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// Make Optional a monad
extension Optional {
    // Scala style, define `flatMap` directly
    func flatMap<U>(f: (a: T) -> Optional<U>) -> Optional<U> {
        switch (self) {
            case .None: return nil
            case .Some(let value): return f(a: value)
        }
    }

    // Haskell style, define `join` and use existing `map` implementation (`fmap` in Haskell terms)
    static func join<U>(s: Optional<Optional<U>>) -> Optional<U> {
        switch (s) {
            case .None: return nil;
            case .Some(let value): return value;
        }
    }

    func bind<U>(f: (a: T) -> Optional<U>) -> Optional<U> {
        return Optional.join(self.map(f))
    }
}