CODEWITHSUNDEEP
swiftgenerics
gragera
gragera

Reputation: 309

Nested type constraints with generics

I'm wondering whether this kind of function definition is possible in swift

Let's say we have this 3 protocols

protocol Foo {
    typealias T

    func get(key: String) -> T
}

protocol Bar {
    typealias T: Cacheable

    func set(item: T)
}

protocol Cacheable: NSCoding {
    func getKey() -> String
}

Then this two classes

class MyClass<T: Cacheable>: Foo, Bar { ... }
class MyOtherClass<T: Cacheable>: Foo, Bar { ... }

Imagine that we want a function that can receive an instance of either one of this classes. I can not figure out how to get the signature right, I have tried this that seems logical but it does not work

func doThing<T<V> : protocol<Foo, Bar> where V:Cacheable>(instance: T) { ... }

Any clue?

Thanks!

Upvotes: 1

Views: 587

Answers (1)

Airspeed Velocity
Airspeed Velocity

Reputation: 40965

You want to put a constraint on the placeholder based on its associated type:

func doThing
  <U: protocol<Foo, Bar> where U.T: Cacheable>
  (instance: U) {
    // ...
}

(You also probably want to avoid giving associated types the name T – that’s best left just for placeholders – in theory you could write T where T.T: Cacheable but its likely to lead to some horrible confusion at some point. If this is a container, the convention is Element)

Upvotes: 2

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