Generator issue: exceptions while initializing arguments

[Brendan Eich]

Allen Wirfs-Brock wrote:
> On Sep 8, 2012, at 3:59 PM, Brendan Eich wrote:
>
>> Here's an attempt at a crucial experiment:
>>
>> js>  var g = 'now'
>> js>  function f(a = g) { yield a; }
>> js>  var it = f()
>> js>  g = 'later'
>> "later"
>> js>  it.next()
>> "later"
>>
>> So I'm wrong, SpiderMonkey inserts the impliciti |yield;| before parameter defaulting. Rats.
>>
>> It may not matter, but I still think early beats late. That Python binds in the definition context may still be relevant, even though Python evaluates parameter default values at function definition evaluation time (way early), insofar as translating the above to Python:
>>
>> Python 2.6.6 (r266:84292, May 28 2011, 19:08:00)
>> [GCC 4.2.1 (Apple Inc. build 5664)] on darwin
>> Type "help", "copyright", "credits" or "license" for more information.
>>>>> g = 'now'
>>>>> def f(a = g):
>> ...   yield a
>> ...
>>>>> it = f()
>>>>> g = 'later'
>>>>> it.next()
>> 'now'
>>
>> does indeed result in 'now'.
>
> I think early beats late and that this concept extends to include any code within the generator function that is involved in setting up the initial state of the generator.

So far, so good, but I think the rest of your message mixes shared 
mutable objects in the heap (a JS feature, not a bug) with generators 
and then tries to patch generators to avoid the consequences of the 
shared-mutable-objects feature.

>     For example, it seems quite reasonable for somebody to try to write something like:
>
> function *dataSnapshot(aCollection) {
>     snapshot = aCollection.captureCurrentState();

(missing var or let -- implicit global...)

>     //initialization complete
>     for (let i=0; i<snapshot.length; i++) yield snapshot[i]
>     throw StopIteration;

This last throw statement is unnecessary and no one should write it.

> }
> ...
>
> lets snappedItr= dataSnapshot(myData);
> ...
> ...
> myData.doABunchOfUpdates();

The contract of dataSnapshot wants a deep copy of aCollection, so it 
must make one. Fortunately if we bind parameters before the implicit 
yield, it can:

function dataSnapshot(aCollection) {
   var snapshot = aColllection.clone();
   return (function* () {
     for (let i = 0; i < snapshot.length; i++)
       yield snapshot[i];
   })();
}

There's no need to add more magic to generators. Just use a function and 
a generator together. Compositionality.

> ...
> ...
> for (let d of snappedItr)  {/* look at old data */}
> ...
>
> Unfortunately, they will see the new updated data not the old data when they iterate.
> What they need is the ability to move the implicit "initialization is complete" yield from the beginning of the body  to the point of the comment I provided.  We don't have a way to do that and , of course, if we did it would complicate things and probably create other ways to mess things up.  The only reason approach that I've come up would be to treat a yield without an associated expression as the initialization complete marker:
>
> function *dataSnapshot(aCollection) {
>     snapshot = aCollection.snapState();
>     yield;

No, this yield; is well-defined and must mean (yield void 0); -- it 
should not be reinterpreted just because it is first.


>     //initialization complete
>     for (let i=0; i<snapshot.length; i++) yield snapshot[i]
>     throw StopIteration;
> }
>
>
> The semantics would probably be if the body does not contain such a yield, then one is auto generated at the top of the function body. So the default would be "late" (possibly include the arguments) but it could be explicitly make "early" to an arbitrary point in the body.

We still have the choice of the implicit yield going after argument 
defaulting, not before. SpiderMonkey implements before, though, and my 
own argument from compositionality, above, makes it more clear to me 
that we should go with implicit-yield-first. This makes contrived 
throwing parameter default expressions throw "late" but if that matters, 
use a function combined with a generator.

In this thread, you and I have agreed on simpler 
scope/hoisting/defaulting/initialisting a la ES1-5. I think on 
reflection that this argues for the implicit yield in a generator 
function going before anything with observable effects, including 
argument defaulting.

/be

>
> Allen
>
>
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