On Fri, Jan 17, 2020 at 03:57:18PM +0000, PICCA Frederic-Emmanuel wrote:
chunkLen :: Chunk -> Int chunkLen (Chunk _ f t) = t - f
chunks :: Int -> [Chunk] -> [[Chunk]] chunks n cs = reverse $ map reverse $ go cs [[]] 0 where go :: [Chunk] -> [[Chunk]] -> Int -> [[Chunk]] go [] _ _ = [] go [x@(Chunk fn f t)] (c:cs') acc = if acc + chunkLen x < n then (x : c) : cs' else go [(Chunk fn (f + n - acc) t)] ([] : ((Chunk fn f (f + n - acc)) : c) : cs') 0 go (x@(Chunk fn f t):xs) (c:cs') acc = if acc + chunkLen x < n then go xs ((x : c) : cs') (acc + chunkLen x) else go ((Chunk fn (f + n - acc) t) : xs) ([] : ((Chunk fn f (f + n - acc)) : c) : cs') 0
To your specific question I would refactor this a bit: -- | Split a list after the first element that reaches a target cumulative weight -- splitWeight :: Int -> (a -> Int) -> [a] -> ([a], [a]) splitWeight target weight xs = (,) <$> reverse . fst <*> snd $ go 0 xs [] where go _ [] acc = (acc, []) go n (h:ts) acc | let w = n + weight h , w < target = go w ts $ h : acc | otherwise = (h : acc, ts) -- | Partition a list into chunks, with each non-final chunk having -- a weight at least equal to the target. -- chunks :: Int -> (a -> Int) -> [a] -> [[a]] chunks target weight = unfoldr gen gen [] = Nothing gen xs = Just $ splitWeight target weight xs Example: λ> chunks 42 id [1..25] [[1,2,3,4,5,6,7,8,9],[10,11,12,13],[14,15,16],[17,18,19],[20,21,22],[23,24],[25]] Your weight function would be:
chunkLen :: Chunk -> Int chunkLen (Chunk _ f t) = t - f
That said, it feels like perhaps you're asking the wrong question. If you want parallelize monadic list reduction over multiple cores, perhaps something out of https://hackage.haskell.org/package/monad-par would meet your needs? You might find https://www.oreilly.com/library/view/parallel-and-concurrent/9781449335939/ a good resource. -- Viktor.