You don't get instant live update of existing variables in a REPL. The programming paradigm described is called Reactive Functional, it's the model popularized

You don't get instant live update of existing variables in a REPL. The programming paradigm described is called Reactive Functional, it's the model popularized by spreadsheets. However, spreadsheets can't create new objects programatically; new libraries for web development (React, Vue.js) are popularizing the model, but so far they're using reactive components just for high-level components, not the whole line-by-line program execution. Bret Victor's "Learnable Programming" concept (linked at the end) shows how you could apply the approach to a whole textual program.