Why does type inference fail for `extract` here? `...

# announcements

[Sam Garfinkel]

Why does type inference fail for extract here?

object Container {
    val contained = mutableMapOf<String, Any>()

    inline fun <reified T> extract(name: String): T = contained[name] as T
}

class Foo {
    val bar: String by lazy {
        Container.extract("Bar")
    }
}

Because lazy is declared to return a String, shouldn’t the inferred type T also be String?

[Fleshgrinder]

The current type inference is not that great, you can try -Xnew-inference (your example works with it).

[Zach Klippenstein (he/him) [MOD]]

What type is it inferring?

[Fleshgrinder]

None, it fails to inferre anything.

[Zach Klippenstein (he/him) [MOD]]

What’s the error you’re getting?

[Zach Klippenstein (he/him) [MOD]]

And where in that snippet?

[Sam Garfinkel]

Why does type inference fail for extract here?

[Zach Klippenstein (he/him) [MOD]]

Sorry, I wasn’t clear. I don’t see anything in the given code that would require any types to be inferred. Is the compiler telling you to write lazy<String>? Sorry if this is obvious, maybe I need another coffee…

[Sam Garfinkel]

The reified T of extract should be inferred from the return value of lazy but isn’t with the current implementation of type inferrence.

[Fleshgrinder]

Inside lazy writing Container.extract<String>("Bar") is where it fails.

[Sam Garfinkel]

@Fleshgrinder Not exactly, to make the type checker happy you need Container.extract<String>("Bar") but my point was Container.extract("Bar)" should be valid because generic type should match the return value of lazy

[Fleshgrinder]

I was just answering to @Zach Klippenstein (he/him) [MOD] because he specifically asked where the additional type is required. I know what's going on. 😉