SE-0346: Lightweight same-type requirements for primary associated types

If anything then this should look like this:

public protocol RangeReplaceableCollection<Element>: Collection<Element> {

This would align with the declaration side of generic classes which like to forward its subclass' generic type parameter to the generic superclass.

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I don’t want to derail this thread with an extended discussion about a downgrade tool. I’ve moved some of these posts to a new discussion thread.

I will say here that, while the Core Team understands the plight of source library maintainers, we are uncomfortable with accepting a restriction that all declaration syntax must be designed around the ease of #ifing it.


Is the problem being addressed significant enough to warrant a change to Swift?

Very much so! I'd like to adopt AsyncSequence in various projects instead of other reactive solutions, but the ergonomics just make it impossible, as depicted in the motivational section of the proposal, without falling back to a boilerplate type-erasure solution, like Combine does.

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Overall +1 , Big improvement on cognitive load when reading and writing generic functions.

Regarding @rethrows — I don’t think it has been explicitly addressed in the proposal or this thread and it probably needs to be.

This proposal does not provide a solution for writing effects constraints for rethrowing protocols. This is a general problem with both opaque result types and generic code that wants to generalize over an effect, independent of primary associated types. John's write up here outlines the limitations on the current form of rethrows and possible language solutions to lift those limitations:

In my opinion, this problem should be solved as part of the rethrowing protocols proposal (or possibly subsumed by a "typed throws" proposal), which hasn't gone through a formal review yet. Depending on the language design for generalizing rethrowing protocols, effects constraints could either compose with opaque result types via keyword, e.g. (strawman!) some throws(ErrorType) AsyncSequence<Element> or via normal same-type constraints. For example, a precise error associated type could become a primary associated type of AsyncSequence, e.g some AsyncSequence<Element, MyError>. If the latter is the solution, we might want a way to default the constraint to Never, similar to how not writing throws means the default is non throwing.

So, the important thing to figure out for this proposal is whether the current design leaves room for these future language features. I can think of a few implications:

  • This proposal does not preclude the ability to add an effects specifier to opaque result types via keyword, e.g. some throws P.
  • If the model for rethrowing protocols transitions to use an associated type, we probably want to leave room for defaulting the constraint on a primary associated type. To me, that suggests that the proposal should remove the ability to add a default type witness for a primary associated type in angle brackets via =. If effects constraints are deemed not common enough to warrant lifting the error associated type to be "primary", perhaps that's motivation for generalized opaque result type constraints, which is also not precluded by the proposed design.
  • Currently, the default effect for code that's generic over a rethrowing protocol conformance is throws. If we want the syntax some AsyncSequence<Element> to default to non-throwing, we should not adopt primary associated types for AsyncSequence until there is a way to express that default. This proposal does not include adopting primary associated types in the standard library (that will be a separate proposal and discussion) so this proposal doesn't preclude that either.

I feel like this example isn't a great fit for the problem being discussed. In my opinion some AsyncSequence<Element> should equal some AsyncSequence<Element, Error> and not some AsyncSequence<Element, Never>. This would be symmetrical to the fact that non-throwingness would require explicit Never or the explicit absence of throws if you look at non-throwing functions etc., while the regular throws without a concrete error type will very likely be the short form for throws(Error). On top of that AsyncSequence in particular would require an associatedtype Failure: Error = Error extension to avoid any breakage.

I think you have that backwards. In general in Swift, things default to not throwing and you have to write something extra to allow them to throw. That is a key part of our effects design: if you default to allowing an effect (like mutating, throws, or async), you have an immediate problem where lots of code and abstract interfaces unintentionally declare that they allow the effect, which can be difficult or impossible to clean up later. In contrast, if you have to opt in to declaring an effect, it’s very likely that you’ll quickly converge to opting in at all the right places, because even a basic implementation will fail to compile otherwise.

That corresponds to types like Task defaulting to an error type of Never. Naively, at least, it seems like AsyncSequence ought to be the same way. I agree that the fact that we’d need to treat bare AsyncSequence as throwing complicates this. Still, that just confirms to me the wisdom of leaving this open as Holly suggests so that we can consider it holistically with whatever language approach we decide on for throwing and non-throwing conformances.


I don't disagree with your reasoning and what Holly was previously analyzing. I just wanted to highlight that AsyncSequence is a bit of a special example that in my opinion does not fit well to describe this problem. Let me put this differently. Yes I agree that if AsyncSequence would already have an associated type for Failure, then it would make total sense to have some AsyncSequence<Foo> the same as some AsyncSequence<Foo, Never>. However since this isn't the case, and as far as my understanding goes on the future extension of this protocol with the missing Failure associated type, we have to provide a default which would mirror the todays behavior, which makes the bare some AsyncSequence a throwing type. Therefore I think it's a bit of an unlucky situation we have now. If I'm totally wrong on my expectations, then feel free to ignore my conversation regarding this particular issue.

