com.github.tonivade.purefun.instances.WriterInstances Maven / Gradle / Ivy

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/*
 * Copyright (c) 2018-2019, Antonio Gabriel Muñoz Conejo 
 * Distributed under the terms of the MIT License
 */
package com.github.tonivade.purefun.instances;

import static java.util.Objects.requireNonNull;

import com.github.tonivade.purefun.Function1;
import com.github.tonivade.purefun.Higher1;
import com.github.tonivade.purefun.Instance;
import com.github.tonivade.purefun.monad.Writer;
import com.github.tonivade.purefun.typeclasses.Monad;
import com.github.tonivade.purefun.typeclasses.Monoid;

public interface WriterInstances {

  static  Monad> monad(Monoid monoid) {
    requireNonNull(monoid);
    return new WriterMonad() {

      @Override
      public Monoid monoid() { return monoid; }
    };
  }
}

@Instance
interface WriterMonad extends Monad> {

  Monoid monoid();

  @Override
  default  Writer pure(T value) {
    return Writer.pure(monoid(), value);
  }

  @Override
  default  Writer flatMap(Higher1, T> value,
      Function1, R>> map) {
    return Writer.narrowK(value).flatMap(map.andThen(Writer::narrowK));
  }
}