[swift-evolution] [Pitch] Nested types in protocols (and nesting protocols in types)

Adrian Zubarev adrian.zubarev at devandartist.com
Mon Oct 17 13:20:59 CDT 2016

Two weeks ago there was a similar pitch here https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon–20161003/027643.html

In general I’m in favor of this feature, but I don’t know any technical issues this might have or does already have.

As I said in the other thread.

I’m for nested everything.

Nested extensions to reduce noise at some point, BUT this should not remove extension A.B { … } completely.

class A {
    class B { … }
    extension B { … } // extends A.B
We already have A.B syntax for extensions, why don’t we allow it for type declarations to reduce nesting (sometimes you don’t want to cluster everything)? Basically something like class A.B { … } would be a shortcut for extension A { class B { … } } and is bounded by the access modifier of A. (This is probably additive.)

Adrian Zubarev
Sent with Airmail

Am 17. Oktober 2016 um 19:59:54, Karl via swift-evolution (swift-evolution at swift.org) schrieb:

I was just doing some googling, turns out there was a discussion about nesting protocols in other types that seemed to go positively a long time ago: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160425/016074.html

I would additionally like to propose that protocols be allowed to contain nested types (including other protocols). Relevant ABI issue is that the standard library contains enums for “FloatingPointRoundingRule”, “FloatingPointClassification” and “FloatingPointSign”. They would probably be better expressed as “FloatingPoint.RoundingRule”, “.Sign", etc.

so to summarise, newly legal would be:

class MyClass {

    protocol Delegate {

and also:

protocol MyProto {

    enum SomeValue {

    protocol Delegate {
        associatedType ExpectedContent

func receive(_: ExpectedContent, for: SomeValue)

        protocol SecondaryTarget {
            func receive(_ : ExpectedContent)

When conforming to a nested protocol, you can just use the name of the protocol:

class Host : MyProto.Delegate {

Except if a protocol in the chain has associated types, then you must use a concrete, conforming type instead (as you would in the first example — MyClass.Delegate):

class SecondaryProcessor : Host.SecondaryTarget {

If we’re good with this, I’ll write up a proposal.

- Karl
swift-evolution mailing list
swift-evolution at swift.org
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