[swift-evolution] New async keyword usage

Jonathan Hull jhull at gbis.com
Fri Aug 25 16:48:21 CDT 2017

To prove (or potentially disprove) my assertion that this is not just sugar, how would you accomplish the following under the current proposal?

	let a = async longCalculationA()
	let b = async longCalculationB() //b doesn’t wait for a to complete before starting
	let c = async longCalculationC() //c doesn’t wait for a or b
	let result = await combineCalculations(a: a, b: b, c: c) //waits until a, b, and c are all available

(Note: this is using my version of async below which doesn’t use futures)


> On Aug 25, 2017, at 2:13 PM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
> I actually really like the idea of using ‘async' to start a computation in a non-blocking way.  It is extremely common in real-world code to want to start a few computations/downloads at once in the background and then use the results together...
> I think full-fledged futures could be a framework thing added on top, but what I would really love to see at the language level is that using ‘async’ just allows you to defer calling ‘await’.  That is, you could get a value back from something called with async, but you would be forced to await that value before it could be used:
> 	var image = async downloadImage()  //Image is type UIImage
> 	//Do something else here
> 	let size = await image.size //The compiler forces me to call await before I can actually use the value
> This looks somewhat similar to a future, but you can’t interact with it as a separate type of object.  The value above is just a UIImage, but with a compiler flag/annotation that forces me to call await on it before it can be accessed/used.  The compiler has a lot more freedom to optimize/reorganize things behind the scenes, because it doesn’t necessarily need to make an intermediate object.
> I don’t think this is just sugar.  It adds expressivity and control for a set of very common use-cases which aren’t fully supported by await alone.
> Thanks,
> Jon
>> On Aug 24, 2017, at 4:40 AM, Trevör ANNE DENISE via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Hello Swift community,
>> I was really interested by the recent Task-based concurrency manifesto and Concrete proposal for async semantics in Swift.
>> Looking at beginAsync() and Futures, I had an idea for a new syntax based on the `async` keyword, I'd love to hear your feedback about this idea:
>> https://github.com/adtrevor/Swift-ideas/blob/master/New%20async%20keyword%20usage.md <https://github.com/adtrevor/Swift-ideas/blob/master/New%20async%20keyword%20usage.md>
>> Would such a syntax make any sense?
>> Thank you ! :)
>> Trevör
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