y combinator

y.cc

// main() from https://github.com/simnalamburt/snippets/blob/master/cpp/y-combinator.cpp  which is licensed under the Apache License 2.0.
// Apache-2.0/MIT. 
// --------

#include <stdint.h>
#include <iostream>

template <typename Func>
auto y(Func f) {
    auto g = [f](auto r){
        return [f,r](auto&&...z){
            return f(r(r),z...);
        };
    };
    return g(g);
}

int main() {
    auto fact = y([](auto fact, uint64_t n)->uint64_t {
        return n == 0 ? 1 : n * fact(n-1);
    });
    auto gcd = y([](auto gcd, uint64_t a, uint64_t b)->uint64_t {
        return b == 0 ? a : gcd(b, a%b) ;
    });
    std::cout << fact(15) << std::endl;
    std::cout << gcd(135, 225) << std::endl;
    return 0;
}