https://hackage.haskell.org/package/stm-containers

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 :)