Validate XML : error on elem disapeared with Node

[gbersac]

I stumbled upon a puzzling error. With a From[Elem] in this code :

case class Commentaire(
  codeMessage: Option[String],
  libelleMessage: String,
  codePriorite: Option[String]
)

object Commentaire {
  implicit val messageRule: Rule[Elem, Commentaire] = From[Elem] { __ =>
    import jto.validation.xml.Rules._

    (
      (__ \ "DossEtude"  \ "codeMessage").read[Option[String]] ~
      (__ \ "DossEtude" \ "libelleMessage").read[String] ~
      (__ \ "DossEtude" \ "prioriteMessage").read[Option[String]]
    )(Commentaire.apply _)
  }

}

I got the following error :

[error] /Users/gbe/projects/nfc-nb/commons/server/src/main/scala/models/OMDEMessage.scala:92: No implicit view available from jto.validation.Path => jto.validation.RuleLike[scala.xml.Elem,Option[String]].
[error]       (__ \ "DossEtude" \ "codeMessage").read[Option[String]] ~

This error disepeared when I replaced Elem by Node :

object Commentaire {
  implicit val fmt = Json.format[Commentaire]

  implicit val messageRule: Rule[Node, Commentaire] = From[Node] { __ =>
    (
      (__ \ "DossEtude" \ "codeMessage").read[Option[String]] ~
      (__ \ "DossEtude" \ "libelleMessage").read[String] ~
      (__ \ "DossEtude" \ "prioriteMessage").read[Option[String]]
    )(Commentaire.apply _)
  }

}

Whether this library should completely support Elem or the user should be notified that the type Elem is not supported by this library.

[OlivierBlanvillain]

The only thing we could do here is improve the error message. Indeed, the library does support Elem! You would just need to implement a few type class instances for this type (as the error suggests), see this section of the doc.

For better error messages, we would need to replace usages of Function1[A, B] with an equivalant custom type (ValidationView[A, B] or something), and annotate it with an @implicitNotFound.

[jto]

Thanks for reporting this.

Creating a new type ValidationView would break compatibility so maybe for validation v3 but not in v2. We could maybe however support Elem too.
As always, PR are welcome @gbersac :)

BTW, there's an extensive test suite here that may help if you need examples of XML validation.