On Wed, Mar 9, 2022 at 9:26 AM Brian Goetz <brian.go...@oracle.com> wrote:
>
> >>
> >> The minimal constraint is that the return type of the constructor MH is
> >> the same type as the argument type of the component MHs.
> >
> > Agreed. The types should match but they shouldn't be considered part
> > of the api. I don't think (correct me if I'm wrong) that we want them
> > to "escape" and be baked into classfiles. The implementation of the
> > anonymous class holding the fields ("holder object") should remain as
> > a hidden implementation detail. One way to do that is to enforce that
> > the holder object is always hidden behind other public types like
> > Object.
>
> Yes. So my question is, who does the laundry? Is it the carrier API (who
> always says Object), or the caller who is going to take the return value of
> the carrier constructor and stick it in an Object? Does it make a
> difference? If I take the constructor MH, and compose it with the component
> MHs, will having an extraneous Object signature make it harder to expose the
> true type (which may be a Q type), or will that routinely and reliably come
> out in the wash anyway?
Having the Carrier api do the laundry ensures that the implementation
details don't leak into MH/MT constantpool entries. And encourages
users not to try to be overly clever and rely on the implementation
details. The more they do that the harder it will be to evolve the
implementation in the future.
The Q-ness should just flow through the APIs - I don't see an extra
cost here. Either it's all used in the same JIT compilation unit and
there's full type visibility or its not and the Q may have to hit the
heap to flow along the callstack. The costs are the same whether the
descriptors say Q or not.
>
> >> It would seem to me that preserving stronger types here dynamically gives
> >> MH combinators more room to optimize?
> >
> > Only the outer edge of the MH chain would need to return (constructor)
> > / take (component) Object. The implementation of the MHs can use a
> > sharper type. I don't think we gain any optimization abilities here
> > by exposing the sharper type - at worst there's an asType operation to
> > check the type but that shouldn't be visible in the performance
> > profile.
>
> OK, so you’re saying it’s fine to slap an Object label on it, as it will come
> off easily when needed.
>
That's my assertion.
