pure_fixpoint_combinator.cpp

// Pure Z combinator in C++14 variadic polymorphic lambdas.

auto fix = [](auto&& f) {
    return [&](auto&& x) {
      return x(x); }([f = static_cast<decltype(f)&&>(f)](auto& x) mutable {
          return [&](auto&&... a) -> decltype(auto) {
              return f([&](auto&&... b) -> decltype(auto) {
                  return x(x)(static_cast<decltype(b)&&>(b)...); },
                  static_cast<decltype(a)&&>(a)...); }; }); };

#include <iostream>
#include <tuple>

int main() {
    std::cout << fix([](auto f, int n) -> int { return n < 2 ? n : n * f(n - 1); })(5) << std::endl;
    fix([](auto f, auto&& a, auto&&... b) {
        std::cout << std::forward<decltype(a)>(a);
        std::get<sizeof...(b) == 0>(std::make_tuple(
            [&](auto&&... b) {
                std::cout << ' ';
                f(std::forward<decltype(b)>(b)...); },
                []{}))(std::forward<decltype(b)>(b)...); })("hello", "world!");
}