I have some type system head-scratching trying to ...

# general-advice

[Jakub Gwóźdź]

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?

[ephemient]

because <*>

[Jakub Gwóźdź]

oh nvm I see now that some class can be MyFuture : Future<Int>, CompletionStage<String> however stupid that seems…

[ephemient]

exactly

[Jakub Gwóźdź]

and I can’t test if (this is CompletionStage<T>) because then “cannot check for erased type”

[ephemient]

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

[ephemient]

you can do it further down

[Jakub Gwóźdź]

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…

[Jakub Gwóźdź]

basically my use-case is for Kafka’s producer send() which returns ancient Future<RecordMetadata> (I’m not sure if it is Completable, tho)