Compile error when using a companion object of a case class as a type parameter

Question

I'm create a number of json messages for spray in scala using case classes. For example:

  case class Foo(name: String, attrs: List[String])
  implicit val fooFormat = jsonFormat2(Foo)
  object Foo {
    case class Invalid(error: String)
  }
  case class Bar(name: String, kv: Map[String, String])
  implicit val barFormat = jsonFormat2(Bar)

In the above snippet, barFormat compiles, but fooFormat does not:

type mismatch; found : Foo.type required: (?, ?) => ? 
 Note: implicit value barFormat is not applicable here because it comes 
 after the application point and it lacks an explicit result type

I don't want to use barFormat in place of fooFormat, and I understand that a case class automatically generates a companion object, but I don't understand why there's a compiler error here, and the error message is difficult for me to decipher. Does anyone know what the problem is here and how to fix it, preferably without removing my Foo companion object?

Answer 1

From your compile error, it looks like jsonFormat2 expects a two-argument function. Do you mean to pass the constructors of Foo and Bar into it? If so, you should do Foo.apply and Bar.apply.