在 Haskell 中制作小写字符串列表

Question

I have a list of strings ["Hi", "hELLO", "nO"]

and I want to convert everything to lowercase. I wrote a function for this:

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

It compiles, but when I do makeSmall ["Hi", "hELLO", "nO"] it gives me this error

<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"] 

I'm trying to understand how I can make my function work for a list of strings, instead of just a string

Answer 1

toLower :: Char -> Char maps a Char to a Char , this means that map toLower will take a single String , and it will return a String .

If you want to handle a list of String s, you should use a second map :

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

or shorter:

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

We thus make a mapper that uses map toLower as map function. This function will thus work on a single String . Since we make thus a mapper for this, we work on a list of String s.