- Beginners - Haskell.org

Confused by GLFW and more
by Britt Anderson 03 Mar '11

03 Mar '11

To: Beginners List I am trying to get a simple program working that will display something on the screen, get user input via joystick until a quit event, and then start a new trial, and repeat this until all the trials have been performed. The problem is that if I try to sequence the trials it hangs on the last trial (screen freezes and I have to kill it), however, if I just execute one after the other it exits fine. I don't understand what is different about doing the commented line versus explicitly invoking two trials (as demonstrated in the snippet below). Here is a snippet ( I am happy to provide more, but I don't know what is needed to diagnose my problem and I don't want to clutter the question): tn = 3 main :: IO () main = do let ps = initPlayers createWindow putStr "Window Initialized\n" let joy = GLFW.Joystick 1 putStr "Got Joystick\n" --sequence_ $ map (trialLoop gain tolerance joy) (trialList ps) trialLoop gain tolerance joy ((trialList ps)!!0) trialLoop gain tolerance joy ((trialList ps)!!1) putStr "Back from Sequence\n" where trialList oldps = [[(h' i) , (p' i)] | i <- [1 ..], i < tn] where x = 0.2 y = 0.2 h' idx = (oldps!!0) p' idx = Prey (Circle (((-1)^idx)*0.2,0.2) ((radius . shape) (oldps!!1))) (clr $ oldps!!1) As an aside, I was wondering if anyone has experience using joysticks with any of the Graphics.UI I have both a logitech and a saitek and I find that although both are found by the Graphics.UI.SDL I cannot get the buttons for the Saitek, but can for logitech. For neither does pollEvents seem to work right, although I can use update and getAxis for both joysticks. It was this unpredictable behaviour that drove me from SDL to GLFW. Any general advice on the UIs for joysticks would be appreciated as well. I am not actually writing a game, but a simple psychophysics experiment (the use of Haskell is just an exercise - normally I would do this [and have been done with it long ago] in python. So, my questions are really focused on trying to use haskell and its libraries for this purpose). Using Arch Linux if that makes any difference. Thanks in advance for any comments. --Britt

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Understanding the type signature of flip$id
by Jeff Lasslett 03 Mar '11

03 Mar '11

Greetings, I have been reading about how to use system.console.getOpt, specifically "Interpreting flags as transformations of an options record" which can be found at http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Console-G… Line 3 of compilerOpts argv = case getOpt Permute options argv of (o,n,[] ) -> return (foldl (flip id) defaultOptions o, n) (_,_,errs) -> ioError (userError (concat errs ++ usageInfo header options)) where header = "Usage: ic [OPTION...] files..." uses (flip id) to resolve a list of functions down to a single record. Looking at the type of flip$id I can see why it works in this context. I just don't understand how passing id to flip results in the following type signature: Prelude> :t flip$id flip$id :: b -> (b -> c) -> c I do understand what flip has done to map here:- Prelude> :t flip$map flip$map :: [a] -> (a -> b) -> [b] map take a function and a list and produces a new list. If map is passed to flip the result is a function that takes a list, then a function and results in a new list. How do we go from flip having this signature: Prelude> :t flip flip :: (a -> b -> c) -> b -> a -> c Prelude> and id having Prelude> :t id id :: a -> a Prelude> to flip$id looking like flip$id :: b -> (b -> c) -> c ??? Thanks, Jeff

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Problems installing plugins?
by Patrick LeBoutillier 02 Mar '11

02 Mar '11

Hi, I'm trying to install the plugins package on Fedora 14 but I get a strange error: [patrickl@fc14x64 /]$ sudo cabal install plugins Resolving dependencies... cabal: cannot configure plugins-1.5.1.4. It requires ghc >=6.10 There is no available version of ghc that satisfies >=6.10 [patrickl@fc14x64 /]$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 One thing probably worth mentioning is that my home directory is shared amongst many servers, some running older Fedora versions (10). Could something in my ~/.cabal be messed up or incompatible between versions? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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feedback on simplifying my Takusen code
by Neil Jensen 02 Mar '11

