在C ++中的定点组合器 [英] Fixed point combinators in C++

问题描述

我对使用固定点组合器的实际示例感兴趣（例如 y-combinator < a> in C ++。你曾经使用过一个固定点组合器 egg 或绑定真实的实时代码？

I'm interested in actual examples of using fixed point combinators (such as the y-combinator in C++. Have you ever used a fixed point combinator with egg or bind in real live code?

我发现这个例子在egg有点密集：

I found this example in egg a little dense:

void egg_example()
{
    using bll::_1;
    using bll::_2;

    int r =
        fix2(
            bll::ret<int>(
                // \(f,a) -> a == 0 ? 1 : a * f(a-1)
                bll::if_then_else_return( _2 == 0,
                    1,
                    _2 * lazy(_1)(_2 - 1)
                )
            )
        ) (5);

    BOOST_CHECK(r == 5*4*3*2*1);
}

你能解释一下这一切是如何工作的吗？

Can you explain how this all works?

有一个很好的简单例子可能使用bind，可能比这个更少的依赖性？

Is there a nice simple example perhaps using bind with perhaps fewer dependancies than this one?

推荐答案

是相同的代码转换为 boost :: bind 注意y组合器及其应用程序网站在main函数。我希望这有助于。

Here is the same code converted into boost::bind notice the y-combinator and its application site in the main function. I hope this helps.

#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <iostream>

// Y-combinator compatible factorial
int fact(boost::function<int(int)> f,int v)
{
  if(v == 0)
    return 1;
  else
    return v * f(v -1);
}

// Y-combinator for the int type
boost::function<int(int)>
    y(boost::function<int(boost::function<int(int)>,int)> f)
{
  return boost::bind(f,boost::bind(&y,f),_1);
}


int main(int argc,char** argv)
{
  boost::function<int(int)> factorial = y(fact);
  std::cout << factorial(5) << std::endl;
  return 0;
}

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