为什么Spray error＆＃34;找不到参数marshaller的隐含值＆＃34;？

Question

我使用spray-json将一些case类转换为json，但是我将转换List转换为json时遇到了麻烦：

case class Foo(id: Int)

object FooJsonSupport extends DefaultJsonProtocol with SprayJsonSupport {
  implicit val fooFormats = jsonFormat1(Foo)
}

//and i have a list with Foo:
val list = List(Foo(1), Foo(2), Foo(3))

// and i want to return json response

post {
  respondWithMediaType(`application/json`) {
    complete(list)
  }
}

运行示例后，我收到错误：

  

无法找到参数marshaller的隐含值：spray.httpx.marshalling.ToResponseMarshaller [scala.collection.immutable.List [Post]]

如何解决？