ex.scala

class FormatEncoder[Encoder <: EncodeResponse[_]] {
  def encoder[A]: Encoder[A] = implicitly[Encoder[A]] // implicit encode must be in scope =/
}

class Formats(encoders: (String, FormatEncoder[_])*, defaultFormat: String) {
  private val formatsHash = encoders.toMap
  private val default = formatsHash(defaultFormat)

  def encodeFor[A](httpReq: HttpRequest): EncodeResponse[A] = formatsHash.getOrElse(???, default).encoder[A]
}

val formats = new Formats (
  "application/json" -> FormatEncoder[EncodeJson],
  "application/xml" -> FormatEncoder[EncodeXml]
)

def hello(name: String) = new Service[HttpRequest, HttpResponse] {
  def apply(req: HttpRequest) = for {
    title <- OptionalParam("title")(req)
  } yield Ok(Response(...))(formats.encodeFor(req))
}