This is a bit naive though. It depends on what you want to do and whether the language you are using offers the required primitives and other things like persis

This is a bit naive though. It depends on what you want to do and whether the language you are using offers the required primitives and other things like persistent functional data structures. Without those, you will find yourself hard-pressed to make FP happen. It is of course usually possible with most languages (except those where primitives are already mutating and therefore infectiously prevent you from writing pure functions), but it might not be idiomatic at all, or might not be feasible to roll all things your own, to replace any mutating basics. For example imagine having to copy a data structure all over the place again and again, because its methods are mutating its internal state. That would be inefficient, much more inefficient than a well written corresponding functional data structure, and it would be ugly code. Are you going to write that extra data structure, when your task is actually something else? Some management breathing down your neck, asking when something will be done? Or not so well versed coworkers complaining about you adding a lot of code that might need to be maintained by them, while they don't understand FP? Do you even have the knowledge to implement that data structure in the first place, or will you need to study a couple of papers and carefully translate their code, if any, i to your language and then verify expected performance in meaningful benchmarks? Lots of problems can arise there in practice.