Playframework 2.2.X scala - How can I use implicit variable in json's Writes

Question

I use Play 2.2 json writes helper : http://www.playframework.com/documentation/2.2.x/ScalaJson

I want to write a json version of a File case class. Here is my writer :

implicit val fileWrites = new Writes[File] {
def writes(file: File) = Json.obj(
  "name" -> file.name,
  "description" -> file.description,
  "contentType" -> file.contentType,
  "lastModify" -> file.lastModify,
  "size" -> file.size,
  "link" -> routes.Document.downloadFromPermalink(file.uuid).absoluteURL(false)
  )
}

However this routes.Document.downloadFromPermalink(file.uuid).absoluteURL(false) need an implicit request: RequestHeader. If I put fileWrites definition in an action with the implicit request in scope, it works.

But I want to define fileWrites to be reusable between action. How can I write it so fileWrites take the implicit request as a parameter ?

Same question for this definition :

implicit val fileWrites: Writes[File] = (
  (JsPath \ "name").write[String] and
  (JsPath \ "description").write[Option[String]] and
  (JsPath \ "contentType").write[String] and
  (JsPath \ "lastModify").write[Date] and
  (JsPath \ "size").write[Long] and
  (JsPath \ "link").write[String])
(f => (f.name, f.description, f.contentType, f.lastModify, f.size, routes.Document.downloadFromPermalink(f.uuid).absoluteURL(false)))

Edit for wingedsubmariner answer :

it's not working with a request in scope :

  def documents(id: Long) = Action { implicit request =>
    Users.findById(id).map { user =>

      //get files from somewhere
      val files = XXX

      //it works if I put fileWrites here but I want a reusable function

      //error on this line : No Json deserializer found for type List[models.File]. Try to implement an implicit Writes or Format for this type.
      val json = Json.toJson(files)
      Ok(json)
    }.getOrElse(NotFound)
  }

  implicit def fileWrites(implicit request: RequestHeader) = new Writes[File] {
    def writes(file: File) = Json.obj(
      "name" -> file.name,
      "description" -> file.description,
      "contentType" -> file.contentType,
      "lastModify" -> file.lastModify,
      "size" -> file.size,
      "link" -> routes.Document.downloadFromPermalink(file.uuid).absoluteURL(false))
  }

Answer 1

Try:

implicit def fileWrites(implicit request: RequestHeader) = new Writes[File] {
  def writes(file: File) = Json.obj(
    "name" -> file.name,
    "description" -> file.description,
    "contentType" -> file.contentType,
    "lastModify" -> file.lastModify,
    "size" -> file.size,
    "link" -> routes.Document.downloadFromPermalink(file.uuid).absoluteURL(false)
  )
}