I have some type system head-scratching trying to figure it out. I want to wrap Java 7 Future into kotlin’s Deferred. There is a Java 8

CompletableFuture.asDeferred()

already, but when the API you’re using is still java7-compatible, tough luck. So far I just went with

inline fun <reified T> Future<T>.asCompletableDeferred(): Deferred<T> =
    CompletableFuture.supplyAsync { this.get() }.asDeferred()

and it works, as far as I can see. But I’d like to omit unnecessary wrapping if

this

is already Completable:

inline fun <reified T> Future<T>.asCompletableDeferred(): Deferred<T> =
    if (this is CompletionStage<*>) this.asDeferred() else CompletableFuture.supplyAsync { this.get() }.asDeferred()

and that is going all hysterical complaining that

this.asDeferred()

is no longer

Deferred<T>

but

Deferred<Any>

Why is that so and how to fix it?

because

<*>

oh nvm I see now that some class can be

MyFuture : Future<Int>, CompletionStage<String>

however stupid that seems…

exactly

and I can’t test

if (this is CompletionStage<T>)

because then “cannot check for erased type”

if you assume that nobody will do that, then go ahead and make a cast

I think going

inline fun <reified T> Future<T>.asCompletableDeferred(): Deferred<T> =
    if (this is CompletableFuture<T>) this.asDeferred() else CompletableFuture.supplyAsync { this.get() }.asDeferred()

is good enough…