Add applicative `fmap` and monadic `bind` to Rust's builtin enums

monadic_builtins.rs

trait MonadicOpt<T> {
    fn fmap<U>(self, f: |T| -> U) -> Option<U>;
    fn bind<U>(self, f: |T| -> Option<U>) -> Option<U>;
}

impl<T> MonadicOpt<T> for Option<T> {
    fn fmap<U>(self, f: |T| -> U) -> Option<U> {
        match self {
            Some(x) => Some(f(x)),
            _       => None,
        }
    }

    fn bind<U>(self, f: |T| -> Option<U>) -> Option<U> {
        match self {
            Some(x) => f(x),
            _       => None,
        }
    }
}

trait MonadicRes<T,E> {
    fn fmap<U>(self, f: |T| -> U) -> Result<U,E>;
    fn bind<U>(self, f: |T| -> Result<U,E>) -> Result<U,E>;
}

impl<T,E> MonadicRes<T,E> for Result<T,E> {
    fn fmap<U>(self, f: |T| -> U) -> Result<U,E> {
        match self {
            Ok(x)  => Ok(f(x)),
            Err(e) => Err(e),
        }
    }

    fn bind<U>(self, f: |T| -> Result<U,E>) -> Result<U,E> {
        match self {
            Ok(x)  => f(x),
            Err(e) => Err(e),
        }
    }
}

// ============================
// Example of using Option bind
// ============================

fn increment(n: int) -> Option<int> {
    if n < 10 {
      Some(n + 1)
    } else {
      None
    }
}

fn display(n: int) -> Option<String> {
    Some(n.to_string())
}

fn main() {
    Some(1).bind(increment).bind(display);  // => 2
    Some(10).bind(increment).bind(display); // => None
    None.bind(increment).bind(display);     // => None
}