[swift-evolution] [Proposal Draft] automatic protocol forwarding

Kevin Ballard kevin at sb.org
Tue Dec 29 16:45:05 CST 2015

On Tue, Dec 29, 2015, at 02:11 PM, Joe Groff wrote:
>> On Dec 29, 2015, at 1:24 PM, Matthew Johnson via swift-evolution <swift-
>> evolution at swift.org> wrote:
>> Hi Kevin,
>> Thanks for taking time to look at the proposal.
>> The technique you show here is not bad, but it has several
>> deficiencies IMO which are addressed by the solution in the proposal.
>> 1. Forwarding should be an implementation detail, not exposed as it
>>    is with this method.
> This could theoretically be managed by access control on protocol
> conformances:
>> public struct Foo: internal SequenceTypeForwarder, public
>> SequenceType { ... }
> though that's even more boilerplatey, and makes it easy to
> accidentally expose more API than you intended to.

That's an interesting approach.

Another workaround for this today is to use an operator (similar to how
the stdlib uses ~> internally for a lot of stuff) coupled with a phantom
type. As an example (using ~> because why not):

// phantom type

struct _SequenceTypeForwarder {}

protocol SequenceTypeForwarder : SequenceType {

typealias ForwardedSequenceType : SequenceType

func ~>(this: Self, _: _SequenceTypeForwarder) -> ForwardedSequenceType


The need for the phantom type is a bit unfortunate (you can't just use
SequenceTypeForwarder.Protocol because it complains about the Self or
associated type requirements issue).

-Kevin Ballard
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