typedef enum ObjectType {
    OBJ_TYPE_FIXNUM,
    OBJ_TYPE_BOOL,
    OBJ_TYPE_NIL,
    OBJ_TYPE_SYMBOL,
    OBJ_TYPE_STRING,
    OBJ_TYPE_PAIR,
    OBJ_TYPE_PROCEDURE,
    OBJ_TYPE_ERR,
} ObjectType;

typedef struct Object {
    ObjectType type;
    union {
        // OBJ_TYPE_FIXNUM
        ssize_t fixnum;

        // OBJ_TYPE_STRING
        struct {
            char *string;
            size_t string_n;
        };

        // OBJ_TYPE_PAIR
        struct {
            struct Object *car;
            struct Object *cdr;
        };

        // OBJ_TYPE_SYMBOL
        struct {
            char *symbol;
            size_t symbol_n;
        };

        // OBJ_TYPE_PROCEDURE
        struct Object *(*proc)(struct Object *args);
    };
} Object;

//
// Singletons.
//

static Object *obj_nil;
static Object *obj_true;
static Object *obj_false;
static Object *obj_err;

//
// Constructors.
//

Object *
alloc_object(ObjectType type) {
    Object *obj = malloc(sizeof(Object));
    obj->type = type;
    return obj;
}

Object *
make_fixnum(ssize_t num) {
    Object *obj = alloc_object(OBJ_TYPE_FIXNUM);
    obj->fixnum = num;
    return obj;
}

Object *
make_procedure(Object *(*proc)(struct Object *args)) {
    Object *obj = alloc_object(OBJ_TYPE_PROCEDURE);
    obj->proc = proc;
    return obj;
}

Object *
make_pair(Object *car, Object *cdr) {
    Object *obj = alloc_object(OBJ_TYPE_PAIR);
    obj->car = car;
    obj->cdr = cdr;
    return  obj;
}

Object *
make_symbol(StringView sv) {
    Object *obj = alloc_object(OBJ_TYPE_SYMBOL);
    obj->symbol = malloc(sizeof(char) * sv.n);
    memcpy(obj->symbol, sv.start, sv.n);
    obj->symbol_n = sv.n;
    return obj;
}

Object *
make_string(void) {
    Object *obj = alloc_object(OBJ_TYPE_STRING);
    obj->string = NULL;
    obj->string_n = 0;
    return obj;
}

void
append_string(Object *obj, const StringView sv) {
    assert(obj != NULL);
    assert(obj->type == OBJ_TYPE_STRING);
    if (sv.n == 0) {
        return;
    }
    obj->string = realloc(obj->string, (obj->string_n + sv.n) * sizeof(char));
    memcpy(obj->string + obj->string_n, sv.start, sv.n);
    obj->string_n += sv.n;
}