import List ( nub )
import Control.SetFunctions

-- Permutation of a list with a functional pattern:
perm :: [a] -> [a]
perm []             = []
perm (xs ++ y : ys) = y : perm (xs ++ ys)

-- Contains a list two elements not in order:
someDescending :: [Int] -> Bool
someDescending (_ ++ [x,y] ++ _) | x > y = True

sort :: [Int] -> [Int]
sort xs | isEmpty (set1 someDescending p) = p
 where p = perm xs


-- Compute the length of a list prefix until the first repeated element.
lengthUpToRepeat :: [Int] -> Int
lengthUpToRepeat (ps ++ [x] ++ _)
  | nub ps == ps && elem x ps
  = length ps + 1

example = lengthUpToRepeat [1,2,3,43,99,2,1,3] --> 6