- Haskell-Cafe - Haskell.org

cabal advice: library paths
by Michael Snoyman 31 Mar '11

31 Mar '11

Hi all, I have a package[1] which uses some system libraries (Qt to be precise). On Linux (and I'm hoping Mac), it's able to use pkg-config to determine which libraries are necessary and where to find all the files. On Windows, I've been less than successful using pkg-config. However, I *was* able to hardcode in the paths[2], and the resulting executables ran without a hitch. I still have two problems: * Obviously hardcoding paths in less than ideal. * Hackage won't allow me to upload this package anymore. It complains: 'extra-lib-dirs: c:\qt\2010.05\qt\lib' directory does not exist. Can anyone recommend a more elegant solution to the problem? Thanks, Michael [1] http://hackage.haskell.org/package/wai-handler-webkit [2] https://github.com/snoyberg/hack-handler-webkit/blob/master/wai-handler-web…

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object oriented technique
by Tad Doxsee 31 Mar '11

31 Mar '11

I've been trying to learn Haskell for a while now, and recently wanted to do something that's very common in the object oriented world, subtype polymorphism with a heterogeneous collection. It took me a while, but I found a solution that meets my needs. It's a combination of solutions that I saw on the web, but I've never seen it presented in a way that combines both in a short note. (I'm sure it's out there somewhere, but it's off the beaten path that I've been struggling along.) The related solutions are 1. section 3.6 of http://homepages.cwi.nl/~ralf/OOHaskell/paper.pdf 2. The GADT comment at the end of section 4 of http://www.haskell.org/haskellwiki/Heterogenous_collections I'm looking for comments on the practicality of the solution, and references to better explanations of, extensions to, or simpler alternatives for what I'm trying to achieve. Using the standard example, here's the code: data Rectangle = Rectangle { rx, ry, rw, rh :: Double } deriving (Eq, Show) drawRect :: Rectangle -> String drawRect r = "Rect (" ++ show (rx r) ++ ", " ++ show (ry r) ++ ") -- " ++ show (rw r) ++ " x " ++ show (rh r) data Circle = Circle {cx, cy, cr :: Double} deriving (Eq, Show) drawCirc :: Circle -> String drawCirc c = "Circ (" ++ show (cx c) ++ ", " ++ show (cy c)++ ") -- " ++ show (cr c) r1 = Rectangle 0 0 3 2 r2 = Rectangle 1 1 4 5 c1 = Circle 0 0 5 c2 = Circle 2 0 7 rs = [r1, r2] cs = [c1, c2] rDrawing = map drawRect rs cDrawing = map drawCirc cs -- shapes = rs ++ cs Of course, the last line won't compile because the standard Haskell list may contain only homogeneous types. What I wanted to do is create a list of circles and rectangles, put them in a list, and draw them. It was easy for me to find on the web and in books how to do that if I controlled all of the code. What wasn't immediately obvious to me was how to do that in a library that could be extended by others. The references noted previously suggest this solution: class ShapeC s where draw :: s -> String copyTo :: s -> Double -> Double -> s -- needs {-# LANGUAGE GADTs #-} data ShapeD where ShapeD :: ShapeC s => s -> ShapeD instance ShapeC ShapeD where draw (ShapeD s) = draw s copyTo (ShapeD s) x y = ShapeD (copyTo s x y) mkShape :: ShapeC s => s -> ShapeD mkShape s = ShapeD s instance ShapeC Rectangle where draw = drawRect copyTo (Rectangle _ _ rw rh) x y = Rectangle x y rw rh instance ShapeC Circle where draw = drawCirc copyTo (Circle _ _ r) x y = Circle x y r r1s = ShapeD r1 r2s = ShapeD r2 c1s = ShapeD c1 c2s = ShapeD c2 shapes1 = [r1s, r2s, c1s, c2s] drawing1 = map draw shapes1 shapes2 = map mkShape rs ++ map mkShape cs drawing2 = map draw shapes2 -- copy the shapes to the origin then draw them shapes3 = map (\s -> copyTo s 0 0) shapes2 drawing3 = map draw shapes3 Another user could create a list of shapes that included triangles by creating a ShapeC instance for his triangle and using mkShape to add it to a list of ShapeDs. Is the above the standard method in Haskell for creating an extensible heterogeneous list of "objects" that share a common interface? Are there better approaches? (I ran into a possible limitation to this approach that I plan to ask about later if I can't figure it out myself.) - Tad

