Check palindromes in Elm

Ossi Hanhinen @ohanhi

October 20, 2017

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I teach Elm to my co-workers at Futurice by live-coding each Friday that I can. After each session, I publish the code produced and write a short post explaining the topics covered that time.

This is the first post in the series.

All code can be found on GitHub.

Day 1: Check that palindrome

For the first day of live-coding on elm-quicks we created a tiny program that can tell whether a phrase is a palindrome or not. We started with the most simple Elm program possible:

import Html

main =
    Html.text "Hello Elm"

Next, we added a potential palindrome and a stub for a function that would check the palindrome-ness of a string.

palindrome =
    "Are we not drawn onward, we few, drawn onward to new era?"

isPalindrome input =
    False

This was then used in main:

main =
    Html.text (toString (isPalindrome palindrome))

toString was needed to turn the boolean value from isPalindrome into a string, which is what Html.text expects to receive.

From then on, we filled out the logic for the function in a let-in structure. The let block is where you can define local variables for a function. The code after in is the actual function body. We also used Debug.log at times to make sure things looked like we thought they should.

The idea for how we can check for palindrome-ness was: turn the phrase into lower case letters only, and then check if it is equal to its reverse. At the live-coding session, I used a silly method to check if a character is a letter. Thanks to one of the participants I found a much nicer way to do this, so the final code looks like this:

isPalindrome input =
    let
        characters =
            String.toList (String.toLower input)

        justLetters =
            List.filter Char.isLower characters

        lettersInReverse =
            List.reverse justLetters
    in
    justLetters == lettersInReverse

The entire code can be found here: day-01.