Code koan: Trivial extension to `URLSession` for decoding responses with `Decodable` in Swift 4

URLSession+Decodable.swift

import Foundation

public protocol DataDecoder {
    func decode<T>(_ type: T.Type, from data: Data) throws -> T where T : Decodable
}

extension JSONDecoder: DataDecoder {}
extension PropertyListDecoder: DataDecoder {}

extension URLSession {
    public func dataTask<T>(with request: URLRequest,
                            decodedAs decodableType: T.Type,
                            using decoder: DataDecoder,
                            completionHandler: @escaping (T?, URLResponse?, Error?) -> Void
        ) -> URLSessionDataTask where T: Decodable {
        return self.dataTask(with: request, completionHandler: { (data, response, error) in
            var decoded: T? = nil
            if let data = data {
                // TODO: don't drop this error
                decoded = try? decoder.decode(decodableType, from: data)
            }
            completionHandler(decoded, response, error)
        })
    }
}

usage.swift

import Foundation

struct Book: Codable {
    let author: String
    let title: String
    let yearReleased: Int
}

let request: URLRequest! = nil
let task = URLSession.shared.dataTask(with: request, decodedAs: Book.self, using: decoder) {
    book, _, _ in
    print(book ?? "nil")
}