Подсчет инверсий: StackOverflowError во Фреге, отлично работает в Haskell

Я пытаюсь подсчитать инверсии для списка чисел. Следующая программа Фреге работает для небольшого набора чисел, но выдает StackOverflowError для 100000 чисел.

import frege.IO

inversionCount [] _ = (0, [])
inversionCount [x] _ = (0, [x])
inversionCount xs n = (count, sorted) where
  count = lcount + rcount + mergecount
  (lcount, lsorted) = inversionCount left lsize
  (rcount, rsorted) = inversionCount right rsize
  (mergecount, sorted) = inversionMergeCount lsorted lsize rsorted rsize (0, [])
  (left, right) = splitAt mid xs
  mid = n `quot` 2
  lsize = mid
  rsize = n - mid

inversionMergeCount xs _ [] _ (acc,sorted) = (acc, reverse sorted ++ xs)
inversionMergeCount [] _ ys _ (acc,sorted) = (acc, reverse sorted ++ ys)
inversionMergeCount (xs@(x:restx)) m (ys@(y:resty)) n (acc, sorted)
  | x < y = inversionMergeCount restx (m - 1) ys n (acc, x:sorted)
  | x > y = inversionMergeCount xs m resty (n - 1) (acc + m, y:sorted)
  | otherwise = inversionMergeCount restx (m - 1) resty (n - 1) (acc, x:y:sorted)

main (fileName:_) = do
  input <- readFile fileName
  let xs = map atoi (lines input)
  println . fst $ inversionCount xs 100000

--Haskell's readFile using Java's Scanner
type JScanner = JScannerT RealWorld

data JScannerT s = native java.util.Scanner where
  native new :: File -> ST s (Exception JScanner)
  native useDelimiter :: JScanner -> String -> ST s JScanner
  native next :: JScanner -> ST s String

readFile f = do
  file <- File.new f
  exceptionScanner <- JScanner.new file
  let scanner = either throw id exceptionScanner
  scanner.useDelimiter "\\Z"
  scanner.next

Тот же код на Haskell работает нормально:

import System.Environment

inversionCount [] _ = (0, [])
inversionCount [x] _ = (0, [x])
inversionCount xs n = (count, sorted) where
  count = lcount + rcount + mergecount
  (lcount, lsorted) = inversionCount left lsize
  (rcount, rsorted) = inversionCount right rsize
  (mergecount, sorted) = inversionMergeCount lsorted lsize rsorted rsize (0, [])
  (left, right) = splitAt mid xs
  mid = n `quot` 2
  lsize = mid
  rsize = n - mid

inversionMergeCount xs _ [] _ (acc,sorted) = (acc, reverse sorted ++ xs)
inversionMergeCount [] _ ys _ (acc,sorted) = (acc, reverse sorted ++ ys)
inversionMergeCount (xs@(x:restx)) m (ys@(y:resty)) n (acc, sorted)
  | x < y = inversionMergeCount restx (m - 1) ys n (acc, x:sorted)
  | x > y = inversionMergeCount xs m resty (n - 1) (acc + m, y:sorted)
  | otherwise = inversionMergeCount restx (m - 1) resty (n - 1) (acc, x:y:sorted)


main = do
  (fileName: _) <- getArgs
  contents <- readFile fileName
  let xs :: [Int]
      xs = map read (lines contents)
  print . fst $ inversionCount xs 100000

Что может быть причиной переполнения стека? Это какая-то функция, не являющаяся хвостовой рекурсией?