Implement static_if using C11 generic selection

static_if.cc

#include <type_traits>
#include <tuple>
#include <iostream>

// Link: https://github.com/aeyakovenko/notes

//count arguments
//COUNT_ARGS :: ... -> Int
#define COUNT_ARGS(...) COUNT_ARGS_(,##__VA_ARGS__,6,5,4,3,2,1,0)
#define COUNT_ARGS_(z,a,b,c,d,e,f,cnt,...) cnt

//call an object
//CALL :: (... -> a) -> (...) -> a
#define CALL(x,y) x y

//HEAD :: [a] -> a
#define HEAD(args) CALL(HEAD_,args)
#define HEAD_(a,...) a

//TAIL :: [a] -> [a]
#define TAIL(args) (CALL(TAIL_,args))
#define TAIL_(a,...) __VA_ARGS__

//UNPACK :: (...) -> ...
#define UNPACK(args) CALL(UNPACK_,args)
#define UNPACK_(...) __VA_ARGS__

#define ISEMPTY(ls) PASTE(ISEMPTY_,CALL_(COUNT_ARGS,PASTE(ISEMPTY_,CALL(COUNT_ARGS,ls))))
#define CALL_(f,a) f(a) 
#define ISEMPTY_0 a,b
#define ISEMPTY_2 1
#define ISEMPTY_1 0

//JOIN :: (...) -> (...) -> (...)
#define JOIN(l1,l2) PASTE(JOIN_,PASTE(ISEMPTY(l1),ISEMPTY(l2)))(l1,l2)
#define JOIN_00(l1,l2) (UNPACK(l1),UNPACK(l2))
#define JOIN_10(l1,l2) l2
#define JOIN_01(l1,l2) l1
#define JOIN_11(l1,l2) ()

//curry a function to it arguments
#define CURRY(f,args) JOIN(f,args)

//evaluate a function object
#define EVAL(f) EVAL_ f 
#define EVAL_(f,...) f(__VA_ARGS__)


//PASTE :: a -> b -> c
//this is really in the preprocessor domain, so hard to type
#define PASTE(aa,bb) PASTE_(aa,bb)
#define PASTE_(aa,bb) aa ## bb


//FOLDR :: (a -> b -> b) -> b -> [a] -> b
#define FOLDR(func,accum,lst)       PASTE(FOLDR_,CALL(COUNT_ARGS,lst))(func,accum,lst)
#define FOLDR_0(func,accum,lst)     accum
#define FOLDR_1(func,accum,lst)     EVAL_FR(CURRY(func,(HEAD(lst), accum)))
#define FOLDR_2(func,accum,lst)     EVAL_FR(CURRY(func,(HEAD(lst),FOLDR_1(func,accum,TAIL(lst)))))
#define FOLDR_3(func,accum,lst)     EVAL_FR(CURRY(func,(HEAD(lst),FOLDR_2(func,accum,TAIL(lst)))))
#define FOLDR_4(func,accum,lst)     EVAL_FR(CURRY(func,(HEAD(lst),FOLDR_3(func,accum,TAIL(lst)))))
#define FOLDR_5(func,accum,lst)     EVAL_FR(CURRY(func,(HEAD(lst),FOLDR_4(func,accum,TAIL(lst)))))
#define FOLDR_6(func,accum,lst)     EVAL_FR(CURRY(func,(HEAD(lst),FOLDR_5(func,accum,TAIL(lst)))))

//the map needs a different eval then the foldr
//i dont fully grok this part yet
#define EVAL_FR(f) EVAL_FR_ f 
#define EVAL_FR_(f,...) f(__VA_ARGS__)

//MAP :: (a -> b) -> [a] -> b
#define MAP(func,lst)  FOLDR((MAP_,func),(),lst)
#define MAP_(func,aa,acc)  JOIN((EVAL_MP(CURRY(func,(aa)))), acc)
#define EVAL_MP(f) EVAL_MP_ f 
#define EVAL_MP_(f,...) f(__VA_ARGS__)


#define static_if(...)	static_if__(__VA_ARGS__, \
    static_if_4, static_if_3)(__VA_ARGS__)
#define static_if__(_1,_2,_3,_4,f,...) f

#define static_if_3(capture, cond, true_branch) \
	_Generic(std::integral_constant<bool, bool(cond)>{}, \
	    std::true_type: []_as_glargs(capture) true_branch, \
	    default: []_as_glargs(capture) {})_as_ids(capture)

#define static_if_4(capture, cond, true_branch, false_branch) \
	_Generic(std::integral_constant<bool, bool(cond)>{}, \
	    std::true_type: []_as_glargs(capture) true_branch, \
	    std::false_type: []_as_glargs(capture) false_branch)_as_ids(capture)

#define _as_ids(lst)	MAP((_as_id), lst)
#define _as_id(a)	_as_id_(a,)
#define _as_id_(a, b)	a ## b

#define _as_glargs(lst)	MAP((_as_glarg), lst)
#define _as_glarg(a)	auto&& _as_id(a)

template <typename T>
void test_strlen(T const& s)
{
	static_if((s), (std::is_same<T, std::string>{}),
	{
		std::cout << s.size() << std::endl;
	},
	{
		std::cout << std::char_traits<char>::length(s) << std::endl;
	});
}

template <int flag>
void test_empty()
{
	static_if((), flag,
	{
		std::cout << "called" << std::endl;
	});
}

void test_multi()
{
	int a, b, c, d, e, f;

	static_if((a, b, c, d, e, f), true,
	{
		std::tie(a, b, c, d, e, f) = std::make_tuple(1, 2, 3, 4, 5, 6);
	});

	std::cout << a << b << c << d << e << f << std::endl;
}


int main()
{
	test_empty<1>();
	test_empty<0>();
	test_multi();
	test_strlen("wor\0d");
	test_strlen(std::string("wor\0d", 5));
}