Tesla interview question

TeslaInterview.hs

{-
My attempt at a Tesla interview question:

Minimum number of characters to delete from a string so that each character
appears unique number of times. Note: You can delete all occurances of
characters.

eg: "aaaabbbb" -> 1 "a" or 1"b" would make "a" and "b" appear unique number of
times.
-}

import Data.List (group, sort)


answer :: String -> Int
answer s = occurListSum - (progressionSum biggest (biggest - (occurListLength - 1)))
 where
  occurList' = occurList s
  (biggest, occurListLength, occurListSum) =
    foldl (\(b, l, su) x -> (max x b, l+1, su+x)) (0,0,0) occurList'
  progressionSum big small = (((big - small + 1) * (small + big)) `div` 2)

  -- A list of character occurrence counts.
  occurList :: String -> [Int]
  occurList = map length . group . sort


tests :: Bool
tests = and $ map tester testCases
 where
  tester :: (String, Int) -> Bool
  tester (input, correctAnswer) = do
    let response = answer input
    if response == correctAnswer
      then True
      else error $ show (input, correctAnswer, response)

  -- (input, answer)
  testCases :: [(String, Int)]
  testCases =
    [ ("aaaabbbbba", 1)
    , ("ab", 1)
    , ("aabbc", 2)
    , ("eaaaabbbbbaeeee", 3)
    , ("aaaabbbbcccceeee", 6)
    , ("aaaaabbbbbccccceeeee", 6)
    , ("aaaaabbbbbccccceeezz", 5)
    , ("aaaaaabbbbbcccccdddddzzzz", 5)
    ]