gistfile1.scala

trait CanReadXML[A] {
  def reads(seq: scala.xml.NodeSeq): Either[String, A]
}

trait Foo { def foo: Int }
trait Bar { def bar: Int }

object Def {
  def fromXML[A: CanReadXML](seq: scala.xml.NodeSeq) =
    implicitly[CanReadXML[A]].reads(seq)
}

implicit val fooCanReadXML = new CanReadXML[Foo] {
  def reads(seq: scala.xml.NodeSeq) = (seq \ "foo").headOption map { x =>
    Right(new Foo {
      def foo = x.text.toInt
    })    
  } getOrElse { Left("no foo") }
}

implicit val barCanReadXML = new CanReadXML[Bar] {
  def reads(seq: scala.xml.NodeSeq) = (seq \ "bar").headOption map { x =>
    Right(new Bar {
      def bar = x.text.toInt
    })    
  } getOrElse { Left("no bar") }
}

implicit val fooBarCanReadXML = new CanReadXML[Foo with Bar] {
  def reads(seq: scala.xml.NodeSeq) =
    (Def.fromXML[Foo](seq), Def.fromXML[Bar](seq)) match {
      case (Right(x), Right(y)) => Right(new Foo with Bar {
        def foo = x.foo
        def bar = y.bar
        override def toString = "FooWithBar(%d, %d)" format(foo, bar)
      })
      case _ => Left("no foo with bar")
    } 
}


Def.fromXML[Foo with Bar](<obj><foo>1</foo><bar>2</bar></obj>)

scala> Def.fromXML[Foo with Bar](<obj><foo>1</foo><bar>2</bar></obj>)
res0: Either[String,Foo with Bar] = Right(FooWithBar(1, 2))