swap04.hs

swap i j xs0 = swap' 0 xs0
    where
      swap' idx [] = []
      swap' idx (x1:xs1) | idx == i  = xs0 !! j : swap''
                         | idx == j  = xs0 !! i : swap''
                         | otherwise = x1       : swap''
          where
            swap'' = swap' (idx+1) xs1