> He clearly hasn't proven anything about his code.
Is that a bad thing? It seems like many people are able to write software that works well and gets the job done without "proving anything" about their code in the way you're describing. Personally, I'm fine with not proving anything if I can deliver quickly and everything works, but I'd like to be convinced otherwise if there's real value there.
As an Emacs user, from my experience the Clojure REPL has much better integration with my editor than any non-LISP I've used, which includes Haskell and Erlang. Most of the time I'm not typing commands into the REPL - instead, I'm evaluating code inline, which means I don't need to jump back and forth from the source file to the REPL. It's a much more interactive and productive experience for me, and I haven't been able to fully replicate anywhere else - though it's possible I haven't discovered the right tools for those other languages yet.
Is that a bad thing? It seems like many people are able to write software that works well and gets the job done without "proving anything" about their code in the way you're describing. Personally, I'm fine with not proving anything if I can deliver quickly and everything works, but I'd like to be convinced otherwise if there's real value there.