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

Adrian Zubarev adrian.zubarev at devandartist.com
Mon Oct 17 13:44:42 CDT 2016

That option should not be disallowed. Here is a simple example you might want to build at some point:

protocol ProtocolName {
    associatedtype AssociatedType

extension ProtocolName where AssociatedType == Int {
    struct InnerType {}

Adrian Zubarev
Sent with Airmail

Am 17. Oktober 2016 um 20:30:58, Karl via swift-evolution (swift-evolution at swift.org) schrieb:

Is your vision that each conforming type would have to provide its own nested type as specified by the protocol?

Or could the protocol itself define a nested type and anything could use it?

protocol FloatingPoint: … {
    enum RoundingRule {
        // Do I put an implementation here?

No, types which are defined inside the protocol are implemented there. Providing your own types to satisfy a conformance is what associated types are for.

If you wanted something like that, you could do it with a nested protocol + associated type:

protocol FloatingPoint {

    protocol _RoundingRule { func round(_ : Super) -> Super }
    associatedType RoundingRule : _RoundingRule

struct Float : FloatingPoint {

    enum RoundingRule : _RoundingRule {
        func round(_ val: Float) -> Float {
            /* switch self, perform rounding… */ 

That brings up an interesting point, though - we would need a way to refer to the outer protocol (I used “Super” here).

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