Wat.hs

-- Exercise 1

-- converts positive Integers to a list of digits.
toDigits :: Integer -> [Integer]
toDigits x = reverse (toDigitsRev x)

toDigitsRev :: Integer -> [Integer]
toDigitsRev x
  | x <= 0 = []
  | otherwise = (mod x 10) : toDigitsRev (div x 10)

Wat2.hs

-- Exercise 1

import Data.List
import Data.Tuple

-- converts positive Integers to a list of digits.
toDigits :: Integer -> [Integer]
toDigits x = reverse (toDigitsRev x)

toDigitsRev :: Integer -> [Integer]
toDigitsRev = unfoldr $ \x ->
  if divMod x 10 == (0, 0)
  then Nothing
  else Just $ swap $ divMod x 10

Wat3.hs

import Data.List
import Data.Tuple

toDigits :: Integer -> [Integer]
toDigits = reverse . toDigitsRev

toDigitsRev :: Integer -> [Integer]
toDigitsRev = unfoldr $ maybeSwap' . splitLastDigit
  where
    maybeSwap' = uncurry maybeSwap
    splitLastDigit = (`divMod` 10)

maybeSwap :: Integer -> Integer -> Maybe (Integer, Integer)
maybeSwap 0 0 = Nothing
maybeSwap x y = Just (y, x)

main = do
  putStrLn $ show $ toDigits 12345

Wat4.hs

import Data.List
import Data.Tuple

toDigits x = reverse $ unfoldr (\y -> let z = swap (divMod y 10) in if z == (0, 0) then Nothing else Just z) x
doubleEveryOther xs = reverse $ map (uncurry (*)) $ zip (reverse xs) (cycle [1, 2])
sumDigits xs = sum $ concatMap (toDigits) xs
validate x = (== 0) $ (`mod` 10) $ sumDigits $ doubleEveryOther $ toDigits x

main = do
  putStrLn $ show $ validate 101071 -- True