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import scala.annotation.tailrec | |
import scala.language.higherKinds | |
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 abstract class Queue[U[_], A, B] { | |
def apply(a: A): Freer[U, B] = { | |
@tailrec | |
def go(tpe: { type T })(arrows: Queue[U, tpe.T, B], value: tpe.T): Freer[U, B] = | |
arrows.view match { | |
case One(f) => f(value) | |
case cons@Cons() => | |
cons.head(value) match { | |
case Pure(value) => go(new { type T = cons.T })(cons.tail, value) | |
case Impure(fa, f) => Impure(fa, f ++ cons.tail) | |
} | |
} | |
go(new { type T = A })(this, a) | |
} | |
def :+[C](f: B => Freer[U, C]): Queue[U, A, C] = Node(this, Leaf(f)) | |
def ++[C](that: Queue[U, B, C]): Queue[U, A, C] = Node(this, that) | |
def view: View[U, A, B] | |
} | |
case class Leaf[U[_], A, B](arrow: A => Freer[U, B]) extends Queue[U, A, B] { | |
lazy val view: View[U, A, B] = One(arrow) | |
} | |
sealed abstract case class Node[U[_], A, B]() extends Queue[U, A, B] { self => | |
type T | |
def left: Queue[U, A, T] | |
def right: Queue[U, T, B] | |
lazy val view: View[U, A, B] = { | |
@tailrec | |
def go[U[_], A, B](tpe: { type T })(left: Queue[U, A, tpe.T], right: Queue[U, tpe.T, B]): View[U, A, B] = | |
left match { | |
case Leaf(value) => Cons(value, right) | |
case node@Node() => go[U, A, B](new { type T = node.T })(node.left, Node(node.right, right)) | |
} | |
go(new { type T = self.T })(left, right) | |
} | |
} | |
object Node { | |
def apply[U[_], A, B, C](f: Queue[U, A, B], g: Queue[U, B, C]): Node[U, A, C] = | |
new Node[U, A, C] { | |
type T = B | |
val left: Queue[U, A, B] = f | |
val right: Queue[U, B, C] = g | |
} | |
} | |
sealed abstract class View[U[_], -A, +B] | |
case class One[U[_], A, B](arrow: A => Freer[U, B]) extends View[U, A, B] | |
sealed abstract case class Cons[U[_], A, B]() extends View[U, A, B] { | |
type T | |
def head: A => Freer[U, T] | |
def tail: Queue[U, T, B] | |
} | |
object Cons { | |
def apply[U[_], A, B, C](h: A => Freer[U, B], t: Queue[U, B, C]): Cons[U, A, C] = | |
new Cons[U, A, C] { | |
type T = B | |
val head: A => Freer[U, B] = h | |
val tail: Queue[U, B, C] = t | |
} | |
} |
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