Dear Cafe, I’m trying to exploit list fusion as provided by GHC (build/foldr). One function that I want to get fusable is this, it splits a list of integeres into maximal monotonous subsequences: streaks :: [Integer] -> [[Integer]] streaks [] = [] streaks (x:xs) = let (this,rest) = oneStreak (x:xs) in this:streaks rest oneStreak :: [Integer] -> ([Integer], [Integer]) oneStreak [x] = ([x],[]) oneStreak l@(x:y:_) = splitWhile2 (\a b -> a `compare` b == x `compare` y) l splitWhile2 :: (Integer -> Integer -> Bool) -> [Integer] -> ([Integer], [Integer]) splitWhile2 p [x] = ([x],[]) splitWhile2 p (x:y:xs) | p x y = let (s,r) = splitWhile2 p (y:xs) in (x:s,r) | otherwise = ([x],y:xs) Now I’d like to implement streaks in terms of build and foldr such that it is subject to list fusion. Especially, when used in concatMap (streaks . func) :: [X] -> [[Integer]] where func :: X -> [Integer] is implemented with buildr, this should ideally remove all intermediate lists. Can this be done with list fusion at all? How would I go about it? If the above example is too complicated, known how it would work for Data.List.group would help me already a lot. Greetings, Joachim -- Joachim "nomeata" Breitner mail@joachim-breitner.de | nomeata@debian.org | GPG: 0x4743206C xmpp: nomeata@joachim-breitner.de | http://www.joachim-breitner.de/