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

[David Sweeris]

> 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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