maybe.c

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

enum result_type {OK, ERROR};

#define _RESULT_TYPE_NAME(a, b) _ ## a ## _ ## b ## _result
#define RESULT_TYPE_NAME(a, b) _RESULT_TYPE_NAME(a, b)
#define RESULT_TYPE(ret_type, err_type) struct RESULT_TYPE_NAME(ret_type, err_type)
#define DEFINE_RESULT_TYPE(ret_type, err_type) \
RESULT_TYPE(ret_type, err_type) \
{ \
	enum result_type type; \
	union \
	{ \
		struct { ret_type value; }; \
		struct { err_type error; }; \
	}; \
}

typedef enum {FIRST_OPERAND_NEGATIVE, SECOND_OPERAND_NEGATIVE, BOTH_OPERANDS_NEGATIVE} errnum_t;
DEFINE_RESULT_TYPE(int, errnum_t);

RESULT_TYPE(int, errnum_t) add_natural_nums(int a, int b)
{
        RESULT_TYPE(int, errnum_t) result;

        if (a < 0 || b < 0)
        {
                result.type = ERROR;
                result.error = BOTH_OPERANDS_NEGATIVE;
                return result;
        }

        if (a < 0)
        {
                result.type = ERROR;
                result.error = FIRST_OPERAND_NEGATIVE;
                return result;
        }

        if (b < 0)
        {
                result.type = ERROR;
                result.error = SECOND_OPERAND_NEGATIVE;
                return result;
        }

        result.type = OK;
        result.value = a + b;
        return result;
}

int main(int argc, char **argv)
{
        /* Check args */
        if (argc < 3)
        {
                printf("usage: %s num1 num2\n", argv[0]);
                return 1;
        }

        /* Calculate sum */
        int a = atoi(argv[1]);
        int b = atoi(argv[2]);
        RESULT_TYPE(int, errnum_t) result = add_natural_nums(a, b);

        /* Check result */
        if (result.type == ERROR)
        {
                if (result.error == BOTH_OPERANDS_NEGATIVE)
                {
                        printf("error: both operands %d and %d were negative!\n", a, b);
                }

                if (result.error == FIRST_OPERAND_NEGATIVE)
                {
                        printf("error: first operand %d was negative!\n", a);
                }

                if (result.error == SECOND_OPERAND_NEGATIVE)
                {
                        printf("error: second operand %d was negative!\n", b);
                }
        }
        else if (result.type == OK)
        {
                printf("%d + %d = %d\n", a, b, result.value);
        }

        return 0;
}