maybe.c

/* generic monad */
typedef struct {
    int   has_value;
    void *value;
} Maybe;

/* Per use-case struct */
typedef struct { Maybe m; } MaybeInt;
typedef struct { Maybe m; } MaybeStr;
typedef struct { Maybe m; } MaybePoint;

static MaybeInt maybe_int(int x) {
    int *p = malloc(sizeof *p); *p = x;
    return (MaybeInt){ .m = { 1, p } };
}

static MaybeInt maybe_int_none(void) {
    return (MaybeInt){ .m = { 0, NULL } };
}

static int maybe_int_get(MaybeInt mi) {
    int v = *(int*)mi.m.value;
    free(mi.m.value);
    return v;
}

int x = maybe_int_get(a);