Nesting and conversion

Remove a level of nesting, or convert to an Option.

Flatten and Transpose are extension methods, in Waystone.Monads.Results.Extensions — add the using. GetOk and GetErr are on Result<TOk, TErr> itself and need nothing.

Flatten

Result<TOk, TErr> Flatten<TOk, TErr>(this Result<Result<TOk, TErr>, TErr> result)

Removes one level of nesting. You get here by calling Map with a function that itself returns a Result.

Result<string, string> start = Result.Ok<string, string>("Storm Weaver");
Result<Result<int, string>, string> output = start.Map(x => CountRunes(x));
Result<int, string> flattened = output.Flatten();

Prefer AndThen, which does both steps at once. This exists for code that grew one method at a time.

This is the single-result Flatten. The one that drops the failures out of a sequence is a different method — see Collections.

Transpose

Option<Result<TOk, TErr>> Transpose<TOk, TErr>(this Result<Option<TOk>, TErr> result)

Turns a result holding an option into an option holding a result.

Result<Option<decimal>, string> calculationResult =
    CreateCalculator(Realm.TalDorei)
        .Map(calculator => calculator.GetToll(100.00m));

Option<Result<decimal, string>> maybeToll = calculationResult.Transpose();

Calling Transpose here declares that the absence of a toll is a valid outcome in your business rules.

On an Err: you get Some(Err(…)) — the failure survives.

Option<Result<T, E>> transposes the other way. See the Option page.

GetOk

Option<TOk> GetOk()

Converts to an Option, keeping the success value and discarding the error.

Result<int, string> ok = Result.Ok<int, string>(1);
Result<int, string> err = Result.Err<int, string>("Error");

Option<int> some = ok.GetOk();
//          ^? Some(1)

Option<int> none = err.GetOk();
//          ^? None()

The error is gone. Use this only when you have already dealt with it, or do not care.

GetErr

Option<TErr> GetErr()

The other direction. Keeps the error and discards the success value.

Option<string> none = ok.GetErr();
//             ^? None()

Option<string> some = err.GetErr();
//             ^? Some("Error")

Going the other way

To convert an Option into a Result, see OkOr and OkOrElse.