CODEWITHSUNDEEP
jsonscalaplayframeworkplayframework-2.5
JulienD
JulienD

Reputation: 7293

Scala Play: how to define a "Json writable class"

I would like to write a function that transforms case classes to Json:

import play.api.libs.json._

def myJson(cc: Product): JsValue = {
  Json.toJson(cc)  // simplified
}

Each case class has an implicit Writes[T], for example:

case class Test(a: Int)
object Test {
  implicit val jsonWrites: Writes[Test] = Json.writes[Test]
}

It is possible to write Json.toJson(new Test(1)) individually, but the myJson function above does not compile because it never knows if cc has an implicit Writes defined.

[How can I write the function signature so that it takes only classes having a Writes implicit?]

Edit: How can I write the function input type so that it corresponds only to classes having a Writes implicit?

I tried this:

trait JsonWritableResult[T <: Product] {
  implicit val jsonWrites: Writes[T]
}

case class Test(a: Int)
object Test extends JsonWritableResult[Test] {
  implicit val jsonWrites: Writes[Test] = Json.writes[Test]
}

def myJson(cc: JsonWritableResult[_ <: Product]): JsValue = {
  Json.toJson(cc)
}

But it says "No Json serializer found for type models.JsonWritableResult[_$2]".

Upvotes: 0

Views: 1667

Answers (2)

JulienD
JulienD

Reputation: 7293

Instead of forcing the case class to have the implicit, it is more clever to force it to override a toJson method from a trait, using or not an implicit defined in the case class. Much simpler and it works. Then my factory can explicit this trait in its output type, and thus I can serialize whatever it outputs.

But since the other answer answered the question that I formulated wrong, I accept it ;)

Upvotes: 0

Henrik Kirk
Henrik Kirk

Reputation: 604

Something like this seems to give you the behavior you want.

import play.api.libs.json.{JsValue, Json, Writes}

trait Product {}

case class Test(a: Int) extends Product
object Test {
  implicit val jsonWrites: Writes[Test] = Json.writes[Test]
}

def myJson[T <: Product](cc: T)(implicit writes: Writes[T]): JsValue = {
  Json.toJson(cc)  // simplified
}

import Test._

myJson(Test(3))

This is not tested in the general case, but in a Worksheet it seems to work.

Upvotes: 2

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