Y-combinator

Y combinator implementation written in C++14.
使用C++ 14实现的Y combinator。

Basically, the Y combinator is something that allows you to write a recursive lambda in a language(e.g. C++ and Java) that doesn't support self-referencing lambda.
简单地说，Y combinator可以让你在不支持lambda自我引用的语言（如C++或Java）中，写出递归的lambda。

For example, if you want to write this function as a lambda:
例如，如果你想将这个函数写为lambda：

void recursion() { 
  recursion(); 
}

You would be wondering how to call it because lambdas don't have names:
你会想，我该如何调用它呢？lambda又没有名字：

[] { how_to_call_myself???(); };

Most people will figure out this, but even this seemingly promising solution won't work, try it yourself! (actually, this trick works in Swift but not for C++ and Java):
大部分人会搞出这个，但即使这种写法看起来挺靠谱，实际上却不行，你可以试试！（实际上这种写法在Swift里可以，但在C++和Java中却不行）

auto recursion = [] { recursion(); };

But with my Y combinator implementation, you could easily do it:
但有了我写的Y combinator，你可以很容易地做到：

#include <Y.h>

int main() {

  // this recursive lambda never returns, it's just calling itself again and again until stack overflow
  // 这个递归的lambda不返回，它只是疯狂地调用他自己直到栈用完
  Y<void>(
    [](auto self) { self(); }
  );

  // this recursive lambda actually does something, it calculates factorial
  // 这个递归的lambda终于干了点正事，它计算阶乘
  unsigned int fac_of_3 = Y<unsigned int>(
    [](auto self, unsigned int n) -> unsigned int {
      if (n == 0)
        return 1;
      else
        return n * self(n - 1);
    },
    3));
  
}

For more information, please visit Source Code.
了解更多信息，请看源代码。

You might be wondering can we implement Y in Java? The answer is no because Java has baby FP ability. (Fxck Java by the way...)
你可能在想，我们可以在Java中实现Y吗？答案是否定的，因为Java的函数式编程能力不够强。（顺便说一句，傻x Java。。。）