On Wed, Oct 5, 2011 at 10:18 AM, John J Barton <[email protected]>wrote:
> > > On Tue, Oct 4, 2011 at 2:33 PM, Bob Nystrom <[email protected]> wrote: > >> >> >> On Tue, Oct 4, 2011 at 2:12 PM, John J Barton < >> [email protected]> wrote: >> >>> >>> >>> On Tue, Oct 4, 2011 at 12:59 PM, Bob Nystrom <[email protected]>wrote: >>> >>>> >>>> A constructor is different from a regular function. Instead of returning >>>> the value that the body of the function returns, it returns a special >>>> newly-created object. >>>> >>> >>> Sorry, already you lost me ;-) I guess you mean the operand of "new"? If >>> so, then the different thing is the operator 'new'. It's "new" that makes >>> the operand a constructor. >>> >> >> It's the yield that makes a function a generator. One is visible at the >> callsite (which means you can *forget* it at the callsite), the other is >> visible at the definition. >> >> >>> >>> >>> >>>> Likewise, a generator is a special function that doesn't return what the >>>> body returns. >>>> >>> >>> But it does not look special. There is nothing similar to 'new' involved >>> in the invocation of the generator. >>> >> >> That's good because it means whether or not a function returns an iterable >> object by using yield or through some other means is an implementation >> detail of the function and doesn't bleed into every callsite. Let's say >> generators *did* have a callsite difference. Now imagine: >> >> function countdown() { >> for (let i = 10; i >= 1; i--) yield i; >> } >> >> let counter = generate countdown(); // <-- in callsite >> for (let i of counter) alert(i); >> alert('boom!'); >> >> This magic generate keyword means the function is to be treated like a >> generator. But then later it turns out that -- is really slow and you want >> to change countdown to: >> >> function countdown() { >> return [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]; >> } >> >> Well, you can't. You'd have to fix each callsite too. Your abstraction has >> leaked. So I think generators do the right thing here. (And, conversely, I >> kind of think constructors do the *wrong* thing.) >> > > We can agree that |new| is a mess even if we don't agree on generators. > > >> >> and the state of the generator is hidden from the developer as far as I >>> can tell. >>> >> >> That's correct. That's often the cost of concision. By analogy: you can do >> a lot of stuff using either an explicit stack data structure or recursion. >> Using recursion is often more concise but then the state is hidden from you >> in the callstack. Sometimes that's a good trade-off, sometimes it's not. >> > > Thanks, that analogy helps confirm my opinion of generators. In my > experience, the good trade-off is recursion on data structures and the > not-good-trade-off is recursion for iteration. So far the only examples > I've seen for generators involve iteration. > > >> >> >>> I think generators are an excellent example of a feature that is well >>> prototyped (in FF JS 1.7+). I think the developer uptake is minimal, outside >>> of the original advocates. I don't hear any clamor for other browsers to >>> implement this feature. >>> >> >> Browsers don't clamor, users do. If it was up to the browsers, they >> wouldn't implement anything. Implementing is hard work! >> > > I've heard no clamor at all. Really this is a niche feature, an aid for the > rare case where conventional iterators are a poor match. > Generators are more than iteration optimizations -- they enable shallow frame continuations, and open the doors to some really nice async programming patterns (just the beginnings of which are demonstrated by Dave Herman's task.js [1]). Again, they're not fully implemented in any engine, so library support is practically non-existent. How are users supposed to clamor for a feature that can't demonstrate itself as advantageous? I've heard plenty of calls for language-level async support -- generators enable this without the well-documented hazards of the alternatives. [1] https://github.com/dherman/taskjs
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