[swift-evolution] [Pitch] merge types and protocols back together with type<Type, Protocol, ...>

Adrian Zubarev adrian.zubarev at devandartist.com
Sat May 14 13:44:18 CDT 2016

Thank you for pointing me to the right reading.

I just read both pages:

I’ll have to rewrite my proposal a bit. Btw. is there any better name for Ceylon `Any<>`? Maybe `Either<>`?

`Any<T, U>` - "Any type that conforms to T and U“ 

Could someone explain to me what existentials suppose to be? I didn’t get that part from the article, where everything else made somehow sense to me. :)

Adrian Zubarev
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Am 14. Mai 2016 bei 20:21:04, Austin Zheng via swift-evolution (swift-evolution at swift.org) schrieb:

Yes, this is theoretically possible, but why is it useful? (I am genuinely curious.) 

If the intention is to allow B to be a user-defined extension point for T, this goes back to the core team's arguments (in the thread about optional protocol requirements) about why having checking for conformance to some requirement at the use site is a suboptimal idea.

If the intention is to make the type system as expressive as possible, the core team has already ruled out a number of features (user-defined variance on generics, generic protocols) because they don't believe in their general applicability.


On Sat, May 14, 2016 at 11:13 AM, Vladimir.S <svabox at gmail.com> wrote:
FWIW, yes, protocols available for struct are known at compile-time, but could be unknown at the *moment of writing* the code.

What I mean:

Step 1. I write source code:

protocol A {}
protocol B {}
struct S:A {}

func f(a: A) {
  if a is struct<S,B> {...} // I expect that S could be conformed to B

Step 2. I give my code to someone, who can do somewhere in his project:

extension S:B{..}

On 14.05.2016 7:06, Austin Zheng via swift-evolution wrote:
1. struct<SomeConcreteStruct, Protocol1, Protocol2>. In this case the
struct<...> representation is unnecessary; the protocols that are available
to the user are known at compile-time, and structs can't have subtypes that
conform to additional protocols like classes can. There is an example
marked "func boo(value: struct<SomeStruct>) /* equivalent to */ func
boo(value: SomeStruct)"; my question is why having more than two ways to
express the same idea makes the language better, easier to use, etc.

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