Propose Try.toEither(Throwable => L)

[eslep]

Thank You

Thank you for all of the work on Vavr! It has quickly become my favorite Java library, and usually the first thing I add to every new project ❤️ Aside from the benefit to my own code, it has also been a great tool to help teach others about FP and get them curious about it.

Proposal / What is this?

I come across cases where I want to convert a Try to an Either while mapping the Failure value onto a Left value.
One example would be safely handling code which isn't mine, then mapping the Throwable onto an error type.

I do this normally either with fold(throwableMapper, identity()) or .toEither().mapLeft(throwableMapper), but find that .toEither(throwableMapper) would be a nice shortcut to replace either case.
I have also a few colleagues who would appreciate the addition from a readability standpoint I think, so I thought I'd suggest it.

Implementation

By my own style, I'd normally just write this as a wrapper around a fold call. However, as a first contribution, I am trying to stick as close as possible to the style I already see in the code here.

By that reasoning, the code style, namings, and Javadoc are all created based on examples I find existing within the file.

Since the toEither(Supplier) implementation assumes the Supplier is pure/will not throw an exception and does not handle such a case, I have followed the same assumption here as well.

[nfekete]

Looks good to me, with a few minor suggestions.

[nfekete]

I'd probably replace the test code in a way that limits the number of comparisons to one, which would be the one in the assertion itself.
You could use a mapper function that extracts the message from the exception and use that as the left value to compare to in the assertion. This would make the code both shorter, and — in my opinion — easier to understand, since it would have a slightly less degree of indirection.

Suggested change

	final Either<Boolean, Object> either = failure.toEither(t ->
	t.getMessage().equals("a certain value")
	);
	assertThat(either).isEqualTo(Either.left(true));
	final Either<Boolean, Object> either = failure.toEither(Throwable::getMessage);
	assertThat(either).isEqualTo(Either.left("a certain value"));

[eslep]

Sure, sounds much better that way 👍 I also felt a bit weird using Throwable => Boolean as my function where many people would maybe expect that to then be Predicate<Throwable>. I can see also how the equals looks like an assertion.

Main point was just to have a clear expected value returned by the function, so the suggestion sounds great

[nfekete]

I'm proposing this change for the documentation so that it is more precise in meaning, because one could legitimately use a Try value to wrap a Throwable instance as a success value.

Suggested change

	* Converts this {@code Try} to an {@link Either}, converting the Throwable (if present)
	* Converts this {@code Try} to an {@link Either}, converting the Throwable (in the failure case)

[eslep]

I had the same impression that the wording was strange, but I kept this to stay consistent with toValidation(Throwable => U).

I would make this doc change to both of these methods then, since your point applies to both cases.