Non-recursive extraction in Lift JSON for-comprehension

Question

I'm using Lift JSON's for-comprehensions to parse some JSON. The JSON is recursive, so e.g. the field id exists at each level. Here is an example:

val json = """
{
  "id": 1
  "children": [
    {
      "id": 2
    },
    {
      "id": 3
    }
  ]
}
"""

The following code

var ids = for {
  JObject(parent) <- parse(json)
  JField("id", JInt(id)) <- parent
} yield id

println(ids)

produces List(1, 2, 3). I was expecting it to product List(1).

In my program this results in quadratic computation, though I only need linear.

Is it possible to use for-comprehensions to match the top level id fields only?

Answer 1

I haven't delved deep enough to figure out why the default comprehension is recursive, however you can solve this by simply qualifying your search root:

scala>  for ( JField( "id", JInt( id ) ) <- parent.children ) yield id
res4: List[BigInt] = List(1)

Note the use of parent.children.