General recursion with Elgot algebras

Hylomorphism.java

import com.jnape.palatable.lambda.adt.Either;
import com.jnape.palatable.lambda.functions.Fn3;
import com.jnape.palatable.lambda.functor.Functor;
import com.jnape.palatable.lambda.functor.builtin.Identity;

import java.util.function.Function;

import static com.jnape.palatable.lambda.adt.Either.left;
import static com.jnape.palatable.lambda.adt.Either.right;

public final class Hylomorphism<A, B, FA extends Functor<A>, FB extends Functor<B>>
        implements Fn3<Function<? super FB, ? extends B>, Function<? super A, ? extends FA>, A, B> {

    @Override
    @SuppressWarnings("unchecked")
    public B apply(Function<? super FB, ? extends B> phi, Function<? super A, ? extends FA> psi, A a) {
        return phi.apply((FB) psi.apply(a).fmap(apply(phi, psi)));
    }

    public static final class Elgot<A, B, FA extends Functor<A>, FB extends Functor<B>>
            implements Fn3<Function<? super FB, ? extends B>, Function<? super A, ? extends Either<B, FA>>, A, B> {

        @Override
        @SuppressWarnings("unchecked")
        public B apply(Function<? super FB, ? extends B> phi, Function<? super A, ? extends Either<B, FA>> psi, A a) {
            return psi.apply(a).match(b -> b, fa -> phi.apply((FB) fa.fmap(apply(phi, psi))));
        }

        public static <A, B, FA extends Functor<A>, FB extends Functor<B>> Elgot<A, B, FA, FB> elgot() {
            return new Elgot<>();
        }

        public static void main(String[] args) {
            Elgot<String, Integer, Identity<String>, Identity<Integer>> elgot = elgot();
            Integer r = elgot.apply(Identity::runIdentity, s -> s.length() > 3 ? left(s.length()) : right(new Identity<>(s + "+"))).apply("");
            System.out.println(r); // prints "4"
        }
    }
}