Comma separated list with Enumerator - json

Question

I've just started working with Scala in my new project (Scala 2.10.3, Play2 2.2.1, Reactivemongo 0.10.0), and encountered a pretty standard use case, which is - stream all the users in MongoDB to the external client. After navigating Enumerator, Enumeratee API I have not found a solid solution for that, and so I solved this in following way:
val users = collection.find(Json.obj()).cursor[User].enumerate(Integer.MAX_VALUE, false)
var first:Boolean = true
val indexedUsers = (users.map(u => {
if(first) {
first = false;
Json.stringify(Json.toJson(u))
} else {
"," + Json.stringify(Json.toJson(u))
}
}))
Which, from my point of view, is a little bit tricky - mainly because I needed to add Json Start Array, Json End Array and comma separators in element list, and I was not able to provide it as a pure Json stream, so I converted it to String steam.
What is a standard solution for that, using reactivemongo in play?

Answer 1

I wrote a helper function which does what you want to achieve:
def intersperse[E](e: E, enum: Enumerator[E]): Enumerator[E] = new Enumerator[E] {
val element = Input.El(e)
override def apply[A](i1: Iteratee[E, A]): Future[Iteratee[E, A]] = {
var iter = i1
val loop: Iteratee[E, Unit] = {
lazy val contStep = Cont(step)
def step(in: Input[E]): Iteratee[E, Unit] = in match {
case Input.Empty ⇒ contStep
case Input.EOF ⇒ Done((), Input.Empty)
case e # Input.El(_) ⇒
iter = Iteratee.flatten(iter.feed(element).flatMap(_.feed(e)))
contStep
}
lazy val contFirst = Cont(firstStep)
def firstStep(in: Input[E]): Iteratee[E, Unit] = in match {
case Input.EOF ⇒ Done((), Input.Empty)
case Input.Empty ⇒
iter = Iteratee.flatten(iter.feed(in))
contFirst
case Input.El(x) ⇒
iter = Iteratee.flatten(iter.feed(in))
contStep
}
contFirst
}
enum(loop).map { _ ⇒ iter }
}
}
Usage:
val prefix = Enumerator("[")
val suffix = Enumerator("]")
val asStrings = Enumeratee.map[User] { u => Json.stringify(Json.toJson(u)) }
val result = prefix >>> intersperse(",", users &> asStrings) >>> suffix
Ok.chunked(result)