A badly written monad in C

optional.c

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

#define OPTIONAL_INIT(t) typedef struct optional_ ## t { short isPresent; t value; } optional_ ## t
#define OPTIONAL(t) optional_ ## t

static short _optional_empty = 0;
struct optional_any { short isPresent; int value; };
static inline struct optional_any *_unit(size_t os, size_t vs, const char *v) {
    struct optional_any *o = malloc(os);
    o->isPresent = 1;
    memcpy(&(o->value), v, vs);
    return o;
}

#define OPTIONAL_UNIT(t, v) ((OPTIONAL(t) *)_unit(sizeof(OPTIONAL(t)), sizeof(t), (const char *)&v))
#define OPTIONAL_EMPTY(t) ((OPTIONAL(t) *)&_optional_empty)
#define OPTIONAL_BIND(t, o, f) (o->isPresent ? f(o->value) : OPTIONAL_EMPTY(t))

/* Declare an optional int type */
OPTIONAL_INIT(int);

OPTIONAL(int) *maybe_double(int arg) {
    int val = arg * 2;
    return OPTIONAL_UNIT(int, val);
}

OPTIONAL(int) *maybe_plus_two(int arg) {
    int val = arg + 2;
    return OPTIONAL_UNIT(int, val);
}

typedef OPTIONAL(int) * (*fn)(int);

/* composition in C: messy. */
static fn f = maybe_double;
static fn g = maybe_plus_two;
OPTIONAL(int) *f_bind_g(int v) {
    return OPTIONAL_BIND(int, f(v), g);
}

/* Helper for right identitiy law */
OPTIONAL(int) *unit(int v) {
    return OPTIONAL_UNIT(int, v);
}

int main(int argc, char **argv) {
    int myval = 5;
    OPTIONAL(int) *optint = OPTIONAL_UNIT(int, myval);

    // left identity: bind(unit(v), f) === f(v)
    OPTIONAL(int) *left1 = OPTIONAL_BIND(int, optint, maybe_double);
    OPTIONAL(int) *left2 = maybe_double(myval);
    assert(left1->value == left2->value);

    // right identity: bind(m, unit) === m
    OPTIONAL(int) *right = OPTIONAL_BIND(int, optint, unit);
    assert(right->value == optint->value);

    // associativity
    OPTIONAL(int) *assoc1 = OPTIONAL_BIND(int, OPTIONAL_BIND(int, optint, maybe_double), maybe_plus_two);
    OPTIONAL(int) *assoc2 = OPTIONAL_BIND(int, optint, f_bind_g);
    assert(assoc1->value == assoc2->value);

    return 0;
}