final class Lazy<T> {
    private(set) lazy var value: T = self.closure()
    private let closure: () -> T

    init(_ closure: @escaping @autoclosure () -> T) {
        self.closure = closure
    }

    func map<U>(transform: @escaping (T) -> U) -> Lazy<U> {
        return Lazy<U>(transform(self.value))
    }

    func flatMap<U>(transform: @escaping (T) -> Lazy<U>) -> Lazy<U> {
        return transform(self.value)
    }
}

// MARK: - Usage

let a = Lazy(42)
    .map { $0 + 10 }
    .flatMap { value in Lazy(String(value)) }
    .value //nothing is evaluated until this line

print(a)