gistfile4.scala

object UserManagementService extends RestHelper {
  ....

  serveJx {
    case Get("api" :: "user" :: id :: _, _) => Full(loadUser(id))
  }
}

trait Convertable {
  def toXml: Node
  def toJson: JValue
}

implicit def cvt: JxCvtPF[Convertable] = {
  case (JsonSelect, c, _) => c.toJson
  case (XmlSelect, c, _) => c.toXml
}

RestHelper.scala

trait RestHelper extends LiftRules.DispatchPF {
...
  /**
   * A function that chooses JSON or XML based on the request..
   * Use with serveType
   */
  implicit def jxSel(req: Req): Box[JsonXmlSelect] = 
    if (jsonResponse_?(req)) Full(JsonSelect)
    else if (xmlResponse_?(req)) Full(XmlSelect)
    else None
}  

----------------------------------------------------------------
object UserManagementService extends RestHelper {
  ...
  override implicit def jxSel(req: Req): Box[JsonXmlSelect] = {
    val prefs = ClientMediaPreference orderedList (req.headers("accept"))
    val preferredMediaType = prefs(0).mediaType + "/" + prefs(0).mediaSubType
    if(preferredMediaType == "text/xml" || preferredMediaType == "application/xml") Full(XmlSelect)
    else if (preferredMediaType == "application/json") Full(JsonSelect)
    else None
  }
}

User.scala

case class User (@Column("first_name") val firstName: String, @Column("last_name") val lastName:String, val email:String)
    extends UserManagementDB with Convertable {
  ....
  def toXml():Node = {
    <user>
      <id>{this.id}</id>
      <firstName>{this.firstName}</firstName>
      <lastName>{this.lastName}</lastName>
      <email>{this.email}</email>
    </user>
  }

  def toJson():JValue = {
    Extraction.decompose(this)
  }
}

UserManagementService.scala

object UserManagementService extends RestHelper {
  ...  
  override implicit def jxSel(req: Req): Box[JsonXmlSelect] = {
    val prefs = ClientMediaPreference orderedList (req.headers("accept"))    
    val preferredMediaType = prefs(0).mediaType + "/" + prefs(0).mediaSubType    
    if(preferredMediaType == "text/xml" || preferredMediaType == "application/xml") Full(XmlSelect)
    else if (preferredMediaType == "application/json") Full(JsonSelect)
    else None
  }
}

object ClientMediaPreference {
  def orderedList(acceptHeader:List[String]):List[Media] = {
    acceptHeader.size match {
      case 0 => Media("application", "xml", 1.0f) :: List() // XML is the default (this can be customized somewhere, if no Accept Header)
      case _ => parseAcceptHeader(acceptHeader)
    }
  }

  private def parseAcceptHeader(acceptHeader:List[String]): List[Media] = {
    val mediaPrefs = acceptHeader(0) split (",") map (_.trim)
    val acceptHeaderVals = for{
      pref <- mediaPrefs
      val media = parseMime(pref)
    } yield media
    acceptHeaderVals.toList.sortWith((e1, e2) => e1.qualityFactor > e2.qualityFactor)
  }

  private def parseMime(header:String):Media = {
    var q = 1.0f //defaults to 1.0
    val parts = header split(";")
    val typeSubType = parts(0) split("/")
    if (parts.length == 2) {
      q = (parts(1) split("="))(1).toFloat
    }
    Media(typeSubType(0), typeSubType(1), q)
  }
}

case class Media(mediaType:String, mediaSubType:String, qualityFactor:Float) {
  override def toString:String = {
    "Media[" + mediaType + "/" + mediaSubType + "; q=" + qualityFactor + "]"
  }
}