com.github.tonivade.purefun.instances.WriterTInstances 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.Kind;
import com.github.tonivade.purefun.transformer.WriterT;
import com.github.tonivade.purefun.typeclasses.Monad;
import com.github.tonivade.purefun.typeclasses.Monoid;

public interface WriterTInstances {
  static  Monad, L>> monad(Monoid monoid, Monad monadF) {
    requireNonNull(monoid);
    requireNonNull(monadF);
    return new WriterTMonad() {

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

      @Override
      public Monad monadF() { return monadF; }
    };
  }
}

@Instance
interface WriterTMonad extends Monad, L>> {

  Monad monadF();
  Monoid monoid();

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

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