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Haskell Weekly News: Issue 175
by Daniel Santa Cruz 31 Mar '11

31 Mar '11

Welcome to issue 175 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of March 20 - 26. You can find the HTML version at: http://bit.ly/gs9Rr8 Announcements Wren Ng Thornton [2]announced the release of unix-bytestring: Unix/Posix-specific functions for ByteStrengs. John Lato [3]announced an incremental update to the ListLike package. "This version adds newtype wrappers for strict and lazy ByteString, CharString, and CharStringLazy..." Mario Blazevic [4]announced the release of incremental-parser, which provides the usual set of Applicative and monad combinators with a few twists that make it unique. Duncan Coutts [5]announced the release of version 0.16.3 of c2hs, the FFI preprocessor tool. Jeff Wheeler [6]announced a new release of Yi, the text editor written in and extensible in Haskell. Christiaan Baaij [7]announced an incremental upgrade to 0.1.3.0 to CLaSH. Mario Blazevic [8]announced updates to monad-parallel, monad-coroutine, and SCC (Streaming Component Combinators). John Millikin [9]announced an update to enumerator to version 0.4.8. Quotes of the Week * Squarism: wo utilizing any tricks, i want 20 miljon rabbits to mate with each other once.. how many mate event will there be * mm_freak: using combinators instead of explicit recursion can be justified with about the same reasoning as using 'while' instead of explicit 'goto' in imperative programming * monochrom: <$> is pronounced Jacquelin von Brionée * geheimdienst: what does a pirate say when starting a sentence, then suddenly being hit in the nadgers? arr- ow! * monochrom: 1985 is way more than a decade ago * copumpkin: matlab is horrible as far as language design goes[.] it's fairly convenient for doing stuff in but I need to shower after every time I use it Top Reddit Stories * Debugging trick From (self.haskell), scored 60 with 11 comments. Read on [10]reddit. * Your friendly Haskell JSON library's encoding performance just got 4x faster From (serpentine.com) scored 45 with 2 comments. Read on [12]reddit. Read the [13]original post. * A monad for deterministic parallelism (ICFP'2011 submission) From (community.haskell.org) scored 41 with 8 comments. Read on [14]reddit. Read the [15]original post. * Haskell servers benchmarked against node/ruby/python/php/java. From (docs.yesodweb.com) scored 36 with 12 comments. Read on [16]reddit. Read the [17]original post. * The Yesod Framework and the Case of the Eight Level Monad Transformer Stack From (docs.yesodweb.com) scored 35 with 16 comments. Read on [18]reddit. Read the [19]original post. * Improving Persistent Performance (Yesod Blog) From (docs.yesodweb.com) scored 29 with 1 comments. Read on [20]reddit. Read the [21]original post. * Fast, clean Haskell bindings to Google's Snappy library From (hackage.haskell.org) scored 22 with 8 comments. Read on [22]reddit. Read the [23]original post. * Open Quark on Github (Haskell like Lang on the JVM) From (github.com) scored 15 with 3 comments. Read on [24]reddit. Read the [25]original post. * EclipseFP 2.0.4 released: supports GHC 7 From (jpmoresmau.blogspot.com) scored 15 with 7 comments. Read on [26]reddit. Read the [27]original post. * Demonstrating a Time Leak in Arrowized FRP From (blog.edwardamsden.com) scored 14 with 12 comments. Read on [28]reddit. Read the [29]original post. Top StackOverflow Answers * [30]Reading large file in haskell ? votes: 19 The construct "seq