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Functional patterns for Java

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package com.jnape.palatable.lambda.monoid.builtin;

import com.jnape.palatable.lambda.adt.Maybe;
import com.jnape.palatable.lambda.functions.Fn1;
import com.jnape.palatable.lambda.functions.specialized.MonoidFactory;
import com.jnape.palatable.lambda.monoid.Monoid;
import com.jnape.palatable.lambda.semigroup.Semigroup;
import com.jnape.palatable.lambda.semigroup.builtin.Absent;

import static com.jnape.palatable.lambda.adt.Maybe.nothing;
import static com.jnape.palatable.lambda.monoid.Monoid.monoid;
import static com.jnape.palatable.lambda.monoid.builtin.First.first;

/**
 * A {@link Monoid} instance formed by {@link Maybe}<A> and a semigroup over A. The
 * application to two {@link Maybe} values is presence-biased, such that for a given {@link Maybe} x
 * and y:
 * 
 *  if x is present and y is absent, the result is x
 *  if x is absent, the result is y
 *  if both x and y are present, the result is the application of the x and y values in
 * terms of the provided semigroup, wrapped in {@link Maybe#just}
 * 
 *
 * @param  the Maybe value parameter type
 * @see Monoid
 * @see Absent
 * @see Maybe
 */
public final class Present implements MonoidFactory, Maybe> {

    private static final Present INSTANCE = new Present<>();

    private Present() {
    }

    @Override
    public Monoid> checkedApply(Semigroup aSemigroup) {
        return monoid((maybeX, maybeY) -> first(maybeX.fmap(x -> maybeY.fmap(aSemigroup.apply(x)).orElse(x)), maybeY),
                      nothing());
    }

    @SuppressWarnings("unchecked")
    public static  Present present() {
        return (Present) INSTANCE;
    }

    public static  Monoid> present(Semigroup semigroup) {
        return Present.present().apply(semigroup);
    }

    public static  Fn1, Maybe> present(Semigroup aSemigroup, Maybe x) {
        return present(aSemigroup).apply(x);
    }

    public static  Maybe present(Semigroup semigroup, Maybe x, Maybe y) {
        return present(semigroup, x).apply(y);
    }
}