void
display(Object *root) {
    if (root == NULL) {
        return;
    }

    switch (root->type) {
        case OBJ_TYPE_FIXNUM: {
            printf("%zd", root->fixnum);
        } break;
        case OBJ_TYPE_BOOL: {
            if (root->boolean) {
                printf("true");
            } else {
                printf("false");
            }
        } break;
        case OBJ_TYPE_NIL: {
            printf("()");
        } break;
        case OBJ_TYPE_STRING: {
            printf("\"%.*s\"", (int)root->string_n, root->string);
        } break;
        case OBJ_TYPE_SYMBOL: {
            printf(":%.*s", (int)root->symbol_n, root->symbol);
        } break;
        case OBJ_TYPE_PAIR: {
            printf("(");
            display_pair(root);
            printf(")");
        } break;
        default: {
            printf("TYPE NOT IMPLEMENTED FOR DISPLAY.");
        } break;
    }
}

Object *
eval(Object *root) {
    if (root == NULL) {
        return NULL;
    }

    switch (root->type) {
        case OBJ_TYPE_FIXNUM:
        case OBJ_TYPE_BOOL:
        case OBJ_TYPE_NIL:
        case OBJ_TYPE_STRING:
        case OBJ_TYPE_SYMBOL: {
            return root;
        } break;
        case OBJ_TYPE_PAIR: {
            if (root->car->type == OBJ_TYPE_SYMBOL) {
                Object *value = find_environment_symbol(root->car);
                if (value == NULL) {
                    printf("error: symbol not found: `");
                    display(root->car);
                    printf("`\n");
                    return NULL;
                }
                if (value->type == OBJ_TYPE_PROCEDURE) {
                    return value->proc(root->cdr);
                }
            }
        } break;
        default: {
            printf("error: can't eval type %d.\n", root->type);
        } break;
    }

    return NULL;
}

Object *
proc_add(Object *args) {
    ssize_t tot = 0;
    while (args->type == OBJ_TYPE_PAIR) {
        Object *car = eval(args->car);
        if (car->type != OBJ_TYPE_FIXNUM) {
            fprintf(stderr, "addition not supported for type %d\n", car->type);
            return NULL;
        }
        tot += car->fixnum;
        args = args->cdr;
    }
    return make_fixnum(tot);
}

Object *
proc_sub(Object *args) {
    if (args->type != OBJ_TYPE_PAIR) {
        fprintf(stderr, "substraction not supported for type %d\n", args->type);
        return NULL;
    }

    // Extract first parameter.
    Object *car = eval(args->car);
    args = args->cdr;
    ssize_t tot = car->fixnum;

    while (args->type == OBJ_TYPE_PAIR) {
        Object *car = eval(args->car);
        if (car->type != OBJ_TYPE_FIXNUM) {
            fprintf(stderr, "substraction not supported for type %d\n", car->type);
            return NULL;
        }
        tot -= car->fixnum;
        args = args->cdr;
    }
    return make_fixnum(tot);
}

Object *
proc_mul(Object *args) {
    ssize_t tot = 1;
    while (args->type == OBJ_TYPE_PAIR) {
        Object *car = eval(args->car);
        if (car->type != OBJ_TYPE_FIXNUM) {
            fprintf(stderr, "multiply not supported for type %d\n", car->type);
            return NULL;
        }
        tot *= car->fixnum;
        args = args->cdr;
    }
    return make_fixnum(tot);
}

Object *
proc_div(Object *args) {
    if (args->type != OBJ_TYPE_PAIR) {
        fprintf(stderr, "substraction not supported for type %d\n", args->type);
        return NULL;
    }

    // Extract first parameter.
    Object *car = eval(args->car);
    args = args->cdr;
    ssize_t tot = car->fixnum;

    while (args->type == OBJ_TYPE_PAIR) {
        Object *car = eval(args->car);
        if (car->type != OBJ_TYPE_FIXNUM) {
            fprintf(stderr, "div not supported for type %d\n", car->type);
            return NULL;
        }
        tot /= car->fixnum;
        args = args->cdr;
    }
    return make_fixnum(tot);
}