This has probably been asked before, but I couldn’t find it from a quick search. Why doesn’t `shift`...

# arrow

[Sam]

This has probably been asked before, but I couldn’t find it from a quick search. Why doesn’t shift return Nothing?
It’s declared as

suspend fun <B> shift(r: R): B

but it could instead be

suspend fun shift(r: R): Nothing

The reason I ask is specifically because of this use case:

var failure: MyFailure? = null
val effect = eagerEffect<MyFailure, String> {
    failure?.let { shift(it) } // "Not enough information to infer type variable B"
    "some result"
}

The fix is to be explicit about the Nothing type, which feels super weird:

val effect = eagerEffect<MyFailure, String> {
    failure?.let { shift<Nothing>(it) }
    "some result"
}

[raulraja]

I think we had encoded it at some point with Nothing but due to how inference worked it was causing issues. It may be possible now. Technically Nothing is more correct than any type because after shift is called all statements and code afterward is unreachable.

[raulraja]

Maybe worth a proof of concept PR to see if tests and everything else passes when shift return is changed to Nothing. If it works maybe we can change it in 2.0

[simon.vergauwen]

This follows the same pattern as emptyList from Kotlin Std. Requiring A rather than Nothing.

IIRC, this is the recommended pattern for inference in Kotlin but I might be mistaken.

[Sam]

That’s a good comparison, I hadn’t considered it 👍. I would also compare it to error(...) which isn’t generic and always returns Nothing. I can see why emptyList uses the generic, but my sense is that shift is more similar to error than to emptyList.

[simon.vergauwen]

but my sense is that shift is more similar to error than to emptyList

Yes, very valid. We hadn't made this comparison before 🤔

Good candidate for Arrow 2.0 improvement as @raulraja said. Feel free to make a ticket for this on the Github repo so we can keep track of it 👍

[Sam]

https://github.com/arrow-kt/arrow/issues/2805

[simon.vergauwen]

Thank you for the very complete issue report @Sam 🙏

[Sam]

https://github.com/arrow-kt/arrow/pull/2839 blob sunglasses