wrong implicit resolution about play-json

[MasseGuillaume]

Well it's supported, here is a counter example:

import play.api.libs.json._

def dontSerialize[T](fallback: T): Format[T] = new Format[T]{
  def writes(v: T): JsValue = JsNull
  def reads(json: JsValue): JsResult[T] = JsSuccess(fallback)
}
def dontSerializeOption[T]: Format[Option[T]] = dontSerialize(None)

class ThisIsNotSerializable()
type WorkArround = Option[ThisIsNotSerializable]
case class Foo(v: WorkArround, field1: Int, field2: Int)
implicit val dontSerializeFur: Format[WorkArround] = dontSerializeOption[ThisIsNotSerializable]
implicit val formatFoo = Json.format[Foo]

[cchantep]

Message is updated in #89

[cchantep]

Hi,

In such case the details of the error is useful:

error: No instance of play.api.libs.json.Format is available for scala.Option[Fur] in the implicit scope (Hint: if declared in the same file, make sure it's declared before)
       implicit val formatFoo = Json.format[Foo]

As indicated in the guide, please note that this tracker is for confirmed issue. Rather use the MailingList to ask question/for help.

It happens that:

1. The case class Fur as-is doesn't make sense, without any property (either at least one property or a case object).
2. A Format[Fur] is not provided, whereas required to resolve the missing Format[Option[Fur]].

e.g.:

import play.api.libs.json._

def dontSerialize[T](fallback: T): Format[T] = new Format[T]{
  def writes(v: T): JsValue = JsNull
  def reads(json: JsValue): JsResult[T] = JsSuccess(fallback)
}
def dontSerializeOption[T]: Format[Option[T]] = dontSerialize(None)

// Does not compile
case class Fur(v: Int)
implicit val requiredFormat: Format[Fur] = Json.format[Fur]
case class Foo(v: Option[Fur])
implicit val dontSerializeFur: Format[Option[Fur]] = dontSerializeOption[Fur]
implicit val formatFoo = Json.format[Foo]

[MasseGuillaume]

1. Edited to make it obvious what Fur is: ThisIsNotSerializable

import play.api.libs.json._

def dontSerialize[T](fallback: T): Format[T] = new Format[T]{
  def writes(v: T): JsValue = JsNull
  def reads(json: JsValue): JsResult[T] = JsSuccess(fallback)
}
def dontSerializeOption[T]: Format[Option[T]] = dontSerialize(None)


class ThisIsNotSerializable()
case class Foo(v: Option[ThisIsNotSerializable], field1: Int, field2: Int)
implicit val dontSerializeFur: Format[Option[ThisIsNotSerializable]] = dontSerializeOption[ThisIsNotSerializable]
implicit val formatFoo = Json.format[Foo]

2. Reported error:

No instance of play.api.libs.json.Format is available for scala.Option[Playground.ThisIsNotSerializable] in the implicit scope (Hint: if declared in the same file, make sure it's declared before)

[cchantep]

... and so ? The error makes sense for me.

Please use the MailingList to ask question/for helper.

[MasseGuillaume]

are you sure ?

implicit val dontSerializeThisIsNotSerializable: Format[Option[ThisIsNotSerializable]] = dontSerializeOption[ThisIsNotSerializable]

No instance of play.api.libs.json.Format is available for scala.Option[Playground.ThisIsNotSerializable]

[cchantep]

The macros are automatically managing Optional case class properties, provided that for a property whose type is Option[T] there are typeclass (Writes and/or Reads) for T.

Overriding the automatic support of Optional properties is not supported (as would be error prone).

Either use the macro in this way, or you need to manage the whole format manually.

Please use the MailingList to ask question/for helper.

[cchantep]

Because the alias prevent it (probably a missing .dealias), anyway the expected behaviour is the automatic one.

[julienrf]

Still, I think the error message is incorrect. What’s missing is a Format[ThisIsNotSerializable] (as opposed to a Format[scala.Option[Playground.ThisIsNotSerializable]], like the error message says)