Using a functional language doesn’t make reality and the complexity of state go away (unless you are writing a compiler or a program/system that fully and purely mathematically controls its inputs and outputs).
A good functional language lets you clearly define, manage, and trace where your code meets reality, though.
That kind-of needs strict static type systems, rather than loosey-goosey dynamic tag checking, though.
That's IMO the main reason statically type checked languages like C++/Java/etc work much better at large scale than runtime tag checked languages like Python/Clojure/etc.
I think "needs strict static type systems" is a hypothesis but one with certainly many anecdotal examples to refute it. Most of the studies that have been done (and admittedly these studies are extremely difficult to do well) show lower or similar bug counts in dynamic languages vs static languages.
If you've built large scale system with statically type checked languages that solve all these problems, I assume you never had any bugs right? Never needed to write any unit tests?
