Luhn

Challenge Overview

Given a number determine whether or not it is valid per the Luhn formula.

Visit the luhn exercise on Exercism to read the full instructions and download the exercise files.

Community Solutions

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The following are the top 3 community solutions by stars:

Solution 1 by @talkingaboutcode (⭐ 1) — View on Exercism
Solution 2 by @ziic (⭐ 0) — View on Exercism
Solution 3 by @wagenert (⭐ 0) — View on Exercism

Source: Exercism fsharp/luhn

💡 View Solution

module Luhn

let (|ValidFormat|InvalidFormat|) (s:string) =
    if s.Trim().Length > 1 && Regex.IsMatch(s, "^[\d\s]*$")
    then ValidFormat (s |> String.filter Char.IsDigit |> Seq.map (fun c -> int c - int '0'))
    else InvalidFormat

let luhnDouble n =
    match (2 * n) with
    | d when d > 9 -> d - 9