If we think about it, the whole point of functional programming is to have immutability and recursive definitions (or inductive definitions) of programs so that they can be analyzed easily. In general, adding additional constraints on your program would make it simpler to analyze but would reduce what you can do with it. Functional programming, of course, adds additional constraints on imperative programming in the form of immutability, that is, you are no longer allowed to reassign a variable. This is done so that the analysis of the program, that is, understanding how the program works, is now simpler. It is also simpler to prove theorems about the programs. However, we also lose some of the things that we could do without such restrictions. It turns out that any program can be rewritten in a functional style in a way to produce the same results. However, no guarantees are made about their performance or complexity in general. So, a functional version...