Polymorphic_Boolean.swift

class Trulean<T> {
    class Empty: Trulean<T> {
        override func onFalse(_ eval: @autoclosure () throws -> T) rethrows -> T {
            return try eval()
        }
    }

    class Full: Trulean<T> {
        let _value: T
        init(_ value: T) {
            _value = value
        }

        override func onFalse(_ eval: @autoclosure () throws -> T) rethrows -> T {
            return _value
        }
    }

    func onFalse(_ eval: @autoclosure () throws -> T) rethrows -> T {
        preconditionFailure()
    }
}

class Falsean<T> {
    class Empty: Falsean<T> {
        override func onTrue(_ eval: @autoclosure () throws -> T) rethrows -> T {
            return try eval()
        }
    }

    class Full: Falsean<T> {
        let _value: T
        init(_ value: T) {
            _value = value
        }

        override func onTrue(_ eval: @autoclosure () throws -> T) rethrows -> T {
            return _value
        }
    }

    func onTrue(_ eval: @autoclosure () throws -> T) rethrows -> T {
        preconditionFailure()
    }
}

extension Bool {

    func onTrue<T>(_ eval: @escaping @autoclosure () throws -> T) rethrows -> Trulean<T> {
        return self ? Trulean<T>.Full(try eval()) : Trulean<T>.Empty()
    }

    func onFalse<T>(_ eval: @escaping @autoclosure () throws -> T) rethrows -> Falsean<T> {
        return self ? Falsean<T>.Empty() : Falsean<T>.Full(try eval())
    }

    func onTrue<T>(_ eval: @escaping @autoclosure () throws -> T) rethrows -> T? {
        return self ? try eval() : nil
    }

    func onFalse<T>(_ eval: @escaping @autoclosure () throws -> T) rethrows -> T? {
        return self ? nil : try eval()
    }

}

let a = 1 > 0

let number: Int = a.onTrue(42).onFalse(0)

let optionalTrueString: String? = a.onTrue("Evaluated!")
let anotherOptionalString: String? = a.onFalse("Not evaluated")