I shouldn't be getting nerd sniped by comment about Maven but here goes. Maven doesn't compose because it is an ad-hoc lisp where every element has different semantics, every element is a different language. This isn't an XML problem, but a way that XML fragments in Maven map to the interpreters of those fragments. Maven could have been an amazingly compact expressive combination of a logic/constraint solver with a declarative language, instead it is PHP written in XML. Maven is declarative only in that it has a textual interface.