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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.Either;
import com.jnape.palatable.lambda.functions.Fn1;
import com.jnape.palatable.lambda.functions.specialized.BiSemigroupFactory;
import com.jnape.palatable.lambda.functions.specialized.SemigroupFactory;
import com.jnape.palatable.lambda.monoid.Monoid;
import com.jnape.palatable.lambda.semigroup.Semigroup;

/**
 * A {@link Semigroup} instance formed by {@link Either#merge} and semigroups over L and R.
 * Like {@link Either#merge}, this is left-biased.
 * 
 * For the {@link Monoid}, see {@link com.jnape.palatable.lambda.monoid.builtin.Merge}.
 *
 * @param  The left parameter type
 * @param  The right parameter type
 * @see Monoid
 * @see Either#merge
 */
public final class Merge implements BiSemigroupFactory, Semigroup, Either> {

    private static final Merge INSTANCE = new Merge();

    private Merge() {
    }

    @Override
    public Semigroup> apply(Semigroup lSemigroup, Semigroup rSemigroup) {
        return (x, y) -> x.merge(lSemigroup::apply, rSemigroup::apply, y);
    }

    @SuppressWarnings("unchecked")
    public static  Merge merge() {
        return INSTANCE;
    }

    public static  SemigroupFactory, Either> merge(Semigroup lSemigroup) {
        return Merge.merge().apply(lSemigroup);
    }

    public static  Semigroup> merge(Semigroup lSemigroup, Semigroup rSemigroup) {
        return Merge.merge(lSemigroup).apply(rSemigroup);
    }

    public static  Fn1, Either> merge(Semigroup lSemigroup, Semigroup rSemigroup,
                                                               Either x) {
        return merge(lSemigroup, rSemigroup).apply(x);
    }

    public static  Either merge(Semigroup lSemigroup, Semigroup rSemigroup, Either x,
                                            Either y) {
        return merge(lSemigroup, rSemigroup, x).apply(y);
    }
}