[robto]

Clojure atoms use STM, though. I've been writing Clojure for almost a decade now, it's not that STM isn't great, it's just that immutable data will carry you a very long way - you just don't need coordinated mutation except in very narrow circumstances. In those circumstances STM is great! I have no complaints. But it just doesn't come up very often.

[eduction]

That’s incorrect. Only refs+dosync use stm. https://clojure.org/reference/refs

Not atoms.

From Hickey’s History of Clojure paper:

“ Taking on the design and implementation of an STM was a lot to add atop designing a programming language. In practice, the STM is rarely needed or used. It is quite common for Clojure programs to use only atoms for state, and even then only one or a handful of atoms in an entire program. But when a program needs coordinated state it really needs it, and without the STM I did not think Clojure would be fully practical.”

https://dl.acm.org/doi/pdf/10.1145/3386321

Atoms do an atomic compare and swap. It’s not the same thing.

[robto]

Haha, I read The Joy of Clojure way back in 2013 and conflated the different reference types with STM. So thanks for mentioning that, I always thought it weird that you'd need STM for vars and atoms too.

That said, I have never used a ref, nor seen one in use outside of a demo blogpost.

[eduction]

Totally! They are rare. Cheers

[eduction]

PS I agree atoms and stm are both solid though — and that you can go a very long way without touching either!

[dwohnitmok]

I would say to the contrary it would come up all the time if the right idioms were in place.

For example, when it comes to concurrent access to a map the Clojure community generally forces a dichotomy, either stick a standard Clojure map in an atom and get fully atomic semantics at the expense of serial write performance or use a Java ConcurrentMap at the expense of inter-key atomicity (or do a more gnarly atom around a map itself containing atoms which gets quite messy quite fast).

Such a stark tradeoff doesn't need to exist! In theory STM gives you exactly the granularity you need where you can access the keys that you need atomicity for and only those keys together while allowing concurrent writes to anything else that doesn't touch those keys (this is exactly how e.g. the stm-containers library for Haskell works that's linked elsewhere).