This is exactly the intention of optional/gradual type systems. You can prototype away just as freely as with a purely dynamic language. Then when you have learned enough to be more definitive you can start putting in types. Dart is a good exponent of this (I think). Its a fully dynamic language but the compiler will catch the majority of static type errors (for types you define) and you can run it in checked mode during testing to catch other type errors. Typescript also has optional types (without runtime checking) but when I tried it it seemed to be harder to mix and match dynamic/static.