No, a TVar is not a mutex guard. A TVar is a software transactional memory (STM) variable. STM works just like a database: you batch together a sequence of operations into a transaction and then execute them. During execution of a transaction, all changes made to the contents of the TVar are stored in a transaction log. If some other transaction occurs during the execution then the whole thing is aborted and re-run.
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.
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 :)
> How it performs is another matter!
I know you say it depends on how much contention one sees but I'm interested in the performance hit. Also, is STM the "standard" (or accepted) way to do async in Haskell?