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

[Tony Allevato]

What's the behavior if you have something like this:

struct Foo<T, U> {
    func foo(bar: Any<T, U>) { ... }
}
let f = Foo<A, A>

...?

More generally, my fear is that being *too* restrictive about banning
redundant types could get you into a situation through generic
metaprogramming where you might *want* that argument to coalesce to just
"A", but if the redundancy check is implemented a certain way, you've just
prevented a perhaps legitimate usage with no easy workaround.

I'm inclined to just say: minimize redundant types automatically and if a
user wants to write something redundant, that's fine—so a variable declared
with type `Any<A, A>` would actually be of type `A`. You might be able to
avoid this if you only check the *literal* type or type parameter name
going into the Any<>, but I'm not sure how much difficulty that is on the
type checking side, and then you'd only catch some of the cases and you'd
still have to be able to minimize.


On Fri, May 20, 2016 at 7:53 AM Adrian Zubarev via swift-evolution <
swift-evolution at swift.org> wrote:

> What I meant here is, that the example should produce an error because it
> is basically `Any<Any<Any<…>>>` which should be banned. It depends on the
> context. Using typealias in general can be allowed but it depends how you
> will use and nest it.
>
> This example is valid:
>
> protocol A {} protocol B {} protocol C {}
>
> typealias AB = Any<A, B>
> typealias ABC = Any<AB, C>
>
> If we ban `Any<Any<...>>` one should not abuse `typealias` to achieve
> that, even if its useless. I’m not sure how hard that would be implement
> that restriction.
>
> The example you was pointing at wouldn’t work anyway because the rule #3
> and rule #4 should raise an error. Thats how I see it.
>
> If I’m not correct please provide an example that makes more sense. :)
>
> --
> Adrian Zubarev
> Sent with Airmail
>
> Am 20. Mai 2016 bei 16:44:35, Matthew Johnson (matthew at anandabits.com)
> schrieb:
>
>
>
> Sent from my iPad
>
>
> On May 20, 2016, at 4:39 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_-6273636555714890771_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.
>
> Why do you think having multiple typealiases for the same type is a
> problem?  The whole point of typealias is to *alias* the name of the
> type.
>
>
>
> 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.
>
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