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. 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
<[email protected]>, wrote:
>
> > On Jun 27, 2017, at 12:39 PM, Jaden Geller via swift-evolution
> > <[email protected]> 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
> > > <[email protected]> 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
> > > > <[email protected]> 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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