No, it doesn't. Pure FP is about pure functions, meaning only that they don't have side-effects (essentially just that you could replace the function call with

No, it doesn't. Pure FP is about pure functions, meaning only that they don't have side-effects (essentially just that you could replace the function call with it's return value without changing the semantics of the program). If a function mutates some global state then that would certainly make it impure but a function which takes an input, mutates it and returns the mutated value is absolutely pure.