The docstring for <!! says "Not intended for use in direct or transitive calls from (go ...) blocks", but what can I expect to go wrong if I do use it there?

you can block your entire go pool and deadlock your app

(he says, from experience)

Glad I don't have to walk where you have 🙏

the go pool is a fixed size thread pool and only expects to be parked by the go, not blocked by other calls. <!! (and other !!) calls can block, ergo that's usually bad, and often only bad like a year after you put it into production. that said, there might be circumstances where you know more (like the chan is a sliding/dropping buffer so never blocks, or whatever)

@winsome you can add -Dclojure.core.async.go-checking=true to your "JVM properties" to help you to avoid <!! calls inside go blocks

(but note that won't protect you from other blocking calls)

and it will complain about !! calls even if they can never block (putting on a newly created chan 1)