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Simple Free monad example
package me.panavtec
import cats.free.Free
import cats.{Id, ~>}
import scala.io.StdIn
object GuessTheSecretNumber {
sealed trait UserInteraction[A]
case object Ask extends UserInteraction[String]
case class Tell(what: String) extends UserInteraction[Unit]
case object ForgetWhatISaid extends UserInteraction[Unit]
implicit def liftFree[A](u: UserInteraction[A]): Free[UserInteraction, A] = Free.liftF(u)
def interpreter = new (UserInteraction ~> Id) {
override def apply[A](fa: UserInteraction[A]): Id[A] = fa match {
case Ask => StdIn.readLine
case Tell(what: String) => println(what)
case ForgetWhatISaid => println(Array.fill(100)("\n").foldLeft("")(_ + _))
}
}
type Input = Free[UserInteraction, String]
def introduceSecretNumber: Input = for {
_ <- Tell("Player One: Input the secret number")
secret <- Ask
_ <- ForgetWhatISaid
} yield secret
def guessSecretNumber: Input = for {
_ <- Tell("Player Two: What is the secret number?")
input <- Ask
} yield input
private def program = {
def untilUserDoesNotGuess(secret: String): (String => Boolean) = input => input != secret
def repeat[A[_], B](free: Free[A, B])(pred: B => Boolean): Free[A, B] =
free.flatMap(a => if (!pred(a)) Free.pure(a) else repeat(free)(pred))
for {
secret <- introduceSecretNumber
_ <- repeat(guessSecretNumber)(untilUserDoesNotGuess(secret))
} yield ()
}
def main(args: Array[String]): Unit = program.foldMap(interpreter)
}
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