How do I get the sums of the digits of a large number in Haskell?

Question

I'm a C++ Programmer trying to teach myself Haskell and it's proving to be challenging grasping the basics of using functions as a type of loop. I have a large number, 50!, and I need to add the sum of its digits. It's a relatively easy loop in C++ but I want to learn how to do it in Haskell.

I've read some introductory guides and am able to get 50! with

sum50fac.hs::

fac 0 = 1
fac n = n * fac (n-1)
x = fac 50
main = print x

Unfortunately at this point I'm not entirely sure how to approach the problem. Is it possible to write a function that adds (mod) x 10 to a value and then calls the same function again on x / 10 until x / 10 is less than 10? If that's not possible how should I approach this problem?

Thanks!

Answer 1

sumd 0 = 0
sumd x = (x `mod` 10) + sumd (x `div` 10)

Then run it:

ghci> sumd 2345
14

UPDATE 1:

This one doesn't generate thunks and uses accumulator:

sumd2 0 acc = acc
sumd2 x acc = sumd2 (x `div` 10) (acc + (x `mod` 10))

Test:

ghci> sumd2 2345 0
14

UPDATE 2:

Partially applied version in pointfree style:

sumd2w = (flip sumd2) 0

Test:

ghci> sumd2w 2345
14

I used flip here because function for some reason (probably due to GHC design) didn't work with accumulator as a first parameter.