Should this be allowed? It does compile fine if I ...

# getting-started

[Ben Woodworth]

Should this be allowed? It does compile fine if I remove the type annotation from acceptsString
I want to say R can be constrained to String by the smart cast but I'm not sure.

interface Accepts<in T> {
    fun accept(value: T)
}

object AcceptsString : Accepts<String> {
    override fun accept(value: String) = println(value)
}

fun <R> example(providesR: () -> R, acceptsR: Accepts<R>) {
    if (acceptsR is AcceptsString) { // so R must be String
        val acceptsString: AcceptsString = acceptsR
        val providedR = providesR()

        acceptsString.accept(providedR)
        //                   ^^^^^^^^^ Type mismatch.
        //                             Required: String
        //                             Found: R
    }
}

[Dominaezzz]

Yes that's looks correct to me. That should indeed be a compiler error.

[Dominaezzz]

AcceptsString can't take R .

[Ben Woodworth]

I guess I'm wondering if it's guaranteed safe for me to do acceptsString.accept(providedR as String) in this case.

(And if so, could Kotlin in the future be made to infer R as String here)

[Dominaezzz]

What does safe mean? If R is not a CharSequence an exception will be thrown if you do that.

[Dominaezzz]

Might be possible in the future with contracts or something.

[Ben Woodworth]

(I just realized I forgot to change CharSequence types back to String in the example after playing around, so fixed that in a code block)

[Ben Woodworth]

If R is not a String an exception will be thrown if you do that.

That's the thing, I don't think it's possible for R to be anything other than a String within that if block

[Ben Woodworth]

And I'm definitely not certain, but I think the fact that the example compiles fine without the : AcceptsString is proof of that

[Dominaezzz]

It shouldn't be possible ideally but type erasure is very much a thing on the JVM.