Play framework: JSON Reads for a single-attribute case class

Question

I'm trying to create a JSON Reads implicit for a case class that contains a single attribute, but I'm getting an error "Reads[Nothing] doesn't conform to expected type". Here's teh codes:

import play.api.libs.functional.syntax._
import play.api.libs.json.Reads._
import play.api.libs.json.{JsPath, Json, Reads}

case class Feedback(message: String)
object Feedback {
  implicit val reads: Reads[Feedback] = (
      (JsPath \ "message").read[String](maxLength[String](2000))
    )(Feedback.apply _)
}

Why isn't this working? If I add extra attributes to the case class and multiple .read calls joined with and it works...

Answer 1

Json combinators doesn't work for single field case class.

You can do the following:

import play.api.libs.json.Reads._
import play.api.libs.json.{__, Reads}

case class Feedback(message: String)
object Feedback {
  implicit val reads: Reads[Feedback] = (__ \ "message")
    .read[String](maxLength[String](2000)).map {message => Feedback(message)}
}

It's because of a limitation in the current Macro implementation. You can read more about it here: Pacal is the writer of this API