Concurrency in Haskell: Fast, Simple, Correct

I thought it was a bit odd that the author claims there’s no mutexes in sight, the TVar is effectively a mutex guard unless I’m misunderstanding this? (I’ve written exactly 0 lines of Haskel). Or is the claim that the lack of ceremony and accidental complexity around threading is the real win for concurrency here?

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.