> So, (re)defining language constructs in a project seems nightmarish to me to support in production and therefore I never even attempted anything serious in a functional programming language.
It can be, but also not. If you isolate them into libraries with clear interfaces, you can kind of avoid that. I think clojure.core.async is an excellent showcase in something you couldn't do in other languages, where asynchronous channels were possible to add to the core language without changing anything in the core compiler itself, and because of the small interface, you can still use it without ending up with nightmares :)
Understood. I was more referring to a case where a project would start defining its own language constructs and if each project would do that then every single project would come with a very high cognitive load.
It can be, but also not. If you isolate them into libraries with clear interfaces, you can kind of avoid that. I think clojure.core.async is an excellent showcase in something you couldn't do in other languages, where asynchronous channels were possible to add to the core language without changing anything in the core compiler itself, and because of the small interface, you can still use it without ending up with nightmares :)