Write a Mapping[A] and get a QueryStringBindable[A] for free

Application.scala

// --- Definition of a query string binder for type A using a Mapping[A]

object Binders {
  implicit def mappingBinder[A](implicit mapping: Mapping[A]) = new QueryStringBindable[A] {
    override def bind(key: String, params: Map[String, Seq[String]]): Option[Either[String, A]] = {
      val data = for {
        (k, ps) <- params
        if k startsWith key
        p <- ps.headOption
      } yield (k.drop(key.length + 1), p)
      if (data.isEmpty) {
        None
      } else {
        Some(mapping.bind(data.toMap).left.map(_ => "Unable to bind object for key '%s'".format(key)))
      }
    }
    override def unbind(key: String, value: A): String = {
      val (map, _) = mapping.unbind(value)
      map.map { case (k, v) => key + "." + k + "=" + URLEncoder.encode(v, "utf-8") }.mkString("&")
    }
  }
}

// --- Usage

case class User(name: String, age: Int)

object User {
  implicit val userMapping: Mapping[User] =
    mapping(
        "name"->nonEmptyText,
        "age"->number.verifying(min(0))
    )(User.apply)(User.unapply)
}

case class Pager(index: Int, size: Int)

object Pager {
  implicit val pagerMapping: Mapping[Pager] =
    mapping(
        "index"->number,
        "size"->number
    )(Pager.apply)(Pager.unapply)
}

object Application extends Controller {

  /* Example of request for the following action:
   * http://foo.com/?user.name=Julien&user.age=26&pager.index=1&pager.size=42
   */
  def index(user: User, pager: Pager) = Action {
    Ok(views.html.index(user, pager))
  }

}

what about mappers using []?

Mappings containing inner repeated mappings should work (though I didn’t tested). E.g.:

/?foo.xs[0]=1&foo.xs[1]=2&foo.xs[2]=3

On the other hand, if you define a Mapping[Foo] and want to bind a List[Foo] on your controller action, your code will compile but the binding will always fail (I guess, I didn’t tested either).