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

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/*
 * Copyright (c) 2018-2020, Antonio Gabriel Muñoz Conejo 
 * Distributed under the terms of the MIT License
 */
package com.github.tonivade.purefun.instances;

import static com.github.tonivade.purefun.Precondition.checkNonNull;
import com.github.tonivade.purefun.Function1;
import com.github.tonivade.purefun.Kind;
import com.github.tonivade.purefun.monad.Writer;
import com.github.tonivade.purefun.monad.WriterOf;
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) {
    return WriterMonad.instance(checkNonNull(monoid));
  }
}

interface WriterMonad extends Monad> {

  static  WriterMonad instance(Monoid monoid) {
    return () -> monoid;
  }

  Monoid monoid();

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

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