[swift-evolution] [Draft] Hasher & HashVisitable

[Vincent Esche]

Pardon my ignorance here, but shouldn’t `foo<T: Protocol>(t: T)` always
monomorphize due to being generic? Isn’t that how `T: …` differs from,
say `foo(t: Protocol)`?

On Tue, Mar 14, 2017 at 7:30 PM, Joe Groff <jgroff at apple.com> wrote:

>
> > On Mar 14, 2017, at 11:27 AM, David Hart <david at hartbit.com> wrote:
> >
> >
> >
> >> On 14 Mar 2017, at 16:41, Joe Groff via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>
> >>
> >>> On Mar 13, 2017, at 8:38 AM, Vincent Esche via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>>
> >>> Source compatibility
> >>>
> >>> Making use of "extending protocols to conform to protocols":
> >>>
> >>> extension Hashable: HashVisitable
> >>> {
> >>>
> >>> func hash<H: Hasher>(_ hasher: inout
> >>> H) {
> >>>
> >>> self.hashValue.hash(&
> >>> hasher)
> >>>   }
> >>> }
> >>
> >> We're unlikely to add this feature soon. It seems reasonable to me to
> instead have `HashVisitable` refine `Hashable` and provide a default
> implementation of `hashValue` using a default hasher. I think we still want
> `Hashable` to be the currency protocol most APIs work with for performance
> in unspecialized code, since we could inline the visitation and hasher
> implementation together inside the specialized `hashValue` witness.
> >
> > Can you explain the performance argument? How does it fare (in your
> opinion) compared to the arguments in the proposal?
> >
> > How about:
> >
> > protocol Hashable {
> >    func hash<H: Hasher>(with hasher: inout H)
> > }
> >
> > extension Hashable {
> >    var hashValue: Int {
> >        var hasher = StdLibDefaultHasher()
> >        hash(with: hasher)
> >        return hash.finish()
> >    }
> > }
>
> For unspecialized code that takes a generic T: Hashable, that will place
> the only dynamic dispatch point on `hash`, so that will place an
> abstraction barrier between the Hasher and Self type being hashed, so would
> likely mean a dynamic call for every component of the value being hashed.
> Having `hashValue` be a dynamic dispatch point allows the hasher to be
> inlined together with the type's visitor implementation.
>
> -Joe
>
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