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

Thorsten Seitz tseitz42 at icloud.com
Tue May 17 03:15:11 CDT 2016

Am 17. Mai 2016 um 08:55 schrieb Brent Royal-Gordon <brent at architechies.com>:

But don't you mean the union type of all possible Collection types when you write Any<Collection>?

No, I mean "an existential capable of holding any Collection".

I agree but I think this is just a generalization of type unions as they are currently possible in Ceylon.

´A | B´ in Ceylon notation (or any<A, B> in proposed Swift notation) is a union type meaning a type capable of holding an A or a B.
Any<Collection> is a type capable of holding any Collection, i.e. something like `forall E. Collection where Element == E`. If I could enumerate all possible values for `E` then I could write this as an explicit type union (which would be very very long, of course :-)

Something like: (Collection where Element == Int) | (Collection where Element == String) | ...

If I write Any<Equatable, Collection>, I mean "an existential capable of holding any Equatable Collection".

Which is a type union of Equatable and the existential representing any Collection.

If I write Any<UITableViewCell, CounterDisplaying>, I mean "an existential capable of holding any CounterDisplaying UITableViewCell".

Which would be written as type union `UITableViewCell | CounterDisplaying` in Ceylon.

If I write Any<Collection where .Element: Equatable>, I mean "an existential capable of holding any Collection with an Equatable Element".

If I write Any<class>, I mean "An existential capable of holding any class instance".

If I write Any, I mean "An existential capable of holding anything".

Union types have nothing to do with it.

IMHO they are a just special case where I can write down all types forming the union explicitly whereas an existential uses a type variable which can assume all types (or all types of a constrained subset).

Actually I like this generalization very much (although I think the type operators `|` and `&` are much easier to read than `Any<>` and `All<>` but they don't generalize to existentials).


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