Erlang isn't purely functional... the time function is an example. To be pure, iirc, a function call have to return the same result everytime, there's a one to

Erlang isn't purely functional... the time function is an example. To be pure, iirc, a function call have to return the same result everytime, there's a one to one mapping for each parameter there's a unique value. time() is an arity 0 (no parameters) and yet it return different value every time you call it depending on the time currently. You can mutate by making a new copy, you modify the old one and save it to a new variable. The old variable is unchanged. If you set x <- 1, you can't change x anymore but you can do y <- x + 1. Your modification is in a new variable. Elixir you can change x to a new value but that's just a new copy, unless you pin it.. You can also keep state by passing an accumulator via function parameter, it's also how you to tail call optimization. It's a weird pattern and I haven't use functional paradigm enough to give one on top of my head.