scala list type mismatch - list

Question

So I had list of tuples like this:
val rooms = List(("Hi", "mom"),("hi", "dad"))
val foo = rooms.map(arg =>{
var fields = List
( new JField("greeting",arg._1),
new JField("recipient",arg._2))
new JObject(fields)})
And there was much happiness in the land, but when I changed list of room like so:
case class Room(greeting:String, recipient:String)
val rooms = List(Room("Hi", "mom"),Room("hi", "dad"))
val foo = rooms.map(arg =>{
var fields = List
( new JField("greeting",arg.greeting),
new JField("recipient",arg.recipient))
new JObject(fields)})
I get:
[error] <file>: type mismatch;
[error] found : scala.collection.immutable.List.type (with underlying type object List)
[error] required: List[blueeyes.json.JsonAST.JValue]
[error] new JArray(fields)
So it appears that the list is now of Object instead of JField as it was before, why is that?

Answer 1

It works if you don't detach the List from its (:
var fields = List(
new JField("greeting", arg.greeting),
new JField("recipient", arg.recipient))
Basically, it's parsing like this:
var fields = List // assign the List companion object
(new JField("greeting", arg.greeting), // construct a tuple with two items
new JField("recipient", arg.recipient)) // ...but don't use or assign it
new JObject(fields) // Make JObject containing the type
The error comes because the JObject constructor expects a JValue but you are passing it fields which has type List.type.