Getting a value from an Option without 'Some' in value

[speterson-zoll]

I have a function that takes in two DateTime parameters and turns them into Option<DateTime>:

public static Validation<Error, bool> ValidateDateRange(Option<DateTime> startDateTime,
      Option<DateTime> endDateTime)
    {
      var result = map(Tuple(startDateTime, endDateTime), (start, end) => (start.IsNone || end.IsNone) || start <= end);

      return result
        ? Success<Error, bool>(result)
        : Fail<Error, bool>(Error.New($"The value of the start date '{startDateTime.GetValue()}' cannot be greater than the end date '{endDateTime.GetValue()}'."));
    }

As you can see, I created an extension method called GetValue based on something I read in another discussion. I made it generic and it looks like this:

public static class GetValues
{
  public static string? GetValue<T>(this Option<T> input) =>
    input.Match(
      Some: x => x!.ToString(),
      None: () => string.Empty);
}

My goal is to include the value of the DateTime in my error message, but I can't seem to get the actual value out with a call to Map. For example, this call: startDateTime.Map(x => x) returns Some(11/15/2022 8:06:36 AM). How do I get rid of Some? The variable x in the Map call shows up as a DateTime object, so why am I still getting Some in my value?

[louthy]

Not sure if I'm entirely parsing what you're asking, but I think there's an alternative route that's more elegant. Really you want to work with the Some state when you know it is a Some state, not try to hack it to be either the value or an empty string after-the-fact:

public static Error InvalidRange(DateTime s, DateTime e) =>
    Error.New($"The value of the start date '{s}' cannot be greater than the end date '{e}'.");

public static Validation<Error, Unit> ValidateDateRange(
    Option<DateTime> startDateTime,
    Option<DateTime> endDateTime) =>
    (startDateTime.Case, endDateTime.Case) switch
    {
        (DateTime s, DateTime e) when s > e => InvalidRange(s, e),
        _ => Success<Error, Unit>(unit),
    };

What this does is it pattern-matches the Option when the start is greater than the end. This is the only failure state I can gather from your example. So, as soon as we know it's wrong, we fail. The subsequent catch of any other value returns Success.

I have also changed it to return unit rather than bool, because in your example I believe you will always have true if the validation is successful. Which means it's a single value, i.e. a unit, not a bool.

This test:

System.Console.WriteLine(ValidateDateRange(DateTime.Now, DateTime.Now.AddDays(1)));
System.Console.WriteLine(ValidateDateRange(DateTime.Now, DateTime.Now.AddDays(-1)));
System.Console.WriteLine(ValidateDateRange(None, DateTime.Now));
System.Console.WriteLine(ValidateDateRange(DateTime.Now, None));

Yields:

Success(())
Fail([The value of the start date '15/11/2022 17:17:04' cannot be greater than the end date '14/11/2022 17:17:04'.])
Success(())
Success(())

[speterson-zoll]

Thanks you once again. I had not made use of the Case property.