Created with Copy to Gist

gist.java

interface Monoid<A> extends Empty<A> {
    A combine(A x, A y);

...

    static <A, B> Monoid<Pair<A, B>> deriveMonoidPair(Monoid<A> A, Monoid<B> B) {
        return new Monoid<>() {
            @Override
            public Pair<A, B> combine(Pair<A, B> x, Pair<A, B> y) {
                return new Pair<>(A.combine(x.getFirst(), y.getFirst()), B.combine(x.getSecond(), y.getSecond()));
            }

            @Override
            public Pair<A, B> empty() {
                return new Pair<>(A.empty(), B.empty());
            }
        };
    }
...
}