x x" is always useless. If y = seq x x and I force y then this forces x then returns x. This is equivalent to y=x and forcing y. Thus "seq forceEval forceEval" does nothing more than "forceEval". The error with your use of a fold is a common one. You are using a fold to perform a count of the bytes in the input. You should be using ... * [31]What to call a function that splits lists? votes: 12 I believe the function you're describing is breakBefore from the list-grouping package. Data.List.Grouping: http://hackage.haskell.org/packages/archive/list-grouping/0.1.1/doc/html/Da… st-Grouping.html ghci:: breakBefore even [3,1,4,1,5,9,2,6,5,3,5,8,9,7,9,3,2,3,8,4,6,2,6] [[3,1],[4,1,5,9],[2],[6,5,3,5],[8,9,7,9,3],[2,3],[8],[4],[6],[2],[6]] Top StackOverflow Questions * [32]Haskell record syntax votes: 16, answers: 1 * [33]Reading large file in haskell? votes: 12, answers: 1 * [34]Haskell laziness - how do I force the IO to happen sooner? votes: 9, answers: 3 * [35]case on monadic value votes: 7, answers: 2 * [36]Simplest way to find which data types are most garbage collected? votes: 7, answers: 2 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15484 3. http://article.gmane.org/gmane.comp.lang.haskell.general/18590 4. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15492 5. http://article.gmane.org/gmane.comp.programming.c2hs/247 6. http://article.gmane.org/gmane.editors.yi.devel/5346 7. http://article.gmane.org/gmane.comp.lang.haskell.general/18592 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/87540 9. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15506 10. http://www.reddit.com/r/haskell/comments/gbd7w/debugging_trick/ 12. http://www.reddit.com/r/haskell/comments/g8s0t/your_friendly_haskell_json_l… 13. http://www.serpentine.com/blog/2011/03/22/a-new-week-a-new-json-performance… 14. http://www.reddit.com/r/haskell/comments/gainp/a_monad_for_deterministic_pa… 15. http://community.haskell.org/~simonmar/papers/monad-par.pdf 16. http://www.reddit.com/r/haskell/comments/g7uia/haskell_servers_benchmarked_… 17. http://docs.yesodweb.com/blog/preliminary-warp-cross-language-benchmarks 18. http://www.reddit.com/r/haskell/comments/ga40w/the_yesod_framework_and_the_… 19. http://docs.yesodweb.com/blog/hamlet-lucius-widgets 20. http://www.reddit.com/r/haskell/comments/gbz43/improving_persistent_perform… 21. http://docs.yesodweb.com/blog/improving-persistent-performance 22. http://www.reddit.com/r/haskell/comments/ga080/fast_clean_haskell_bindings_… 23. http://hackage.haskell.org/package/snappy 24. http://www.reddit.com/r/haskell/comments/g7vcj/open_quark_on_github_haskell… 25. https://github.com/levans/Open-Quark 26. http://www.reddit.com/r/haskell/comments/g8du0/eclipsefp_204_released_suppo… 27. http://jpmoresmau.blogspot.com/2011/03/eclipsefp-204-released-supports-ghc-… 28. http://www.reddit.com/r/haskell/comments/g943x/demonstrating_a_time_leak_in… 29. http://blog.edwardamsden.com/2011/03/demonstrating-time-leak-in-arrowized.h… 30. http://stackoverflow.com/questions/5410777/reading-large-file-in-haskell/54… 31. http://stackoverflow.com/questions/5438558/what-to-call-a-function-that-spl… 32. http://stackoverflow.com/questions/5367167/haskell-record-syntax 33. http://stackoverflow.com/questions/5410777/reading-large-file-in-haskell 34. http://stackoverflow.com/questions/5373883/haskell-laziness-how-do-i-force-… 35. http://stackoverflow.com/questions/5395517/case-on-monadic-value 36. http://stackoverflow.com/questions/5410345/simplest-way-to-find-which-data-…

