Hey guys, I'm trying to migrate some functionality...

# arrow

[Dasyel Willems]

Hey guys, I'm trying to migrate some functionality from IO to the new suspend way
We're calling a third-party method which returns a CompletableFuture<Void> and before we handled it in the following way

async {
    run {
        doSomethingAsync() //this returns the CompletableFuture<Void>
    }
}

How do I correctly go from that completableFuture to a suspend function which returns an Either<MyError, Unit>

[Dasyel Willems]

Either.catch {
    doSomethingAsync().await()
}.mapLeft { MyError() }.unit()

this is as far as I got, but I'm not sure if await() is the correct way to go

[simon.vergauwen]

Hey @Dasyel Willems,

I think to correctly wire CompletableFuture into suspend you want the following implementation. Which is optimised to return immediately in case the CompletableFuture is already finished, and adds support for cancellation.

suspend fun <A> CompletableFuture<A>.await(): A =
  if (isDone) {
    try {
      get()
    } catch (e: ExecutionException) {
      throw e.cause ?: e // unwrap original cause from ExecutionException
    }
  } else cancellable { callback ->
    whenComplete { res, exception ->
      if (exception == null) callback(Result.success(res))
      else callback(Result.failure((exception as? CompletionException)?.cause ?: exception))
    }

    CancelToken { cancel(false) }
  }

And then you can safely use the snippet you shared in the second snippet. You could also update the above implementation to take a (exception: Throwable) -> E lambda and return Either<E, A> immediately.

This method is not available yet in Arrow Fx Coroutines, but would be a welcomed contribution 🙂

[Kristian]

Thanks Simon!