I think if you have to run a whole program to understand a small part of it, you've already lost. The most valuable tools are a REPL that can execute units of y

I think if you have to run a whole program to understand a small part of it, you've already lost. The most valuable tools are a REPL that can execute units of your code, and a language that enforces purity and immutability so that local reasoning is more likely to be sufficient.