March 2010 - Haskell-Cafe

how to listen on a specific IP using the network library
by Jeremy Shaw 16 Mar '10

16 Mar '10

Hello, I would really like to modify happstack so that you can specific what IP address to listen on. So I think I want a function like: listenOnAddr :: SockAddr -> IO Socket The problem is that the user might want to specify IPv4 or IPv6 address. But support for IPv6 is only conditionally compiled into the network library. I can not figure out how to write my code so that it will compile regardless of whether network was compiled with ipv6 enabled. I got this far: -------------------------------- import qualified Control.Exception as Exception import Network.BSD (getProtocolNumber) import Network.Socket listenOnAddr :: Family -> SocketType -> SockAddr -> IO Socket listenOnAddr fam typ sockAddr = do proto <- getProtocolNumber "tcp" Exception.bracketOnError (socket fam typ proto) (sClose) (\sock -> do setSocketOption sock ReuseAddr 1 bindSocket sock sockAddr listen sock maxListenQueue return sock ) --------------------------------------- But I am not sure how to modify it to calculate 'fam' and 'typ' from 'sockAddr' with out making my code depend on IPv6 being enabled. If I was adding this function directly to the network package, there would be no problem. I could just use: #if defined(IPV6_SOCKET_SUPPORT) #endif But in a 3rd party library IPV6_SOCKET_SUPPORT is not defined. I could copy the autoconf code into my library -- but there is no guarantee that my library and the network library were compiled with the same value for IPV6_SOCKET_SUPPORT. In happstack we use a really horrible trick involving template haskell where we see if the SockAddrInet6 constructor exists at compile time and conditionally compile different versions of the code that way. But it is really ugly. We defined supportsIPv6 here: http://patch-tag.com/r/mae/happstack/snapshot/current/content/pretty/happst… And then use it in acceptLite here: http://patch-tag.com/r/mae/happstack/snapshot/current/content/pretty/happst… It wouldn't be that bad if we could actually use the [| |] mechanism. But we can't because it won't compile when ipv6 support is not enabled due to '(S.SockAddrInet6 _ _ ha _) -> showHostAddress6 ha'. Any suggestions? At this point I am thinking that the best solution is to add a new function to network, since it is not trivial to do it anywhere else in a portable way, and it seems like a fairly useful and common operation? - jeremy

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Can't cabal install darcs because hashed-storage needs 'System'
by Daniel McAllansmith 16 Mar '10

16 Mar '10

Hello. Short story: $ cabal install --global --constraint="old-time>=1.0.0.3.1" --reinstall -v darcs /tmp/hashed-storage-0.4.71127/hashed-storage-0.4.7/Setup.hs:13:7: Could not find module `System': Use -v to see a list of the files searched for. This is using GHC 6.12.1, cabal-install 0.8.0 and Cabal 1.8.0.2 How do I get darcs installed? Longer story: I've been trying to install darcs 2.4 built against a patched version of old- time, 1.0.0.3.1 is effectively 1.0.0.4 just that it's patched locally because 1.0.0.4 hasn't been released to hackage yet. I ran into various problems with global vs. user packages, no version of base being valid and a few other bits and pieces. So I am trying a global install and have had to reinstall Cabal (and various other packages), presumably because it was built against old-time 1.0.0.3. Looking at the hashed-storage code it imports System (system, exitWith). Is that old base-3 code? The debug messages show: selecting base-3.0.3.2 (installed) and 4.2.0.0 (installed) and /usr/local/bin/ghc --make /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/Setup.hs -o /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/dist/setup/setup -odir /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/dist/setup -hidir /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/dist/setup -i -i/tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7 -package Cabal-1.8.0.2 so I'm not really sure what version of base it's actually trying to compile hashed-storage against. Any further information I can provide? Thanks Dan

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ANNOUNCE: tupleinstances-0.0.1
by Diego Echeverri 16 Mar '10

16 Mar '10

Hello. TupleInstances It's a really small library that provides instances for Functor, Applicative, Foldable and Traversable for tuples of the form (a,a...a). It uses newtypes to wrap the tuples and Template Haskell to generate the code. Here's an example of these instances in action: example6 :: (Int, Int) -> [(Int, Int)] example6 p = map unT $ liftA2 (+) (T2 p) <$> moves where moves = tail $ traverse (\x -> [0,x+1,x-1]) (pure 0 :: T2 Int) -- > Result with (1,1): [(1,2),(1,0),(2,1),(2,2),(2,0),(0,1),(0,2),(0,0)] More examples here: http://github.com/diegoeche/tupleinstances/raw/master/README PS: This is also my first package uploaded to Hackage. I would appreciate any feedback. Best Regards, Diego Echeverri

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definition of sum
by TeXitoi 15 Mar '10

