Hi all, I've written some code to generate set partitions: import Control.Arrow import Data.List -- pSet' S generates the power set of S, with each subset paired -- with its complement. -- e.g. pSet' [1,2] = [([1,2],[]),([1],[2]),([2],[1]),([],[1,2])]. pSet' :: [a] -> [([a],[a])] pSet' [] = [([],[])] pSet' (x:xs) = mp first ++ mp second where mp which = map (which (x:)) psRest psRest = pSet' xs -- partitions S generates a list of partitions of S. -- e.g. partitions [1,2,3] = [[[1,2,3]],[[1,2],[3]],[[1,3],[2]],[[1],[2,3]],[[1],[2],[3]]]. partitions :: [a] -> [[[a]]] partitions [] = [[]] partitions (x:xs) = (pSet' xs) >>= ((x:) *** partitions >>> uncurry (map . (:))) However, this version of partitions generates duplicates when given a multiset, for example: *Combinatorics> partitions [1,1,2] [[[1,1,2]],[[1,1],[2]],[[1,2],[1]],[[1],[1,2]],[[1],[1],[2]]] The partition [[1,2],[1]] is generated twice (order doesn't matter). I'd like to write a version of partitions which generates duplicate-free output even for input multisets, but haven't come up with a good method yet. Any ideas? -Brent PS Yes, this is for Project Euler. =)