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I have never seen Refs(STM) being used in a Clojure open source project, looks like they are great in theory but in practice they are another story, on other hand, core.async is much more prevalent, I do hear a lot about the use of Haskell's STM thought.

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Yeah I think there is a very real cost to going the STM route when you can have arbitrary side-effects, so it's a lot harder for that tradeoff to be worth it than in haskell. That's not to say there aren't any downsides to a language that prevents uncontrolled side-effects, but being able to make more guarantees is one of it's greatest upsides.

In your run of the mill webapp you rarely have state outside the db and caches. What little you have can often be modeled with a single agent or atom. So no need for refs.

What you do have in webapps is synchronization between concurrent processes. For that the STM primitives don't offer much. The core.async on the other hand is a great fit.

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