如何运行列表并为每个条目返回一些字符串？

Question

如何浏览列表并为每个元素返回一些内容？

这是一个小例子：

stack = [("ax","bx"),("cx","dx")]

test :: [(String, String)] -> String
test [] = ""
test (x:xs) = do
    reverse (fst x)
    test xs

我该怎么做？

Answer 1

我不太确定你要做什么。如果要反转每个元组的第一个元素并将结果返回到列表中：

stack = [("ax","bx"),("cx","dx")]

test :: [(String, String)] -> [String]
test [] = []
test (x:xs) = reverse (fst x) : test xs

如果您希望将结果作为单个字符串，请尝试：

stack = [("ax","bx"),("cx","dx")]

test :: [(String, String)] -> String
test [] = ""
test (x:xs) = reverse (fst x) ++ test xs

要将函数应用于列表的每个元素，您可能还需要查看map。

Answer 2

test = concatMap (reverse . fst)

Answer 3

test :: [(String, String)] -> String
test = foldr (\x r -> reverse (fst x) ++ r) ""