CODEWITHSUNDEEP
swiftgenerics
smartin
smartin

Reputation: 25

Declare a function in Swift that returns a closure with a generic T.Type

we are trying to make a function in Swift , that returns a closure. The return type is as follows:

func completionBlock() -> (Response<T, NSError>) -> ()

where Response itself is a struct with 2 generics. The second will be always NSError, but the first depends on a switch.

Declaration of Response:

public struct Response<Value, Error: ErrorType> { ...

What we want to achieve is a function that will return a closure with a type that depends on a value of variable. Depending on this value, and with a switch, it specifies one type or another, the only requirement for the generic parameter is that it must conform to a specific protocol, but we can't find a solution.

What we got so far is:

class BaseClass {
    var endpoint: Int
    ... 

    func completionBlock() -> (Response<T, NSError>) -> () {

        switch endpoint
        {
        case 1:
            return getHandleResponseClosure(Car.self)
        case 2:
            return getHandleResponseClosure(Truck.self)
        }
    }

    func getHandleResponseClosure<T: Mappable>(_: T.Type) -> (Response<T, NSError>) -> () {

        let closure = { (_: Response<T, NSError>) -> () in
            // ...
        }
        return closure
    }

}

But this don't compile: "Use of undeclared Type T" at func completionBlock(), the type at this point is generic, it only requires to conform to protocol Mappable, but we don't know how to specify this.

Thanks!!, Greetings

Upvotes: 1

Views: 971

Answers (1)

Gregory Higley
Gregory Higley

Reputation: 16558

While there are other problems with your code, the fundamental one is that generic types in method signatures must be declared directly after the name of the method. Take a look at completionBlock below. This code compiles and works.

struct Response<T, E: Error> {

}

struct Car {}
struct Truck {}

class BaseClass {

    func completionBlock<T>() -> ((Response<T, NSError>) -> ()) {
        return { response in /* What goes here? */ }
    }

}

let c = BaseClass()
let r: (Response<Car, NSError>) -> () = c.completionBlock()

On a side note, I would replace struct Response<T, E: Error> with…

enum Response<T> {
case success(T)
case error(Error)
}

Upvotes: 0

Related Questions