But try/catch makes swallowing errors explicit, you have to catch it to swallow it. With promises it's the opposite situation, errors will be swallowed unless you explicitly handle the rejection. At least the runtimes have grown up to show console output when there's an unhandled rejection, but man it was pretty dark for a while.
> Does the `x = foo()` block/implicitly await?
Yes, that's what I'd expect, because `foo` returns a promise.
> Unless you're just saying that the bug would be apparent as soon as you ran the code, but the syntax checker wouldn't flag it for you?
It may or may not be apparent even from running the code, consider the following:
let data
try {
data = readFile('some-file.csv', 'utf8')
.split('\n')
.filter(l => !!l)
.map(l => l.split(','))
} catch (e) {
data = []
}
It's not particularly good code, granted, but it's also not a contrived example. What's the bug? Well, assuming `readFile` is sync, and `some-file.csv` is a nice csv file with no surprises in it, this should make data an array filled with data from said file. If the file is empty, it's just going to yield an empty array. If the file can't be read for some reason and `readFile` throws an error, it'll be an empty array.
Now make `readFile` return a promise instead and `data` will always be an empty array, regardless of whether the file exists, can be read, or otherwise all conditions for a happy path are met. Why? No `split` method on the promise.
The problem isn't promises of course, it's the fact that we're doing too much in a try clause, or at least not dealing with specific errors properly. But let's be honest – who hasn't seen (or even written?) code like this in the past?
It might've even worked fantastically well for a long time, years even, until someone comes around and changes `readFile` to be async, for reasons, and now it breaks in a subtle way and it's fun and games trying to find why `data` is always an empty array even though it seems everything should be fine. Actually, it doesn't even have to be `readFile` that changes, it might be a function that it depends on, causing bugs further up the call stack. It happens, no matter how semantic our versioning is.
If however the runtime would always implicitly await return values, and you'd have to explicitly mark statements as async to break out of that, then this code would continue to function regardless of whether `readFile` or one of its dependencies returns a promise or the actual file contents, because the runtime would deal with it. As it is now, you have to go and change all code that calls `readFile` to be async, so you can await, meaning anything further up the stack also needs to be async, so you can await. It's a bit of a foot gun I think.
In any case, it's just a thought and at best a half baked one at that. I just find the async/await semantics to be backwards, and while I've used it for a few years at this point I still keep running into dumb situations like the above. Maybe I'm just a bad programmer, I'm certainly not excluding that as a possibility. :o)
> With promises it's the opposite situation, errors will be swallowed unless you explicitly handle the rejection.
Yes, I've been bitten by this too, it's certainly a drawback with the design of Promises. I wouldn't describe it as magic though, since both the implementation and the impetus are easy to understand.
> Actually, it doesn't even have to be `readFile` that changes, it might be a function that it depends on, causing bugs further up the call stack.
I don't think it's possible for readFile to become async without at least some change to it (possibly just adding 'async' and 'await' keywords, but at least some change), except maybe a rare case of a tail call.
> even though it seems everything should be fine
In what way would it seem that everything should be fine? If readFile() were changed from sync to async, wouldn't every single call to readFile() in the entire codebase need to be changed, just like if readFile() were changed from returning a string to returning a File object? It's not like most or even any at all of the calls to readFile() wouldn't need changing, then I could see how it might seem like everything would be fine.
> breaks in a subtle way
But this isn't a subtle bug, it completely breaks as soon as readFile() is changed from sync to async, right? No testing, automated or manual, of this codepath would work at all after the change, right? It's not like a cursory smoke test of this codepath seems to work fine, then I could see how the bug could seem subtle.
> I still keep running into dumb situations like the above. I still keep running into dumb situations like the above
You don't seem like a bad programmer, which is why I'm skeptical of the example you gave.
> I wouldn't describe it as magic though, since both the implementation and the impetus are easy to understand.
That's fair, magic may have been a bit hyperbolic.
> [...] just like if readFile() were changed from returning a string to returning a File object
But that would change the semantics of the function, it literally changes the return type. My point is that adding `async` really just changes the meachanics of the function, not the semantics. If my function returned a string before, and I add `async`, it'll still return a string; just eventually. As a caller, I don't really care, I just want the darn string.
ometimes, as a caller, I do care, and that's exactly why I think have the caller decide when to run something async makes more sense. (There's a whole other discussion that could be had here about how JS promises are a poor async abstraction anyhow, but I digress.)
> But this isn't a subtle bug, it completely breaks as soon as readFile() is changed from sync to async, right?
No it's definitely subtle. In the example I gave, the code would use the default empty array value when there's an error reading the file, for whatever reason. For the happy path, it'll work just fine, though it probably wouldn't deal with invalid input very well. Change the mechanics of readFile to async though and it'll always return the default value, even though the semantics of readFile stays the same. It still returns a string, just eventually, but because the code expects a string, it'll always break because it gets a promise instead. Add `await` and it'll be fine, but now whatever function that code is in is async, and whatever function calls that needs to also `await`, ad nauseam.
> You don't seem like a bad programmer, which is why I'm skeptical of the example you gave.
Hey, thanks! :o)
To your point though, it's definitely representative of the kind of code I come across on a regular basis. Many a times have I had to help colleagues debug this kind of issue, and many a times have I shot myself in the foot in similar ways. In any case, JS async/await semantics are set in stone now, and it's probably too dynamic a language for something like implicit await to work (performantly) anyhow, as previously mentioned.
I appreciate you taking the time to discuss, it's nice being challenged on the actual topic, without it devolving into ad hominem nonsense. There are still good corners of the internet after all!
> Does the `x = foo()` block/implicitly await?
Yes, that's what I'd expect, because `foo` returns a promise.
> Unless you're just saying that the bug would be apparent as soon as you ran the code, but the syntax checker wouldn't flag it for you?
It may or may not be apparent even from running the code, consider the following:
It's not particularly good code, granted, but it's also not a contrived example. What's the bug? Well, assuming `readFile` is sync, and `some-file.csv` is a nice csv file with no surprises in it, this should make data an array filled with data from said file. If the file is empty, it's just going to yield an empty array. If the file can't be read for some reason and `readFile` throws an error, it'll be an empty array.Now make `readFile` return a promise instead and `data` will always be an empty array, regardless of whether the file exists, can be read, or otherwise all conditions for a happy path are met. Why? No `split` method on the promise.
The problem isn't promises of course, it's the fact that we're doing too much in a try clause, or at least not dealing with specific errors properly. But let's be honest – who hasn't seen (or even written?) code like this in the past?
It might've even worked fantastically well for a long time, years even, until someone comes around and changes `readFile` to be async, for reasons, and now it breaks in a subtle way and it's fun and games trying to find why `data` is always an empty array even though it seems everything should be fine. Actually, it doesn't even have to be `readFile` that changes, it might be a function that it depends on, causing bugs further up the call stack. It happens, no matter how semantic our versioning is.
If however the runtime would always implicitly await return values, and you'd have to explicitly mark statements as async to break out of that, then this code would continue to function regardless of whether `readFile` or one of its dependencies returns a promise or the actual file contents, because the runtime would deal with it. As it is now, you have to go and change all code that calls `readFile` to be async, so you can await, meaning anything further up the stack also needs to be async, so you can await. It's a bit of a foot gun I think.
In any case, it's just a thought and at best a half baked one at that. I just find the async/await semantics to be backwards, and while I've used it for a few years at this point I still keep running into dumb situations like the above. Maybe I'm just a bad programmer, I'm certainly not excluding that as a possibility. :o)
(Apologies for the wall of text.)