Quite a few generics related questions here recent...

# getting-started

[David Kubecka]

Quite a few generics related questions here recently so I will add my own as well 🙂 I'm trying to create an extension on an interface/class from Spring's WebTestClient and cannot get the types right:

// this is (basically) from WebTestClient
class RequestHeadersSpec<S : RequestHeadersSpec<S>> {
    fun header(headerName: String, headerValue: String): RequestHeadersSpec<*> = this
}

// this is my extension
fun <S : RequestHeadersSpec<S>> S.customHeader(headerValue: String): RequestHeadersSpec<*> =
	header("custom-header", headerValue)

fun main() {
    val spec = RequestHeadersSpec()
    spec.customHeader("value")
}

Basically I want the customHeader extension to have the same receiver and output type as header. I'm getting "receiver type mismatch" on the call site, however.

https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMi4wLjIwLVJDIiwicGxhdGZvcm0iOiJqYXZhIiwiYX[…]BlYygpXG4gICAgc3BlYy5jdXN0b21IZWFkZXIoXCJ2YWx1ZVwiKVxufSJ9

[Sam]

I'd be tempted to just use apply, and let Spring worry about the awkward generics

fun <T : RequestHeadersSpec<*>> T.customHeader(headerValue: String): T =
  apply { header("custom-header", headerValue) }

[David Kubecka]

Ah, so you also find the types not entirely correct/convenient? Out of curiosity, what's the exact root cause of the awkwardness in your opinion and how to avoid it?

[Sam]

Just type variance I think. This works:

fun <S : RequestHeadersSpec<out S>> S.customHeader(headerValue: String): RequestHeadersSpec<*> =
  header("custom-header", headerValue)

But trying to make a 'self type' with generics is always a mess, and if you're already using extension functions, apply is an easy way out.

[David Kubecka]

Hmm, the apply way works but I don't exactly understand how:
• first, you need to use wildcard base type RequestHeadersSpec<*>
• second, what exactly apply brings to the game wrt types?

[David Kubecka]

Ah, I can answer the second question myself 🙂 By using apply we are completely ignoring the problematic typing of headers and just returning the receiver type