Maybe处理技巧

发表于 分类于 ，

判断多个Maybe都不为Nothing

case of 嵌套

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processValues :: Maybe String -> Maybe String -> Maybe String -> IO ()
processValues ma mb mc = do
    case ma of
        Nothing -> responseFail "ma 为空"
        Just a -> case mb of
            Nothing -> responseFail "mb 为空"
            Just b -> case mc of
                Nothing -> responseFail "mc 为空"
                Just c -> responseSuccess "成功"

模式匹配

将多个Maybe组成元组用于模式匹配

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processValues :: Maybe String -> Maybe String -> Maybe String -> IO ()
processValues ma mb mc = 
	case (ma, mb, mc) of 
	(Nothing, _, _) -> responseFail "ma 为空" 
	(_, Nothing, _) -> responseFail "mb 为空" 
	(_, _, Nothing) -> responseFail "mc 为空" 
	(Just a, Just b, Just c) -> responseSuccess "成功"

traverse

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processValues :: Maybe String -> Maybe String -> Maybe String -> IO ()
processValues ma mb mc = do
    let values = [ma, mb, mc]
    let result = traverse id values  -- 遍历每个 Maybe 值，保持原有的结构
    case result of
        Nothing -> responseFail "有空值"
        Just _ -> responseSuccess "成功"

MaybeT

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processValues :: Maybe String -> Maybe String -> Maybe String -> IO ()
processValues ma mb mc = do
    runMaybeT $ do
        a <- MaybeT (maybe (responseFail "ma为空" >> return Nothing) (return . Just) ma)
        b <- MaybeT (maybe (responseFail "mb为空" >> return Nothing) (return . Just) mb)
        c <- MaybeT (maybe (responseFail "mc为空" >> return Nothing) (return . Just) mc)
        liftIO $ responseSuccess "成功"
    return ()

下面这句将Maybe String转为IO (Maybe String)，如果为Nothing将会打印提示

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maybe (responseFail "ma为空" >> return Nothing) (return . Just) ma)

MaybeT包裹IO (Maybe String)使得遇到Nothing后短路