随手撸了一个可以写递归lambda的Y函数

最近学习C++11的variadic template argument，终于可以摆脱用fpmacro模板来复制一大堆代码的做法了，好开心。这个例子的main函数用lambda写了一个斐波那契数列的递归计算函数。跟以往不同的是，在Y函数的帮助下，这个lambda表达是可以成功看到自己，然后递归调用。当然这仍然需要用普通的C++递归来实现，并不是λ-calculus那个高大上的Y Combinator。

#include <functional>

#include <memory>

#include <iostream>

#include <string>

using namespace std;

template<typename TResult, typename ...TArgs>

class YBuilder

{

private:

    function<TResult(function<TResult(TArgs...)>, TArgs...)> partialLambda;

public:

    YBuilder(function<TResult(function<TResult(TArgs...)>, TArgs...)> _partialLambda)

        :partialLambda(_partialLambda)

    {

    }

    TResult operator()(TArgs ...args)const

    {

        return partialLambda(

            [this](TArgs ...args)

            {

                return this->operator()(args...);

            }, args...);

    }

};

template<typename TMethod>

struct PartialLambdaTypeRetriver

{

    typedef void FunctionType;

    typedef void LambdaType;

    typedef void YBuilderType;

};

template<typename TClass, typename TResult, typename ...TArgs>

struct PartialLambdaTypeRetriver<TResult(__thiscall TClass::*)(function<TResult(TArgs...)>, TArgs...)>

{

    typedef TResult FunctionType(TArgs...);

    typedef TResult LambdaType(function<TResult(TArgs...)>, TArgs...);

    typedef YBuilder<TResult, TArgs...> YBuilderType;

};

template<typename TClass, typename TResult, typename ...TArgs>

struct PartialLambdaTypeRetriver<TResult(__thiscall TClass::*)(function<TResult(TArgs...)>, TArgs...)const>

{

    typedef TResult FunctionType(TArgs...);

    typedef TResult LambdaType(function<TResult(TArgs...)>, TArgs...);

    typedef YBuilder<TResult, TArgs...> YBuilderType;

};

template<typename TLambda>

function<typename PartialLambdaTypeRetriver<decltype(&TLambda::operator())>::FunctionType> Y(TLambda partialLambda)

{

    return typename PartialLambdaTypeRetriver<decltype(&TLambda::operator())>::YBuilderType(partialLambda);

}

int _tmain(int argc, _TCHAR* argv[])

{

    auto fib = Y([](function<int(int)> self, int index)

    {

        return index<2

            ?1

            :self(index-1)+self(index-2);

    });

    for (int i = 0; i < 10; i++)

    {

        cout << fib(i) << " ";

    }

    cout << endl;

}