[swift-evolution] [swift-evolution-announce] [Review] SE-0089: Replace protocol<P1, P2> syntax with Any<P1, P2>

[Dave Abrahams]

on Wed May 25 2016, Matthew Johnson <swift-evolution at swift.org> wrote:

> Sent from my iPad
>
>> On May 25, 2016, at 12:10 PM, Jordan Rose via swift-evolution
>> <swift-evolution at swift.org> wrote:
>> 
>> 
>>>> On May 25, 2016, at 05:27, Brent Royal-Gordon via swift-evolution
>>>> <swift-evolution at swift.org> wrote:
>>>> 
>>>> AFAIK an existential type is a type T with type parameters that
>>>> are still abstract (see for example
>>>> https://en.wikipedia.org/wiki/Type_system#Existential_types),
>>>> i.e. have not been assigned concrete values.
>>> 
>>> My understanding is that, in Swift, the instance used to store
>>> something whose concrete type is unknown (i.e. is still abstract),
>>> but which is known to conform to some protocol, is called an
>>> "existential". Protocols with associated values cannot be packed
>>> into normal existentials because, even though we know that the
>>> concrete type conforms to some protocol, the associated types
>>> represent additional unknowns, and Swift cannot be sure how to
>>> translate uses of those unknown types into callable members. Hence,
>>> protocols with associated types are sometimes called
>>> "non-existential".
>>> 
>>> If I am misusing the terminology in this area, please understand
>>> that that's what I mean when I use that word.
>> 
>> We’re not consistent about it, but an “existential value” is a value
>> with protocol or protocol composition type. My mnemonic for this is
>> that all we know is that certain operations exist (unlike a generic
>> value, where we also have access to the type). John could explain it
>> more formally. We sometimes use “existentials” as a (noun) shorthand
>> for “existential value”.
>> 
>> In the compiler source, all protocols and protocol compositions are
>> referred to as “existential types”, whether they have associated
>> types or not. Again, a protocol asserts the existence (and
>> semantics) of various operations, but nothing else about the
>> conforming type. (Except perhaps that it’s a class.) All protocols
>> are thus “existential types” whether or not the language supports
>> values having that type.
>> 
>> It is incorrect to say that protocols with associated types (or
>> requirements involving Self) are “non-existential”.
>
> I haven't heard people using this term myself, but I imagine they
> probably mean "can't form an existential value with the protocol".
> There certainly appears to be a lot of confusion in the community with
> many not realizing that this is a temporary limitation of the
> implementation, not a necessary fact.

As far as I know there is no known way to make protocols with Self
requirements usefully “existentiable,” or whatever you want to call it.
So unless I'm missing something, in this respect, the limitation is not
temporary at all.

-- 
-Dave