[swift-evolution] await keyword "scope"

Adam Kemp adam.kemp at apple.com
Mon Aug 28 15:09:12 CDT 2017

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).

I’m not a fan of the ambiguity of applying the await keyword to an entire statement. I know some people think this is a good thing, but to me it’s just obfuscating important information.

Further, if you’re going beyond a single expression then why you would stop at the statement level? Why not make it apply to the whole block or even a whole function? Why require the keyword at all? It doesn’t appear to be adding any value if it doesn’t specify exactly where the suspension point is. “Somewhere on this line” can get rather vague.

async/await can be a huge win for clarity, but it also comes with the downside of having to think a bit more about what can happen at these suspension points. I feel like it should be a goal to make it very clear where those suspension points are so that we can more easily reason about them. That’s why I prefer restricting it to apply to a single expression. It’s very clear where the function gets suspended, which means it’s clearer where you need to be concerned about things possibly happening in between your code. Consider this, for example:

await decodeImage(loadWebResource(self.webProfilePath), loadWebResource(self.imagePath))

If webProfilePath and imagePath are vars then they could change in between those two calls. If you can’t see that these calls are suspending then you might not know to cache them up-front to ensure consistency:

let webProfilePath = self.webProfilePath
let imagePath = self.imagePath
await decodeImage(await loadWebResource(webProfilePath), await loadWebResource(imagePath))

I think a general guideline we should use when considering how this feature should work is to ask whether it makes bugs more likely or less likely. The goal should be to reduce bugs while simplifying code. If we simplify the code to the point where we’re making some bugs too subtle then we may be doing more harm than good.
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