[Haskell-cafe] How to do this in FP way?

[Magicloud Magiclouds]

Hello,
    I am getting familiar with FP now, and I have a "program design" kind of
question.
    Say I have something like this in C:
static int old;
int diff (int now) { /* this would be called once a second */
  int ret = now - old;
  old = now;
  return ret;
}
    Because there is no "variable" in Haskell. So how to do this in a FP
way?

Thanks.
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