Not a program, but the concept of functional programming and pure functions. It makes it a lot easier to think about code, and also makes it easier to test and parallelize code.
Although be warned, it can also have the "side-effect" (functional programming pun!) of making you somewhat insufferable as you try to convince everyone around you that functional programming is amazing.
Although be warned, it can also have the "side-effect" (functional programming pun!) of making you somewhat insufferable as you try to convince everyone around you that functional programming is amazing.