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@paulp
Created June 3, 2016 23:19
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Another in the endless parade.
package p
/** Super basic map/flatMap fusion ADT.
*/
sealed trait View[A] {
def map[B](f: A => B): View[B] = CView(this, Mapped(f))
def flatMap[B](f: A => View[B]): View[B] = CView(this, FlatMap(f))
def foreach(f: A => Unit): Unit = this match {
case IdView(xs) => xs foreach f
case Optimize(xs) => xs foreach f
case CView(xs, Mapped(g)) => xs foreach (g andThen f)
case CView(xs, FlatMap(g)) => xs foreach (x => g(x) foreach f)
}
}
final case class IdView[A, B](xs: Vector[A]) extends View[A]
final case class CView[A, B](prev: View[A], op: Op[A, B]) extends View[B]
sealed trait Op[A, B]
final case class FlatMap[A, B](f: A => View[B]) extends Op[A, B]
final case class Mapped[A, B](f: A => B) extends Op[A, B]
/** Super simple peephole optimizer.
*
* Can you spot the type error? Don't worry, neither can Scala.
*/
object Optimize {
def unapply[A](v: View[A]): Option[View[A]] = v match {
case CView(CView(xs, Mapped(f)), Mapped(g)) => Some(CView(xs, Mapped(f andThen g)))
case CView(CView(xs, FlatMap(f)), FlatMap(g)) => Some(CView(xs, FlatMap(f andThen g)))
case _ => None
}
}
/** No warnings, just throw the ClassCastException.
*/
object Run {
def view[A](xs: A*): View[A] = IdView(xs.toVector)
def main(argv: Array[String]): Unit = {
view(1, 2, 3) map (_ + 1) map (_ + 1) foreach println // ok
view(1, 2, 3) flatMap (x => view(x)) foreach println // ok
// java.lang.ClassCastException: p.IdView cannot be cast to java.lang.Integer
view(1, 2, 3) flatMap (x => view(x)) flatMap (x => view(x)) foreach println
}
}
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