15 Mar '10

After programming as an exercice the sum function, my version is faster than the Data.List's version. Looking at the source code, Data.List uses a foldl and not a foldl'. foldl' seems faster and allows to use very big lists. So, why is foldl used by Data.List for sum? -- Guillaume Pinot http://www.irccyn.ec-nantes.fr/~pinot/ « Les grandes personnes ne comprennent jamais rien toutes seules, et c'est fatigant, pour les enfants, de toujours leur donner des explications... » -- Antoine de Saint-Exupéry, Le Petit Prince () ASCII ribbon campaign -- Against HTML e-mail /\ http://www.asciiribbon.org -- Against proprietary attachments

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HTTP package freezes on Windows 7
by Phil 15 Mar '10

15 Mar '10

Hi, I'm using GHC 6.12.1 on Windows 7. I've built the latest Network package using Haskell's MinGW and installed HTTP package on top of this. The code below builds fine, but on execution it just sits there grabbing ever increasing amounts of memory. It's a simplified call that I've got working fine in Linux. Is this a known issue? Anyone else had success using HTTP from Windows? Thanks, Phil. import qualified Network.HTTP as HTTP main :: IO () main = do x <- HTTP.simpleHTTP(HTTP.getRequest "http://maps.google.com/maps/api/geocode/json?address=London&sensor=false") print x

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How to do the "permutation and combination" thing?
by Magicloud Magiclouds 15 Mar '10

15 Mar '10

Hi, For example, I have this: list1 = [a, b, c] list2 = [d, e, f] list3 = [g, h, i] Now I want: [ [(a, d, g), (b, e, h), (c, f, i)] , ... ] -- a list that contains all the combinations. How to do it pretty? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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How to define instance for type synonym with parameter.
by Vasyl Pasternak 15 Mar '10

15 Mar '10

Hello, I am start with example. Suppose I have the following type synonym: type SomeMonad a = ErrorT String (ReaderT String IO) a this is monad, and I want to make it instance of Applicative, so, the obvious way is to write the following: instance Applicative SomeMonad where pure = return (<*>) = ap GHCi warns me, that I have to use -XTypeSynonymInstances option to allow this construction, but than I have following error: Type synonym `SomeMonad' should have 1 argument, but has been given 0 In the instance declaration for `Applicative SomeMonad' Neither `instance Applicative (SomeMonad a)` nor `instance Applicative SomeMonad a` help. But works the following: instance Applicative (ErrorT String (ReaderT String IO)) where pure = return (<*>) = ap Which is the same (from my point of view). Could anyone tell me what is going on, and how to declare SomeMonad as instance of Applicative ? Thanks, Vasyl

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Space leak
by Arnoldo Muller 15 Mar '10

