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

[Nevin Brackett-Rozinsky]

I like it!

I haven’t thought through the ramifications regarding associated types yet
though.

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?
    }
}

Nevin



On Mon, Oct 17, 2016 at 1:59 PM, Karl via swift-evolution <
swift-evolution at swift.org> wrote:

> 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
>
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>
>
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