Dealing with properties where null is valid

[speterson-zoll]

I'm still learning the library, but I get stuck on things. In my case I have a DTO class with this property:

public string? EventDateTime { get; set; }

For this property, a null is perfectly valid; the date is not required. However, I still need to parse/validate the string to be sure it's a proper DateTime. I wrote this parser:

public static Validation<Error, DateTime> ParseNullableDateTime(string? dateTime) =>
        new(notnull(dateTime)
            ? parseDateTime(dateTime).ToValidation(Error.New($"{dateTime} is not a valid Date"))
            : null);

The Parser works just fine for a valid date; it returns a properly parsed DateTime. It also works fine for an invalid date string; it returns a Fail with the error message shown above. However, the null business is an issue. I get a Fail with an empty list, but the return value also shows a Success with the default DateTime:

[0] = {ValidationData<Error, DateTime>} ValidationData(Fail, 1/1/0001 12:00:00 AM, [])
 Fail = {Lst<Error>} []
 State = {Validation.StateType} Fail
 Success = {DateTime} 1/1/0001 12:00:00 AM 

I was hoping that a null would count as a Success, but I see now that that can't happen.

I also tried changing the string? type to Option<string> and parsing that, but I get a Value is null exception:

public static Validation<Error, DateTime?> ParseNullableDateTime(Option<string> value) =>
        value.Match(
            Some: d => DateTime.TryParse(d, out var date)
                ? Success<Error, DateTime?>(date)
                : Fail<Error, DateTime?>(Error.New($"{d} is not a valid Date")),
            None: () => null);

Any advice on how to validate a string? property to a DateTime is welcome.

[YuriiNskyi]

I'm not sure why do you still need to return nullable DateTime, please correct me, if I'm mistaken and you really need to do this.

Anyway, IMHO the clearest solution is something like this:

public static Validation<Error, DateTime> ParseNullableDateTime(string? dateTime)
    => Prelude.Optional(dateTime)
        .Bind(Prelude.parseDateTime)
        .ToValidation(Error.New($"{dateTime} is not a valid Date"))

This signature is kind of strange:

Validation<Error, DateTime?>

Forget about null when you have already valid DateTime, you don't need to bother with nulls anymore.

[sparerd]

I think you have it almost right with your last example. The Value is null exception you are getting is because LanguageExt forbids nulls unless explicitly allowed (using the ____Unsafe functions). In this case, the Validation type does not have any overloads for accepting nulls.

Rather than propagating the nullable type, consider going all-in on using the alternate value monads. Changing the return type from Validation<Error, DateTime?> to Validation<Error, Option<DateTime>> provides the same information but with better safety. Now instead of a Success case with a null value, we give it None.

public static Validation<Error, Option<DateTime>> ParseNullableDateTime(Option<string> value) =>
	value.Match(
		Some: d => DateTime.TryParse(d, out var date)
			? Success<Error, Option<DateTime>>(date)
			: Fail<Error, Option<DateTime>>(Error.New($"{d} is not a valid Date")),
		None: () => Success<Error, Option<DateTime>>(None));

And if you do ever really need a nullable DateTime, you can still MatchUnsafe() on your Option<DateTime> and map the None to null.