generics in swift protocols

Question

I am trying to migrate the Visitor pattern from my (old) java code to swift. So far I have a generic FIFO (working fine).

protocol FiFo {
    associatedtype U
    func enqueue(_ : U)
    func dequeue() -> U
}

I also want to tell the instance of the FIFO only to accept an instance of a visitor that uses the same type for the generic as the instance of the FIFO.

protocol Visitor {
    associatedtype T
    func processValue(_ value : T)
}
protocol FiFo {
    associatedtype U
    func enqueue(_ : U)
    func dequeue() -> U
    func visit(_ visitor : Visitor<U>)
}

Here I am facing:

Cannot specialize non-generic type 'Visitor'

Any hints? Thanks!

Thanks for your answers - I would love to mark both helpful. And thanks for the general hint that there is a difference between generics vs associated types (in swift). — Thomas, Aug 19 '21 at 16:50
Does this answer your question? [How to create generic protocols in Swift?](https://stackoverflow.com/questions/24469913/how-to-create-generic-protocols-in-swift) — pkamb, Aug 19 '21 at 17:30
@RTXGamer: Re your edit: The backticks are for `code`, not for general emphasis. “Visitor pattern” or ”fifo” is not code. — Martin R, Aug 19 '21 at 17:32
@pkamb: I've found that other question, but I wasn't sure at all how to apply that solution to the problem of two protocols. The part when `where` comes in was not that clear to me in the other post. — Thomas, Aug 19 '21 at 17:53

score 3 · Answer 1 · answered Aug 19 '21 at 16:14

3

You can add a constraint to an associated type:

protocol Visitor {
    associatedtype T
    func processValue(_ value : T)
}

protocol FiFo {
    associatedtype U
    associatedtype V: Visitor where V.T == U
    func enqueue(_ : U)
    func dequeue() -> U
    func visit(_ visitor: V)
}

answered Aug 19 '21 at 16:14

Rob

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Sorry, I had not seen your answer appearing. Now I wonder if there is a difference between our suggestions or if they are equivalent. – Martin R Aug 19 '21 at 16:17
3

@MartinR - No worries. Re differences b/w generic and associated type, it is the standard homogeneous vs heterogeneous issues: E.g., should there be more than one `FiFo` method that takes a `Visitor`, having multiple methods with `where V.T == U`, they don’t necessarily need to be the same type, whereas associated type can enforce that). If it really is only one generic method with `Visitor`, your generic pattern is more concise. – Rob Aug 19 '21 at 16:32

score 3 · Accepted Answer · answered Aug 19 '21 at 16:15

3

You can make the visit method generic: Accept any visitor whose associated type T is the fifo's type U:

protocol FiFo {
    associatedtype U
    func enqueue(_ : U)
    func dequeue() -> U
    func visit<V: Visitor>(_ visitor : V) where V.T == U
}

answered Aug 19 '21 at 16:15

Martin R

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generics in swift protocols

2 Answers2