02 Mar '11

Hello, beginner here, looking for some ideas on how to simplify how I use the Takusen database library to query a database and return types. In the working code below, I'm able to query the database and through several functions get a list of simple types. I've tried playing around with the query2Iteratee and getAccounts' functions to simplify the code and reduce it to a single function, but keep getting hung up on compiler errors due to type issues. Is there a better way to write this, or is it best to keep the various functions quite separate? (in my real project, I have the database access in a separate module from the list of custom types and the code to convert to the Account type). Thanks in advance, Neil import Database.Sqlite.Enumerator import Database.Enumerator data Account = Account {account_id :: Int} deriving (Eq, Show) query2Iteratee :: (Monad m) => Int -> [Int] -> m (IterResult [Int]) --query2Iteratee :: (Monad m) => Int -> IterAct m [Int] query2Iteratee a accum = result' (a:accum) getAccounts' :: IO [Int] getAccounts' = do let dbh = connect "performance_data.sqlite" withSession dbh (do r <- doQuery (sql "SELECT account_id FROM ACCOUNT;") query2Iteratee [] return r ) getAccounts :: IO [Account] getAccounts = do accountsList <- getAccounts' let r = map conv accountsList return r conv :: Int -> Account conv a = Account {account_id=a} main = do accounts <- getAccounts' putStrLn (show accounts)

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HUnit - testing for failed pattern match
by Xavier Shay 26 Feb '11

26 Feb '11

Hello, Given the following contrived function: giveMeThree x | x == 3 = True How do I test with HUnit that 2 is invalid input? giveMeThreeTests = [ "Likes 3" ~: True ~=? giveMeThree 3 , "Dislikes 2" ~: ..... -- what? ] I couldn't find anything mentioned in the HUnit user guide. References to appropriate documentation appreciated. Cheers, Xavier

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Can't reproduce the build failure of my package on hackage
by Daniel Seidel 25 Feb '11

25 Feb '11

Hi, a while ago I uploaded a package to Hackage ( http://hackage.haskell.org/package/SBench ). I checked it with ghc-6.12.3 and it worked well. The uploaded version was tried to compile via ghc-7 and failed with the following log: cabal: dependencies conflict: haskell98-1.1.0.1 requires time ==1.2.0.3 however time-1.2.0.3 was excluded because gnuplot-0.4.1 requires time ==1.1.* Now, on Hackage, the latest haskell98 version is 1.1.0.0 and it does, at least directly, *not* depend on time. To figure out the problem I installed ghc-7.0.1 and tried to install my package. It worked. Can anyone give me a hint why compilation fails on Hackage and where haskell98-1.1.0.1 comes from? I want to upload a new version because I needed to adjust to the '-rtsopts' flag in ghc 7. Before doing so it would be great to ensure the new upload will compile on Hackage. A way might be to restrict haskell98 to be less or equal to 1.1.0.0, but I think it's not a good way. Cheers, Daniel.

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GTK Haskell and Error on Win32, anyone see the issue?
by Berlin Brown 23 Feb '11

