Proposal: Ensure null with Maybe

[DerStimmler]

Currently, the ToResult method on Maybe returns a success result if the Maybe has a value, and a failure result if there is no value.

But what if it should be the other way around?

---

Lets say I want to create a report, but only if no report already exists.

Right now I would probably do something like this:

await Result
      .Success()
      .Map(() => service.FindAsync(id)) //returns Report?
      .Ensure(report => report is null, "There is already a report available.")
      .Tap(_ => service.CreateReport())
      ...

But this comes with some overhead because I have to start the chain with an empty success result to then map the nullable value and ensure its null using the Ensure method.

I think it would be more appropriate and compact to use Maybe and its ToResult method since I'm dealing with a nullable.

But to keep the chain running if the Maybe has no value, I basically need an inverted ToResult method that returns a success result if there is no value. E.g:

await Maybe
      .From(service.FindAsync(id))
      .ToInvertedResult("There is already a report available.") //returns success result if Maybe has no value, otherwise returns failure result with error message
      .Tap(() => service.CreateReport())
      ...

---

What do you think about this addition?

ToInvertedResult is just an example name because I can't think of anything better. 😄

[vkhorikov]

Yeah, sounds useful enough. Feel free to submit a PR. Can't think of a better name than ToInvertedResult either :)

[bothzoli]

Hey @DerStimmler, @vkhorikov,
I'm happy to give this one a shot in the coming days.

[bothzoli]

If I'm correct with my thinking, this will return a vanilla Result for Maybe<T> and a UnitResult<E> when using a custom failure.
Because I cannot populate a T from a failed Maybe<T> for it to be Result<T> (or populate a T for a Result<T, E> from a Maybe<T,E>).

But I believe it would probably make sense to add overrides which could do just that, like:
Result<T> ToInvertedResult<T>(in this Maybe<T> maybe, string errorMessage, Func<string, T> compensate)
or
Result<T, E> ToInvertedResult<T, E>(in this Maybe<T> maybe, string errorMessage, Func<E, T> compensate)

See #588, any thoughts?

[DerStimmler]

Hi @bothzoli,
thanks for your help.

I think you are right with Result and UnitResult<E> as return types.

I don't think we need the compensate func since there is already a Compensate operation.
Then it would look like this:

Maybe.From("Foo")
  .ToInvertedResult("Bar")
  .Compensate(error => ...)

Since we can create Task<Maybe<T>> from async values the ToInvertedResult methods need async variants.

await Maybe.From(Task.FromResult("Foo"))
  .ToInvertedResult("Bar");

Here are some async examples for the normal ToResult method for reference.

[bothzoli]

Hey @DerStimmler,
Thanks for the quick feedback. Yeap I'll make sure that the async use-case is also covered, I just wanted to make sure I understood the issue correctly (hence the PR still in draft).
I'll try to tie up all the loose ends in the coming days and issue the PR.
Cheers

[bothzoli]

@DerStimmler I've finalised the PR, feel free to review.

[DerStimmler]

Great, I'll have a look at it.