dynamic scope emulation by reader monad

gistfile1.scala

import scalaz._
import Scalaz._
import util.continuations._
import collection.mutable.LinkedHashMap

object DynamicScopeEmulation extends App {

  //see also: https://gist.github.com/1338773
  object ReifyReflect {
    def reify[M[_] : Monad, A](body: => A @cps[M[A]]): M[A] =
      reset{val result: A = body; implicitly[Monad[M]].pure[A](result)}

    implicit def monad2reflect[M[_] : Monad, A](action: M[A]) = new {
      def reflect[B]: A @cps[M[B]] =
	shift{(k: A => M[B]) => implicitly[Monad[M]].bind(action, k)}
    }
  }

  object ReaderMonad {
    type Env = LinkedHashMap[String, Any]

    class Reader[A](f: Env => A) {
      def apply(env: Env): A = f(env)
    }

    implicit def readerMonad: Monad[Reader] = new Monad[Reader] {
      def pure[A](x: => A) = new Reader(_ => x)
      def bind[A, B](a: Reader[A], f: A => Reader[B]): Reader[B] = {
	new Reader[B](env => f(a(env))(env))
      }
    }

    def ask = new Reader(identity)
    def local[A](f: Env => Env, c: Reader[A]) =
      new Reader(e => c(f(e)))
  }

  //reference page: https://sites.google.com/site/scalajp/home/scala-tips/dynamic_variable
  object JsonBuilder {
    import ReifyReflect._
    import ReaderMonad._
    
    def json(x: => Env @cps[Reader[Env]]): Env = {
      reify(x).apply(LinkedHashMap())
    }

    class ArrowOperatorExtension(key: String) {
      def ->(value: Any): (String, Any) @cps[Reader[Env]] = {
	val result = (key, value)
        val status = ask.reflect[Env]
	status += result
	result
      }
    }
  
    def %(children: => (String, Any) @cps[Reader[Env]] ): Env @cps[Reader[Env]] = {
      local(_ => LinkedHashMap(), reify{
	children
	ask.reflect[Env]
      }).reflect[Env]
    }
  
    def $(elements: Any*): List[Any] = elements.toList

    implicit def string2ArrowOperatorExtension(key: String)
      : ArrowOperatorExtension = {
	new ArrowOperatorExtension(key)
      }
  }

  import JsonBuilder._

  val obj = json{%{
    "X" -> 1
    "Y" -> 2
    "Z" -> 4
    "nums" -> $(1, 2, 3, 4, 5,
      %{"hoge" -> 1; "foo" -> 2})
  }}

  println(obj)
}