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/*
 * Copyright 2014 Shape Security, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.shapesecurity.functional.data;

import com.shapesecurity.functional.Unit;

import org.jetbrains.annotations.NotNull;

public interface Monoid extends Semigroup {
  public static final UnitIdentity UNIT = new UnitIdentity();
  public static final IntegerAdditive INTEGER_ADDITIVE = new IntegerAdditive();
  public static final IntegerMultiplicative INTEGER_MULTIPLICATIVE = new IntegerMultiplicative();
  public static final StringConcat STRING_CONCAT = new StringConcat();

  @NotNull
  T identity();

  public static class UnitIdentity extends Semigroup.UnitIdentity implements Monoid {
    protected UnitIdentity() {
      super();
    }

    @NotNull
    @Override
    public final Unit identity() {
      return Unit.unit;
    }
  }

  public static class IntegerAdditive extends Semigroup.IntegerAdditive implements Monoid {
    protected IntegerAdditive() {
      super();
    }

    @NotNull
    @Override
    public final Integer identity() {
      return 0;
    }
  }

  public static class IntegerMultiplicative extends Semigroup.IntegerMultiplicative implements Monoid {
    protected IntegerMultiplicative() {
      super();
    }

    @NotNull
    @Override
    public final Integer identity() {
      return 1;
    }
  }

  public static class StringConcat extends Semigroup.StringConcat implements Monoid {
    protected StringConcat() {
      super();
    }

    @NotNull
    @Override
    public final String identity() {
      return "";
    }
  }
}