This is a reason why I like languages that have REPLs, and perhaps an advantage of dynamically typed, and especially functional languages that I feel is often not emphasized enough (judging by the recent discussions on the issue).

I love how I can just open up IEx (Elixir) and work my way through various functions in my app interactively. If something doesn't work as expected, I change the code and run the 'recompile' command. Then, once things work and eventually stabilize, I add typespecs for at least some of the benefits of an explicitly typed language. In some cases I might make (unit) tests part of the process, but that depends on the situation.