Quicksort in Haskell, from Learn You a Haskell for Great Good! Optimization courtesy of @saolsen.

quicksort.hs

quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort [x] = [x]
quicksort (x:xs) =
    let smallerOrEqual = [a | a <- xs, a <= x]
        larger = [a | a <- xs, a > x]
    in quicksort smallerOrEqual ++ [x] ++ quicksort larger

-- using filter
quicksort' :: (Ord a) => [a] -> [a]
quicksort' [] = []
quicksort' [x] = [x]
quicksort' (x:xs) =
    let smallerOrEqual = filter (<= x) xs
        larger = filter (> x) xs
    in quicksort' smallerOrEqual ++ [x] ++ quicksort' larger