Turkish NLP: Split a word to its syllables

syllablize.hs

import Data.Maybe (isNothing, fromJust)
import Data.Char (isAlpha, toLower)
import Data.List (findIndex)

-- charAt xs i = Just x, where x character at index i
--               Nothing, is i is out of bounds
charAt :: String -> Int -> Maybe Char
charAt xs i = if length xs > i then Just (xs !! i) else Nothing

-- isVowel x = True if x is a vowel,
--             False otherwise
isVowel :: Char -> Bool
isVowel x = x `elem` "aeiıoöuüâêîû"

-- substring x y xs = Java's substring
substring :: Int -> Int -> String -> String
substring x y = drop x . take y

-- syllablize xs = list of Turkish syllables of xs
syllablize :: String -> [String]
syllablize s
  | '\'' `elem` s = concatMap syllablize [takeWhile (/='\'') s, dropWhile (/='\'') s]
  | isNothing firstVowelIndex = [xs]

  | any isNothing [afterVowel 1] = [xs]
  | isVowel(fromJust $ afterVowel 1) =
      substring 0 (fVI + 1) xs : syllablize(substring (fVI + 1) len xs)

  | any isNothing [afterVowel 2] = [xs]
  | isVowel(fromJust $ afterVowel 2) =
      substring 0 (fVI + 1) xs : syllablize(substring (fVI + 1) len xs)

  | any isNothing [afterVowel 3] = [xs]
  | isVowel(fromJust $ afterVowel 3) =
      substring 0 (fVI + 2) xs : syllablize(substring (fVI + 2) len xs)

  | lastPart `elem` ["str", "ktr", "ntr", "nsp"] =
      substring 0 (fVI + 2) xs : syllablize(substring (fVI + 2) len xs)
  | otherwise =
      substring 0 (fVI + 3) xs : syllablize(substring (fVI + 3) len xs)
  where xs = (filter isAlpha . map toLower) s
        firstVowelIndex = findIndex isVowel xs
        fVI = fromJust firstVowelIndex
        len = length xs
        lastPart = substring (len + 1) (len + 4) xs
        afterVowel i = fromJust $ fmap (charAt xs . (+i)) firstVowelIndex

Ported from http://www.serkanucpinar.net/ttstext-to-speech/turkce-heceleme-algoritmasi/
Added apostrophe as a definite syllable separator, for words containing "ع" sound.
Added "nsp" as an exception.

Enhanced version: https://github.com/joom/Guguk/blob/master/src/Guguk/Syllabification.hs
The tests for it (they all pass): https://github.com/joom/Guguk/blob/master/tests/Syllabification.hs