我正在嘗試使用更高級別的順序函式來獲取 Maybe 值串列并回傳 int 型別的最大值，但由于某種原因，我只能回傳一個 Maybe 值。例如這有效：
max' iL = foldl maxMaybe Nothing iL
maxMaybe Nothing y = Just y
maxMaybe (Just x) y = Just (max x y)
然而，這并不像它所說的那樣它仍然期望一個 Maybe 值，但它正在為結果生成一個 int ：
max' iL = foldl maxMaybe (minBound::Int) iL
maxMaybe Nothing y = y
maxMaybe (Just x) y = (max x y)
uj5u.com熱心網友回復：
對于第二個實作，您對累加器使用了錯誤的型別，因為foldl' 的累加器是其函式的第一個引數 - 這里是maxMaybefunction-，我們必須在 中交換引數maxMaybe：
max' :: Foldable t => t (Maybe Int) -> Int
max' = foldl maxMaybe (minBound::Int)
maxMaybe :: Ord a => a -> Maybe a -> a
maxMaybe y Nothing = y
maxMaybe y (Just x) = max x y
這個函式現在可以作業了，但我認為這個方法不是找到最大值的好方法，我更喜歡第一個實作，因為你可以對它的結果進行模式匹配。
我建議將第一個實作更改為這個，因為 asfoldlr對于 haskell foldr更可取，并且Just (Just a)是多余且丑陋的：
maxWithFoldlr = (Foldable t, Ord a) => t (Maybe a) -> Maybe a
maxWithFoldlr = foldlr maxMaybe Nothing
maxWithFoldl :: (Foldable t, Ord a) => t (Maybe a) -> Maybe a
maxWithFoldl = foldl maxMaybe Nothing
maxMaybe :: Ord a => Maybe a -> Maybe a -> Maybe a
maxMaybe Nothing y = y
maxMaybe x Nothing = x
maxMaybe (Just x) (Just y) = Just $ max x y
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