[swift-evolution] [Proposal] Disallow redundant `Any<...>` constructs

[Austin Zheng]

I think you should submit this for review, but I also think you should take
the part of your older proposal to add class support to Any<...> and submit
it as a separate proposal. (I mean, the part where you can define things
like "Any<UIViewController, Protocol>" or "Any<class, Protocol>".)

Yes, it is additive, but even getting that feature into Swift 3 would be an
enormous benefit if it can be implemented easily. And the core team is
probably better positioned than anyone else to determine whether that's
true.

Austin

On Fri, May 20, 2016 at 2:39 AM, Adrian Zubarev via swift-evolution <
swift-evolution at swift.org> wrote:

> This is a follow up proposal to SE-0095
> <https://github.com/apple/swift-evolution/blob/master/proposals/0095-any-as-existential.md> which
> should be considered for Swift 3 if SE-0095 will be accepted.
>
> Here is the formatted draft:
> https://github.com/DevAndArtist/swift-evolution/blob/master/proposals/nnnn-ban-redundancy-in-any-existential.md
>
> Please provide your feedback in this thread, and don’t make a race who is
> making a better proposal on the exact same topic.
>
> If you spot any types or other mistakes I’d be happy to see you pointing
> me to them. ;)
>
> --
> Adrian Zubarev
> Sent with Airmail
>
> Disallow redundant Any<...> constructs
>
>    - Proposal: SE-NNNN
>    <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md>
>    - Author: Adrian Zubarev <https://github.com/DevAndArtist>
>    - Status: Awaiting review <#m_3447501318802576264_rationale>
>    - Review manager: TBD
>
> Introduction
>
> This is a follow up proposal to SE–0095
> <https://github.com/apple/swift-evolution/blob/master/proposals/0095-any-as-existential.md>,
> if it will be accepted for Swift 3. The current concept of Any<...>
> introduced in SE–0095 will allow creation of redundant types like Any<A>
> == A. I propose to disallow such redundancy in Swift 3 to prevent
> breaking changes in a future version of Swift.
>
> Swift-evolution thread: [Proposal] Disallow redundant Any<...> constructs
> Motivation
>
> If SE–0095 will be accepted there will be future proposals to enhance its
> capabilities. Two of these will be *Any-type requirement* (where *type*
> could be class, struct or enum) and *Class requirement*. Without any
> restrictions these will introduce more redundancy.
>
> As said before it is possible to create redundant types like Any<A> == A
> or endless shadowed redundant nesting:
>
> typealias A_1 = Any<A>
> typealias A_2 = Any<A_1>
> typealias A_3 = Any<A_2>
> /* and so on */
>
> This proposal should ban redundancy right from the beginning. If there
> might be any desire to relax a few things, it won’t introduce any breaking
> changes for Any<...> existential.
> Proposed solution
>
>    1.
>
>    If empty Any<> won’t be disallowed in SE–0095, we should disallow
>    nesting empty Any<> inside of Any<...>.
>    2.
>
>    Disallow nesting Any (type refers to current typealias Any = protocol<>)
>    inside of Any<...>.
>    3.
>
>    Disallow Any<...> containing a single Type like Any<Type>.
>
>    The first three rules will ban constructs like Any<Any<>, Type> or Any<Any,
>    Type> and force the developer to use Type instead.
>    4. Disallow nesting a single Any<...> inside another Any<...>.
>       - e.g. Any<Any<FirstType, SecondType>>
>    5.
>
>    Disallow same type usage like Any<A, A> or Any<A, B, A> and force the
>    developer to use A or Any<A, B> if A and B are distinct.
>    6.
>
>    Disallow forming redundant types when the provided constraints are not
>    independent.
>
>    // Right now `type` can only be `protocol` but in the future Any<...>
>    // could also allow `class`, `struct` and `enum`.
>    // In this example `B` and `C` are distinct.
>    type A: B, C {}
>
>    // all following types are equivalent to `A`
>    Any<A, Any<B, C>>
>    Any<Any<A, B>, C>
>    Any<Any<A, C>, B>
>    Any<A, B, C>
>    Any<A, B>
>    Any<A, C>
>
>    -
>
>       If all contraints form a known Type provide a Fix-it error
>       depending on the current context. If there is more than one Type,
>       provide all alternatives to the developer.
>       -
>
>       Using Any<...> in a generic context might not produce a Fix-it
>       error:
>
>       protocol A {}
>       protocol B {}
>       protocol C: A, B {}
>
>       // there is no need for `Fix-it` in such a context
>       func foo<T: Any<A, B>>(value: T) {}
>
>
> Impact on existing code
>
> These changes will break existing code. Projects abusing Any<...> to
> create redundant types should be reconsidered of usings the equivalent
> Type the compiler would infer. One would be forced to use A instead of
> Any<A> for example. A Fix-it error message can help the developer to
> migrate his project.
> Alternatives considered
>
>    - Leave redundancy as-is for Swift 3 and live with it.
>    - Deprecate redundancy in a future version of Swift, which will
>    introduce breaking changes.
>
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
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