Swift: cannot convert value of type 'SomeType<T>' to expected argument type 'SomeType<_>'

Question

I am using generics to build a paging library where I have code similar to this one (simplified):

struct PageData<T> {
    let item: T
    let pageNumber: Int
}

protocol PagedServiceProvider: class {
    func fetchPage<ItemType>(page: Int, completion: @escaping (PageData<ItemType>?) -> Void)
}


class TestPagedServiceProvider<ItemType>: PagedServiceProvider {
    var pageData: PageData<ItemType>?

    func fetchPage<ItemType>(page: Int, completion: @escaping (PageData<ItemType>?) -> Void) {
        completion(pageData) // COMPILATION ERROR HERE
    }
}

the call to completion(pageData) produces the following error:

Cannot convert value of type PageData<ItemType>? to expected argument type PageData<_>?

This workaround casting with as? gets rid of the error:

class TestPagedServiceProvider<ItemType>: PagedServiceProvider {
    var pageData: PageData<ItemType>?

    func fetchPage<ItemType>(page: Int, completion: @escaping (PageData<ItemType>?) -> Void) {
        completion(pageData as? PageData<ItemType>)
    }
}

Also if var pageData: PageData<ItemType>? is declared as a local variable inside the fetchPage<ItemType>... method, the error is gone as well.

NOTE: This gist provides additional context.

I am confused as to why the compiler requires an explicit cast (that looks redundant to me in this case).

Could it be a matter of placeholder types scope (the ItemType in the fetchPage<ItemType>... method is not the same as the ItemType for the TestPagedServiceProvider<ItemType> class)?

I tried giving them different names and using where clauses to no avail.

Answer 1

The following code worked for me.

struct PageData<T> {
    let item: T
    let pageNumber: Int
}

protocol PagedServiceProvider: class {
    associatedtype T
    var pageData: PageData<T>? { get set }
    func fetchPage(page: Int, completion: @escaping (PageData<T>?) -> Void)
}


class TestPagedServiceProvider: PagedServiceProvider {

    var pageData: PageData<String>?

    func fetchPage(page: Int, completion: @escaping (PageData<String>?) -> Void) {
        completion(pageData)
    }

}

Answer 2

I think the below code will work for you. I have passed String as the Generic type while instantiating the PageData object.

func fetchPage<String>(page: Int, completion: @escaping (PageData<String>?) -> Void) {

        let page = PageData<String>(item: "Item", pageNumber: 1)
        completion(page)
}

Answer 3

Your type signature says that the caller can pass any type as ItemType, but then you require the closure to accept a PageData<String>? .

fetchPage is not actually generic (it can't accept types chosen by the caller), so you can't claim it is. What you mean is to remove the <ItemType> and say what you require:

func fetchPage(page: Int, completion: @escaping (PageData<String>?) -> Void) {
              ^^                                          ^^^^^^

This, however, breaks your PagedServiceProvider protocol, because that protocol requires that TestPagedServiceProvider accept absolutely any ItemType, and it doesn't. You'll need to decide what you really mean. You might mean a protocol with an associated type here, which is a type that the implementation gets to pick rather than the caller.

protocol PagedServiceProvider: class {
    associatedtype ItemType
    func fetchPage(page: Int, completion: @escaping (PageData<ItemType>?) -> Void)
}

This opens up some headaches, however, since now PagedServiceProvider cannot easily be put in an Array (it's no longer a concrete type). How you deal with this depends heavily on what problem you're actually solving. There is no general solution that is always appropriate.