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.