sum.c

#include <stdio.h>
#include <stdlib.h>

int sum (int n) {
    if (n <= 0)
        return 0;
    else
        return n + sum(n - 1);
}

int sumLoop (int n) {
    int sum = 0;
    for (int i = n; i > 0; i--)
        sum += i;
    return sum;
}

void main (int argc, char *argv[]) {
    int n = atoi(argv[1]);

    printf("sum(%d) is %d\n", n, sum(n));
    printf("sumLoop(%d) is %d\n", n, sumLoop(n));
}

sum.hs

sum' :: Integer -> Integer
sum' 0 = 0
sum' x = x + sum' (x - 1)