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Subject: Re: [boost] [next gen future-promise] What to call the monadicreturn type?
From: Peter Dimov (lists_at_[hidden])
Date: 2015-05-25 16:11:34

Avi Kivity wrote:
> On 05/25/2015 09:04 PM, Peter Dimov wrote:
> > Niall Douglas wrote:
> >
> >> I've got everything working except the sequence:
> >>
> >> promise<int> p;
> >> p.set_value(5);
> >> return p.get_future().get();
> >>
> >> This should reduce to a mov $5, %eax, but currently does not for an
> >> unknown reason. I'm just about to go experiment and see why.
> >
> > I'm really struggling to see how all that could work. Where is the
> > result stored? In the promise? Wouldn't this require the promise to
> > outlive the future<>? This doesn't hold in general. How is it to be
> > guaranteed?
> In seastar, we achieved this by reserving space for the result in both
> future and promise, and by having future and promise track each other (so
> they can survive moves).

Doesn't this introduce a race between ~promise and ~future? ~future checks
if( promise_ ), ~promise checks if( future_ ), and things get ugly.

Fixing that requires synchronization, which makes mov $5, %eax impossible.

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