Haskell- How to keep track of counter in recursive function

Question

I'm having some difficulty trying to keep track of a counter variable in my function.

I created a function that takes a number as a single parameter and recursively multiplies the number by two, summing all the numbers multiplied by two together, the code below makes it more clear what I intend to do:

sum :: Float -> Float
sum x = x + sum (2*x)

However, the difficulty I am facing, is that I want the function to recurse only ten times. So I want it to stop once ten numbers have been added together. I tried using a counter to keep track of how many times the function has recursed, but to no avail.

This is what I tried:

sumTen :: Float -> Float
sumTen x n
    where
      n=10
       |n == 0 = x
       |otherwise = x + sumTen (2*x) (n-1)

I realize the code above does not work, counter n would be given the value ten through every recursive call, meaning it will never reach the base case n == 0.

What makes this so difficult is that sumTen must be called using one parameter. In the code above, I tried to give the function a default parameter n with a predetermined value, but it clearly does not work.

Can someone help me make the recursive function stop after 'n' recursive calls?

Answer 1

You can use an auxiliary function:

sumNRec x 0     = 0 
sumNRec x times = x + sumNRec (x*2) (times - 1)

sumTen x = sumNRec x 10