Convert 2D function to 1D function

For Education Purposes

I have a function which takes the derivative of a given input function.

//In analysis class 
double analysis::differentiateBest(std::function<double(double)> fn, double x)
{
    return derivative_1_Richardson_6O(fn,  x);
}

The way I call this function is by using a lambda in place of the std::function

analysis al;
std::cout << al.differentiateBest([](double x) { return x*x*x*x*x*x; }, 2)

I want to extend the derivative to Higher Dimension. ie.Take a gardient.

// matrix<T> my class for holding matrices , implemented as 1D std::vector
matrix<double> analysis::gradient_2D(std::function<double(double, double)> fn,double x, double y)
{
    matrix<double> R(2, 1);

//DOUBT
    std::function<double(double xVar)> fnX = fn( xVar, y); //ERROR
    // creates a new fn which holds y constant and allows x to vary  

    std::function<double(double yVar)> fnY = fn( x, yVar); //ERROR
    // creates a new fn which holds x constant and allows y to vary 

    R(1, 1) = differentiateBest(fnX,x); // Takes derivative in X direction , holding y constant 
    R(1, 2) = differentiateBest(fnY,y);  // Takes derivative in Y direction , holding x constant 

return R; 
}

So the way I will call gradient is

analysis al; 
matrix<double> R = al.gradient_2D([](double x,double y) { return x*x*x*y + y; }, 2 ,3) ; 
// Takes the gradient of lambda function at the point  2 , 3 

This is what I want to do. But I get error in VS at this line

    std::function<double(double xVar)> fnX = fn( xVar, y); 
    std::function<double(double yVar)> fnY = fn( x, yVar);

I want to make the 2D function into a 1D function , by fixing the x or y value. The x or y values are fixed at the point at which we want to take the gradient.

So my question is how can I convert a std::function taking 2 variables into a function taking a single variable .

PS- I know it will be easier to use a library , but I would like to learn to do this myself , hence educational at top . -Thank you