[swift-evolution] await keyword "scope"

[Adam Kemp]

> On Aug 28, 2017, at 11:19 PM, Chris Lattner <clattner at nondot.org> wrote:
> 
> 
>> On Aug 28, 2017, at 1:09 PM, Adam Kemp via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> I decided to split this out to its own thread because it seems orthogonal to other issues being discussed.
>> 
>> When I read this line from the proposal:
>> 
>> await decodeImage(dataResource.get(), imageResource.get())
>> 
>> It’s not clear to me where the asynchronous call is. There are three function calls on that line. Which ones might actually suspend? You can’t tell by looking at it because there’s only one await keyword (the answer is all 3).
> 
> Yes, this is a reasonable concern.  We debated it heavily in the Swift 2 timeframe when introducing error handling (as other’s have pointed out, it works the same way).
> 
> This is a tradeoff between the readability benefits of marking non-obvious control flow vs the readability disadvantage of having noise in the code. Requiring marking on every async or throwing call is particularly bad in the case of chaining.  Compare:
> 
>    let x = try (try (try a.foo()).bar()).baz()
> vs:
>    let x = try a.foo().bar().baz()
> 
> In the Swift 2 timeframe, we decided that in many cases, it is mostly obvious what APIs can throw, so one marker is enough.  That said, there ARE potentially confusing cases, and some programmers may want to be more explicit about marking.  This is why the compiler allows you to explicitly mark subexpressions if you’d like.
> 
> I believe that this design has served the community well, and I haven’t heard of serious problems with it.  I’m pretty confident that async following the same model will have similar success.
> 
> -Chris
> 

I think that decision makes sense for try/throws, but I feel like the await keyword is fundamentally different from that. The pitfalls of not understanding how the code is transformed and how it will behave at runtime are much greater with await than with try.

If you have a line of code with multiple expressions that can throw then the consequences of not knowing which particular one threw the error are minor. In most cases it doesn’t matter, and you would handle a given error the same regardless of which subexpression threw the error.

With await the function is actually broken up into pieces, and unrelated code can run in between those pieces on the same thread and/or the same queue. That has a much higher potential of leading to subtle bugs if you can’t see where those breaks are.

That’s why I think that it is important for every one of those locations to be explicitly marked so that it is very clear where the breaks in the function are, and thus where other code can run.
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