On the other hand, a generation of world-class programmers were raised on the REPL and interactivity (e.g., Lisp and Smalltalk programmers from the 70s-80s). A

On the other hand, a generation of world-class programmers were raised on the REPL and interactivity (e.g., Lisp and Smalltalk programmers from the 70s-80s). And interactivity can be an extremely effective way of experimenting and learning how things work and fit together so that you can rapidly develop a mental model of your core primitives and how to put them together in different ways. It's also part of the appeal of TDD. Small iterative loops with rapid feedback.