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[ghci] unknown symbol in base-unicode-symbols-0.2.1.2 ...
by John Obbele 30 Mar '11

30 Mar '11

Hi everyone ! I'm having a problem in GHCi when loading modules relying on the base-unicode-symbols package. My prompt gives me the following message: >> ghci $ GHCi, version 7.0.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :m +System.USB Prelude System.USB> newCtx Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package deepseq-1.1.0.2 ... linking ... done. Loading package bytestring-0.9.1.10 ... linking ... done. Loading package text-0.11.0.5 ... linking ... done. Loading package base-unicode-symbols-0.2.1.2 ... linking ... <interactive>: /home/john/.cabal/lib/base-unicode-symbols-0.2.1.2/ghc-7.0.2/HSbase-unicode-symbols-0.2.1.2.o: unknown symbol `__stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziCategoryziUnicode_' ghc: unable to load package `base-unicode-symbols-0.2.1.2' Prelude System.USB> I thought this was due to me screwing around cabal/ghc-pkg caches but after a fresh install of Fedora15-alpha / cabal update / cabal install usb I'm still getting the same error. Moreover, a quick `nm HSbase-unicode-symbols-*.o | grep ControlziCategoryziUnicode` give me the following output. >> nm HSbase-unicode-symbols-0.2.1.2.o | grep -C 4 'ControlziCategoryziUnicode_' 0000000000000070 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziApplicativeziUnicode 0000000000000040 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziApplicativeziUnicode_ 0000000000000120 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziArrowziUnicode 00000000000000d8 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziArrowziUnicode_ U __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziCategoryziUnicode_ 00000000000001b8 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziMonadziUnicode 0000000000000188 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_ControlziMonadziUnicode_ 0000000000000250 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_DataziBoolziUnicode 0000000000000220 T __stginit_basezmunicodezmsymbolszm0zi2zi1zi2_DataziBoolziUnicode_ Could someone please explain to me what's happening ? regards, /John

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Accessing to a Haskell type in C
by Yves Parès 30 Mar '11

30 Mar '11

Hello, To access a haskell type from C code, do we have to make this type instance of Storable (and so to also define an homomorph structure on the C side), or does FFI specifies some C function to enable C code to access the fields, or even some GHC-specific functions? (Even if I'd prefer a portable way) I mean, if I have the type: data Character = Character { pos :: (Float, Float), size :: Float, sprite :: Image } Do I compulsorily have to double its definition on C side: stuct CCharacter { float x; float y; float size; Image sprite; } and then make Character instance of Storable, which will make the program *copy* Characters to CCharacters each time that I must call a C function to, say, draw a character?

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Re: [Haskell-cafe] ArrowLoop and streamprocessors
by Patai Gergely 30 Mar '11

30 Mar '11

> So loop really doesn't seem to help here, but I couldn't find another > way either to feed outputs back into the system. > What I need is: > Either A B ~> Either C B -> A ~> C > > Does such a thing exist? At this point you don't really have enough structure to define such a feedback loop. Since you have streams of different rates, you need to find some common ground to merge them. The path taken by Reactive is to assign a timestamp to events, and ensure that merging preserves time ordering. Of course you'll immediately run into the problem of simultaneous occurrences, and it depends on the context how you want to resolve them. Also, what you're trying to achieve looks similar to Peakachu, so you might want to check it out, especially the Program abstraction [1]. It even has a feedback combinator with a slightly different signature. Gergely [1] http://hackage.haskell.org/packages/archive/peakachu/0.3.0/doc/html/FRP-Pea… -- http://www.fastmail.fm - mmm... Fastmail...

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CURL and upload a file
by Evgeny Dzhurinsky 30 Mar '11

30 Mar '11

Hello! I am trying to find out the way of uploading a file using HTTP multipart/form-data. I created simple application aimed to upload an image to imgpaste.com. The source is below: ========================================================= module ImgPaste.Upload where import Control.Monad import Network.Curl import Data.ByteString import Text.Regex.PCRE import Text.Regex.PCRE.ByteString type LocalCtx = ( ByteString, ByteString, ByteString, [ByteString] ) type UploadResult = Either UploadError ByteString data UploadError = UploadError { message :: String, response :: ByteString } deriving (Show) extractResponse :: CurlResponse_ [(String,String)] ByteString -> ByteString extractResponse = respBody uploadFile :: String -> IO (ByteString) uploadFile fileName = initialize >>= withCurlDo . (flip uploadFileWithCurl fileName) uploadFileWithCurl :: Curl -> String -> IO ( ByteString ) uploadFileWithCurl curl fileName = do liftM extractResponse $ do_curl_ curl "http://imgpaste.com/" [CurlPost True, CurlHttpPost postData, CurlVerbose True ] where postData = [ HttpPost "upfile" Nothing ( ContentFile fileName ) [] Nothing, makeFormPost "submit" "Upload", makeFormPost "keep" "a" ] makeFormPost name value = HttpPost name Nothing (ContentString value) [] Nothing -- <input type="text" name="copyfield" size="31" value="http://imgpaste.com/tmp/123456.png" /> extractUrl :: ByteString -> UploadResult extractUrl src = match (src =~ "<input type=\"text\" name=\"copyfield\" size=\"31\" value=\"([^\"]+?)\" />" :: LocalCtx) where match (_,_,_,[x]) = Right x match _ = Left $ UploadError "Can not parse content." src pasteImage :: String -> IO ( UploadResult ) pasteImage = liftM extractUrl . uploadFile ========================================================= however when invoking the function: pasteImage "image.png" no data is posted to the server. The protocol dump looks like: ========================================================= POST / HTTP/1.1 Host: imgpaste.com Accept: */* Content-Length: 241 Expect: 100-continue Content-Type: multipart/form-data; boundary=----------------------------630d154aaf3b ========================================================= so the request data is not sent. Can somebody please advice, what am I doing in a wrong way? Thank you in advance! -- regards Eugene Dzhurinsky

