查找并返回第一个包含 char 的字符串（Haskell）答案

【问题标题】：Find and return first string which contains char (Haskell)查找并返回第一个包含 char 的字符串（Haskell）
【发布时间】：2021-10-24 14:58:55
【问题描述】：

我正在尝试编写一个循环遍历字符串列表的函数，如下所示：

["     |     ","     #     ","     *     ","     |     ","     #     "]

我正在尝试查找并返回包含'|', '#', or '*' 的第一个元素，具体取决于用户的选择。

我想出了这个解决方案：

test :: Char -> [String] -> Maybe Char
test c (x:xs) = find (== c) $ concatMap (filter (/=' ')) (reverse xs)

我得到这个输出：

ghci> test '*' ["     |     ","     #     ","     *     ","     |     ","     #     "]
Just '*'

找到元素，但我希望它返回元素而不改变它（删除空格）。如何在不过滤空格的情况下做到这一点？

这是我想要的输出：

ghci> test '*' ["     |     ","     #     ","     *     ","     |     ","     #     "]
"     *     "

【问题讨论】：

你想要的输出值的类型是什么？
我希望输出是字符串类型
是的，我知道，这是一个指导性问题。 :)

标签： haskell find list-comprehension

【解决方案1】：

由于您已使用list-comprehension 标记您的问题，

test :: Eq a => a -> [[a]] -> Maybe [a]
test c xs = listToMaybe [s | s <- xs, elem c s]

应该做的工作。

> test '*' ["     |     ","     #     ","     *     ","     |     ","     #     "]
Just "     *     "

> test '-' ["     |     ","     #     ","     *     ","     |     ","     #     "]
Nothing

> :i listToMaybe
listToMaybe :: [a] -> Maybe a   -- Defined in `Data.Maybe'

【讨论】：