Does JDK8's CompletableFuture class satisfy the Monad laws? Yes, it does.

jdk8_completablefuture_monad_laws.java

/**
 * ```
 * Does JDK8's CompletableFuture class satisfy the Monad laws?
 * =================================================
 *   1.  Left identity:  true
 *   2.  Right identity: true
 *   3.  Associativity:  true
 *
 * Yes, it does.
 * ```
 *
 * For more info on the monad laws, see:
 *   [1] http://learnyouahaskell.com/a-fistful-of-monads#monad-laws
 *   [2] http://eed3si9n.com/learning-scalaz/Monad+laws.html
 *   [3] http://en.wikipedia.org/wiki/Monad_(functional_programming)#Monad_laws
 *   
 * Note: The original code for this tests monad laws of Optional class. 
 * See: https://gist.github.com/ms-tg/7420496
 *
 * @author Marc Siegel <[email protected]>, Manuel Mauky <[email protected]>
 */
import java.util.concurrent.CompletableFuture;
import java.util.function.Function;

class jdk8_completablefuture_monad_laws {
    public static void main (String[] args) throws java.lang.Exception
    {
        System.out.println("");
        System.out.println("Does JDK8's CompletableFuture class satisfy the Monad laws?");
        System.out.println("=================================================");
        System.out.println("  1.  Left identity:  " + satisfiesLaw1LeftIdentity());
        System.out.println("  2.  Right identity: " + satisfiesLaw2RightIdentity());
        System.out.println("  3.  Associativity:  " + satisfiesLaw3Associativity());
        System.out.println("");
        System.out.println(satisfiesLaw1LeftIdentity()
                           && satisfiesLaw2RightIdentity()
                           && satisfiesLaw3Associativity()
                           ? "Yes, it does."
                           : "No, it doesn't.");
    }

    // Input values for the monad law tests below
    static int value = 42;
    static CompletableFuture<Integer> monadicValue = CompletableFuture.completedFuture(value);

    static Function<Integer, CompletableFuture<Integer>> f = n -> CompletableFuture.completedFuture(n * 2);

    static Function<Integer, CompletableFuture<Integer>> g = n -> CompletableFuture.completedFuture(n * 5);

    static Function<Integer, CompletableFuture<Integer>> f_flatMap_g = n -> f.apply(n).thenCompose(g);

    /**
     * Monad law 1, Left Identity
     *
     * From LYAHFGG [1] above:
     *   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
     */
    public static boolean satisfiesLaw1LeftIdentity() throws Exception {
        return CompletableFuture.completedFuture(value).thenCompose(f).get().equals(f.apply(value).get());
    }

    /**
     * Monad law 2, Right Identity
     *
     * From LYAHFGG [1] above:
     *   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.
     */
    public static boolean satisfiesLaw2RightIdentity() throws Exception {
        return monadicValue.thenCompose(CompletableFuture::completedFuture).get().equals(monadicValue.get());
    }

    /**
     * Monad law 3, Associativity
     *
     * From LYAHFGG [1] above:
     *   The final monad law says that when we have a chain of monadic function
     *   applications with >>=, it shouldn’t matter how they’re nested.
     */
    public static boolean satisfiesLaw3Associativity()  throws Exception {
        return monadicValue.thenCompose(f).thenCompose(g).get().equals(monadicValue.thenCompose(f_flatMap_g).get());
    }
}