is it possible to do quicksort of a list with only one passing?

You mean something like this?

quicksort [] = []
quicksort (x:xs) = quicksort less ++ (x : equal) ++ quicksort more
  where (less, equal, more) = partition3 x xs

partition3 _ [] = ([], [], [])
partition3 x (y:ys) =
  case compare y x of
    LT -> (y:less, equal, more)
    EQ -> (less, y:equal, more)
    GT -> (less, equal, y:more)
  where (less, equal, more) = partition3 x ys

Note that this isn’t really quicksort, as the real quicksort is in-place.

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