April 10 2014 Meeting Notes

[Allen Wirfs-Brock]

On Apr 24, 2014, at 7:49 AM, Erik Arvidsson wrote:

> I completely agree. Adding return() will make adoption suffer.

I don't think that should be the high order-bit on this decision.

There are ways to implement finally unwinding that have zero to very low dynamic setup cost so there is only a significant cost when unwinding actually occurs. This would probably be a good thing for implementations to move towards.

If (and it's still an open question) it make sense semantically to call "return" on for-of initiated  generators on unwind, then that is what we should do. Implementations can catch up, but specified semantics is forever.

Allen


> 
> On Thursday, April 24, 2014 4:05:14 AM, Andreas Rossberg <rossberg at google.com> wrote:
> On 15 April 2014 18:06, Allen Wirfs-Brock <allen.wirfsbrock at gmail.com> wrote:
> >>> AWB: We _could_ add a `return()` method.
> >>> ... It's a bigger change, but we could make for-of invoke `return()` on exit
> >>> or completion
> >>
> >> This would be a _significant_ cost. One reason we got rid of the
> >> StopIteration protocol was the performance cost incurred by having to
> >> wrap all loops into implicit try-statements, which are costly. This
> >> proposal asks for re-introducing the same cost.
> >
> > Essentially it means that all generator based loops need to be wrapped in a
> > finally. Whether or not this has a significant cost or is a equivalent to an
> > exception handler depends upon implementation specific details of the
> > runtime design.
> 
> I'm very doubtful, given how VMs currently handle these features. I'm
> pretty sure the consequence will be that the golden new future with
> for-of and generators will be significantly slower than manual for-in
> or for-;; loops for a long time, if not forever. And consequently,
> adoption would suffer.
> 
> And FWIW, it would have similar effects on yield* and generator
> expressions as well.
> 
> (We already see a similar effect with higher-order array functions: we
> get regular complaints that they are slower than for-loops, but there
> is only so much we can do given their more expensive spec'ed
> semantics.)
> 
> /Andreas
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