[swift-evolution] New async keyword usage

[Jonathan Hull]

But then it would need to be called with await under the current proposal, meaning that either:

• b would await the calculation of a

	beginAsync {
		let a = await longCalculationA()
		let b = await longCalculationB() //This only calculates when a is finished
	}

• or b would be executed while a is awaiting, but a and b would be in different scopes

	beginAsync{
		let a = await longCalculationA()
	}
	beginAsync{
		let b = await longCalculationB() //We can’t see ‘a’ anymore
	}
	//We can’t see ‘a’ or ‘b’ here to use them

We could, also implement some sort of future, and then re-write our functions to take advantage of it, but this misses out on numerous compiler optimizations and requires our functions to be written with futures in mind.  In my example, the functions can just be written as async, and they don’t care whether they are called with async or await.

Thanks,
Jon

> On Aug 25, 2017, at 3:13 PM, Florent Vilmart <florent at flovilmart.com> wrote:
> 
> be doesn't wait if it's defined as 
> 
> func longCalculationB() async -> SomeType 
> 
> which would be helpful if it's a long calculation, 
> 
> in the case it's
> 
> func longCalculationB() -> SomeType 
> 
> That would probably be valid to put the async keyword front even though I would not be a big fan of that as you'd be executing on an indefinite queue a calculation that may not be thread safe.
> 
> async would be in that case a wrapper around dispatch_async  + semaphore
> 
> 
> On 25 août 2017 18:08 -0400, Jonathan Hull <jhull at gbis.com>, wrote:
>> Why wouldn’t b wait for a in this example?  If it is using futures, those aren’t available in the current proposal.
>> 
>>> On Aug 25, 2017, at 3:02 PM, Florent Vilmart <florent at flovilmart.com <mailto:florent at flovilmart.com>> wrote:
>>> 
>>> Probably with:
>>> 
>>> let a = longCalculationA() 
>>> let b = longCalculationB() //b doesn’t wait for a to complete before starting
>>> let c = longCalculationC() //c doesn’t wait for a or b
>>> let (aResult, bResult, cResult) = await Future.collect(a, b, c) //waits until a, b, and c are all available
>>> 
>>> On 25 août 2017 17:48 -0400, wrote:
>>>> 
>>>> 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
>> 

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