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Regarding two platforms build error.
by Serguey Zefirov 30 Mar '11

30 Mar '11

Haskell Platform 2010.1 with ghc 6.12.1 worked quite well. Problem solved. ;)

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Dynamically linked executables
by Anakim Border 30 Mar '11

30 Mar '11

Hi, I'm trying to understand if it's possible to build dynamically linked executables under Mac OS X (10.6) with GHC 7. Looking here: http://hackage.haskell.org/trac/ghc/wiki/SharedLibraries/PlatformSupport it seems like the support is already present. Inside ghc-7.1.20110325 sources (the latest development snapshots as I write), I find the following: # Do we support shared libs? PlatformSupportsSharedLibs = $(if $(filter $(TARGETPLATFORM),\ i386-unknown-linux x86_64-unknown-linux \ i386-unknown-freebsd x86_64-unknown-freebsd \ i386-unknown-openbsd x86_64-unknown-openbsd \ i386-unknown-mingw32 \ i386-unknown-solaris2 \ i386-apple-darwin powerpc-apple-darwin),YES,NO) which seems to indicate the contrary. GHC bug tracker, finally, doesn't show any open ticket on the matter. Anyone has more details about this? Thanks, AB

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Two Haskell Platforms on single machine.
by Serguey Zefirov 30 Mar '11

30 Mar '11

I had to use two Haskell Platforms at once in the Windows environment. We use Haskell Platform 2011.1 as our main build platform. It provide real benefits for code with GADTs so we ported most of our code there. Right now we cannot switch back or it would be quite a regress. We also have some heavy type level computation code in one of our programs so we have to use Haskell Platform 2010.2 with ghc 6.12.3 as 7.0.2 is quite slow at that. And now I have some strange error (it didn't appear when I used only one of Haskell platforms): ---------------------------------- ghc-6.12.3 -O2 -odir objs -hidir objs -stubdir objs -o CGen.exe -fcontext-stack=100 --make CGen.hs Linking CGen.exe ... realgcc.exe: USERNA: Invalid argument C:\PROGRA~1\HASKEL~1\201020~1.0\lib\..\mingw\bin\windres: "C:\PROGRA~1\HASKEL~1\ 201020~1.0\lib\..\mingw\bin\gcc exited with status 1 ---------------------------------- When I looked at the dump in hex, I encountered that "USERNA" string contains "USERNAM^H" byte sequence, actually. I cannot look at ghc -v5 as I get another error: ---------------------------------- Glasgow Haskell Compiler, Version 6.12.3, for Haskell 98, stage 2 booted by GHC version 6.10.4 Using binary package database: C:\PROGRA~1\HASKEL~1\201020~1.0\lib\package.conf.d\package.cache ... ... (I deleted much of the lines) ... Deleting: *** Deleting temp dirs: Deleting: <stderr>: hPutChar: invalid argument (character is not in the code page) ---------------------------------- I use localized version of Windows XP so it appears that ghc cannot print some name with russian characters in it. And here I stuck. I cannot find source of one error because of another error. Could anyone help me?

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