networking.swift

import Foundation

struct Resource<A> {
    var request: URLRequest
    var parse: (Data) throws -> A
}

extension Resource where A: Decodable {
    init(get url: URL) {
        self.init(request: URLRequest(url: url)) { data in
            try JSONDecoder().decode(A.self, from: data)
        }
    }
}

extension URLSession {
    func load<A>(_ r: Resource<A>, callback: @escaping (Result<A, Error>) -> ()) {
        dataTask(with: r.request) { data, response, err in
            callback(Result {
                if let e = err { throw e }
                guard let d = data else { fatalError() }
                return try r.parse(d)
            })
        }.resume()
    }
}

struct Country: Codable {
    var alpha2Code: String
    var name: String
    var population: Int
}

let sample = Resource<[Country]>(get: URL(string: "https://restcountries.eu/rest/v2/all")!)
URLSession.shared.load(sample) { res in print(res) }