score:9

Accepted answer

As mentioned in the play-framework documentation here

Format[T] is just a mix of the Reads and Writes traits and can be used for implicit conversion in place of its components.

Format is a combination of Reads[T] and Writes[T]. So you can define a single implicit Format[T] for type T and use it to read and write Json instead of defining a separate implicit Reads[T] and Writes[T] for type T. Thus if you already have Reads[T] and Writes[T] defined for your type T then Format[T] is not required and vice versa.

One advantage with Format is that you can define a single Implicit Format[T] for your type T instead of defining two separate Reads[T] and Writes[T] if they are both symmetrical (i.e. the Reads and Writes). So Format makes your JSON Structure definition less repetitive. For example you can do something like this

implicit val formater: Format[Data] = (
    (__ \ "id").format[Int] and
    (__ \ "name").format[String] and
    (__ \ "value").format[String]
  ) (Data.apply, unlift(Data.unapply))

Instead of this.

implicit val dataWriter: Writes[Data] = (
    (__ \ "id").write[Int] and
    (__ \ "name").write[String] and
    (__ \ "file_type").write[String]
  ) (Data.apply)

implicit val dataWriter: Reads[Data] = (
    (__ \ "id").read[Int] and
    (__ \ "name").read[String] and
    (__ \ "file_type").read[String]
  ) (unlift(Data.unapply))

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