斯卡拉列表類型不匹配

Question

所以我有過這樣的元組的列表：

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)}) 

了。在土地多的快樂，但是當我改變了房間列表，像這樣：

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)}) 

我得到：

[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) 

因此，看起來該列表現在是對象而不是以前的JField，爲什麼？

Answer 1

，如果你不分離從(的List它的工作原理：

var fields = List(
    new JField("greeting", arg.greeting), 
    new JField("recipient", arg.recipient)) 

基本上，它的解析如下：

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 

的錯誤出現，因爲JObject構造函數需要一個JValue但你通過它fields其類型List.type。