You can use
digits = map (`mod` 10) . reverse . takeWhile (> 0) . iterate (`div` 10)
or for reverse order
rev_digits = map (`mod` 10) . takeWhile (> 0) . iterate (`div` 10)
The iterate part generates an infinite list dividing the argument in every step by 10, so 12345 becomes [12345,1234,123,12,1,0,0..]. The takeWhile part takes only the interesting non-null part of the list. Then we reverse (if we want to) and take the last digit of each number of the list.
I used point-free style here, so you can imagine an invisible argument n on both sides of the "equation". However, if you want to write it that way, you have to substitute the top level
digits n = map(`mod` 10) $ reverse $ takeWhile (> 0) $ iterate (`div`10) n