[swift-evolution] [Pitch] Improving unspecified generic usability

Logan Shire logan.shire at gmail.com
Tue Aug 8 03:44:01 CDT 2017

Thanks for the feedback!

Félix, sorry about the confusion between FooStruct and FooProtocol - I'll
refer to them as such moving forwards.

David, I don't believe you should be able to cast an [FooStruct<String>] to
an [FooStruct<Any>] because those are both valid specifications. If Generalized
implemented, that would be another story, but that's outside the scope of
this proposal. I do believe you should be able to cast [FooStruct<String>]
to [FooStruct], and that you should be able to flatMap [FooStruct] into
[FooStruct<Any>] with as?, but all of the casts would fail and you would be
left with an empty array.

In regards to the Named protocol, yes, that is the current idiomatic
approach to solving this problem (along with making a function
unnecessarily generic and then using the generic type as a constraint). I
want to avoid jumping through those hoops. We'd essentially be synthesizing
the Named protocol with the same name as the non-generic version of the
type. I.e. FooStruct<T>: FooStruct

Félix, I believe the above answers some of your questions, but in regards
to protocols with associated types, I'd imagine it would work the same way.
If FooProtocol has an associated type T, there would be another protocol,
FooProtocol, without the associated type. (behind the scenes its garbled
name would be different)

Also, an aside, it would be nice if protocols could use the generic syntax
for their associated type constraints. I.e. "FooProtocol with T = Int"
could be expressed as FooProtocol<T: Int>. It feels strange that we have
two different syntaxes for essentially the same language construct. At the
very least, I want some way to cast a value to a protocol type with an
associated value. E.g. "if let grassEater = any as? Animal where Food =

Elviro, yes, the generalized existentials would help a lot here, but that's
outside the scope of what I'm proposing. In the near term I'd like to be
able to use a generic type's non-generic interface, casting to and from it.
See the above discussion regarding the Named protocol. Essentially we'd be
synthesizing the Named protocol, but where the type's name is the same as
the non-generic version of the type name.

FooStruct<String> as FooStruct // works
FooStruct as? FooStruct<String> // works
FooStruct as? FooStruct<Any> // Compiles but conversion fails, becomes nil
FooStruct<String> as? FooStruct<Any> // Compiles but conversion fails,
becomes nil

Let me know if you have any other questions!


On Tue, Aug 8, 2017 at 9:43 AM Félix Cloutier <felixcloutier at icloud.com>

> I'm going to separate your examples into FooStruct and FooProtocol for
> clarity.
> I agree that generics tend to propagate virally and I remember that at
> some point I wanted type erasure, though I don't remember for what exactly.
> The solution for `sayHi`, right now, is to make that one generic too:
> func sayHi<T>(to foo: T) where T: FooProtocol {
>     print("hi \(foo.name)")
> }
> The "let foos: [FooStruct] = [FooStruct(name: "Int",
> value: 2), FooStruct(name: "Double", value: 2.0)]" part can't work for
> structs because arrays require each element to have the same size (but it
> could work for classes).
> Even then, you couldn't infer the type to [FooClass<Any>] because
> contravariance isn't permissible in that situation: doing so would allow
> you to assign any Any to a FooClass's value.
> Another problem that this would have to solve is that once you lose the
> associatedtype that came with the protocol, there is nothing you can do to
> recover it; you currently can't express "FooProtocol with T = Int" as a
> type that you can cast to, so you would only be able to pass the instance
> to functions that don't have constraints on T.
> But all in all, with my current understanding of the issue, I think that
> I'm favorable to the idea.
> Félix
> Le 7 août 2017 à 19:35, David Sweeris via swift-evolution <
> swift-evolution at swift.org> a écrit :
> On Aug 7, 2017, at 3:00 PM, Logan Shire via swift-evolution <
> swift-evolution at swift.org> wrote:
> One of my longstanding frustrations with generic types and protocols has
> been how hard it is to work with them when their type is unspecified.
> Often I find myself wishing that I could write a function that takes a
> generic type or protocol as a parameter, but doesn’t care what its generic
> type is.
> For example, if I have a type:
> struct Foo<T> {
>     let name: String
>     let value: T
> }
> or:
> protocol Foo {
>     associatedtype T
>     var name: String { get }
>     var value: T { get }
> }
> And I want to write a function that only cares about Foo.name, I’d like to
> be able to:
> func sayHi(to foo: Foo) {
>     print("hi \(foo.name)")
> }
> But instead I get the error, “Reference to generic type Foo requires
> arguments in <…>”
> Also, when you want to have a polymorphic array of generic types, you
> can’t:
> let foos: [Foo] = [Foo(name: "Int", value: 2), Foo(name: "Double", value:
> 2.0)]
> And if you remove the explicit type coercion, you just get [Any]
> let foos = [Foo(name: "Int", value: 2), Foo(name: "Double", value: 2.0)]
> I wish that could be inferred to be [Foo].
> What happens if you try to say "foos: [Foo<Any>] = ..."?
> I’d like to propose being able to use the non-generic interface of a type
> normally.
> I.e. if you have a type Foo<T>, it is implicitly of type Foo as well. The
> type Foo could be used like any other type.
> It could be a parameter in a function, a variable, or even the generic
> type of another type (like a Dictionary<String, Foo>)
> The only restriction is that if you want to call or access, directly or
> indirectly, a function or member that requires the generic type,
> the generic type would have to be known at that point.
> Foo<T> should be able to be implicitly casted to Foo wherever you want,
> and Foo could be cast to Foo<T> conditionally.
> Initializers would still obviously have to know the generic type, but
> given the above example, you should be able to:
> let names = foos.map { $0.name }
> However, you could not do the following:
> let foos = [Foo]()
> Because the initializer would need to know the generic type in order to
> allocate the memory.
> Let me know what you think!
> The idiomatic solution would be to create a `Named` protocol with a `var
> name: String {get}` property, and write your function like `func sayHi(to
> foo:Named) {...}`. However, this `Named`protocol is really pretty trivial
> -- its purpose is simply to "degenericify" a generic type, not to provide
> any semantic meaning. Perhaps an analogy could be drawn between such
> "trivial protocols" and how we sometimes view tuples as "trivial structs"?
> Dunno, maybe I'm just trying to turn two trees into a forest, but this
> kinda smells like it might be part of a bigger issue, and if it is I'd
> rather tackle that and then see if we still need to address anything here.
> +1, either way, though.
> - Dave Sweeris
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> swift-evolution at swift.org
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