Rewriting the main sum method

We can optimize the main sum method by making it tail-recursive. This can be done with an accumulator as follows:

def sum(f: Int => Int, a: Int, b: Int) = {
     def loop(a: Int, acc: Int): Int =
          if (a>b) acc
          else loop(a+1, f(a) + acc)
     loop(a, 0)
}