如何用 Haskell 中的守卫替换大小写或匹配模式？

Question

我正在尝试（作为学习的一部分）用带有警卫的表达式替换表达式的大小写。 这是一个带有外壳的 function

myList :: [a] -> String
myList xs = case xs of []  -> "empty"
                       [x] -> "one"
                       xs  -> "more"

我已经设法编写了一个 function 与 where 和匹配模式

myList' :: [a] -> String
myList' xs = someF xs 
where someF []  = "empty"
      someF [x] = "one"
      someF xs  = "more"

但是，我无法制作另一个使用警卫 (|) 的 function myList''。 我是 Haskell 的新手，如果有人能帮助我解决这个问题，我将不胜感激。 先感谢您。 铝

Answer 1

您可以使用模式防护，例如：

myList'' :: [a] -> String
myList'' xs
    | [] <- xs = "empty"
    | [_] <- xs = "one"
    | otherwise = "more"

或者您可以使用null:: Foldable f => fa -> Bool和drop:: Int -> [a] -> [a]检查列表是否为空：

myList'' :: [a] -> String
myList'' xs
    | null xs = "empty"
    | null (drop 1 xs) = "one"
    | otherwise = "more"