Created
May 2, 2017 19:40
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Free Monad
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| import scalaz.Monad | |
| import scalaz._ | |
| sealed trait Free[F[_], A] { | |
| def pure(a: A): Free[F, A] = Free.Pure(a) | |
| def lift(f: F[A]): Free[F, A] = Free.Suspend(f) | |
| def flatMap[B](f: A => F[B]): Free[F, B] = Free.Bind[F, A, B](this, f) | |
| def foldMap[G[_] : Monad, E](nt: F ~> G): G[A] = this match { | |
| case Free.Pure(a) => Monad[G].pure(a) | |
| case Free.Suspend(fa) => nt(fa) | |
| case bind: Free.Bind[F, E, A] => // in order to fix the types, otherwise Bind[F, Any] | |
| Monad[G].bind(bind.target.foldMap(nt)) { | |
| (e: E) => lift(bind.f(e)).foldMap(nt) | |
| } | |
| } | |
| } | |
| object Free { | |
| final case class Pure[F[_], A](a: A) extends Free[F, A] | |
| final case class Suspend[F[_], A](fa: F[A]) extends Free[F, A] | |
| final case class Bind[F[_], A, B](target: Free[F, A], f: A => F[B]) extends Free[F, B] | |
| } |
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