WriterT examples

WriterT.scala

  // See http://stackoverflow.com/a/11951799

  // Just Writer:
  
  def calc1a: Writer[List[String], Int] = Writer("doing calc" :: Nil, 11)
  def calc1b: Writer[List[String], Int] = Writer("other calc" :: Nil, 22)
  
  val r1 = for {
    a <- calc1a
    b <- calc1b
  } yield a + b
  println(r1.run)
  // (List(doing calc, other calc),33)
  
  
  
  // Writer inside Option:
  
  def calc2a: WriterT[Option, List[String], Int] = WriterT((("doing calc" :: Nil) -> 11).point[Option])
  def calc2b: WriterT[Option, List[String], Int] = WriterT((("other calc" :: Nil) -> 22).point[Option])
  //def calc2a: WriterT[Option, List[String], Int] = WriterT(Some((("doing calc" :: Nil) -> 11)))
  //def calc2b: WriterT[Option, List[String], Int] = WriterT(None: Option[(List[String], Int)])
  val r2 = for {
    a <- calc2a
    b <- calc2b
  } yield a + b
  println(r2.run)
  // Some((List(doing calc, other calc),33))
  
  
  
  // Writer outside Option:
  
  type Logger[A] = Writer[List[String], A]  // source had Logger[+A]
  def calc3a = OptionT[Logger, Int](Writer("doing calc" :: Nil, Some(11)))
  def calc3b = OptionT[Logger, Int](Writer("other calc" :: Nil, None))

  val r3 = for {
    a <- calc3a
    b <- calc3b
  } yield a + b
  println(r3.run.run)
  // (List(doing calc, other calc),None)