ValidationExample.cs

using System.Numerics;
using LanguageExt;
using LanguageExt.Common;
using LanguageExt.Traits;
using static LanguageExt.Prelude;

namespace ValidationExamples;


// Credit card number
public record CardNumber(Seq<int> Number)
{
    public override string ToString() =>
        $"{Number}";
}

// Expiry date
public record Expiry(int Month, int Year) :
    IAdditionOperators<Expiry, Expiry, Expiry>,
    IComparisonOperators<Expiry, Expiry, bool>
{
    public static readonly Expiry OneMonth = new (1, 0); 
    
    public static Expiry operator +(Expiry left, Expiry right)
    {
        var m = left.Month + right.Month;
        var y = left.Year + right.Year;
        while (m > 12) m -= 12;
        return new Expiry(m, y);
    }

    public static bool operator >(Expiry left, Expiry right) =>
        left.Year > right.Year ||
        left.Year == right.Year && left.Month > right.Month;

    public static bool operator >=(Expiry left, Expiry right) =>
        left.Year > right.Year ||
        left.Year == right.Year && left.Month >= right.Month;

    public static bool operator <(Expiry left, Expiry right) =>
        left.Year < right.Year ||
        left.Year == right.Year && left.Month < right.Month;
    
    public static bool operator <=(Expiry left, Expiry right) =>
        left.Year < right.Year ||
        left.Year == right.Year && left.Month <= right.Month;

    public static Expiry Now
    {
        get
        {
            var now = DateTime.Now;
            return new Expiry(now.Month, now.Year);
        }
    }

    public static Range<Expiry> NextTenYears =>
        LanguageExt.Range.fromMinMax(Now, Now + new Expiry(0, 10), new Expiry(1, 0));

    public override string ToString() =>
        $"{Month}/{Year}";
}

// CVV code
public record CVV(int Number)
{
    public override string ToString() =>
        $"{Number}";
}

// Complete credit card details
public record CreditCardDetails(CardNumber CardNumber, Expiry Expiry, CVV CVV)
{
    public static CreditCardDetails Make(CardNumber cardNo, Expiry expiry, CVV cvv) =>
        new (cardNo, expiry, cvv);
    
    public override string ToString() =>
        $"CreditCard({CardNumber}, {Expiry}, {CVV})";
}

public static class CreditCard
{
    public static void Test()
    {
        Console.WriteLine(Validate("4560005094752584", "12-2024", "123"));
        Console.WriteLine(Validate("00000", "00-2345", "WXYZ"));
    }
    
    public static Validation<Error, CreditCardDetails> Validate(string cardNo, string expiryDate, string cvv) =>
        fun<CardNumber, Expiry, CVV, CreditCardDetails>(CreditCardDetails.Make)
           .Map(ValidateCardNumber(cardNo))
           .Apply(ValidateExpiryDate(expiryDate))
           .Apply(ValidateCVV(cvv))
           .As();

    static Validation<Error, CardNumber> ValidateCardNumber(string cardNo) =>
        (ValidateAllDigits(cardNo), ValidateLength(cardNo, 16))
            .Apply((digits, _) => digits.ToSeq())
            .Bind(ValidateLuhn)
            .Map(digits => new CardNumber(digits))
            .As()
            .MapFail(e => Error.New("card number not valid", e));    

    static Validation<Error, Expiry> ValidateExpiryDate(string expiryDate) =>
        expiryDate.Split(['\\', '/', '-', ' ']) switch
        {
            [var month, var year] =>
                from my in ValidateInt(month) & ValidateInt(year)
                let exp = new Expiry(my[0], my[1])
                from _  in ValidateInRange(exp, Expiry.NextTenYears )
                select exp,
            
            _ => Fail(Error.New($"expected expiry-date in the format: MM/YYYY, but got: {expiryDate}"))
        };

    static Validation<Error, A> ValidateInRange<A>(A value, Range<A> range)
        where A : IAdditionOperators<A, A, A>,
                  IComparisonOperators<A, A, bool> =>
        range.InRange(value)
            ? Pure(value)
            : Fail(Error.New($"expected value in range of {range.From} to {range.To}, but got: {value}"));

    static Validation<Error, CVV> ValidateCVV(string cvv) =>
        fun<int, string, CVV>((code, _) => new CVV(code))
           .Map(ValidateInt(cvv).MapFail(_ => Error.New("CVV code should be a number")))
           .Apply(ValidateLength(cvv, 3).MapFail(_ => Error.New("CVV code should be 3 digits in length")))
           .As(); 

    static Validation<Error, EnumerableM<int>> ValidateAllDigits(string value) =>
        value.AsEnumerableM()
             .Traverse(CharToDigit)
             .As();

static Validation<Error, int> ValidateInt(string value) =>
    ValidateAllDigits(value).Map(_ => int.Parse(value));

    static Validation<Error, string> ValidateLength(string value, int length) =>
        ValidateLength(value.AsEnumerableM(), length)
            .Map(_ => value);

    static Validation<Error, K<F, A>> ValidateLength<F, A>(K<F, A> fa, int length)
        where F : Foldable<F> =>
        fa.Count() == length
            ? Pure(fa)
            : Fail(Error.New($"expected length to be {length}, but got: {fa.Count()}"));

    static Validation<Error, int> CharToDigit(char ch) =>
        ch is >= '0' and <= '9'
            ? Pure(ch - '0')
            : Fail(Error.New($"expected a digit, but got: {ch}"));
    
    static Validation<Error, Seq<int>> ValidateLuhn(Seq<int> digits)
    {
        int checkDigit = 0;
        for (int i = digits.Length - 2; i >= 0; --i)
        {
            checkDigit += ((i & 1) is 0) switch
                          {
                              true  => digits[i] > 4 ? digits[i] * 2 - 9 : digits[i] * 2,
                              false => digits[i]
                          };
        }

        return (10 - checkDigit % 10) % 10 == digits.Last
                   ? Pure(digits)
                   : Fail(Error.New("invalid card number"));
    }
}

@iblazhko Thanks for pointing this out. I have moved this into Samples/CreditCardValidation in the main repo and updated it. I've also updated the blog article that pointed to this gist to point to the repo instead.