[swift-evolution] Universal Equatability, Hashability, and Comparability

[Haravikk]

While I appreciate the idea behind the proposal, I think I’m a -1 to it. Java has required equality and hashable as part of its base Object class, but I frequently encountered classes that had very poor implementations for these, or never bothered to provide one; arguably they didn’t need to, which is fine, but it kind of went against the whole idea.

Swift has some pretty nifty features that also make this redundant, for example, I’ve been working on some ordered collection types; my natural inclination was to require that values be Comparable, however this actually limits the usefulness of the collection (or requires values to be wrapped somehow). Instead I decided to accept values of any type, and also take a closure (same as used to sort an array).

However, with generic constraints I can still provide a default closure for Comparable types like so:

// Sort Comparable elements in ascending order if no closure is provided.
extension OrderedCollection where Self.Generator.Element:Comparable {
	init<S:SequenceType where S.Generator.Element == Generator.Element>(elements:S) {
		self.init(isOrderedBefore: { $0 < $1 }, elements: elements)
	}
}

(the same feature also lets me implement ArrayLiteralConvertible for Comparable arrays, though I have to provide a default initialiser producing a fatal error for the rest)


It’s a bit of a weird thing to get your head around at first, but you can solve a lot of problems in a similar way, without having to place overly strict requirements on the types that you can accept, removing the need for all types to conform to anything.

> On 9 Mar 2016, at 08:30, Austin Zheng via swift-evolution <swift-evolution at swift.org> wrote:
> 
> As Brent pointed out, adding this sort of support opens a whole can of worms. Large parts of the standard library would silently become unsound.
> 
> As well, in my experience people who have had trouble using (e.g.) Equatable with heterogeneous collections are often trying to do type-unsound things. Maybe Swift should support a separate notion of heterogenous equality for comparisons between Equatable types (and one of the POP WWDC talks actually sketched out an outline of how this might be done), but that's different from making Equatable universal. In addition, I think Swift 3's proposed support for conditional protocol conformance will make creating principled heterogeneous collections easier, which should ease some of the burden.
> 
> Best,
> Austin
> 
>> On Mar 9, 2016, at 12:17 AM, David Hart <david at hartbit.com <mailto:david at hartbit.com>> wrote:
>> 
>> 
>> On 08 Mar 2016, at 23:15, Austin Zheng via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> I would prefer Equatable and Hashable to remain opt-in, and for us to add better support for automatic deriving of implementation.
>> 
>> On 08 Mar 2016, at 23:57, Zach Waldowski via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> I completely agree with Austin here. Automatic derivation (perhaps through the same mechanisms Joe is talking about) would be a nice enhancement, but I find it refreshing and advantageous for simple value types to have very little automatic behavior.
>> 
>> Pedantically I agree with both of you, but from a very pragmatic point of you, I think it's very important to point out what Joe said about how this could reduce one of the most frustrating aspects of Swift, when people work with heterogeneous arrays and try to conform to Equatable:
>> 
>>> that would solve many of the common problems people currently have trying to work with heterogeneous containers.
>> 
>> 
>> 
> 
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