My Haskell "Hello World" Practice.

aabb.hs

import Data.List

toListNums :: Int -> [Int]
chrToInt :: Char -> Int
chrToInt chr = read ([chr])
toListNums num = if length rs == 4 then rs else 0:rs where rs = map chrToInt (show num)

ifDistinct :: (Eq a) => [a] -> Bool
ifDistinct [] = True
ifDistinct (x:px) = x `notElem` px && ifDistinct px

noRepeatingDegit :: Int -> Bool
noRepeatingDegit n = if n < 100 then False else ifDistinct lst where lst = toListNums n

countMatch :: (Eq a) => [a] -> [a] -> Int
countMatch [] b = 0
countMatch a [] = 0
countMatch (a0:a) (b0:b) = if a0 == b0 then 1 + rest else rest where rest = countMatch a b

getAB :: Int -> Int -> (Int, Int)
getAB expectNum check = if noRepeatingDegit expectNum && noRepeatingDegit check
  then (getA, getCommon - getA)
  else error "both expectNum and check must not contain repeating digits."
  where getA = countMatch (toListNums expectNum) (toListNums check)
        getCommon = length (filter (`elem` (toListNums expectNum)) (toListNums check))

abFilter :: Int -> (Int, Int) -> Int -> Bool
abFilter expectNum (a, b) check = a == expectA && b == expectB
  where (expectA, expectB) = getAB expectNum check

initialList = filter noRepeatingDegit [0..9999]

askConstraint :: Int -> IO (Int, Int, Int)
askConstraint query = do
  putStrLn ("I ask you: " ++ (intercalate "" (map show (toListNums query))))
  putStrLn "  Type your response in the format _a_b"
  line <- getLine
  if length line /= 4 then error "Invalid input." else do {return 0}
  let [sa, _, sb, _] = line
  let a = read [sa]
  let b = read [sb]
  return (query, a, b)

game :: [Int] -> [(Int, Int, Int)] -> IO [Int]
game remaining constraints = do
  np <- (askConstraint chooseRandom)
  let nCons = np : constraints
  let matchAllConstraints = \guess -> all (\(num, a, b) -> abFilter guess (a, b) num) nCons
  let flt = (filter matchAllConstraints remaining)
  putStrLn ("Progress: " ++ (show (length flt)))
  ret <- if length flt > 1 then game flt nCons else do {return flt}
  return ret
    where remExceptCons = filter (\n -> all (\(query, _, _) -> query /= n) constraints) remaining
          chooseRandom = if length remExceptCons > 0 then head remExceptCons else error "Nothing left to choose from."

main = do
  gResult <- game initialList []
if length gResult /= 0 then putStrLn (show gResult) else putStrLn "Failed."