[swift-evolution] Proposals: (1) Forbidding custom `==` for value types, (2) `dispatch` keyword, (3) `default`-result for methods with `Self`, and (4) Poor-Mans-Existentials

[Xiaodi Wu]

As mentioned earlier, NaN != NaN, demonstrating that an Equatable instance
that does not always equal itself is not "radical." Plainly, your proposal
is unworkable.


On Mon, Jul 18, 2016 at 13:48 Johannes Neubauer via swift-evolution <
swift-evolution at swift.org> wrote:

>
> > Am 18.07.2016 um 14:01 schrieb Johannes Neubauer via swift-evolution <
> swift-evolution at swift.org>:
> >
> >
> >> Am 18.07.2016 um 13:52 schrieb Johannes Neubauer via swift-evolution <
> swift-evolution at swift.org>:
> >>
> >>>
> >>> Am 18.07.2016 um 13:05 schrieb L. Mihalkovic <
> laurent.mihalkovic at gmail.com>:
> >>>
> >>> IMHO implementing your proposal would close the door on some of the
> things you do when building in-memory dbs (T == U -> TRUE for T not related
> to U), which if swift remains for small apps is not a terrible loss, but
> may be more of an issue for one day doing big-data with it.
> >>
> >> You talk about reference types now, right? I proposed a `default`
> keyword, which (in a pattern matching fashion) would catch all calls to T
> == U for which no implementation exists (so this is exactly when T != U).
> You could of course change for a given type hierarchy the `default` result
> to `true` if appropriate.
> >
> > This formulation can be misleading: I mean `a == b` where `a: T` and `b:
> U` and `T != U`. Due to dynamic dispatch even: `a.dynamicType == T &&
> b.dynamicType == U && T != U`.
>
> But I think, for such a radical different semantic than the normal
> interpretation of equality I think I wouldn’t use the `Equatable`-protocol
> at all, but implement a custom protocol with a custom operator.
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