play json写子类给出了模糊隐含值的错误

Question

我正在使用play框架,并有一个抽象类:

 abstract class Base{...}

在伴随对象中有自己的隐式JSON编写器

object Base {
   implicit val baseWrites: Writes[Base] = (...)(unlift(Base.unapply))
}

我是这个抽象类的子类:

case class SubClass{...}

在其伴随对象中也有自己的隐式JSON编写器

object SubClass {
   implicit val subClassWrites: Writes[SubClass] = (...)(unlift(SubClass.unapply))
}

当我尝试使用Json.toJson(SubClass)序列化子类对象时,我收到一个错误:

[error]  both value subClassWrites in object SubClass of type => play.api.libs.json.
Writes[models.SubClass]
[error]  and value baseWrites in object Base of type =>        
play.api.libs.json.Writes[models.Base]
[error]  match expected type play.api.libs.json.Writes[models.SubClass]
[error]  Ok(Json.toJson(SubClass.find(id)))

有没有办法消除歧义？

Answer 1

您遇到了碰撞,因为Writes它具有逆变类型参数A:

trait Writes[-A] extends AnyRef

这意味着它Writes[Base]是 - 的子类Writes[SubClass]- 您可以Writes[Base]在Writes[SubClass]需要的地方使用.

问题出在这里:

val base: Base = new SubClass(...)
val jsBase = Json.toJson(base)

所以Writes[Base]应该能够序列化一个实例SubClass.你可以ADT在这种情况下使用:

sealed trait Base
object Base {
  implicit val baseWrites: Writes[Base] = 
    new Writes[Base]{
      def writes(o: Base): JsValue = o match {
        case s: SubClass => SubClass.writes.writes(s)
        case s: SubClass2 => SubClass2.writes.writes(s)
      }
    }
}

case class SubClass(...) extends Base
object SubClass {
  val writes: Writes[SubClass] = (...)(unlift(SubClass.unapply))
}

case class SubClass2(...) extends Base
object SubClass2 {
  val writes: Writes[SubClass2] = (...)(unlift(SubClass2.unapply))
}

使用sealed关键字,如果match不是详尽无遗,您将收到警告.