Maybe Monad in C

#include <stdio.h>

#define PP_NARG(...) \
         PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
#define PP_NARG_(...) \
         PP_ARG_N(__VA_ARGS__)
#define PP_ARG_N( \
          _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
         _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
         _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
         _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
         _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
         _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
         _61,_62,_63,N,...) N
#define PP_RSEQ_N() \
         63,62,61,60,                   \
         59,58,57,56,55,54,53,52,51,50, \
         49,48,47,46,45,44,43,42,41,40, \
         39,38,37,36,35,34,33,32,31,30, \
         29,28,27,26,25,24,23,22,21,20, \
         19,18,17,16,15,14,13,12,11,10, \
         9,8,7,6,5,4,3,2,1,0

#define CAT(a, ...) PRIMITIVE_CAT(a, __VA_ARGS__)
#define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__

#define IIF(c) PRIMITIVE_CAT(IIF_, c)
#define IIF_0(t, ...) __VA_ARGS__
#define IIF_1(t, ...) t

#define COMPL(b) PRIMITIVE_CAT(COMPL_, b)
#define COMPL_0 1
#define COMPL_1 0

#define INC(x) PRIMITIVE_CAT(INC_, x)
#define INC_0 1
#define INC_1 2
#define INC_2 3
#define INC_3 4
#define INC_4 5
#define INC_5 6
#define INC_6 7
#define INC_7 8
#define INC_8 9
#define INC_9 9

#define DEC(x) PRIMITIVE_CAT(DEC_, x)
#define DEC_0 0
#define DEC_1 0
#define DEC_2 1
#define DEC_3 2
#define DEC_4 3
#define DEC_5 4
#define DEC_6 5
#define DEC_7 6
#define DEC_8 7
#define DEC_9 8

#define CHECK_N(x, n, ...) n
#define CHECK(...) CHECK_N(__VA_ARGS__, 0,)
#define PROBE(x) x, 1,

#define NOT(x) CHECK(PRIMITIVE_CAT(NOT_, x))
#define NOT_0 PROBE(~)

#define BOOL(x) COMPL(NOT(x))
#define IF(c) IIF(BOOL(c))

#define EAT(...)
#define EXPAND(...) __VA_ARGS__
#define WHEN(c) IF(c)(EXPAND, EAT)

#define EMPTY()
#define DEFER(id) id EMPTY()
#define OBSTRUCT(...) __VA_ARGS__ DEFER(EMPTY)()
#define EXPAND(...) __VA_ARGS__

#define EVAL(...)  EVAL1(EVAL1(EVAL1(__VA_ARGS__)))
#define EVAL1(...) EVAL2(EVAL2(EVAL2(__VA_ARGS__)))
#define EVAL2(...) EVAL3(EVAL3(EVAL3(__VA_ARGS__)))
#define EVAL3(...) EVAL4(EVAL4(EVAL4(__VA_ARGS__)))
#define EVAL4(...) EVAL5(EVAL5(EVAL5(__VA_ARGS__)))
#define EVAL5(...) __VA_ARGS__

#define MAYBE_BIND(count, condition, x1, x2, ...) ({ \
        typeof(x1) __res0 = (x1); \
        condition(__res0) ? __res0 : \
            IF(count) \
            ( \
                OBSTRUCT(MAYBE_BIND_INDIRECT) () \
                ( \
                    DEC(count), condition, x2, __VA_ARGS__ \
                ), \
                (x2) \
            ) \
        ; \
    })

#define MAYBE_BIND_INDIRECT() MAYBE_BIND

#define MAYBE(condition, x1, ...) (EVAL(MAYBE_BIND(DEC(PP_NARG(__VA_ARGS__)), condition, x1, __VA_ARGS__)))

#define IS_ERROR(x) ((x) < 0)

int f1() {
    printf("f1()\n");
    return 0;
}
int f2() {
    printf("f2()\n");
    return -1;
}
int f3() {
    printf("f3()\n");
    return 2;
}
int f4() {
    printf("f4()\n");
    return 5;
}

int main(void) {

    int res = MAYBE(IS_ERROR,
            f1(),
            f2(),
            f3(),
            f4()
        );
    printf("res = %i\n\n", res);
    // f1()
    // f2()
    // res = -1

    int a, b, c;
    res = MAYBE(IS_ERROR,
            a = f1(),
            b = f3(),
            c = f4(),
            a + b + c
        );
    printf("res = %i\n", res);
    // f1()
    // f3()
    // f4()
    // res = 7

    return 0;
}