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

[Johannes Neubauer]

Dear Xiaodi,

> Am 18.07.2016 um 20:55 schrieb Xiaodi Wu <xiaodi.wu at gmail.com>:
> 
> As mentioned earlier, NaN != NaN, demonstrating that an Equatable instance that does not always equal itself is not "radical." Plainly, your proposal is unworkable.

1. this is a basic internal type, so it can have a special behavior, since it is a well-designed data type created by the language designers (since there is no need to bootstrap swift from the first bits this is OK).
2. when I made my proposal I didn’t expect that there is such a hard wish for doing things that for sure do **not** comply to the contract of equality (e.g. in order to work with dictionaries and sets). If you allow to implement something like NaN != NaN in custom code, you allow them to implement equality, that is **not reflexive**.

Swift (AFAIK) has three goals:

1. simplicity
2. performance
3. safety

Allowing things like NaN != NaN in custom value types **without** even flagging it with a keyword like `iknowthisisdangerousbutiknowwhatido`, is against goal 3.

All the best
Johannes
> 
> 
> 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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