MonadicOptional.java

import java.util.Optional;
import java.util.function.Function;

/**
 * A Gist prooving that Java 8's Optional type satisfies the Monadic laws
 *
 * Based on the idea originally by Marc Siegel <[email protected]>, at:
 * https://gist.github.com/ms-tg/7420496
 *
 * However, updated to the final/released version of Java 8 (Marcs version was based on pre-release
 * versions, which had no lambda notation, and some bugs in the JDK, which had to be worked around)
 *
 * In Java 8 Optional Monad:
 *    - of Value T (m T) = Optional<T>
 *    - return (T -> m T) is: Optional<T>.of()
 *    - bind (m T >>= (T -> m V) is : Optional<T>.flatmap()
 *
 * See:
 *  http://learnyouahaskell.com/a-fistful-of-monads#monad-laws
 *
 *  @author Jano Janahan <[email protected]>
 */
public class MonadicOptional {

    private static Function<Integer, Optional<String>> f =
        x -> Optional.of(x.toString());

    private static Function<String, Optional<String>> g =
        x -> Optional.of(x + " items");

    private static Function<Integer, Optional<String>> fThenG =
        i -> f.apply(i).flatMap(g);

    /**
     * Law 1: Left identity
     * The first monad law states that if we take a value, put it in a default context with return and
     * then feed it to a function by using >>=, it's the same as just taking the value and applying
     * the function to it. To put it formally:
     *
     *      return x >>= f is the same damn thing as f x
     */
    public static void law11LeftIdentity() {
        final Integer value = 2;
        Optional<String> lhs = Optional.of(value).flatMap(f);
        Optional<String> rhs = f.apply(value);
        System.out.println("Law 1: Left Identity : " + (lhs.equals(rhs) ? "applies": "does not apply"));
    }

    /**
     * Law 2: Right Identity
     * The second law states that if we have a monadic value and we use >>= to feed it to return,
     * the result is our original monadic value. Formally:
     *
     *     m >>= return is no different than just m
     */
    public static void law2RightIdentity() {
        Optional<String> monadicValue = Optional.of("A String");
        Optional<String> rhs = monadicValue.flatMap(Optional::of);
        System.out.println("Law 2: Right Identity : " + (monadicValue.equals(rhs) ? "applies": "does not apply"));
    }


    /**
     * Law 3: Associativity
     *
     * The final monad law says that when we have a chain of monadic function applications with >>=,
     * it shouldn't matter how they're nested. Formally written:
     *     Doing (m >>= f) >>= g is just like doing m >>= (\x -> f x >>= g)
     */
    public static void law3Associativity() {
        Optional<Integer> monadicValue = Optional.of(23);
        Optional<String> lhs = monadicValue.flatMap(f).flatMap(g);
        Optional<String> rhs = monadicValue.flatMap(fThenG);
        System.out.println("Law 3: Associativity : " + (lhs.equals(rhs) ? "applies": "does not apply"));
    }


    public static void main(String[] args) {
        law11LeftIdentity();
        law2RightIdentity();
        law3Associativity();
    }
}