> Repositories with no mutable reference usages: 7,245 (63%)

> Truly bananas. Clojure libraries really do have less state. It probably isn't surprising if you've been programming in clojure for any length of time, but it's pretty wild to see the data back this up. I don't think I would have believed this 10 years ago.

Truly interesting and impressive, although I'll note that there are plenty of ways (most of them via Java interop) to introduce mutability of some kind into a Clojure program besides the first-class reference types (atoms/refs/agents/volatiles.)

I'd be interested in an analysis in what percentage of Clojure functions are truly referentially transparent, but that's difficult (if not impossible) to determine statically.

Fair point, I tried to word that statistic accurately without being too verbose.

> I'd be interested in an analysis in what percentage of Clojure functions are truly referentially transparent, but that's difficult (if not impossible) to determine statically.

Absolutely, I'm not sure it's possible or even practical to measure functional purity in an absolute sense, but there's definitely room for improvement:

- measure Java Interop

- measure usage of mutation functions (eg. swap!, set!, binding, send, vswap!, alter, etc)

- measure (def ^:dynamic *my-var*) usages

My intuition is that the numbers wouldn't change that much, but I'd rather have the data than guess.

Isn't the meta-lesson of the history of programming languages from the past 60 years, basically that great language design should take into account the ecosystem level?

The success or failure of a language has really depended on the health of its interaction with its community and ecosystems, much more so than narrow technical merit of the language.

Name a language, and its history bears this out. (Clojure as well.)

Like others, I've been telling Clojure newbies something like: When you need Clojure STM, default to using an atom unless you really know you need to use something else.

In Haskell I can add a handful of lines to parallelize an "embarrassingly parallel" computation. For example, there are 66,960,965,307 atomic lattices on 6 atoms. A 20 core Mac Studio can figure this out by reverse search in just over four minutes; divvy the work up into piles, and have everyone count the work in their pile. For the problems I care about, everyone's looking for needles in a haystack; they can report what they found with no concern for what anyone else is doing.

So what's the dumbest "try this first" approach in Clojure?

All of these options are, IME, relatively easy to drop in to some extant data processing pipeline to parallelize it, and probably require a similar level of finagling to what you're used to in Haskell.

[0] (->> some-lazy-seq (pmap ...) (reduce ...)) goes pretty far, but nesting/composing pmaps or doing i/o doesn't always work particularly well since the JVM [currently] uses OS threads rather than something like the lightweight threads GHC provides.

[1] https://clojure.org/reference/reducers

[2] https://github.com/clj-commons/claypoole

[3] https://github.com/aphyr/tesser

For example right now in Clojure, when you have concurrent access to a map, you're forced to choose between either atomic, but entirely serial writes (wrap the map in an atom) or per-key concurrency, but no inter-key atomicity (either use nested atoms or use ConcurrentHashMap).

But this false dilemma has all the hallmarks of complection. It's an all-or-nothing choice brought on by an overly coarse idea of atomicity. You could instead use a map structure built on top of STM to get exactly the amount of atomicity you need. If you need two keys to be modified in the same transaction then they get modified atomically. If you need another key to be modified in parallel, then that can happen. The amount of atomicity you need is specified dynamically and on the fly, instead of bound inextricably to a predetermined choice.

I find myself wishing for this kind of tool whenever I have concurrent write contention on a map. Yes I usually bite the bullet and just accept forced serial writes to an atom, but I do so begrudgingly.

Other ecosystems with more ergonomic STM systems use this to great effect (e.g. Haskell's stm-containers library: https://hackage.haskell.org/package/stm-containers).

I guess it depends upon what you mean by "per key", but if your map is a ref, and the values are refs, you can get per-value concurrency with inter-value atomicity.

You couldn't concurrently add new keys though, since that's changing the map's ref.

I think single atom with serial writes is still better. Firstly, I think it’s more performant still, serial updates on atom are fast. And secondly, it’s much easier to reason about such code, much easier to test and debug it.

> When you need Clojure STM, you usually don't, there is usually another way. But if you really, really do need it, default to using an atom unless you really know you need to use something else.

    |              | ref | agent | atom | var |
    | coordinated  | x   |       |      |     |
    | asynchronous |     | x     |      |     |
    | retriable    | x   |       | x    |     |
    | thread-local |     |       |      | x   |

I do like that 63% (!!) of clojure repos have no mutable references at all - that tracks very strongly with my experience. And that the average number of mutable references is less than 2! Immutability can carry you a long ways, and I love that I can trust that contract. On the other hand, it's nice that I can opt in to mutation really easily if I need it.

I wonder how that would skew things.