This can take any ordinary Haskell data structure and give you a lock-free concurrent data structure with easy-to-use transactional semantics. How it performs is another matter! That depends on the amount of contention and the cost of re-playing transactions.
This library is full of STM-oriented data structures. They perform better than a simple `TVar (Map k v)`.
It's kind of a fun trick actually. The stock Map is just a tree. The STM Map is also a tree [1] but with TVars at each node. So this helps a lot with contention - you only contend along a "spine" instead of across the whole tree, which is O(log n).
[1] Technically a HAMT a la unordered-containers - trie, tree, you get the idea :)