Composability Example of Software Transactional Memory

Question

One of the major advantages of software transactional memory that always gets mentioned is composability and modularity. Different fragments can be combined to produce larger components. In lock-based programs, this is often not the case.

I am looking for a simple example illustrating this with actual code. I'd prefer an example in Clojure, but Haskell is fine too. Bonus points if the example also exhibits some lock-based code which can't be composed easily.