Hey folks, I’m facing a `ClassCastException` that ...

# http4k

[edrd]

Hey folks, I’m facing a ClassCastException that I’m almost sure is being caused by a Kotlin compiler bug but I’m having a hard time confirming it’s not in http4k, particularly due to my lack of familiarity with the lens concepts. Can you help me figure this out so I know who I should report this to?

Here’s the smallest reproducer I could get:

import org.http4k.core.*
import org.http4k.lens.contentType

fun main() {
	class CustomResponse : Response by Response(Status.OK)

	CustomResponse()
		.contentType(ContentType.APPLICATION_JSON)
		.let { println(it) } // Oddly enough, .let(::println) doesn't cause the bug
}

It throws:

java.lang.ClassCastException: class org.http4k.core.MemoryResponse cannot be cast to class ReproducerKt$main$CustomResponse (org.http4k.core.MemoryResponse and ReproducerKt$main$CustomResponse are in unnamed module of loader 'app')
at ReproducerKt.main(Reproducer.kt:11)

I’m running it with Kotlin 2.0.0 and I was also able to reproduce it with 1.9.21 to ensure it’s not related to K2. http4k version is 5.21.1.0

[dave]

This is because the with statement is taking MemoryResponse as a generic and not Response. You can fix it by upcasting to the Response

fun main() {
    class CustomResponse : Response by Response(Status.OK)

    val resp: Response = CustomResponse()
    resp
        .with(CONTENT_TYPE of ContentType.APPLICATION_JSON)
        .let { println(it) }
}

[dave]

alternatively, this will do what you want without problems:

class CustomResponse : Response by Response(Status.OK) {
    fun with(vararg modifiers: (Response) -> Response) = (this as Response).with(*modifiers)
}

[edrd]

Thanks, I applied it to my non-reproducer code and it worked.

However, I don’t get why it’s inferring MemoryResponse given Response.Companion.invoke returns Response.

[dave]

It's not that - the val resp without the cast would be of type CustomResponse. It's the with function which is blowing up when applying the lens.

[edrd]

Can we consider that as a bug?

[edrd]

(not a rethorical question — really trying to understand)

[dave]

Marginal - in the past we haven't really considered the Response to be user-extendable. What exactly is the purpose of you extending it with CustomResponse? (It might be that there is a more idiomatic way to achieve the same thing.)

[edrd]

I want to return this custom response, which contains some additional data, over to one of my filters. There, I check the type of the response and if it’s a custom response, I add some additional headers, etc. My type check is smart casting the response to this extended type and that’s where it blows up.
I read briefly about RequestContexts and that looks like an alternative but I wanted to try my approach since it’s simpler.

[dave]

yeah- RequestContexts is the idiomatic way to do this TBH.

[edrd]

Gotcha. Thanks for the help.