A.java

package pack1;

interface Eff<A, B> {
    B f(A a);
}

abstract class POne<A> {
    abstract A _1();

    static <A> POne<Hooray<A>> sequence(final Hooray<POne<A>> as) {
        return new POne<Hooray<A>>() {
            public Hooray<A> _1() {
                return as.map(POne.<A>__1());
            }
        };
    }

    static <A> Eff<POne<A>, A> __1() {
        return POne::_1;
    }

}

final class Hooray<A> {
    public <B> Hooray<B> map(final Eff<A, B> f) {
        return null;
    }
}