Should this example throw or `shift("boom")`? ```c...

# arrow

[phldavies]

Should this example throw or shift("boom")?

class Test : FreeSpec({
    "should this throw?" {
        effect {
            result { error("💥") }.bind { "boom" }
        }.toEither() shouldBe Either.Left("boom")
    }
})

Currently it throws. Either.catch({ "boom" }) { error("💥") }.bind() shifts as you’d expect.

[phldavies]

on a similar note, should `EffectScope`’s Result<B>.bind use shift(transform(throwable.nonFatalOrThrow()))?

[simon.vergauwen]

This should not be throwing but the assertion should pass.

[simon.vergauwen]

@phldavies, I think I know what you're seeing. result { } doesn't catch, and this works.

class Test : FreeSpec({
    "should this throw?" {
        effect {
            runCatching { error("💥") }.bind { "boom" }
        }.toEither() shouldBe Either.Left("boom")
    }
})

[simon.vergauwen]

Does that make sense?

[phldavies]

Yeah I think that’s what I ended up doing. I’m not entirely sure what the need for result is with runCatching available and the bind being on Result<T>. The only question I still have is if .bind should be aware of fatal/cancellation exceptions or not

[phldavies]

The Either.catch equivalent does use nonFatalOrThrow - should Result.bind?

[phldavies]

result {} allows you to shift your Throwable instead of throw it, which I assume has better performance.

either {
    result<Int> { shift(IllegalStateException("💥")) }.bind { "boom" }
} shouldBe Either.Left("boom")

does shift as expected.

[phldavies]

With this in mind, I can kind of understand the reasoning to have a result {} computation that handles shifted throwables but unexpected are propagated. It was just surprising. If it’s expected to catch and shift any thrown throwables though, I’m not sure it has any extra utility over runCatching{} , unless I’m missing something

[simon.vergauwen]

Ah okay, I see your confusion and it makes perfect sense! Perhaps we can clarify this in documentation somewhere.
I would say the difference comes from Either<Throwable, A> vs Result<A>.

The Kotlin Std exposes Result<A> and has explicitly stated in the passed that it on purpose catches all exceptions, also fatal ones. Arrow felt that changing those semantics on an existing type was confusing, so we left Result<A> as is.

In contrast people can use Either<Throwable, A> which offers similar semantics as Result<A> with the only different that it does take into account FatalError just like all other data types and operators in Arrow.

[simon.vergauwen]

So with that in mind Result.bind should not check nonFatalOrThrow, whilst Either.catch does.

[simon.vergauwen]

This difference comes from runCatching vs Either.catch rather than bind.

[phldavies]

Ah ok, understood - it now makes sense why nonFatalOrThrow is not used in Result.bind. 🙏