February 2009 - Haskell-Cafe

differences between Data.Array and Data.Vector
by Manlio Perillo 01 Mar '09

01 Mar '09

Hi. In Hackage there are some packages named "*array*", and others named "*vector*". What are the differences? Is available a guide to the various data structures available in Haskell? Thanks Manlio Perillo

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memory-efficient data type for Netflix data - UArray Int Int vs UArray Int Word8
by Kenneth Hoste 01 Mar '09

01 Mar '09

Hello, I'm having a go at the Netflix Prize using Haskell. Yes, I'm brave. I kind of have an algorithm in mind that I want to implement using Haskell, but up until now, the main issue has been to find a way to efficiently represent the data... For people who are not familiar with the Netflix data, in short, it consist of roughly 100M (1e8) user ratings (1-5, integer) for 17,770 different movies, coming from 480,109 different users. The idea I have in mind requires fast access to all the ratings for a particular user, so I was thinking about an IntMap in terms of user ids, which maps to movie id/rating pairs somehow. To see if I could efficiently represent the data set in this way, I wrote a small Haskell program (attached) which uses the following data type: type data = IntMap (UArray Int Word8) -- map of user IDs to ratings (lacks movie IDs) For convenience, and because I've been discussing this with various people in #haskell @ IRC, the code is also available here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=1671#a1671 . I'm aware that the way in which the UArray's are constructed (i.e. by adding a single element each time), is far from efficient performance-wise, but that's not the point here... By reformatting the raw data, I can easily read in the data more efficiently. The issue I ran into is that changing the data type to the following, doesn't yield any significant different in memory usage. type data = IntMap (UArray Int Int) -- use Int instead of Word8 for a user rating Someone (dolio) @ #haskell suggested that maybe UArray is not byte packed for Word8, which would cause little difference with a UArray containing Int's, but someone else (dons @ #ghc) was able to tell me it _is_ byte packed. Does anyone know why the Word8 version is not significantly better in terms of memory usage? greetings, Kenneth PS: My adventures on trying to tackle the Netflix Prize problem with Haskell can be followed at http://boegel.kejo.be. -- Kenneth Hoste ELIS - Ghent University email: kenneth.hoste(a)elis.ugent.be blog: http://www.elis.ugent.be/~kehoste/blog website: http://www.elis.ugent.be/~kehoste

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Problem with haddock - it dislikes my comments
by Colin Paul Adams 01 Mar '09

01 Mar '09

Having just read the Haddock manual , I am haddocking (may I copyleft that verb?) my code. But half-way through this data type, I discover that haddock rejects my non-haddock comments, although GHC is fine: -- | Non-interactive representation of a Chu Shogi move data Move = Pass Piece Coordinate -- ^ Double move to empty square and back to origin | Igui Piece Coordinate -- ^Capture without moving Piece Coordinate -- Captured piece | Double_move Piece -- ^ Other double moves Coordinate -- Initial location Coordinate -- First destination (Maybe Piece) -- Capture? Coordinate -- Second destination (Maybe Piece) -- Capture? | Capture Piece Coordinate Piece Coordinate Bool -- Promote? Bool -- Declined to promote | Move Piece Coordinate Coordinate Bool Bool How can I document the components of each constructor? -- Colin Adams Preston Lancashire

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MPTC inheritance
by Derek Gladding 01 Mar '09

01 Mar '09

Please forgive me if I'm still mentally contaminated by the OO way of seeing (and discussing) the universe, but I'm trying to figure out how to "inherit an interface" from a multi-parameter type class. I have a Graph class that's parameterisable by Node and Edge type: class (Node a, Edge b) => Graph a b where (lots of stuff that you can do with Graph a b) Now, I'd like to build a FooGraph on top of this that adds additional capabilities: class (Graph a b) => FooGraph a b where (lots of additional stuff) but this isn't allowed (kind mismatch). Of couse, I can do: class (Node a, Edge b) => FooGraph a b but this means that I have to manually replicate the Graph a b operations in the FooGraph a b class definition, which is (a) work that the machine should (?) be able to do for me, and (b) fragile. Any pointers / wisdom would be very much appreciated. - Derek

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Bug in HLint or haskell-src-exts?
by Martin Huschenbett 28 Feb '09

28 Feb '09

Hello Haskellers, I've got the following piece of code type Memory addr value = Map.Map addr value type Stack item = [item] data MachineState addr value item = MachineState { memory :: Memory addr value , stack :: Stack item } deriving (Show) newtype Machine addr value item m a = Machine { unMachine :: StateT (MachineState addr value item) m a } deriving (Monad, MonadState (MachineState addr value item), MonadTrans, MonadIO) and HLint complains: Machine.hs:20:31: Parse failure, Parse error This is exactly the position of the opening bracket after MonadState. Why can GHC parse this and HLint cannot? Regards, Martin.

