Common Lisp lets you do that, but Clojure doesn't - I don't know about Ruby, but I expect not. That said, you have the wrong mental model. You don't run the who

Common Lisp lets you do that, but Clojure doesn't - I don't know about Ruby, but I expect not. That said, you have the wrong mental model. You don't run the whole program and set breakpoints. Instead you directly interact with your program-space. Imagine you have a function that retrieves something from an API. So you execute that function and see what it returns. You change that data around a little to fit your usecase, and then hand it off to the function that acts on the API result. That will produce more data; maybe this data is wrong, so you change the function, recompile it and load it into the program space, and try again with the same data you just changed (which is still right there in the variable you defined). Does that make it clearer?