Scala-Play Json：如何为任何鞍架定义写入？

Question

I have a Scala-Play web services application that does a bit of computations that generate Saddle frames and need to dump those Saddle frames into Json. So I define a frameWrites like this:

object JsonUtils {
  implicit def frameWrites[RX, CX, E] = new Writes[Frame[RX, CX, E]] {
    override def writes(frame: Frame[RX, CX, E]): JsValue = {
      val json: JsArray = Json.arr(
        (0 until frame.numRows).map { i =>
          Json.obj(
            frame.rowIx.at(i).toString ->
              (0 until frame.numCols).map { j =>
                Json.obj(
                  frame.colIx.at(j).toString -> frame.at(i, j).toString
                )
              }
          )
        })
        json
      }
  }
}

and then attempt to use it like this:

import utils.JsonUtils._

def computingAction = Action {
   val pnlStatistics: Frame[String, String, Double] = ???
   Ok(pnlStatistics) 
}

but then always get the error Cannot write an instance of org.saddle.Frame[String,String,Double] to HTTP response. Try to define a Writeable[org.saddle.Frame[String,String,Double]] Cannot write an instance of org.saddle.Frame[String,String,Double] to HTTP response. Try to define a Writeable[org.saddle.Frame[String,String,Double]]

To be really explicit I have also tried defining as part of JsonUtils :

implicit def frameSSDWrites = frameWrites[String, String, Double]

but this one also doesn't get picked up ...

UPDATE invoking the writer explicitly works:

Ok(frameWrites.writes(results("PnlStatistics")))

Answer 1

A Play result input must be some content that can be put in a HTTP response.

Your action does not know that you want to write it specifically as JSON, so you need to return Ok(Json.toJson(pnlStatistics)) .

Let us see how this work:

• The type of pnlStatistics is a Frame , so you can serialize it to JSON, using your implicit Writes[Frame] , so Json.toJson accepts it as parameter. It then returns a JsValue .

• Since there is also an implicit Writeable[JsValue] in scope, Ok then accepts this value as content for your response (and will also add the Content-Type header).

If you forget the explicit JSON conversion, Play has no way to determine how you want the frame handled in the response.