Hi, This question is purely out of curiousity - I'm no expert on this. I wrote the following code: sum1 :: [Int] -> Int sum1 = foldl (+) 0 foldl' :: (a -> b -> a) -> a -> [b] -> a foldl' f v l = foldr (\x -> \fld -> \v' -> fld $ f v' x) id l v sum2 :: [Int] -> Int sum2 = foldl' (+) 0 main :: IO () main = do n <- readLn; print $ sum1 [1..n]; print $ sum2 [1..n] and ran it through ghc with: ghc Folds.hs -ddump-simpl -O I was expecting the two calls to be optimized to something equivalent, or even for the result of a single calculation to be printed twice. However, peering at the Core output, it seems that the two sum functions exist separately and have different structure (sum1 appears to have a simple recursive definition, while sum2 involves two lamba expressions). I did all this because I read a paper introducing folds that showed how you can express foldl in terms of foldr and giving the derivation. Unfortunately I don't have the paper/derivation, but I believe my foldl' is correct (that may be the problem, of course - that sum1 and sum2 are not equivalent either because of a coding error, or because of some subtlety in the types, although I carefully gave them explicit types to try and avoid that). The derivation wasn't that complicated (althugh I find it simpler to simply write the fold down). If it's not an error on my part, why doesn't ghc do the same kind of transformation itself? Or am mistaken in thinking that if derivation is easy then the revrese transformation as an optimisation should also be easy? Thanks, Andrew -- http://www.acooke.org/andrew