It’s a good direction to follow, but it can only get you so far. Some pieces of code do naturally evolve into a functional formalism, while others are inherently imperative. Usually, the top levels of your program (event loop) are imperative and deal with stateful “devices” like IO, the screen and storage subsystems. The “leaf” functions from the call graph can be functional, but you still can’t reason about the whole program when it is imperative at the top.