15 Mar '10

Hello, I am learning haskell and I found a space leak that I find difficult to solve. I've been asking at #haskell but we could not solve the issue. I want to lazily read a set of 22 files of about 200MB each, filter them and then I want to output the result into a unique file. If I modify the main function to work only with one input file, the program runs without issues. I will call this version A. Version B uses a mapM_ to iterate over a list of filenames and uses appendFile to output the result of filtering each file. In this case the memory usage grows sharply and quickly (profiles show constant memory growth). In less than a minute, memory occupation will make my system hang with swapping. This is version B: ------------------------------- Program B -------------------------------------------------------------------------------------------------------------------- import Data.List import System.Environment import System.Directory import Control.Monad -- different types of chromosomes data Chromosome = C1 | C2 | C3 | C4 | C5 | C6 | C7 | C8 | C9 | C10 | C11 | C12 | C13 | C14 | C15 | C16 | C17 | C18 | C19 | CX | CY | CMT deriving (Show) -- define a window type Sequence = [Char] -- Window data data Window = Window { sequen :: Sequence, chrom :: Chromosome, pos :: Int } -- print a window instance Show Window where show w = (sequen w) ++ "\t" ++ show (chrom w) ++ "\t" ++ show (pos w) -- Reading fasta files with haskell -- Initialize the main = do -- get the arguments (intput is [input, output, windowSize] <- getArgs -- get directory contents (only names) names <- getDirectoryContents input -- prepend directory let fullNames = filter isFastaFile $ map (\x -> input ++ "/" ++ x) names let wSize = (read windowSize)::Int -- process the directories mapM (genomeExecute output wSize filterWindow) fullNames -- read the files one by one and write them to the output file genomeExecute :: String -> Int -> (Window -> Bool) -> String -> IO () genomeExecute outputFile windowSize f inputFile = do fileData <- readFile inputFile appendFile outputFile $ fastaExtractor fileData windowSize f -- isFastaFile :: String -> Bool isFastaFile fileName = isSuffixOf ".fa" fileName -- fasta extractor (receives a Fasta String and returns a windowed string ready to be sorted) -- an example on how to compose several functions to parse a fasta file fastaExtractor :: String -> Int -> (Window -> Bool) -> String fastaExtractor input wSize f = printWindowList $ filter f $ readFasta wSize input -- MAIN FILTER that removes N elements from the strings! filterWindow :: Window -> Bool filterWindow w = not (elem 'N' (sequen w)) -- print a window list (the printing makes it ready for output as raw data) printWindowList :: [Window] -> String printWindowList l = unlines $ map show l -- read fasta, remove stuff that is not useful from it -- removes the readFasta :: Int -> [Char] -> [Window] readFasta windowSize sequence = -- get the header let (header:rest) = lines sequence chr = parseChromosome header in -- We now do the following: -- take window create counter remove newlines map (\(i, w) -> Window w chr i) $ zip [0..] $ slideWindow windowSize $ filter ( '\n' /= ) $ unlines rest slideWindow :: Int -> [Char] -> [[Char]] slideWindow _ [] = [] slideWindow windowSize l@(_:xs) = take windowSize l : slideWindow windowSize xs -- Parse the chromosome from a fasta comment -- produce a more compact chromosome representation parseChromosome :: [Char] -> Chromosome parseChromosome line | isInfixOf "chromosome 1," line = C1 | isInfixOf "chromosome 2," line = C2 | isInfixOf "chromosome 3," line = C3 | isInfixOf "chromosome 4," line = C4 | isInfixOf "chromosome 5," line = C5 | isInfixOf "chromosome 6," line = C6 | isInfixOf "chromosome 7," line = C7 | isInfixOf "chromosome 8," line = C9 | isInfixOf "chromosome 10," line = C10 | isInfixOf "chromosome 11," line = C11 | isInfixOf "chromosome 12," line = C12 | isInfixOf "chromosome 13," line = C13 | isInfixOf "chromosome 14," line = C14 | isInfixOf "chromosome 15," line = C15 | isInfixOf "chromosome 16," line = C16 | isInfixOf "chromosome 17" line = C17 | isInfixOf "chromosome 18" line = C18 | isInfixOf "chromosome 19" line = C19 | isInfixOf "chromosome X" line = CX | isInfixOf "chromosome Y" line = CY | isInfixOf "mitochondrion" line = CMT | otherwise = error "BAD header" -------------------------------- End of program B ------------------------------------------------------------------------------------------------ -------------------------------- Program A --------------------------------------------------------------------------------------------------------- If instead of the main function given above I use the following main function to process only one input file, things work OK for even the largest files. Memory usage remains constant in this case. main = do -- get the arguments [input, output, windowSize] <- getArgs -- keep the input stream inpStr <- readFile input let wSize = (read windowSize)::Int writeFile output $ fastaExtractor inpStr wSize filterWindow It is not easy for me to see why is Haskell keeping data in memory. Do you have any idea why program B is not working? Thank you for your help! Arnoldo Muller

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Happstack basic question
by Dmitry V'yal 15 Mar '10

15 Mar '10

Hello haskellers, I want to host a simple happstack application behind a reverse proxy. So ideally would be to bind it to localhost only. According to http://hackage.haskell.org/packages/archive/happstack-server/0.4.1/doc/html… Conf datatyle has only Port field. Does it mean, there is currently no way to prevent binding happstack to all available interfaces? Regards, Dmitry

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darcs hacking sprint (ZuriHac) info!
by Eric Kow 15 Mar '10

15 Mar '10

Hi everybody, Just some quick information for those of you who are interested in working on Darcs at ZuriHac Pre-sprint meeting - Fri 12:00; pizzeria ----------------------------------------- We'll be meeting for lunch at Friday noon at the Pizzeria Santa Lucia Niederdorf (Marktgasse 21), near the City Backpacker hostel. Details on http://wiki.darcs.net/Sprints/2010-03 This will be a great chance for introductions (especially for new or potential Darcs hackers). On the agenda: - prioritising ProbablyEasy bugs for new Darcs hackers - plans for the sprint (news, other meetings, discussions, etc) Get your laptops ready! ----------------------- You'll have more fun if you can hit the ground running, so please make sure you can do all of these things: 1. darcs get --lazy http://darcs.net 2. cabal configure; cabal build 3. darcs send (your patch should wind up on the tracker) #3 is especially important. Here's the quick start 3a. create a test patch (see http://darcs.net/report.html ) 3b. see http://wiki.darcs.net/Msmtp 3c. darcs send 3d. wait 10 minutes 3e. check the patch tracker http://bugs.darcs.net (Patches > Show Open) Travel reimbursements possible ------------------------------ Thanks to a successful fundraising effort, we will be able to reimburse travel costs for folks working on Darcs. Do let me know if you're interested, and please save your receipts! If you have electronic tickets, I think that's fine (perhaps even better); please email them to me. More information on the wiki --------------------------- Information about the sprint, including meetings and maps will be on http://wiki.darcs.net/Sprints/2010-03 It may be a good idea to darcs get --lazy http://wiki.darcs.net before you leave so that you have an up to date version of the wiki :-) Thanks to our donors for making this sprint possible and to Johan and the Zurich/Google crew for setting up this massive hackathon. Whee! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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