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

[Austin Zheng]

I actually like "any<P1, P2>". It does provide that very distinctive visual
signal that any<> is not a generic type, and that 'any' is not itself a
type, but rather a special keyword for constructing an existential:

Array<Int>  // a generic type, Array, containing integers
any<P1, P2> // a protocol composition of two protocols

In this case, would we want to support "any<>" in addition to Any? The
parsing issues should go away, since these are two different identifiers.

Austin



On Fri, May 20, 2016 at 12:04 PM, Matthew Johnson <matthew at anandabits.com>
wrote:

>
> On May 20, 2016, at 2:00 PM, Austin Zheng via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> 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, what is your thought on switching to `any` rather than `Any` since
> it does not behave like a user-defined generic type?  The convention is for
> types to be uppercase and keywords to be lowercase.  This falls more into
> the category of a keyword and has its own behavior distinct from the
> behavior of all generic types.  Making it stand out syntactically will help
> to make that clear.
>
>
> 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
>>    - 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.
>>
>>
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>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
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