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Re: [Haskell-cafe] Performance question
by haskell＠kudling.de 28 Feb '09

28 Feb '09

Hi, thanks for your input. >You can get reasonable numeric performance out of GHC, but you need to >work at it. There is some advice in the GHC manual at >http://www.haskell.org/ghc/docs/latest/html/users_guide/faster.html I am using -O2 and strictness already. Currently i can only imagine to define a data type in order to use unboxed Ints instead of the accumulator tuple. The thing is that i don't see in the profile output yet what to improve. There are some allocations going on in "main", but i don't know what causes it. >The first thing I would do is replace your >isInCircle :: (Floating a, Ord a) => (a,a) -> Bool >with >isInCircle :: (Double, Double) -> Bool Can you point me to why that matters? > >Ben. > > > >On 26/02/2009, at 8:53 PM, haskell(a)kudling.de wrote: > >> Hi, >> >> i have compared a C++ implementation with a Haskell implementation >> of the Monte Carlo Pi approximation: >> >> http://lennart.kudling.de/haskellPi/ >> >> The Haskell version is 100 times slower and i wonder whether i do >> something obvious wrong. >> >> Profiling says that the majority of the time is spend in "main". But >> i have no idea where. >> >> Can someone give me a hint? >> >> Thanks, >> Lenny >> >> >

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Coming up with a better API for Network.Socket.recv
by Johan Tibell 28 Feb '09

28 Feb '09

Hi all, I find it quite inconvenient to use the `recv` function in Network.Socket as it throws an exception when reaching EOF and there's no way to check whether EOF has been reached before calling `recv`. This means that all calls to `recv` needs to be wrapped in an exception handler. I've been thinking about changing the version of `recv` that's included in the network-bytestring library [1] so it behaves differently from the one in the network library. Before I do so I thought I should see if we can reach a consensus on what a nicer definition of `recv` would look like. My current thinking is that it would mimic what C/Python/Java does and return a zero length ByteString when EOF is reached. I'm also interested in understanding the reasons behind the design of the `recv` function in the network library. More generally, I'm interested in discussing the pros and cons of the current Haskell I/O library design where the different read functions throw EOF exceptions and you have to call e.g. hIsEOF before reading from a Handle. 1. http://github.com/tibbe/network-bytestring Cheers, Johan

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controlling timeout for Network.Socket.connect - how?
by Belka 28 Feb '09

28 Feb '09

Hello, communion people! I have a problem and ask for an advice. I'm dealing with sockets on *Linux* platform (Network.Socket). The problem is that I can't fully control timeout for (connect :: Socket -> SockAddr -> IO ()) operation. On my system the timeout is - 3 seconds - I want to be able to change that in run-time. Well I managed to find out how to make it LESS THAN 3 seconds - using System.Timeout. But how to make timeout bigger (for example 9 seconds) is a mystery. (Notice: in order to achieve 9 seconds timeout - just repeating *connect* 3 times won't be effective for long-slow-way-connections. So it's not a solution.) The source code of Network.Socket.connect, taken from darcs: --------------------------------- -- Connecting a socket -- -- Make a connection to an already opened socket on a given machine -- and port. assumes that we have already called createSocket, -- otherwise it will fail. -- -- This is the dual to $bindSocket$. The {\em server} process will -- usually bind to a port number, the {\em client} will then connect -- to the same port number. Port numbers of user applications are -- normally agreed in advance, otherwise we must rely on some meta -- protocol for telling the other side what port number we have been -- allocated. connect :: Socket -- Unconnected Socket -> SockAddr -- Socket address stuff -> IO () connect sock@(MkSocket s _family _stype _protocol socketStatus) addr = do modifyMVar_ socketStatus $ \currentStatus -> do if currentStatus /= NotConnected then ioError (userError ("connect: can't peform connect on socket in status " ++ show currentStatus)) else do withSockAddr addr $ \p_addr sz -> do let connectLoop = do r <- c_connect s p_addr (fromIntegral sz) if r == -1 then do rc <- c_getLastError case rc of 10093 -> do -- WSANOTINITIALISED withSocketsDo (return ()) r <- c_connect s p_addr (fromIntegral sz) if r == -1 then (c_getLastError >>= throwSocketError "connect") else return r _ -> throwSocketError "connect" rc else return r connectBlocked = do #if !defined(__HUGS__) threadWaitWrite (fromIntegral s) #endif err <- getSocketOption sock SoError if (err == 0) then return 0 else do ioError (errnoToIOError "connect" (Errno (fromIntegral err)) Nothing Nothing) connectLoop return Connected --------------------------------- I know that controlling timeout is somehow connected to select(2) (I'm currently investigating this matter...), but it's not in the Network or Network.Socket libs (but in the libs that they FFI with). Hope I won't have to rewrite these low-level functions.... >__< Could anybody, please share some experience on how to adjust timeout for *connect*? Thanks in advance, Best regards, Belka -- View this message in context: http://www.nabble.com/controlling-timeout-for-Network.Socket.connect---how-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Weekly News: Issue 107 - February 28, 2009
by Brent Yorgey 28 Feb '09

