Created
December 26, 2015 02:23
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import scala.language.higherKinds | |
import scala.language.existentials | |
sealed trait Freer[F[_], A] { | |
def map[B](f: A => B): Freer[F, B] = flatMap(a => Pure(f(a))) | |
def flatMap[B](f: A => Freer[F, B]): Freer[F, B] = | |
this match { | |
case Pure(a) => f(a) | |
case Impure(fa, g) => Impure(fa, g :+ f) | |
} | |
} | |
case class Pure[F[_], A](a: A) extends Freer[F, A] | |
case class Impure[F[_], A, B](fa: F[A], f: Queue[F, A, B]) extends Freer[F, B] | |
sealed trait Queue[F[_], A, B] { | |
def :+[C](f: B => Freer[F, C]): Queue[F, A, C] = Node(this, Leaf(f)) | |
def ++[C](q: Queue[F, B, C]): Queue[F, A, C] = Node(this, q) | |
def apply(a: A): Freer[F, B] = { | |
@scala.annotation.tailrec | |
def go(q_a: (Queue[F, T, B], T) forSome { type T }): Freer[F, B] = | |
q_a match { | |
case (q, a) => | |
q.view match { | |
case One(f) => f(a) | |
case Cons(f, q) => | |
f(a) match { | |
case Pure(v) => go(q, v) | |
case Impure(f, r) => Impure(f, r ++ q) | |
} | |
} | |
} | |
go(this, a) | |
} | |
def view: View[F, A, B] = | |
this match { | |
case Leaf(f) => One(f) | |
case Node(l, r) => | |
@scala.annotation.tailrec | |
def go(x_v: (Queue[F, A, T], Queue[F, T, B]) forSome { type T }): View[F, A, B] = | |
x_v match { | |
case (Leaf(f), y) => Cons(f, y) | |
case (Node(l, r), y) => go(l -> Node(r, y)) | |
} | |
go(l -> r) | |
} | |
} | |
case class Leaf[F[_], A, B](f: A => Freer[F, B]) extends Queue[F, A, B] | |
case class Node[F[_], A, B, C](left: Queue[F, A, B], right: Queue[F, B, C]) extends Queue[F, A, C] | |
sealed trait View[F[_], A, B] | |
case class One[F[_], A, B](f: A => Freer[F, B]) extends View[F, A, B] | |
case class Cons[F[_], A, B, C](f: A => Freer[F, B], q: Queue[F, B, C]) extends View[F, A, C] |
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