Writer Monad example 1

writer-monad-example-1.scala

import cats.data.Writer
import cats.instances.vector._ // for Monoid
import cats.syntax.applicative._ // for pure
import cats.syntax.writer._
import scala.concurrent._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._

type Logged[A] = Writer[Vector[String], A]


/**
  * Example of using Writer Monad to produce seperate logs for two different computations where the logs would otherwise
  * be interleaved
  */
def slowly[A](body: => A) =
  try body finally Thread.sleep(100)

def factorial(n: Int): Logged[Int] = {
  for {
    ans <- if(n == 0) {
      1.pure[Logged]
    } else {
      slowly(factorial(n - 1).map(_ * n))
    }
    _ <- Vector(s"fact $n $ans").tell
  } yield ans
}

val Vector((logA, ansA), (logB, ansB)) =
  Await.result(Future.sequence(Vector(
    Future(factorial(3).run),
    Future(factorial(5).run)
  )), 5.seconds)

// logA: Vector[String] = Vector(fact 0 1, fact 1 1, fact 2 2, fact 3 6)
// ansA: Int = 6
// logB: Vector[String] = Vector(fact 0 1, fact 1 1, fact 2 2, fact 3 6, fact 4 24, fact 5 120)
// ansB: Int = 120