[swift-evolution] New async keyword usage

[Jonathan Hull]

> On Aug 25, 2017, at 3:38 PM, Trevör Anne Denise <trevor.annedenise at icloud.com> wrote:
> 
> =============================================================
>> Jonathan Hull jhull at gbis.com <http://gbis.com/> 
> 
>> 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.
> 
> 
> As for the message of Wallacy I'd be interested the pros and cons of hiding the implementation details ! :)
> 
> 
>> 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
> 
> 
> Would this be implemented differently than with Futures? I don't have much experience with concurrency, but I don't see how this would be handled differently than by using Futures, internally ? (at least for this case)
> 

It looks/behaves very similar to futures, but would potentially be implemented differently.  The main difference is that the resulting type is actually the desired type (instead of Future<Type>) with a compiler flag saying that it needs to call await to be used.  Behind the scenes, this could be implemented as some sort of future, but the compiler has a lot more freedom to rearrange things to be much more efficient because there is no affordance for the user to introspect or cancel. So for example, it might actually change:

	let image = async downloadImage()
	let size = await image.size

to:

	let size = await downloadImage().size

This would depend on the other code around it, but the compiler has much more freedom to avoid creating intermediate values, or even to create different types of intermediate values which are more efficient for the situation at hand.

Given that as a base, it would be trivial to create a framework offering true Futures (which allow cancelling, etc…)

Thanks,
Jon

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