Format a string in Elm

Question

I have a list of string and generate it to HTML dynamically with li tag. I want to assign that value to id attribute as well. But the problem is the string item has some special characters like :, ', é, ... I just want the output to include the number(0-9) and the alphabet (a-z) only.

// Input:
listStr = ["Pop & Suki", "PINK N' PROPER", "L'Oréal Paris"]

// Output: 
result = ["pop_suki", "pink_n_proper", "loreal_paris"] ("loral_paris" is also good)

Currently, I've just lowercased and replace " " to _, but don't know how to eliminate special character.

Many thanks!

Answer 1

Instead of thinking of it as eliminating special characters, consider the permitted characters – you want just lower-case alphanumeric characters.

Elm provides Char.isAlphaNum to test for alphanumeric characters, and Char.toLower to transform a character to lower case. It also provides the higher function String.foldl which you can use to process a String one Char at a time.

So for each character:

• check if it's alphanumeric
• if it is, transform it to lower case
• if not and it is a space, transform it to an underscore
• else drop the character

Putting this together, we create a function that processes a character and appends it to the string processed so far, then apply that to all characters in the input string:

transformNextCharacter : Char -> String -> String
transformNextCharacter nextCharacter partialString =
    if Char.isAlphaNum nextCharacter then
        partialString ++ String.fromChar (Char.toLower nextCharacter)
    else if nextCharacter == ' ' then
        partialString ++ "_"
    else
        partialString

transformString : String -> String
transformString inputString =
    String.foldl transformNextCharacter "" inputString

Online demo here.

Note: This answer simply drops special characters and thus produces "loral_paris" which is acceptable as per the OP.