getNext := iterableThing.Iterator()
for {
next, ok := getNext()
if !ok {
break
}
...
}
vs.
for next := range iterableThing.Iterator() {
...
}
One advantage is that it's slightly shorter, which matters for very common patterns--people complain about `err != nil` after all. Another advantage is there isn't another variable for everyone to name differently. Another advantage is that everyone can't do the control flow slightly differently either.
Only newbies tend to complain about `err != nil` in my experience. After a certain point it just clicks and they get used to it. There's a cadence to Go code (do the thing, check the error, do the thing, check the error) that is easy to read once you're used to it, but looks horribly verbose when you're coming from an exceptions-based language.
Go has simplicity as a design goal. Part of that simplicity is not adding all the features we can think of to the language. Just because it would provide slightly leaner syntax for a relatively small group of use cases isn't a good reason to add new language features (imho, from my 10+ years using Go and watching it evolve).
If we can implement this using current language features, but it's complex and messy to implement, then it's a great case for a new library, possibly even inclusion in the standard library. If we can't implement this at all using current language features, then maybe it's a case for a new language feature to enable this. If we can implement it relatively cleanly using current language features, then we're good and don't need to do anything. This seems like a "can implement, but not very cleanly" case, which would be a great justification for a library, but not a language feature. Again, imho.
I'm used to `err != nil`, but it doesn't mean I like it. It's a lot of what amounts to boilerplate in a language that is mercifully short of boilerplate elsewhere. This is doubly true when you want custom error types, and need to start unpacking values from your custom error struct.
>This seems like a "can implement, but not very cleanly" case, which would be a great justification for a library, but not a language feature.
First sentence of OP:
> This post is about why we need a coroutine package for Go
Then in the girst section after the intro:
> Later, I will argue for an optimized implementation provided directly by the runtime, but that implementation should be indistinguishable from the pure Go definition.
Personally, I don't really see anything wrong with explicit error values, and checking them against nil.
The point I was trying to make is that some amount of people see the benefit in removing much smaller boilerplate. And so presumably at least as many people should see the benefit of removing a larger amount of boilerplate.
I'm just talking about the benefits here. There is also a cost to expanding the language. The costs might outweigh the benefits, but it doesn't mean that the benefits don't exist.
> There is also a cost to expanding the language. The costs might outweigh the benefits, but it doesn't mean that the benefits don't exist.
Agreed. I think this is where the discussion will focus. I'm glad that historically the Go team have weighted the costs heavily, and hope they continue to do so.
I guess I'm in the minority but boilerplate really doesn't bother me. Going the other way gets too much "magic" and we lose the ability to fine-tune behaviour.
Not to nitpick this specifically but as a generic reminder not all complaints are worthy of shifting the trajectory of a massively popular programming language.
Balancing "worthy" and "unworthy" changes is really hard both in the community and discussions like this one. I don't envy the teams that have to do it.
If we're going for readability instead of trying to stick to the case presented above, I'd go with:
for i.HasNext() {
current := i.GetNext()
...
}
And just mutate the internal state of the struct. Probably throw it behind some sort of interface like:
type Iterator[T any] interface { // feel free to strip out the generic
HasNext() bool // if you really only have one type
GetNext() T // you do this with
}
I think this still loses to ranging over a function, but I still think it should be compared to what you can do with the current language as opposed to what was originally posted.
The err!=nil is exactly what you don’t want to mention and the perfect example why you shouldn’t listen to all the close-minded takes some people might have.
The OP is about converting a push iterator into a pull iterator efficiently by using coroutines. This provides the best of both worlds, simple iterator implementation and flexible use by its caller.
Besides what everyone else said, the obvious advantage is the latter builds in the logic for exiting the loop once getNext has run out of elements in the slice/map. Your former example will need a final step that's like:
...
if getNext() == nil {
break
}
...
This isn't a huge boon, and is mostly a matter of style. But I prefer the latter because it's logic that gets handled with the range builtin, so my code can be more about application logic, rather than muddling in breaking out of loop logic.
None of the proposals submit your idea of writing things differently. The article proposes an implementation that is fully doable and usable with current spec and no breaking changes.
The point of coroutines is that they are little contexts that serve to be called
- many times
- sometimes with different parameters
- change state
- might be interrupted by callers
- might interrupt callers themselves
All of this can be done by other means. Just like any sorting can be done by copy pasting the same code, but generics make it less tedious. That's the same idea here. Some problems can be implemented as interleaving coroutines, and their definition is simple enough that you want to write it all in the body of some CreateCoroutine() function instead of taking out another struct with 5 almost empty functions. It will not solve all problems, but can more clearly separate business logic and orchestration.
The only way to do control flow in getNext, consistent with how you partially defined it, is to panic. That means there is some important code wrapping the whole loop body that you left out.
Horrible overhead. If the loop does something simple, like summing integers, 99% of time will be spent switching between goroutines.
From TFA:
"Because scheduling is explicit (without any preemption) and done entirely without the operating system, a coroutine switch takes at most around ten nanoseconds, usually even less. Startup and teardown is also much cheaper than threads."
"For this taxonomy, Go's goroutines are cheap threads: a goroutine switch is closer to a few hundred nanoseconds"
"Finally, the most natural channel based implementation is… slow. By a factor of nearly 50x. Buffering does help though, reducing that to a factor of 25x."
And even if it's a toy program and you don't care about performance, it's not as simple as just:
for {
next := <-channel
...
}
You have to set up the channel and the goroutine that feeds it, you need to safely close the channel when the iteration is done (but not before, unless you like panics), you need to deal with panics inside the goroutine and possibly support cancellation if the iteration breaks early (unless you love memory leaks).
If you try to implement this pattern by hand, you are all too likely to make fatal mistake, and this is doubly true in the hands of an inexperienced programmer.
I appreciate the fact that Russ wrote this long post, gradually implementing `coro.New()` and improving its functionality and safety — and only in the very end, we get a short paragraph about performance. Good performance is important to make this feature attractive to use, but if the feature is clunky and error-prone, it wouldn't be worth much, even with great performance.
I'm responding to a comment asking for a justification for sugar for function call based iteration. My comment is an attempt to draw a parallel to the need for sugar for channel based iteration. I'm not trying to suggest channel based iteration as an alternative to coroutines.
