def square(x):
      return x*x

    def derivative(f, h):  # approximate numerical derivative
      def df(x):
        return (f(x+h) - f(x)) / h
      return df

    print square(3)  # prints 9
    print derivative(square,0.0001)(3)   # prints approximately 6

You could write something like "derivative" in C with function pointers,
but it would be much messier.