> On 14 Mar 2017, at 19:30, Joe Groff via swift-evolution
> <[email protected]> wrote:
>
>
>> On Mar 14, 2017, at 11:27 AM, David Hart <[email protected]> wrote:
>>
>>
>>
>>> On 14 Mar 2017, at 16:41, Joe Groff via swift-evolution
>>> <[email protected]> wrote:
>>>
>>>
>>>> On Mar 13, 2017, at 8:38 AM, Vincent Esche via swift-evolution
>>>> <[email protected]> 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
> _______________________________________________
> swift-evolution mailing list
> [email protected]
> https://lists.swift.org/mailman/listinfo/swift-evolution
Couldn’t we solve that by adding hashValue with the protocol requirements (and
with a default implementation)? IIRC, the standard library already does this
for things like map. They are defined as requirements even though they are
completely optional in practice.
protocol Hashable {
func hash<H: Hasher>(with hasher: inout H)
var hashValue: Int { get }
}
extension Hashable {
var hashValue: Int {
var hasher = StdLibDefaultHasher()
hash(with: hasher)
return hash.finish()
}
}
- Karl
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