I said it's not possible to pattern match against the headers in this thread: http://groups.google.com/group/liftweb/browse_thread/thread/474b3e768ea1c949/e7f8036e3b40d51e?hl=en&lnk=gst&q=accept#e7f8036e3b40d51e  and then again in this thread: http://groups.google.com/group/liftweb/browse_thread/thread/b00185cec49a167/8d4d5f22fd7d0de7?hl=en&lnk=gst&q=accept#8d4d5f22fd7d0de7

Have I not made the technical issues clear?

As an interesting footnote, it is possible to pull out a particular header, but it's not clean. You have to bump through a stable identifier first:

scala> object header {
     |   def apply(which: String): HeaderExtractor = new HeaderExtractor(which)
     |   class HeaderExtractor(which: String) {
     |     def unapply(in: List[(String, Any)]): Option[Any] = in.find(_._1 == which)
     |   }
     | }
defined module header

scala> val x = header("Boo")
x: header.HeaderExtractor = header$HeaderExtractor@272b72f4

scala> val y = ("Foo", 1)::("Bar", 2)::("Boo", 3)::Nil
y: List[(java.lang.String, Int)] = List((Foo,1), (Bar,2), (Boo,3))

scala> y match { case x(a) => a; case _ => "no idea" } 
res1: Any = (Boo,3)

I don't mean to comment on the REST feature, just thought it might be interesting that you can do this.

Yes, it's possible to write a custom extractor and it's possible to write code that vends custom extractors.  But in the case you gave, there's no way to chain the extraction:

val accept = header("Accept")
val contentType = header("Content-Type")

list match {
  case accept("text/xml") :: contentType("text/xml") =>
}

And even if it were, you'd need a second extractor to test against "text/xml" because it could be "text/xml", "TEXT/XML" or "text/xml; charset=utf-8", etc.

Until there are language level curried extractors (basically building the extractor in the pattern based on parameters to the extractor), there's no savings to try to pattern match across an unordered List[String -> String], Map[String, String], etc.

Put another way, if you're going to go through the effort to build an extractor, you can compose your own extractor the same way RestHelper does it:  lazy val MyExtractor = new TestGet with MyHeaderMatcher  where you write imperative code to determine if the headers match correctly.