28 Feb '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090228 Issue 107 - February 28, 2009 --------------------------------------------------------------------------- Welcome to issue 107 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Google Summer of Code 2009 - mentors wanted!. Eric Kow [2]reminded everyone that darcs will be applying to be a mentoring organisation for the Google Summer of Code project 2009, and is seeking people to serve as mentors. You need not be a darcs expert to serve as a mentor! c2hs 0.16.0. Duncan Coutts [3]announced the release of [4]c2hs version 0.16.0, a tool which assists in the development of Haskell bindings to C libraries by extracts interface information from C header files and generating Haskell code with foreign imports and marshaling. The major change in this release is that it now uses the [5]Language.C library. X Haskell Bindings 0.2. Antoine Latter [6]announced the 0.2.* series release of the [7]X Haskell Bindings, mostly aimed at making the API prettier. The goal of XHB is to provide a Haskell implementation of the X11 wire protocol, similar in spirit to the [8]X protocol C-language Binding (XCB). text and text-icu, fast and comprehensive Unicode support using stream fusion. Bryan O'Sullivan , on behalf of the Data.Text team, [9]announced the release of preview versions of two new packages: [10]text 0.1, providing fast, packed Unicode text support with modern stream fusion, and [11]text-icu 0.1, which augments text with comprehensive character set conversion support and normalization (and soon more), via bindings to the ICU library. Boston Area Haskell User's Group, first meeting on Saturday February 28th. Shae Matijs Erisson [12]announced the first meeting of the [13]Boston Area Haskell User's Group this Saturday, February 28, at 2pm. Directions are on the web page; you can also check out the [14]mailing list. New version of Hieroglyph released: Hieroglyph 1.1 is on hackage. Jeff Heard [15]announced the release of a new version of [16]Hieroglyph; the biggest change is that it now uses Russell O'Connor's excellent [17]Data.Colour library. bug fix for regex-tdfa, version 0.97.4 (and "regex-ast"). ChrisK [18]announced another bug fix release for the [19]regex-tdfa package, which fixes another tricksy bug. This is now the only known regex library that passes the entire test suite. Hac5: April 17-19, Utrecht -- Book Now!. Sean Leather [20]announced some updated information regarding the [21]5th Haskell Hackathon. The executive summary: hotel or hostel accommodation may be hard to come by, so book your room now, and check out the website for lots of useful information. pkgenv - disposable, isolated pkg environments. Paolo Losi [22]announced a tool, [23]pkgenv, which facilitates setting up isolated, disposable package environments. Blog noise [24]Haskell news from the [25]blogosphere. * JP Moresmau: [26]A high-level GUI library for Haskell. * Magnus Therning: [27]More fun with Cabal, visualising dependencies. Giant graphs are fun! * Magnus Therning: [28]A no-no in my book (found in Cabal). * Bryan O'Sullivan: [29]Finally! Fast Unicode support for Haskell. Data.Text is here! * happstack.com: [30]Happstack now outputs Apache Combined Logs. * GHC / OpenSPARC Project: [31]Benchmarking. * Christophe Poucet (vincenz): [32]Fibonacci in the Type System. * Eric Kow (kowey): [33]inkscape layers. * Magnus Therning: [34]Another reason to create distro-specific packages for Haskell modules. * boegel: [35]Using Haskell to win the Netflix Prize. * Magnus Therning: [36]Simple Cabal parsing. * GHC / OpenSPARC Project: [37]Bugfixes and pretty graphs. * Jeff Heard: [38]New version of Hieroglyph released.. * Chris Done: [39]Vgrabbj hacking. * Conal Elliott: [40]Paper: Beautiful differentiation. Another draft paper from Conal. * happstack.com: [41]Momentum builds as we approach Happstack 0.2. * Luke Palmer: [42]RTS Research. * GHC / OpenSPARC Project: [43]Sanity. * Darcs: [44]darcs weekly news #18. * Don Stewart (dons): [45]Can Haskell give me a pony?. The answer is: Yes! Haskell can do that. * Neil Mitchell: [46]Hoogle package search. * Alex Mason: [47]AVar changes. * Xmonad: [48]Using gimp under xmonad. It can actually be quite nice, despite what you might think. * FP-Syd: [49]Sydney FP Group: FP-Syd #12.. * Eric Kow (kowey): [50]implementing join in terms of (>>=). * GHC / OpenSPARC Project: [51]Triage. * Benjamin L. Russell: [52]Learning Haskell through Category Theory, and Adventuring in Category Land: Like Flatterland, Only About Categories. Quotes of the Week * lilac: haskell's learning curve is