Nanolibrary for a stateful pimp-my-library pattern.

stateful-pimp.scala

package object stateful {

  import scala.collection.mutable

  trait Stateful {
    private val cache = mutable.HashMap.empty[Int, Any]

    // TODO use some reasonable HashMap implementation here
    // TODO how slow is the reflection here, perform microbenchmarks?
    protected[stateful] def getStateOrElseUpdate(d: Decorate[_])(s: => d.State): d.State =
      cache.getOrElseUpdate(System.identityHashCode(d.getClass), s).asInstanceOf[d.State]
  }

  // For the companion object
  trait Decorate[T <: Stateful] {

    protected val self: T

    // TODO think about a reasonable name
    protected[stateful] type State
    protected def newState: State

    protected val state: State = self.getStateOrElseUpdate(this)(newState)
  }
}


object client {

  import stateful._

  class Foo(n: Int) extends Stateful {
    def bar = n
  }

  implicit class FooOps(protected val self: Foo) extends Decorate[Foo] {
    // can hide this neither inside pckg stateful, nor make it protected.
    // are we forced to leak the statetype???
    type State = FooState
    protected def newState = new FooState(self.bar)

    def inc = { state.x += 1; self }
    def get = state.x
  }
  class FooState(var x: Int)

  val f = new Foo(13)

  println(f.bar)

  println(f.inc.inc.inc.get)
}