So how is creating a nursery that matches the lifetime and visibility of the page different from not using a nursery at all?
In real browsers, the UI act of closing a tab and the completion of cleaning up all of its resources are already clearly separated. I can see that when I terminate a browser with a lot of tabs and the main window has entirely closed while the browser runs at 100% CPU for quite a few more seconds. Plus a browser can probably hard-kill running Javascript code with some reasonable effectiveness. (Not sure. Asynchronous exceptions are very, very hard.)
So I don't think that's a disqualifier.
"So how is creating a nursery that matches the lifetime and visibility of the page different from not using a nursery at all?"
Well, as I've said in one of the reddit conversations, bear in mind the entire purpose here is not to enable something that was previously impossible, but to constrain us from using primitives in their most powerful form. The big thing is that if you had a nursery-based system, and the page had a nursery associated with it, and the page's render routine returned, you'd know that all the page threads must necessarily be terminated. You can't know that with the same confidence now, because the programming style does not permit that level of confidence by construction.
Browsers are a bit of a pathological case on a lot of levels, though, and probably not a great mental example, because you can assume a high degree of competence, concern, and skill in the people programming the browser, and they do things like use static analysis all the time and even in the "worst" cases, develop entire new programming languages to write browsers in. So you are probably justified in saying "But jerf, I'm pretty confident the browsers are already cleaning up their stuff without this stuff." The real question is, how does this look for Joe Programmer and his ability to work in the domain of multithreaded programming, which is well known and widely acknowledged to be very difficult, by constraining what mistakes he can make?
One of the other angles you can look at this with is, if I have two pieces of correct code and I compose them together, are they still correct? The nursery system says that with my nursery, I can call other code that uses them, and the composition attained over that function call is also correct and does not leak resources. We do not get that guarantee with some other primitives. We do with others. There's a lot of experimentation still going on in this field. I'm not saying this approach is guaranteed to be correct, but it's one of the more plausible claims to being a primitive as basic as "if" is relative to "goto". Compare with Software Transactional Memory, which as nice as it may be, is wildly more complicated than "if" no matter how you slice it.
