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Functional patterns for Java

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

import com.jnape.palatable.lambda.adt.Maybe;
import com.jnape.palatable.lambda.functions.Fn1;
import com.jnape.palatable.lambda.functions.builtin.fn3.LiftA2;
import com.jnape.palatable.lambda.functions.specialized.SemigroupFactory;
import com.jnape.palatable.lambda.monoid.builtin.Present;
import com.jnape.palatable.lambda.semigroup.Semigroup;

/**
 * A {@link Semigroup} instance formed by {@link Maybe}<A> and a semigroup over A. The
 * application to two {@link Maybe} values is absence-biased, such that for a given {@link Maybe} x
 * and y:
 * 
 *  if x is absent, the result is x
 *  if x is present and y 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 Semigroup
 * @see Present
 * @see Maybe
 */
public final class Absent implements SemigroupFactory, Maybe> {

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

    private Absent() {
    }

    @Override
    public Semigroup> checkedApply(Semigroup aSemigroup) {
        return LiftA2., Maybe>liftA2(aSemigroup)::apply;
    }

    @SuppressWarnings("unchecked")
    public static  Absent absent() {
        return (Absent) INSTANCE;
    }

    public static  Semigroup> absent(Semigroup semigroup) {
        return Absent.absent().apply(semigroup);
    }

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

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