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

[Howard Lovatt]

Presumably a DefaultValueHashable protocol with default implementation of
Hashable and Equatable for value types and similarly DefaultObjectHashable
for objects could be provided in the standard library (using some deep
integration into the compiler or a to be added reflection ability). Then it
would be easy to opt in.

On Thursday, 10 March 2016, Sean Heber via swift-evolution <
swift-evolution at swift.org> wrote:

> > We can also allow opt-in by adopting a protocol, eg `struct
> SimpleStruct: DefaultEquatable`. DefaultEquatable would tell the compiler
> to auto-generate an equatable implementation. Similar for DefaultHashable.
> >
> > In the C# world you’d actually do something like this with an attribute.
> I know user-defined attributes aren’t on the table right now but some more
> built-in ones would be sufficient: @defaultEquatable, @defaultHashable, etc.
> > They could even take a list of properties to ignore if such a feature
> were useful: `@defaultEquatable(ignored: [currentDate, randomInt])`.
>
> Might be interesting if you could pass in parameters when you declare
> conformance, like:
>
> struct MyType : Equatable(default) {}
>
> If “default” was missing, you’d have to supply the relevant ==/equals
> function or whatever as usual, but if it was there you’d get automatically
> generated stuff based on documented rules.
>
> Maybe it could even be extensible so you can define protocol extensions
> that are “named” and only apply if you declare conformance with the
> relevant name:
>
> protocol Bar {
>   func barFunction()
> }
>
> extension Bar(foo) {
>   func fooFunction() { }
> }
>
> struct Type1 : Bar(foo) {
>   func barFunction() {}
> }
>
> struct Type2 : Bar {
>   func barFunction() {}
> }
>
> A Type1 instance would also have a fooFunction(), but a Type2 instance
> would not.
>
> Using this you could have multiple default implementations defined for the
> same protocol depending on circumstances.
>
> extension Bar(scenerio1) {
>   func barFunction() { print(“1”) }
> }
>
> extension Bar(scenerio2) {
>   func barFunction() { print(“2") }
> }
>
> struct Thing1 : Bar(scenerio1) {}
> struct Thing2 : Bar(scenerio2) {}
>
> let a: Bar = Thing1()
> let b: Bar = Thing2()
>
> a.barFunction() -> “1”
> b.barFunction() -> “2”
>
> :)
>
> l8r
> Sean
>
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>


-- 
-- Howard.
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