haskell - 在 Haskell 中制作小写字符串列表

Question

我有一个字符串列表 ["Hi", "hELLO", "nO"]我想将所有内容都转换为小写。我为此写了一个函数:

import Data.Char(toLower)
makeSmall xs = map toLower xs

它可以编译，但是当我这样做时 makeSmall ["Hi", "hELLO", "nO"]它给了我这个错误

<interactive>:2:12: error:
    • Couldn't match expected type ‘Char’ with actual type ‘[Char]’
    • In the expression: "Hi"
      In the first argument of ‘makeSmall’, namely
        ‘["Hi", "hELLO", "nO"]’
      In the expression: makeSmall ["Hi", "hELLO", "nO"]

<interactive>:2:17: error:
    • Couldn't match expected type ‘Char’ with actual type ‘[Char]’
    • In the expression: "hELLO"
      In the first argument of ‘makeSmall’, namely
        ‘["Hi", "hELLO", "nO"]’
      In the expression: makeSmall ["Hi", "hELLO", "nO"]

<interactive>:2:25: error:
    • Couldn't match expected type ‘Char’ with actual type ‘[Char]’
    • In the expression: "nO"
      In the first argument of ‘makeSmall’, namely
        ‘["Hi", "hELLO", "nO"]’
      In the expression: makeSmall ["Hi", "hELLO", "nO"] 

我试图了解如何使我的函数适用于字符串列表，而不仅仅是字符串

Answer 1

toLower :: Char -> Char 映射一个 Char到 Char ，这意味着 map toLower会走单String ，它将返回一个 String .
如果要处理String 的列表s，你应该使用第二个 map :

makeSmall :: [String] -> [String]
makeSmall xs = map (map toLower) xs

或更短:

makeSmall :: [String] -> [String]
makeSmall = map (map toLower)

因此，我们制作了一个使用 map toLower 的映射器。作为 map 功能。因此，此功能将适用于单个 String .由于我们为此制作了一个映射器，因此我们处理了 String 的列表。 s。