Loan Pattern in Swift

Using.swift

/// See also
/// - http://www.ne.jp/asahi/hishidama/home/tech/scala/sample/using.html
/// - https://qiita.com/piyo7/items/c9be1f39bcfea43a778a

protocol Closer {
    func close()
}

struct Loan {
    @discardableResult
    static func using<T: Closer, U>(resource: T, process: (T) throws -> U) -> U? {
        defer { resource.close() }
        do {
            return try process(resource)
        } catch {
            print("error: \(error)")
            return nil
        }
    }
}

/// ex.1

enum MyError: Error {
    case hoge(String)
}

class Source: Closer {
    func read(string: String) throws -> Int {
        if let number = Int(string) {
            return number
        } else {
            throw MyError.hoge("整数で入力して下さい")
        }
    }
    
    func close() {
        print("close")
    }
}

let source = Source()
Loan.using(resource: source) { try $0.read(string: "123a") }
// error: hoge("整数で入力して下さい")
// close

/// ex.2

class Indicator: Closer {
    func show() {
        print("show")
    }
    
    func dismiss() {
        print("dismiss")
    }
    
    func close() {
        dismiss()
    }
}

let indicator = Indicator()
Loan.using(resource: indicator) { $0.show() }
// show
// dismiss