Checking String is a Palindrome Case Insensitive in Haskell

Question

I'm trying to write a function that checks if a string is a Palindrome (case insensitive) in Haskell. To make sure capital case characters don't cause an issue, I first convert the string to lower case with toLower (https://hackage.haskell.org/package/text-2.0/docs/Data-Text.html#v:toLower). Then I check if that is equal to the reversed version of the same thing. Here's my code:

{-# LANGUAGE OverloadedStrings #-}
import Data.Text (toLower)


isPalindrome :: String -> Bool
isPalindrome xs = toLower xs == toLower $ reverse xs

I realized toLower uses Text instead of String and we need to use {-# LANGUAGE OverloadedStrings #-} to overload String to Text. But this does not compile.

I replicated this in GHCI as well. I first used :set -XOverloadedStrings to make sure to overload strings. Then I run this:

Prelude Data.Text> toLower $ reverse "sdssSDSS"

<interactive>:92:11: error:
    • Couldn't match expected type ‘Data.Text.Internal.Text’
                  with actual type ‘[Char]’
    • In the second argument of ‘($)’, namely ‘reverse "sdssSDSS"’
      In the expression: toLower $ reverse "sdssSDSS"
      In an equation for ‘it’: it = toLower $ reverse "sdssSDSS"

I don't understand why this doesn't work. If I first run reverse "sdssSDSS" and copy paste the result into toLower it works without an issue.

Answer 1

leftaroundabout has already explained to you how to solve your problem but I have one small addition. If you want to use regular Char -> Char functions with a Text type, you can easily do it using Lens and unpacked ISO (it's convenient but slow :)).

{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import qualified Data.Text as T

useFun1 :: (String -> String) -> T.Text -> T.Text
useFun1 = over unpacked

useFun2 :: (Char -> Char) -> T.Text -> T.Text
useFun2 = over (unpacked . traversed)