scala-json unserialize in scala.js - scala

Question

I'm trying to convert JSON from Ajax request to Case class in Scala.js using scala-json https://github.com/MediaMath/scala-json
Here is my classes:
sealed trait Result
sealed trait Error extends Result
sealed trait Msg extends Result
case class MsgData(msg: Seq[String], args: Seq[Int]) extends Msg
case class CommentError(#name("obj.comment") comment: Seq[MsgData]) extends Error
Here is how I'm trying to convert:
import json._
implicit val msgDataAcc = ObjectAccessor.create[MsgData]
implicit val commentErrorAcc = ObjectAccessor.create[CommentError]
println("here2")
val errors = JValue.fromString(req.responseText).toObject[CommentError]
println("here3")
This code just silently dies on string with conversion and "here3" never printed to console.
Here is my JSON from server:
{"obj.comment":[{"msg":["error.minLength"],"args":[10]}],"obj.name":[{"msg":["error.path.missing"],"args":[]}]}
What I'm doing wrong? How to fix this?

Answer 1

So I'm guessing this is scala-js. Any exceptions that happen at the top level (entry point) of a scala-js application aren't always echoed out correctly (depending on environment/browser), if you wrap the whole thing in a Try and print out the stack trace during the catch, you should successfully see exception being thrown.
Main issue above is that you need to define 'accessors' for the case classes. There's 2 ways of doing this, one works out of the box by adding an implicit for each type, the other way requires macro-paradise and gives you a much simpler way of defining accessors for case classes.
Here is the normal non-macro-paradise way:
case class MsgData(msg: Seq[String], args: Seq[Int]) extends Msg
object MsgData {
implicit val acc = ObjectAccessor.create[MsgData]
}
case class CommentError(#name("obj.comment") comment: Seq[MsgData]) extends Error
object CommentError {
implicit val acc = ObjectAccessor.create[CommentError]
}
The implicits can be placed anywhere (following the general rule for scala implicits). Placing them in the companion object is the best way to guarantee the implicit can be found anywhere, without special imports or anything needed.
This is 'less magical' than other libs like circe that use shapeless to automatically derive factories, sometimes in a bloated way. scala-json aims to keep the accessor visible for extension purposes, but this does lead to some explicit boilerplate.
This can be reduced down using macro-paradise:
#accessor case class MsgData(msg: Seq[String], args: Seq[Int]) extends Msg
#accessor case class CommentError(#name("obj.comment") comment: Seq[MsgData]) extends Error
This does the exact same thing as the above code, we just leverage macro-paradise to add the implicit 'acc' field to the companion object automatically.