[swift-evolution] [Concurrency] async/await + actors

[Chris Lattner]

> On Sep 5, 2017, at 7:31 PM, Eagle Offshore via swift-evolution <swift-evolution at swift.org> wrote:
> 
> OK, I've been watching this thing for a couple weeks.
> 
> I've done a lot of GCD network code.  Invariably my completion method starts with
> 
> dispatch_async(queue_want_to_handle_this_on,....)
> 
> Replying on the same queue would be nice I guess, only often all I need to do is update the UI in the completion code.
> 
> OTOH, I have situations where the reply is complicated and I need to persist a lot of data, then update the UI.
> 
> So honestly, any assumption you make about how this is supposed to work is going to be wrong about half the time unless....
> 
> you let me specify the reply queue directly.
> 
> That is the only thing that works all the time.  Even then, I'm very apt to make the choice to do some of the work off the main thread and then queue up the minimal amount of work onto the main thread.

I (think that I) understand what you’re saying here, but I don’t think that we’re talking about the same thing.  

You seem to be making an argument about what is most *useful* (being able to vector a completion handler to a specific queue), but I’m personally concerned about what is most *surprising* and therefore unnatural and prone to introduce bugs and misunderstandings by people who haven’t written the code.  To make this more concrete, shift from the “person who writes to code” to the “person who has to maintain someone else's code”:

Imagine you are maintaining a large codebase, and you come across this (intentionally abstract) code:

	foo()
	await bar()
	baz()

Regardless of what is the most useful, I’d argue that it is only natural to expect baz() to run on the same queue/thread/execution-context as foo and bar.  If, in the same model, you see something like:

	foo()
	await bar()
	anotherQueue.async {
		baz()
	}

Then it is super clear what is going on: an intentional queue hop from whatever foo/bar are run on to anotherQueue.

I interpret your email as arguing for something like this:

	foo()
	await(anotherQueue) bar()
	baz()

I’m not sure if that’s exactly the syntax you’re arguing for, but anything like this presents a number of challenges:

1) it is “just sugar” over the basic model, so we could argue to add it at any time (and would argue strongly to defer it out of this round of discussions).

2) We’d have to find a syntax that implies that baz() runs on anotherQueue, but bar() runs on the existing queue.  The syntax I sketched above does NOT provide this indication.

-Chris