How to recursively build up a json lens - scala

Question

I'm using json-lenses and trying to recursively search for a branch of a Json object, something akin to:
import spray.json._
import spray.json.lenses.JsonLenses._
import DefaultJsonProtocol._
import scala.annotation.tailrec
#tailrec
def getLens(json: JsValue, lensAcc: Lens = Nil, depth: Int = 0): Lens = {
if (depth > 10) {
throw new IllegalStateException("Recursive search for lens exhausted.")
}
val thisLens = lensAcc / 'rec / element(0)
json.extract[JsObject](thisLens / 'properties.?) match {
case None => getLens(json, thisLens, depth + 1)
case _ => thisLens
}
}
val json = """{"rec": [{"rec": [{"properties": {}}]}]}""".parseJson.asJsObject
val myLens = getLens(json)
which isn't quite right. I'm relatively new to Scala and can't figure out how to fix this.

Answer 1

This should work:
def nextLens = 'rec / element(0)
#scala.annotation.tailrec
final def getLens(json: JsObject, lensAcc: Lens[Id] = nextLens, depth: Int = 0): Lens[Id] = {
if (depth > 10) {
throw new IllegalStateException("Recursive search for lens exhausted.")
}
json.extract[JsObject](lensAcc / 'properties.?) match {
case None => getLens(json, lensAcc / nextLens, depth + 1)
case _ => lensAcc
}
}
Test:
scala> val json = JsonParser("""{"rec": [{"rec": [{"properties": {}}]}]}""").asJsObject
json: spray.json.JsObject = {"rec":[{"rec":[{"properties":{}}]}]}
scala> getLens(json)
res6: spray.json.lenses.Lens[spray.json.lenses.Id] = spray.json.lenses.JsonLenses$$anon$1#448cd79c
scala> json.extract[JsObject](res6)
res9: spray.json.JsObject = {"properties":{}}
I don't know if there is something like an empty lens...