like this: | * cowardlydragon: [from a reddit comment thread] Don't get me started on monad. What is that, a man with a single testicle? About the Haskell Weekly News New editions are posted to [53]the Haskell mailing list as well as to [54]the Haskell Sequence and [55]Planet Haskell. [56]RSS is also available, and headlines appear on [57]haskell.org. To help create new editions of this newsletter, please see the information on [58]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [59]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.haskell.org//pipermail/haskell-cafe/2009-February/056798.html 3. http://www.haskell.org//pipermail/haskell/2009-February/021052.html 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/c2hs 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/language%2Dc 6. http://article.gmane.org/gmane.comp.lang.haskell.general/16888 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xhb 8. http://xcb.freedesktop.org/ 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/53780 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/text 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/text-icu 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/53761 13. http://www.haskell.org/haskellwiki/Boston_Area_Haskell_Users%27_Group 14. http://groups.google.com/group/bostonhaskell 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/53549 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hieroglyph 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/colour 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/53525 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex%2Dtdfa 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/53510 21. http://haskell.org/haskellwiki/Hac5 22. http://article.gmane.org/gmane.comp.lang.haskell.cafe/53448 23. http://bitbucket.org/pao/pkgenv/raw/488bfe8e58dd/pkgenv 24. http://planet.haskell.org/ 25. http://haskell.org/haskellwiki/Blog_articles 26. http://jpmoresmau.blogspot.com/2009/02/high-level-gui-library-for-haskell.h… 27. http://therning.org/magnus/archives/534 28. http://therning.org/magnus/archives/523 29. http://www.serpentine.com/blog/2009/02/27/finally-fast-unicode-support-for-… 30. http://blog.happstack.com/2009/02/26/happstack-now-outputs-apache-combined-… 31. http://ghcsparc.blogspot.com/2009/02/benchmarking.html 32. http://cpoucet.wordpress.com/2009/02/26/fibonacci-in-the-type-system/ 33. http://koweycode.blogspot.com/2009/02/inkscape-layers.html 34. http://therning.org/magnus/archives/522 35. http://boegel.kejo.be/?q=node%2F8 36. http://therning.org/magnus/archives/514 37. http://ghcsparc.blogspot.com/2009/02/bugfixes-and-pretty-graphs.html 38. http://vis.renci.org/jeff/2009/02/24/new-version-of-hieroglyph-released/ 39. http://chrisdone.com/blog/html/2009-02-25-vgrabbj.html 40. http://conal.net/blog/posts/paper-beautiful-differentiation/ 41. http://blog.happstack.com/2009/02/23/momentum-builds-as-we-approach-happsta… 42. http://lukepalmer.wordpress.com/2009/02/24/rts-research/ 43. http://ghcsparc.blogspot.com/2009/02/sanity.html 44. http://blog.darcs.net/2009/02/darcs-weekly-news-18.html 45. http://donsbot.wordpress.com/2009/02/22/can-haskell-give-me-a-pony/ 46. http://neilmitchell.blogspot.com/2009/02/hoogle-package-search.html 47. http://axman6.homeip.net/blog/2009/02/avar-changes.html 48. http://xmonad.wordpress.com/2009/02/21/using-gimp-under-xmonad/ 49. http://www.mega-nerd.com/erikd/Blog/FP-Syd/fp-syd-12.html 50. http://koweycode.blogspot.com/2009/02/implementing-join-in-terms-of.html 51. http://ghcsparc.blogspot.com/2009/02/triage.html 52. http://dekudekuplex.wordpress.com/2009/01/16/learning-haskell-through-categ… 53. http://www.haskell.org/mailman/listinfo/haskell 54. http://sequence.complete.org/ 55. http://planet.haskell.org/ 56. http://sequence.complete.org/node/feed 57. http://haskell.org/ 58. http://haskell.org/haskellwiki/HWN 59. http://code.haskell.org/~byorgey/code/hwn/

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And another request on how to get started - JSON package
by Gü?nther Schmidt 28 Feb '09

28 Feb '09

Hi, Sigbjorn has contributed tons of very useful packages and particularly quite a few package that are especially useful for the handful of us haskell-windows users. Right now though I could do with an example for how to actually use his JSON package. Since it's actively maintained and already there I'd like to avoid starting from scratch and re-implement the whole thing by using the samples from RWH. I had hoped that RWH and his package are 1:1 alas they are not. Günther

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