Hi all, On the other day I noticed that we could optimize 'sequence' more. I needed it for my monadic parser. Below is my small experiment. Sequence from standard library needs 2.3s to finish (and additional stack space), my version uses only 0.65s and default stack. Is my version better or am I missing something obvious? -- standard sequence1 :: Monad m => [m a] -> m [a] sequence1 ms = foldr k (return []) ms where k m m' = do { x <- m; xs <- m'; return (x:xs) } -- accumulator version sequence2 :: Monad m => [m a] -> m [a] sequence2 ms = sequence' [] ms where sequence' vs [] = return (reverse vs) sequence' vs (m:ms) = m >>= (\v -> sequence' (v:vs) ms) main = do let l = map return [1..1000000] w <- sequence1 l print (sum w) return () gracjan@home:~/some_faster> time ./Some1 +RTS -K100M 500000500000 real 0m2.318s user 0m2.284s sys 0m0.032s gracjan@home:~/some_faster> time ./Some2 500000500000 real 0m0.652s user 0m0.592s sys 0m0.052s -- Gracjan