Cats Eval example

Ev.scala

import cats.Eval

object Ev extends App{

   // eager, method call
  val always: Eval[Int] =Eval.always{
    println("running always")
    123
  }
    // lazy, memoized
    // at most once
  val later: Eval[Int] =Eval.later{
    println("running later")
    124
  }
    // super eager, value. Note this outputs even before "About to run"
    // exactly once
  val now: Eval[Int] =Eval.now{
    println("running now")
    125
  }

  println("About to run")

  println(always.value)
  println(always.value)

  println(later.value)
  println(later.value)

  println(now.value)
  println(now.value)

  println("about to memoize")
  val memo: Eval[Int] =always.memoize
  println("memoized")

  println(memo.value)
  println(memo.value)

  object MutualRecursion {
    def even(n: Int): Eval[Boolean] = {
      println(s"even $n")
      Eval.always(n == 0).flatMap {
        case true => Eval.True
        case false => odd(n - 1)
      }
    }

    def odd(n: Int): Eval[Boolean] = {
      println(s"odd $n")
      Eval.always(n == 0).flatMap {
        case true => Eval.False
        case false => even(n - 1)
      }
    }
  }

  // stack safe, n could be massive, would still not blow stack
  println(MutualRecursion.even(4).value)

}

ouput.txt

running now
About to run
running always
123
running always
123
running later
124
124
125
125
about to memoize
memoized
running always
123
123
even 4
odd 3
even 2
odd 1
even 0
true