[swift-evolution] [Proposal] Associated Type and Generic One-to-One Mapping

Tue Jun 27 17:47:33 CDT 2017

On Tue, Jun 27, 2017 at 16:44 David Moore via swift-evolution <
swift-evolution at swift.org> wrote:

> There are a few ways in which this issue could be addressed. The first,
> and most desirable approach in my opinion, would be full inferencing of a
> given associatedtype and a generic. An example of the foregoing would be
> the following:
>
> protocol Foo {
>     associatedtype ABC
> }
>
> struct Bar<ABC> : Foo {
>     // Would normally require some sort of workaround to complete the
> typealias implementation of the protocol’s associatedtype, but it should be
> inferred in this case because there is no explicit typealias statement.
> }
>
> The inferencing would indeed take place in the above example. But now
> let’s consider the possibility of breaking source compatibility, as in, is
> this compatible or not?
>
> struct Bar<ABC>: Foo {
>     typealias ABC = ArbitraryType // An inference would not take place
> here because there’s nothing to infer. The associated type `ABC` is
> explicitly defined as being `ArbitraryType`.
> }
>
> The above method shall be referred to as Option #1. It would seem like
> Option #1 is source compatible, in addition to solving the original
> problem, and potentially introduces some connivence to protocol
> conformance. For all existing implementations of associatedtypes and so on,
> I don’t think Option #1 affects them at all, unless I’m completely missing
> something.
>

As I mentioned already, this is source breaking for any type Bar that
conforms to Foo where Bar.T is currently distinct from the inferred type of
Foo.T, because the inference would become ambiguous.

In addition, it cannot be right to infer this automatically, as it would be
saying that a typealias named T and an associated type that happens also to
be named T can be assumed to have the same semantics even during
retroactive conformance. This is essentially stringly typing.

More generally, I continue not to understand the motivation. You mentioned
that you wished for there to be a distinction between associated types and
generic type inferences, and this seems to be the opposite of that. Jaden
offered another example, but I am not sure I appreciate why this feature is
desirable or even necessary in that scenario. In Jaden’s example, as in
yours, there is nothing in the protocol itself that demands that
Optional.Wrapped must be the same as OptionalType.Wrapped; this is, rather,
something that is the choice of the author who conforms the type to the
protocol, at the point of conformance. I fail to see why
OptionalType.Wrapped cannot simply have another name, such as WrappedType.

So Option #1 seems like a good candidate.
>
> Onto Option #2, the idea of using base types to express more explicit
> definitions to solve the initial problem. When it comes to using a base
> type to correctly disambiguate these types of situations, it may be
> familiar to some who like a more concrete implementation, although it
> eliminates possible convenience, but still requires knowledge. Options #2
> would look something like the following.
>
> protocol Foo {
> associatedtype ABC
> }
>
> struct Bar<ABC>: Foo {
> typealias Foo.ABC = Bar.ABC // Quite explicit and communicates the
> solution clearly.
> }
>
> Options #2, as you can see above, would also be source compatible because
> it would not impose on already defined typealias’ or other implementations.
> This could be an opt-in feature. However, I don’t know if it is as nice as
> just pure inference, which doesn’t seem too much more difficult than this
> would be, but I’m not sure about that.
>
> There is also a third option (referred to as Option #3), which could be
> the combination of both Option #1 and Option #2. However, there may be some
> syntactically-based issues that can arise in such a situation.
>
> Let me know what everyone thinks about these possible solutions, they are
> by no means concretely defined as of this time, but they could be useful.
>
> On Jun 27, 2017, 2:08 PM -0400, Matthew Johnson via swift-evolution <
> swift-evolution at swift.org>, wrote:
>
>
> On Jun 27, 2017, at 12:39 PM, Jaden Geller via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> I’ve run into this issue many times in the real world as well. For
> example, consider the following protocol:
>
> protocol OptionalType {
>     associatedtype Wrapped
> }
>
> It is not possible to conform `Optional` to this protocol because its
> generic type is already named `Wrapped`. Only when the associated type can
> be inferred is conformance possible.
>
> I definitely think we need a solution, but I don’t know what that solution
> should be.
>
>
> I agree.  I have run into this as well and have been frustrated by it.  It
> isn’t clear to me what the best solution is but I’d love to see one that
> could make it into a 4.x release.
>
>
> Cheers,
> Jaden Geller
>
> On Jun 23, 2017, at 3:52 PM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> There could be source-breaking implications for such a feature, especially
> with retroactive conformance. Therefore, I think this could be very tricky
> and I'd want to be convinced that the benefits are very great to risk such
> a disturbance. Here, I think the problem is rather mild, and here's why:
>
> It is true that, in your example specifically, renaming T to U is the only
> solution (that I know of, anyway). However, for any "serious" protocol P,
> there's likely to be a required property of type P.T, or a function that
> takes an argument of type P.T or returns a value of type P.T. Therefore,
> implementing that requirement in Bar with a corresponding
> property/argument/return value of type Bar.T would generally do the trick.
>
> Have you got any real-world examples where you're running into this issue?
>
> On Fri, Jun 23, 2017 at 17:03 David Moore via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> Hello Swift Evolution,
>>
>> This may have already been discussed before, but I just came across a
>> bothersome language aspect which reminded me to propose a solution.
>> Currently, if we want to add generics to a protocol the only way to do so
>> is with associated types. I am quite fine with the current approach with
>> respect to those semantics.
>>
>> There is, however, a weakness that is built in with using associated
>> types. That weakness is the lack of associated type and generic inference.
>> To be more clear about what I mean, take the following as an example.
>>
>> protocol Foo {
>>     associatedtype T
>> }
>>
>> The foregoing protocol is quite basic, but uses an associated type with
>> the name “T.” Giving the associated type that name will illustrate the
>> dilemma encountered later on down the pipeline.
>>
>> struct Bar<T> : Foo {
>>     // What am I supposed to do? The name is used for both the generic
>> and the type alias Foo needs for conformance.
>>     typealias T = T // Error!
>> }
>>
>> The above illustrates a situation where we want to connect the generic,
>> which is supposedly named appropriately, and the protocol’s associated
>> type. There is no elegant solution for this at the moment. All I could do
>> is the following.
>>
>> struct Bar<U> : Foo {
>>     typealias T = U // Not nearly as readable.
>> }
>>
>> Now, there may be a few ways to go about adding support for generic
>> inference. The compiler as it is already does some awesome inference get
>> when it comes to generics, so why not take it a step further? I propose the
>> introduction of a keyword, or anything else that could work, to specify
>> explicitly what a given type alias declaration would do when it comes to
>> inferencing. Requiring a keyword would ensure source compatibility remains
>> intact, and it would also make the code more readable.
>>
>> I don’t know if this would be something that people could find useful,
>> but I surely would. The implicit mapping of an associated type and a given
>> generic by their names, would be a natural development.
>>
>> Let me know if this is just useless, or if could be a potential feature.
>>
>> Thank you,
>> David Moore
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>>
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