Partial application with a C++ lambda?

Question

EDIT: I use curry below, but have been informed this is instead partial application.

I\'ve been trying to figure out how one would write a curry function in C++, and i a

Answer 1

A lot of the examples people provided and that i saw elsewhere used helper classes to do whatever they did. I realized this becomes trivial to write when you do that!

#include  // for declval
#include 
#include 

using namespace std;

template< class F, class Arg >
struct PartialApplication
{
    F f;
    Arg arg;

    constexpr PartialApplication( F&& f, Arg&& arg )
        : f(forward(f)), arg(forward(arg))
    {
    }

    /* 
     * The return type of F only gets deduced based on the number of arguments
     * supplied. PartialApplication otherwise has no idea whether f takes 1 or 10 args.
     */
    template< class ... Args >
    constexpr auto operator() ( Args&& ...args )
        -> decltype( f(arg,declval()...) )
    {
        return f( arg, forward(args)... );
    }
};

template< class F, class A >
constexpr PartialApplication partial( F&& f, A&& a )
{
    return PartialApplication( forward(f), forward(a) );
}

/* Recursively apply for multiple arguments. */
template< class F, class A, class B >
constexpr auto partial( F&& f, A&& a, B&& b )
    -> decltype( partial(partial(declval(),declval()),
                         declval()) )
{
    return partial( partial(forward(f),forward(a)), forward(b) );
}

/* Allow n-ary application. */
template< class F, class A, class B, class ...C >
constexpr auto partial( F&& f, A&& a, B&& b, C&& ...c )
    -> decltype( partial(partial(declval(),declval()),
                         declval(),declval()...) )
{
    return partial( partial(forward(f),forward(a)), 
                    forward(b), forward(c)... );
}

int times(int x,int y) { return x*y; }

int main()
{
    printf( "5 * 2 = %d\n", partial(times,5)(2) );
    printf( "5 * 2 = %d\n", partial(times,5,2)() );
}