TypeSafeTags.scala

  def toProps(in : Attr[_,_]*) = {
    val p = js.Dictionary.empty[Any]
    in.foreach(t => if(t != null) p.update(t.key, t.value))
    p
  }

  def div(attrs: Attr[DivAttr,_]*)(children: ReactNode*) = React.createElement("div", toProps(attrs :_*), children : _*)
  def input(attrs: Attr[InputAttr,_]*)(children: ReactNode*) = React.createElement("input", toProps(attrs :_*), children : _*)


  sealed trait ElemAttr
  final class DivAttr extends ElemAttr
  final class InputAttr extends ElemAttr
  final class FormAttr extends ElemAttr


  class Attr[-AttrType <: ElemAttr,ValueType](val key: String, val value: ValueType)

  class Key[ValueType, AttrType <: ElemAttr](name : String) {
    def :=(value: ValueType): Attr[AttrType,ValueType] = new Attr[AttrType,ValueType](name,value)
  }


  val id: Key[String, ElemAttr] = new Key("id")
  val className: Key[String, ElemAttr] = new Key("className")
  val value: Key[String, InputAttr] = new Key("value")
  // this can be used in input/form tag
  val accept: Key[String, InputAttr with FormAttr] = new Key("value")


  val d = div(id := "hello", className := "hello")()
  val i = input(id := "hello", className := "hello", accept := "sdsad")()
  
  /**
  [error]  found   : sri.playground.web.Dom.Attr[sri.playground.web.Dom.InputAttr with sri.playground.web.Dom.FormAttr,String]
[error]  required: sri.playground.web.Dom.Attr[sri.playground.web.Dom.InputAttr, _]
[error]   val i = input(id := "hello", className := "hello", accept := "sdsad")()
[error]                                                             ^
[error] one error found
*/