What is the correct syntax for calling a generic function that generates a publisher?

Question

What is the correct syntax of calling the following fetchURL function?

func fetchURL<T: Decodable>(_ url: URL) -> AnyPublisher<T, Error> {
     URLSession.shared.dataTaskPublisher(for: url)
    .map(\.data)
    .decode(type: T.self, decoder: JSONDecoder())
    .eraseToAnyPublisher()
}

I'm confused here.

let url = URL(string:"http://apple.com")
let publisher = fetchURL<[String].self>(url)????

Answer 1

You can't call a generic function by specifying its concrete type directly as you would, for example, with a struct - Swift needs to infer it.

Since T only appears in the return value, Swift can only infer its type based on the type that you're assigning the return value to, so you need to be explicit about it:

let publisher: AnyPublisher<[String], Error> = fetchURL(url)

This is rather inconvenient, so a better approach is to add a Type parameter as a function's argument, which Swift would now use to infer the concrete type T:

func fetchURL<T: Decodable>(_ t: T.Type, url: URL) -> AnyPublisher<T, Error> {
   // ...
}

let publisher = fetchURL([String].self, url: url)

^{For example, JSONDecoder.decode uses the same approach}

---

As suggested in comments, you can also specify a default value for the type, so you could omit it if the type can be otherwise inferred:

func fetchURL<T: Decodable>(_ t: T.Type = T.self, url: URL) -> AnyPublisher<T, Error> {
   // ...
}

let pub1: AnyPublisher<[String], Error> = fetchURL(url: url)
let pub2 = fetchURL([String].self, url: url)