by the time you have your code in its final state (i.e. you're done experimenting) and shaken out the bugs, your types are mostly static; they're just implicitly so. adding annotations and a typechecker helps you maintain that state and catch the few places where type errors might still have slipped through despite all your tests (e.g. lesser-used code paths that need some rare combination of conditions to hit them all but will pass an unexpected type through the call chain when you do). it is very unlikely that you will introduce bugs at this point.