Thank you for posting this.

I always looked at these features of being able to extend the language beyond some commonly accepted practices as detrimental to the language. I've spent way too much time debugging issues with operator overloading or complex templates (C++), or obscure side effects in DSLs. 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.

But... looks like the professional community knows this and so maybe it's time to take a deeper dive :)