John,
My question is, how do I preserve the ordering of entities in my list and still be able to assign the proper order values to each entity? Is there an efficient way to do this? How else might I improve my orderByPriority algorithm.
Straightforwardly, you could do it like this: import List (sortBy) type Priority = Int type Order = Int order :: [Priority] -> [(Priority, Order)] order = unlabel . sortOnLab . assignOrder . sortOnPrio . label where label = zip [0 ..] unlabel = map snd sortOnLab = sortBy (\(l, _) (m, _) -> compare l m) sortOnPrio = sortBy (\(_, p) (_, q) -> compare p q) assignOrder = \xs -> zipWith (\(l, p) o -> (l, (p, o))) xs [0 ..] For instance:
order [5, 2, 3, 7] [(5,2),(2,0),(3,1),(7,3)]
HTH, Stefan