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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.

pad
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EBM
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1 Answers1

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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.

pad
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  • Thanks. Can you explain me the meaning of ref 0 ? – EBM Apr 03 '12 at 23:13
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    See the link I refer to in my answer. Basically `ref 0` is a record `{mutable contents: int}` with `contents` initialized by `0` and `contents` could be reassigned later. – pad Apr 03 '12 at 23:21