I wrote a couple macros that record data transiting through code at runtime (it's in Clojure, so basically almost every function is pure, returning what they produce as if it was water flowing out of a faucet), stores these intermediary results in a file, and finally display these values in the code itself, as comments, just below the call-site that produced them.

You can then, for a given call-site, choose to "load" these recorded computations, which will change the displayed comments, both below this call site and all the other instrumented call-sites that are downstream to it, even for code sitting in other source files.

It's a bit fragile and needs more polishing but it's a lot more convenient than any type system that will always get in the way, be not powerful enough, and it allows me to see what kind of data flows in my program without running it. Because I record everything and display the result not at compile-time but at coding time, in the same window, alongside the rest of the code. I don't understand why this was never done (to the best of my knowledge). Biggest limit I encounter is that Clojure doesn't provide any mean to identify areas in my code that are not pure.