fn fib(n: int, f: fn (num: i64) -> bool) -> (i64, int) {
    if n < 0 {
        // Let these variables be mutated, otherwise too slow
        let mut n1:i64 = 0, n2:i64 = -1, i:int = 0, tmp:i64;
        while i > n {
            f(n1);
            tmp = n1-n2;
            if (tmp > 0 && n2 > 0) { //Detect overflow
                io::println("\nReached the limit of i64, halting");
                return (n1, i);
            }
            n1 = n2;
            n2 = tmp;
            i -= 1;
        }
        (n1+n2, n)
    } else if n > 0 {
        // And these variables
        let mut n1:i64 = 0, n2:i64 = 1, i:int = 0, tmp:i64;
        while i < n {
            f(n1);
            tmp = n1+n2;
            if (tmp < 0) { //Detect overflow
                io::println("\nReached the limit of i64, halting");
                return (n1, i);
            }
            n1 = n2;
            n2 = tmp;
            i += 1;
        }
        (n2-n1, n)
    } else {
        f(0);
        (0,1)
    }
}

fn main() {
    let args = os::args();
    let n = if args.len() == 1 {
        10
    } else if args.len() > 1 {
        // Convert from a string
        match (int::from_str(args[1])) {
            Some(num)   => num,
            None        => 10 //Fall back to default
        }
    } else {
        /* Required to use the if as an expression.
         * We know that args.len() is always >= 1, the compiler
         * does not. fail lets it know that we can't get past here.
         */
        fail ~"No arguments given, somehow...";
    };

    /* Use the loop protocol to be able to do things
     * with the sequence given, in this case, print them out.
     * The loop itself returns a tuple with where it got to and
     * what the number is.
     */
    let (result, n) = for fib(n) |num| {
        //print out the sequence
        io::print(fmt!("%? ", num));
    };

    io::println(fmt!("\nThe %dth fibonacci number is: %?", n, result));
}