read Json into Scalaz Tree - scala

Question

Scala newbie here.
I use Play to provide a json API for reading and writing a directory like structure. Therefore I use Scalaz.Tree, which provides ways of traversing, updating and rebuilding the Tree.
Formatting the Tree into json works well:
case class File(id: String = BSONObjectID.generate.toString(), name: String, noteBookId: String = null)
implicit val fileFormat: Format[File] = Json.format[File]
implicit def treeWrites: Writes[Tree[File]] =
new Writes[Tree[File]] {
def writes(o: Tree[File]) = o match {
case Node(file, children) => Json.obj(
"name" -> file.name,
"id" -> file.id,
"children" -> JsArray(children.map(Json.toJson(_))),
"notebookId" -> file.noteBookId
)
}
}
Reading json into a Tree however, fails
implicit def treeReads: Reads[Tree[File]] = (
//(__ \ "children").lazyRead(Reads.seq[File](treeReads)) and
(__ \ "children").read[Tree[File]] and
(__ \ "name").read[String] and
(__ \ "notebookid").read[String] and // <-- this is line 41, where the error message points at!!
(__ \ "id").read[String]
)(apply _)
implicit val treeFormat: Format[Tree[File]] = Format(treeReads, treeWrites)
The error I am getting:
[error] /home/dikken/Development/core-service-spaas/app/models/dirTree.scala:41: overloaded method value apply with alternatives:
[error] [B](f: B => (scalaz.Tree[model.treedir.File], String, String, String))(implicit fu: play.api.libs.functional.ContravariantFunctor[play.api.libs.json.Reads])play.api.libs.json.Reads[B] <and>
[error] [B](f: (scalaz.Tree[model.treedir.File], String, String, String) => B)(implicit fu: play.api.libs.functional.Functor[play.api.libs.json.Reads])play.api.libs.json.Reads[B]
[error] cannot be applied to ((=> Nothing) => scalaz.Tree[Nothing])
[error] (__ \ "id").read[String] and
[error] ^
[error] one error found
[error] (compile:compile) Compilation failed
Does this mean I have to pattern match on a case where I have a Tree of Nothing? And how excatly should I do that?
Any help appreciated! Tx!

Answer 1

I'm going to assume that apply _ is actually File.apply _, which cannot work here. File.apply accepts the parameters of the case class File (of which there are three). With JSON combinators, it is trying to pass the four parameters above to File.apply, which does not mix. It also does not produce a Tree[File]. What you need to do is replace File.apply with a method that accepts (children, notebookid, name, id) as parameters, and produces a Tree[File].
Here's a somewhat crude one that will work:
def jsonToTree(children: Seq[Tree[File]], name: String, notebookid: String, id: String): Tree[File] =
Tree.node(File(id, name, notebookid), children.toStream)
The Reads will now look more like this:
implicit def treeReads: Reads[Tree[File]] = (
(__ \ "children").lazyRead[Seq[Tree[File]]](Reads.seq(treeReads)).orElse(Reads.pure(Nil)) and
(__ \ "name").read[String] and
(__ \ "notebookid").read[String] and
(__ \ "id").read[String]
)(jsonToTree _)
You were closer with the commented out line as well. Because this is a recursive structure, we need to use lazyRead.
Testing:
val js = Json.parse("""{
"id": "1",
"name": "test",
"notebookid": "abc",
"children": [
{
"id": "2",
"name": "test222",
"notebookid": "ijk"
},
{
"id": "3",
"name": "test333",
"notebookid": "xyz"
}
]
}""")
scala> val tree = js.as[Tree[File]]
tree: scalaz.Tree[File] = <tree>
scala> tree.rootLabel
res8: File = File(1,test,abc)
scala> tree.subForest
res9: Stream[scalaz.Tree[File]] = Stream(<tree>, ?)
This can also be done (certainly in different ways) without combinators, as well (provided there is an implicit Reads[File] available):
implicit def treeReads: Reads[Tree[File]] = new Reads[Tree[File]] {
def reads(js: JsValue): JsResult[Tree[File]] = {
js.validate[File] map { case file =>
(js \ "children").validate[Stream[Tree[File]]].fold(
_ => Tree.leaf(file),
children => Tree.node(file, children)
)
}
}
}