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.) 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. > 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! In languages that do have yield (Python, C#, and Lua are the ones I know) it's used pretty frequently and without complaint. Either way, "class" deserves at least as much investigation. > Agreed! - bob
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