How do I save and change the value of a variable in OCaml?

This may be a very newbie question, but I didn't find the answer. I need to store, for example a list and later replace it with another, under the same pointer.

Solution

It can be done via references:

let fact n =
  let result = ref 1 in (* initialize an int ref *)
  for i = 2 to n do
    result := i * !result (* reassign an int ref *)
   done;
  !result

You do not see references very often because you can do the same thing using immutable values inside recursion or high-order functions:

let fact n =
   let rec loop i acc =
      if i > n then acc
      else loop (i+1) (i*acc) in
   loop 2 1

Side-effect free solutions are preferred since they are easier to reason about and easier to ensure correctness.