I've used Control.Parallel and Control.Parallel.Strategies extensively in the past, and I thought I knew what I was doing. I declared the following function: findSupernodes' :: S.Set Name -> Int -> Tree -> Protein.Tree -> S.Set Name findSupernodes' set size (Node i _ _ s il ir) (Protein.Node _ pl pr _ g) | S.size g > size = (Name (fromIntegral i)) `S.insert` set | otherwise = leftNodes `S.union` rightNodes where leftNodes = (findSupernodes' set size il pl) rightNodes = (findSupernodes' set size il pl) findSupernodes' set size (Leaf _ _) (Protein.Gene _ _ _) = set and then simply wrote the following as an initial stab at parallelizing it, because actually calling this function causes the program to execute an absurd number of string comparisons: findSupernodes = findSupernodes' S.empty findSupernodes' :: S.Set Name -> Int -> Tree -> Protein.Tree -> S.Set Name findSupernodes' set size (Node i _ _ s il ir) (Protein.Node _ pl pr _ g) | S.size g > size = (Name (fromIntegral i)) `S.insert` set | otherwise = leftNodes `par` rightNodes `seq` leftNodes `S.union` rightNodes where leftNodes = (findSupernodes' set size il pl) rightNodes = (findSupernodes' set size il pl) findSupernodes' set size (Leaf _ _) (Protein.Gene _ _ _) = set The thing is, these two functions don't return the same thing. The parallel version is returning an empty set, while the sequential version returns what it should return. Any clue what I did wrong? -- Jeff