json - How to write Reads[T] and Writes[T] in scala Enumeration (play framework 2.1)

Question

I'm a little bit lost with the new ScalaJson Feature in Play Framework 2.1. I would like to write Reads and Writes in my Enumeration.

Here is my code :

object EnumA extends Enumeration {
 type EnumA = Value
 val VAL1, VAL2, VAL3 = Value

def parse(str:String) : EnumA = {
    str.toUpperCase() match {
         case "VAL1" => VAL1
         case "VAL2" => VAL2
         case "VAL3" => VAL3
         case _ => null
    }
}}

Any idea ?

Thanks.

Answer 1

Short answer: use something like Play Enumeration Utils.

Long answer, instead of putting a Reads in your enum, you can create a re-useable Reads for Enumeration types:

object EnumA extends Enumeration {
  type EnumA = Value
  val VAL1, VAL2, VAL3 = Value
}

object EnumUtils {
  def enumReads[E <: Enumeration](enum: E): Reads[E#Value] = new Reads[E#Value] {
    def reads(json: JsValue): JsResult[E#Value] = json match {
      case JsString(s) => {
        try {
          JsSuccess(enum.withName(s))
        } catch {
          case _: NoSuchElementException => JsError(s"Enumeration expected of type: '${enum.getClass}', but it does not appear to contain the value: '$s'")
        }
      }
      case _ => JsError("String value expected")
    }
  }
}

Then when you want to parse something to an enum, create an implicit Reads for your specific Enum type in scope:

import some.thing.EnumUtils
implicit val myEnumReads: Reads[EnumA.Value] = EnumUtils.enumReads(EnumA)

val myValue: EnumA.Value = someJsonObject.as[EnumA.Value]

or

val myValue: EnumA.Value = someJsonObject.asOpt[EnumA.Value].getOrElse(sys.error("Oh noes! Invalid value!"))

(It's considered bad form to use null in Scala.)

Writing enums as JsValues is simpler:

object EnumUtils {
  ...
  implicit def enumWrites[E <: Enumeration]: Writes[E#Value] = new Writes[E#Value] {
    def writes(v: E#Value): JsValue = JsString(v.toString)
  }
}

Then just import that into scope before you attempt to write an enum (or pass it explicitly to the toJson function:

import EnumUtils.enumWrites
val myEnumJson: JsValue = Json.toJson(EnumA.VAL1)

You can similarly make a function to create a Format object combining both Reads and Writes:

object EnumUtils {
  ....
  implicit def enumFormat[E <: Enumeration](enum: E): Format[E#Value] = {
    Format(EnumReader.enumReads(enum), EnumWriter.enumWrites)
  }
}