Why this produces compiler error in Combine?

You cannot initialise an AnyPublisher with a closure accepting a Subscriber. You can only initialise an AnyPublisher from a Publisher. If you want to create a custom Publisher that emits a single random Int as soon as it receives a subscriber and then completes, you can create a custom type conforming to Publisher and in the required method, receive(subscriber:), do exactly what you were doing in your closure.

struct RandomNumberPublisher: Publisher { typealias Output = Int typealias Failure = Never func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input { subscriber.receive(Int.random(in: 0...Int.max)) subscriber.receive(completion: .finished) } } 

Then in your task method, you simply need to create a RandomNumberPublisher and then type erase it.

func task() -> AnyPublisher<Int, Never> { return RandomNumberPublisher().eraseToAnyPublisher() } 

If all you want is a single random value, use Just

fun task() -> AnyPublisher<Int, Never> { return Just(Int.random(in: 0...Int.max)).eraseToAnyPublisher() } 

Sidenote: don't use Int(arc4random()) anymore.

You're likely better off wrapping this in a Future publisher, possibly also wrapped with Deferred if you want it to response when subscriptions come in. Future is an excellent way to wrap external async API calls, especially ones that you can't fully control or otherwise easily adapt.

There's an example in Using Combine for "wrapping an async call with a Future to create a one-shot publisher" that looks like it might map quite closely to what you're trying to do.

If you want it to return more than a single value, then you may want to compose something out of PassthoughSubject or CurrentValueSubject that gives you an interface of -> AnyPublisher<YourType, Error> (or whatever you're looking for).