[swift-evolution] [Draft] Hasher & HashVisitable

[Karl Wagner]

> On 14 Mar 2017, at 19:30, Joe Groff via swift-evolution <swift-evolution at swift.org> 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
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20170314/e84805c7/attachment.html>