使用高阶函数在 Haskell 中查找 Maybe 值列表的最大值

Question

我正在尝试使用更高级别的顺序函数来获取 Maybe 值的列表和 return 一个 int 类型的最大值，但由于某种原因我只能 return 一个 Maybe 值。例如，这有效：

max' iL = foldl maxMaybe Nothing iL
maxMaybe Nothing  y = Just y 
maxMaybe (Just x) y = Just (max x y)

然而，这并不像它所说的那样，它仍然需要一个 Maybe 值，但它正在为结果生成一个 int:

max' iL = foldl maxMaybe (minBound::Int) iL
maxMaybe Nothing  y = y 
maxMaybe (Just x) y = (max x y)

Answer 1

对于第二种实现，您使用了错误的累加器类型，因为 foldl 的累加器是其函数的第一个参数 - 这里是 maxMaybe 函数-，我们必须交换参数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

此函数现在可用，但我认为此方法不是查找最大值的好方法，我更喜欢第一种实现，因为您可以对其结果进行模式匹配。

我建议将第一个实现更改为这个，因为 foldlr 更适合 haskell foldr 而 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

使用高阶函数在 Haskell 中查找 Maybe 值列表的最大值

Finding max of a list of Maybe values in Haskell using Higher Order Functions

haskell

higher-order-functions

maybe