所有内容，然后在内部 do block 中执行所有计算，最后检查是否为 Nothing。

module Main where
import Control.Monad.Trans.Maybe


getAnswer expected = MaybeT $ do
       l <- getLine
       if l == expected
       then return $ Just l
       else return $ Nothing

main = do
    y <- runMaybeT $ do a <- getAnswer "a"
                        b <- getAnswer a
                        return b
    case y of Nothing  -> putStrLn "failure"
              (Just _) -> putStrLn "correct"

使用 liftIO 和 Alternative 类型类的另一个版本:

module Main where
import Control.Monad.Trans.Maybe
import Control.Monad.IO.Class
import Control.Applicative


getAnswer expected = MaybeT $ do
  l <- getLine
  if l == expected
    then return $ Just l
    else return $ Nothing

main = do
    _ <- runMaybeT $ do a <- getAnswer "a"
                        b <- getAnswer a
                        liftIO $ putStrLn "correct" 
                   <|> do liftIO $ putStrLn "failure"
    return ()

但是使用许多电梯操作并不是很优雅。