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Free Monad example with Cats and Scalaz
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| import cats.free.Free | |
| import cats.~> | |
| import cats._, cats.std.all._ | |
| import cats.Functor | |
| // https://github.com/non/cats/blob/master/docs/src/main/tut/freemonad.md | |
| sealed trait Command[+Next] | |
| case class Person(name: String) extends AnyVal | |
| case class Push[Next](person: Person, next: Next) extends Command[Next] | |
| case class Pop[Next](onResult: Person => Next) extends Command[Next] | |
| object CatsFreeMonadExample { | |
| implicit val functor: Functor[Command] = new Functor[Command] { | |
| def map[A, B](fa: Command[A])(f: A => B): Command[B] = fa match { | |
| case Push(p, next) => Push(p, f(next)) | |
| case Pop(next) => { | |
| def func(person: Person): B = f(next(person)) | |
| Pop(func) | |
| } | |
| } | |
| } | |
| def push(p: Person): Free[Command, Unit] = Free.liftF(Push(p, ())) | |
| def pop: Free[Command, Person] = Free.liftF(Pop(identity)) | |
| val logic: Free[Command, Person] = for { | |
| _ <- push(Person("Alice")) | |
| _ <- push(Person("Bob")) | |
| person <- pop | |
| } yield person | |
| var stack = Seq.empty[Person] | |
| val invoker = new (Command ~> Id) { | |
| override def apply[A](e: Command[A]): Id[A] = e match { | |
| case Push(p, next) => { | |
| stack = Seq(p) ++ stack | |
| println(s"put $p") | |
| next | |
| } | |
| case Pop(next) => { | |
| val head = stack.head | |
| println(s"pop $head") | |
| stack = stack.tail | |
| next(head) | |
| } | |
| } | |
| } | |
| // To run the logic with invoker | |
| val result = logic.foldMap(invoker) | |
| println(result) | |
| } |
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| import scalaz.Free | |
| import scalaz._ | |
| import scalaz.syntax._ | |
| import scalaz.std.list._ | |
| import scalaz.syntax.traverse._ | |
| // http://eed3si9n.com/learning-scalaz/Free+Monad.html | |
| sealed trait Command[+Next] | |
| case class Person(name: String) extends AnyVal | |
| case class Push[Next](person: Person, next: Next) extends Command[Next] | |
| case class Pop[Next](onResult: Person => Next) extends Command[Next] | |
| object ScalazFreeMonadExample { | |
| implicit val commandFunctor: Functor[Command] = new Functor[Command] { | |
| def map[A, B](fa: Command[A])(f: A => B): Command[B] = fa match { | |
| case Push(p, next) => Push[B](p, f(next)) | |
| case Pop(next) => { | |
| def func(person: Person): B = f(next(person)) | |
| Pop[B](func) | |
| } | |
| } | |
| } | |
| def push(p: Person): Free[Command, Unit] = Free.liftF(Push(p, ())) // Next is Unit | |
| def pop: Free[Command, Person] = Free.liftF(Pop(identity)) // Next is Person | |
| val logic: Free[Command, Person] = for { | |
| _ <- push(Person("Alice")) | |
| _ <- push(Person("Bob")) | |
| person <- pop | |
| } yield person | |
| var stack = Seq.empty[Person] | |
| def invoker[A](p: Free[Command, A]): A = { | |
| p.resume.fold({ | |
| case Push(p, next) => { | |
| stack = Seq(p) ++ stack | |
| invoker(next) | |
| } | |
| case Pop(next) => { | |
| val head = stack.head | |
| println(s"pop $head") | |
| stack = stack.tail | |
| invoker(next(head)) | |
| } | |
| }, | |
| {r: A => r}) | |
| } | |
| // To run the logic with invoker | |
| val result = invoker(logic) | |
| println(result) | |
| } | |
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