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Scala example source code file (WriterUsage.scala)

This example Scala source code file (WriterUsage.scala) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" TM.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

address, baker, create, finished, holmes, list, logger, london, move, person, street, string, watson, writer

The WriterUsage.scala Scala example source code

package scalaz.example

object WriterUsage extends App {

  import scalaz._
  import std.list._
  import syntax.writer._
  import syntax.semigroup._ // for |+|
  import syntax.apply._     // for |@|

  type Logger[A] = Writer[List[String], A]

  case class Person(name: String, age: Int, address: Option[Address] = None)
  case class Address(street: String, city: String)

  val drWatson =
    for {
      watson <- Person("Watson", 40).set(List("Create Watson"))
      address <- Address("Baker Street", "London").set(List("Create address."))
      _ <- List("tell lets us log something in between. Writer[List[String], Unit]").tell
      moveWatson <- watson.copy(address = Some(address)).set(List("Move to new address."))
    } yield moveWatson

  // print log
  drWatson.written.map(println)

  // get value
  drWatson.value.map(println)

  val sherlockHolmes =
    for {
      holmes <- Person("Holmes", 40).set(List("Create Holmes"))
      address <- Address("Baker Street", "London").set(List("Create address."))
      moveHolmes <- holmes.copy(address = Some(address)).set(List("Move to new address."))
    } yield (moveHolmes)

  // map lets you map over the value side
  val mapValue: Logger[Option[Address]] = sherlockHolmes.map(x => x.address)
  mapValue.value.map(println)

  // with mapWritten you can map over the written side.
  val mapWritten: Logger[Person] = sherlockHolmes.mapWritten(_.map(entry => "[LOG] " + entry))
  mapWritten.written.map(println)

  // with mapValue you can map over both sides
  val mValue: Logger[Option[Address]] = sherlockHolmes.mapValue { case (log, p) => (log :+ "Extracting address", p.address) }
  mValue.written.map(println)

  // with :++> you can append to the log side of things
  val resultAppend: Logger[Person] = sherlockHolmes :++> List("Finished", "--- new Person ready ---")
  resultAppend.written.map(println)

  // with :++>> you can append using a function
  val resultFappend: Logger[Person] = sherlockHolmes :++>> { x => List("Finished", "--- new Person " + x + " ready ---") }
  resultFappend.written.map(println)

  // <++: and <<++: work like :++>, :++>> only to prepend information
  val resultPrepend: Logger[Person] = sherlockHolmes.<++:(List("Starting to create a Person"))
  resultPrepend.written.map(println)

  // reset your log to zero
  val logNoGood: Logger[Person] = sherlockHolmes.reset
  logNoGood.written.map(println)

  // Writer is an applicative, you can easily combine different results.
  val combined: Logger[List[Person]] = (sherlockHolmes |@| drWatson) { List(_) |+| List(_) }
  combined.written.map(println)
}

Other Scala examples (source code examples)

Here is a short list of links related to this Scala WriterUsage.scala source code file:

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