如何遍歷列表併爲每個條目返回一些字符串？

Question

我該如何運行列表併爲每個元素返回一些內容？

這裏有一個小例子：

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

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

我怎樣才能讓這個？

Answer 1

我不太清楚你想要做什麼。如果你想扭轉每個元組的第一個元素，並以列表返回結果：

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

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

如果你想要的結果作爲一個字符串，請嘗試：

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

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

要應用功能列表中的每個元素，你可能也想看看map。

Answer 2

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

？

Answer 3

test = concatMap (reverse . fst)