Json.format, No unapply or unapplySeq function found for generics

[Zhao Hongwei]

I extract a method for controller like:

def validateInput[A](json:JsValue)(func: A => Future[Result]):Future[Result]= {
  implicit val format = Json.format[A]
  json.validate[A].fold(
    errors => {
      Future(BadRequest(JsError.toJson(errors)))
    },
    validObj =>
      func(validObj)
  )
}

when compiling the console shows:

[error] D:\program\ymgl\back\app\controllers\Security.scala:114: No unapply or unapplySeq function found

[error]     implicit val format = Json.format[A]

[error]                                      ^

[error] D:\program\ymgl\back\app\controllers\Security.scala:115: No Json deserializer found for type A. Try

 an implicit Reads or Format for this type.

[error]     json.validate[A].fold(

[error]                  ^

[error] two errors found

Does anyone can help me on this issue?

thanks,

henry

[Greg Methvin]

It doesn't work because Json.format is a macro. It looks at the class at compile time and generates the code to serialize the JSON object to/from your class. Since A could be any type here this won't work.

What you probably want to do is accept an implicit Reads[A], which will allow json.validate[A] to work:

def validateInput[A: Reads](json:JsValue)(func: A => Future[Result]): Future[Result]

Then remove your Json.format[A] and declare your implicit Format for each type you want to serialize in its companion object, or wherever it's convenient for you.

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Greg Methvin

Tech Lead - Play Framework

Lightbend, Inc.

[Greg Methvin]

Also there probably is some kind of performance hit you take from creating the Format every time. It's best to declare it as a val, e.g.

object User {

  implicit val format: Format[User] = Json.format[User]

}

[henry z]

Thanks Greg,  I done it under your suggestion and move the val into an object:

def validateInput[A : Reads ](json:JsValue)(func: A  => Future[Result]):Future[Result]= {
  import models.JsonFormats._
  json.validate[A].fold(

    errors => {
      Future(BadRequest(JsError.toJson(errors)))
    },
    validObj =>
      func(validObj)
  )
}

JsonFormats

object JsonFormats {
  import play.api.libs.json.Json

  implicit val objectIdFormat = Json.format[ObjectId]

}

best regards,

henry z

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Greg Methvin

Tech Lead - Play Framework

Lightbend, Inc.

[Greg Methvin]

The "import models.JsonFormats._" inside validateInput won't have any effect. The implicit Format has to be available at the call site of validateInput. I assume you already figured this out if it's working for you but just wanted to say that for the benefit of anyone else reading this.

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