[swift-evolution] [Discussion] Generic protocols

Adrian Zubarev adrian.zubarev at devandartist.com
Sat Dec 3 13:57:18 CST 2016

To which code ‘above’ do you refer exactly?

Adrian Zubarev
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Am 3. Dezember 2016 um 20:29:05, Xiaodi Wu (xiaodi.wu at gmail.com) schrieb:

On Sat, Dec 3, 2016 at 12:02 PM, Daniel Leping via swift-evolution <swift-evolution at swift.org> wrote:
@Adrian, my comments inline below. Hope this helps.

On Sat, Dec 3, 2016 at 6:22 PM, Adrian Zubarev <adrian.zubarev at devandartist.com> wrote:
There is one thing that I want to point out with pitched syntactic sugar. GenericMyProtocolName<U> will also let you reuse T within a generic type, where currently we cannot nest protocols into types. Even if we could, it’s not clear if nested declarations like protocol MyTProtocolName : MyProtocolName where U == T would be possible, so that T from the outer generic type is accessible.

Autogenerating the only possible generic protocols from protocols with associated types as a syntactic sugar to reduce spawning of types like IntFoo (as previously showed) seems reasonable to me. It does not break the current protocol system, but solves the first generic protocol feature from the generics manifesto.

There are still a few more questioned to answer:

How dow we want the generated parameter list to look like?
 The most obvious thing is to do something like Foo<Alias1 = Int, Alias2 = String>. Otherwise we'll have to add some sort of associatedtype sorting before; which doesn't sound good and just adds more complexity. Any other options?
Can we drop Generic prefix without collisions?
I'm pretty sure we could just use existing protocols. All the namings can be compiler generated as a function from protocol and arguments.
Does this syntactic sugar improves the stdlib or affect ABI?
Don't see any implications. Assuming current stdlib doesn't use the feature. It's rather an addition than change.
There are probably some more questions I cannot foresee by myself. Feel free to jump in the discussion. ;)

PS: Yes this syntactic sugar does create an alternative way of creating ‘kinda’ the same protocol twice, but it reuses the pattern more naturally.

protocol Foo { associatetype Inner : SomeOtherProtocol }

// Assuming String and Int conforms to SomeOtherProtocol
protocol StringFoo : Foo where Inner == String {}   
protocol IntFoo : Foo where Inner == Int {}

I thought I understood the initial topic of discussion, but I am lost here. What is unsatisfactory about the code you wrote above and why do we need new sugar for it?
// VS.

// autogenerated   
procotol GenericFoo<Inner : SomeOtherProtocol> : Foo { … }

// usage

// or
typealias StringFoo = GenericFoo<String>
typealias IntFoo = GenericFoo<Int>
// The usage I propose is just do
typealias StringFoo = Foo<Inner = String>

//also in an inheritance like this:
class FooClass : Foo<Inner = String>, Foo<Inner = Int> {
//My question here is, though: how do we access Inner from FooClass

//Like this?
typealias InnerInt = Foo<Inner = String>.Inner

Adrian Zubarev
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Am 3. Dezember 2016 um 16:43:43, Daniel Leping (daniel at crossroadlabs.xyz) schrieb:

In general I'm very
positive with the idea of generic protocols. This discussion is
more about syntactic sugar, though I really like where it

Off topic:

IMO, we should revisit approaches others already use for conflicts
resolution. I personally tend to get something similar to Scala
traits. Should fit POT smoothly and naturally.

On Sat, 3 Dec 2016 at 16:30 Adrian Zubarev <adrian.zubarev at devandartist.com> wrote:
If I’m not mistaken here, extension Foo where T == Int will have an error of redeclaration foo anyways.

Adrian Zubarev
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Am 3. Dezember 2016 um 15:22:56, Adrian Zubarev (adrian.zubarev at devandartist.com) schrieb:

extension Foo where T == Int {
     func foo() {
          self.bar(o: 42) // calls a function that accepts an Int

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