A monad which captures dynamic method invocations against multiple targets (for use in implementing object builders / matchers)

RecorderSpec.scala

package com.youdevise.lofty

import org.specs2.mutable._
import scala.language.dynamics

trait State[S, A] {
  val runState: Function[S, (S, A)]
  def flatMap[B](f: A => Function[S, (S, B)]):Function[S, (S, B)] = (state:S) => {
    val (newState, value) = runState(state)
    f(value)(newState)
  }

  def map[B](f: A => B): Function[S, (S, B)] = flatMap { (a:A) => (s:S) => (s, f(a)) }
}

case class RecordedCall(target:RecorderTarget, methodName:String, parameters:Seq[Any])

case class Recording(val calls:Seq[RecordedCall])

object Recording {

  val emptyRecording = Recording(Seq.empty[RecordedCall])

  def newTarget(): State[Recording, RecorderTarget] = new State[Recording, RecorderTarget] {
    val runState: Function[Recording, (Recording, RecorderTarget)] = (recording:Recording) => (recording, new RecorderTarget())
  }
}

import Recording._

class RecorderTarget() extends Dynamic {
  def applyDynamic(name:String)(args:Any*):State[Recording, RecorderTarget] = {
    val target = this
    new State[Recording, RecorderTarget] {
      val runState: Function[Recording, (Recording, RecorderTarget)] = (recording:Recording) => (
        new Recording(recording.calls :+ RecordedCall(target, name, args.toSeq)),
        target)
    }
  }
}

class RecorderSpec extends Specification {

  """A recorder""" should {
    """Capture all method calls within the recorder monad""" in {
      val recorder = for {
        t1 <- newTarget()
        _  <- t1.hey()
        _  <- t1.iJustMet("you")
        _  <- t1.andThis("is", "crazy")
        t2 <- newTarget()
        _  <- t2.receive(t1)
      } yield (t1, t2)

      val (recording:Recording, (t1: RecorderTarget, t2: RecorderTarget)) = recorder(emptyRecording)

      recording.calls must beEqualTo(Seq(RecordedCall(t1, "hey", Seq()),
                                         RecordedCall(t1, "iJustMet", Seq("you")),
                                         RecordedCall(t1, "andThis", Seq("is", "crazy")),
                                         RecordedCall(t2, "receive", Seq(t1))))
    }
  }

}