A Result type example for C

result-example.c

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

#define OUTCOME_SUCCESS 0

typedef double DivResultOk;

// TODO For a generic error handling, you coud add a polimorphic interface here. This could help for generec error handling and error chaining.
// TODO Downcasting could be a challenge, but still possible.
typedef const char* DivResultErr;

// Tagged union
typedef struct {
    enum {
        DIV_OUTCOME_SUCCESS = OUTCOME_SUCCESS,
        DIV_OUTCOME_ERR_DIV_BY_ZERO,
        DIV_OUTCOME_ERR_OVERFLOW,
        DIV_OUTCOME_ERR_OVERFLOW2,
        // other errors here...
    } outcome;
    union {
        DivResultOk value;
        DivResultErr err;
    };
} DivResult;

static DivResult divide_by(int x, int y) {
    if (y == 0) {
        return (DivResult){
            .outcome = DIV_OUTCOME_ERR_DIV_BY_ZERO,
            .err = "divide_by: cannot divide by zero",
        };
    }
    if (x > 1000 || y > 1000) {
        return (DivResult){
            .outcome = DIV_OUTCOME_ERR_OVERFLOW2,
            .err = "divide_by: overflow: numbers too high",
        };
    }
    return (DivResult){
        .outcome = DIV_OUTCOME_SUCCESS,
        .value = (double) x / y,
    };
}

int main() {
    {
        int x = 10;
        int y = 0;
        DivResult result = divide_by(x, y);
        if (result.outcome != OUTCOME_SUCCESS) {
            fprintf(stderr, "error: %s\n", result.err);
        } else {
            printf("ok: %d / %d == %lf\n", x, y, result.value);
        }
    }
    {
        int x = 10;
        int y = 1001;
        DivResult result = divide_by(x, y);
        switch(result.outcome) {
            case OUTCOME_SUCCESS:
                // noop
                break;
            case DIV_OUTCOME_ERR_DIV_BY_ZERO:
            case DIV_OUTCOME_ERR_OVERFLOW:
                fprintf(stderr, "error: %s\n", result.err);
                break;
            default:
                fputs("error: assertion failure: unrecognized error on DivResult outcome.\n", stderr);
                assert(false);
        }
        printf("ok: %d / %d == %lf\n", x, y, result.value);
    }
    {
        int x = 10;
        int y = 5;
        DivResult result = divide_by(x, y);
        if (result.outcome != OUTCOME_SUCCESS) {
            fprintf(stderr, "error: %s\n", result.err);
        } else {
            printf("ok: %d / %d == %lf\n", x, y, result.value);
        }
    }
    return 0;
}