I don't think an absence is explicit. In fact, I think a quality expressed by the absence of some trait is the definition of implicit.

Well, that is all what we have, so it's no wonder no one suggests using a feature that does not exist.
On the other hand, the query "relation over co-equal types" was not even meant to be about a existing or hypothetical feature, but about the concept.

Sorry to pick a specific example — there are some more, and I don't think speculation about what people might want is a good argument. On the other hand, there a quite some people who definitely want generic protocols, and I would not assume that this is not an informed opinion.

There has been some demand for examples in this thread, and I realized one compelling case is still missing (or maybe I just did not see it... or it is not that compelling :grimacing:):

The whole family of Any-types (AnyIterator, AnyPublisher...) and manual conversion is just needed because the necessary generic parameter can't be applied to a protocol.
If we had that capability, all those glue types could be removed, and instead all actual types could (automatically) conform to generic protocol. That protocol would not have associated types, so you would not suffer from their limitations.
That is, you could define and use properties like

let numbers: AnyIteratorProtocol<Int>
// ....
   numbers = someIntArray.makeIterator()

The primary reason why AnyIterator<Element> et al is necessary to manually write as a generic struct is because you cannot write an existential type any IteratorProtocol<Element> where the Element associated type is constrained and known statically. Extending this exact feature of primary associated type constraints to existential types, which is a future direction of this proposal, would allow that to be expressed without manually writing a type-erasing wrapper type that preserves the primary associated type. There is still an issue that the existential type any IteratorProtocol<Element> does not itself conform to IteratorProtocol, but existential opening solves many use cases that would otherwise need conformances for existential types.

  • What is your evaluation of the proposal?


  • Is the problem being addressed significant enough to warrant a change to Swift?

Yes — this is a nice ergonomic change that makes generics easier to read and write, and takes a significant step towards allowing generalized existentials.

  • Does this proposal fit well with the feel and direction of Swift?

Yes. The syntax here is familiar because it's reused from generic types. Some people on here have opposed this syntax on the basis that this would be the most obvious syntax for generic protocols. However, generic protocols have been considered an unlikely addition to the language since before the release of Swift 3, in part due to the fact that associated types can usually be used for the same thing. (Case in point: String instances are able to be used as a Collection<Character>, Collection<UTF8.CodeUnit>, Collection<UTF16.CodeUnit>, or Collection<Unicode.Scalar> with the help of a few wrapper types.)

Alternative syntaxes like Collection<.Element == String> are pointlessly verbose — it would be like having to use Array<.Element == String> to refer to an array of strings.

  • If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?

I have not used any languages with a similar feature.

  • How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

I've been following discussions relating to the feature and I've read the proposal itself.

One question: would it be an ABI-breaking change if a library compiled in library evolution mode were to replace a function that has the signature

func secondElement<C: Collection>(of collection: C) -> C.Element

with a function that has the signature

func secondElement<E>(of collection: some Collection<E>) -> E

? The second version of this function looks much clearer to me, but LibraryEvolution.rst states that you're not allowed to reorder generic parameters in a function. What's the story for existing libraries?

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Yes, it would be ABI breaking. The first function's generic signature is

<C where C : Collection>

The second is

<E, C where C : Collection, C.Element == E>

The calling conventions differ -- the first one passes a single type metadata for C followed by a witness table for C : Collection, the second one passes two type metadata for E and C, followed by the witness table for C : Collection. Generic signature minimization does not delete generic parameters, even though in this case we could theoretically recover the type metadata for E from C.Element (that is, looking up the associated type metadata for Collection.Element from the witness table for C : Collection).


Out of sheer curiosity, will this make it more difficult for the standard library developers to take advantage of this proposal? Something like, I don't know, shipping in the binary several versions of updated methods, legacy ones along with updated ones (assuming the mangled names are distinct)?

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I am unreasonably excited about this. When manually coded, one-off AnyFoo types are no longer necessary, it will be a beautiful day in the land of Swift.


The first review of SE-0346 is now over. The Core Team went over the feedback we received, and we have decided to accept most of the proposal as written. The community did point out a few things that the Core Team agrees ought to be changed in the proposal, and the authors have accordingly revised it. Therefore, SE-0346 has been put back into review. The Core Team's full decision can be found in the new review thread.

I'd like to thank you all for your work in this thread. I know this is a contentious topic that many people feel strongly about, and I think this review has nonetheless stayed remarkably productive and even-keeled.

John McCall
Review Manager