クイックソートが動かない

quicksort.hs

#!/usr/bin/env runghc

class QuickSortable a where
  pickPivot :: [a] -> a
  pickPivot (top : _) = top
  before :: Ord a => a -> [a] -> [a]
  before pivot arr = filter (\n -> n < pivot) arr
  after :: Ord a => a -> [a] -> [a]
  after pivot arr = filter (\n -> n >= pivot) arr

instance Ord a => QuickSortable a

quicksort :: Ord a => [a] -> [a]
quicksort [] = []
quicksort arr = sortedBefore ++ [pivot] ++ sortedAfter
  where
    pivot = pickPivot arr
    sortedBefore = quicksort (before pivot arr)
    sortedAfter = quicksort (after pivot arr)

main = do
  print (quicksort ([1, 2, 4, 2, 7, 9, 6] :: [Int]))