23 Feb '11

I am following the steps outlined here but I am still getting errors related to my install of gtk Here is one of the first errors: tup\setup.exe ... Configuring cairo-0.12.0... setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: z, cairo This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. http://www.haskell.org/haskellwiki/Gtk2Hs Here is the output: C:\Documents and Settings\Bot User>echo %LIB% C:\Program Files\Gtk+\lib C:\Documents and Settings\Bot User>echo %PKG_CONFIG_PATH% C:\Program Files\Gtk+\lib\pkgconfig;C:\ Program Files\Gtk+\include\libglade-2.0;C:\Program Files\libxml2\libxml2-dev_2.7.7-1_win32\lib\pkgconfig I got the install of GTK and everything worked: http://ftp.gnome.org/pub/GNOME/binaries/win32/glade3/3.6/glade3-3.6.7-with-… Microsoft Windows XP [Version 5.1.2600] C:\Documents and Settings\Bot User>cabal update Downloading the latest package list from hackage.haskell.org Version: C:\Documents and Settings\Bot User>ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 C:\Documents and Settings\Bot User>cabal install gtk2hs-buildtools Resolving dependencies... Configuring gtk2hs-buildtools-0.12.0... Preprocessing executables for gtk2hs-buildtools-0.12.0... ... hs\gtk2hsC2hs-tmp\GenBind.o ) [41 of 41] Compiling Main ( c2hs\toplevel\Main.hs, dist\build\gtk2hs C2hs\gtk2hsC2hs-tmp\Main.o ) Linking dist\build\gtk2hsC2hs\gtk2hsC2hs.exe ... Installing executable(s) in C:\Documents and Settings\Bot User\Application Data\cabal\bin Here is where it fails: C:\Documents and Settings\Bot User>cabal install gtk Resolving dependencies... C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\cairo-0.12.03164\cairo-0.12.0\Gtk2HsSetup.hs: 25: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when build ing (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\cairo-0 .12.03164\cairo-0.12.0\Gtk2HsSetup.hs, C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\cairo- 0.12.03164\cairo-0.12.0\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\cairo-0 .12.03164\cairo-0.12.0\Setup.hs, C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\cairo-0.12.0 3164\cairo-0.12.0\dist\setup\Main.o ) Linking C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\cairo-0.12.03164\cairo-0.12.0\dist\se tup\setup.exe ... Configuring cairo-0.12.0... setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: z, cairo This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\glib-0.12.03164\glib-0.12.0\Gtk2HsSetup.hs:25 : warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when buildin g (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\glib-0. 12.03164\glib-0.12.0\Gtk2HsSetup.hs, C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\glib-0.1 2.03164\glib-0.12.0\dist\setup\Gtk2HsSetup.o ) [2 of 2] Compiling Main ( C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\glib-0. 12.03164\glib-0.12.0\Setup.hs, C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\glib-0.12.0316 4\glib-0.12.0\dist\setup\Main.o ) Linking C:\DOCUME~1\BOTBER~1\LOCALS~1\Temp\glib-0.12.03164\glib-0.12.0\dist\setu p\setup.exe ... Configuring glib-0.12.0... setup.exe: Missing dependencies on foreign libraries: * Missing C libraries: gobject-2.0, glib-2.0, intl This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. cabal: Error: some packages failed to install: cairo-0.12.0 failed during the configure step. The exception was: ExitFailure 1 gio-0.12.0 depends on glib-0.12.0 which failed to install. glib-0.12.0 failed during the configure step. The exception was: ExitFailure 1 gtk-0.12.0 depends on glib-0.12.0 which failed to install. pango-0.12.0 depends on glib-0.12.0 which failed to install. C:\Documents and Settings\Bot User> -- Berlin Brown (berlin dot brown at gmail.com) http://botnode.com http://berlinbrowndev.blogspot.com/

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Help with GHC Packages Installation on Fedora 14
by Hong Yang 23 Feb '11

23 Feb '11

Hi, I installed a Fedora 14 virtual machine on my home Windows desktop. Then I tried to install GHC packages on Fedora by typing "yum install ghc" as root, and got the following error: "Error: Cannot retrieve repository metadata (repomd.xml) for repository: fedora. Please verify its path and try again." What should I do now to fix this error? Thanks, Hong

2 1

Very basic question on monads
by blackcat＠pro-ns.net 23 Feb '11

23 Feb '11

I'm working through the "Write Yourself a Scheme" wikibook and having trouble with one of the exercises. For the function parseNumber :: Parser LispVal, both parseNumber = liftM (Number . read) $ many1 digit parseNumber' = do digits <- many1 digit return $ (Number . read) digits work. But parseNumber'' = many1 digit >>= liftM read >>= liftM Number complains "couldn't match expected type Char against inferred type [Char]". Is liftM engaging the list monad? How do I fix this?

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Having trouble defining type correctly
by Chris Bolton 22 Feb '11

22 Feb '11

Ok, so here's my current code: http://paste.pocoo.org/show/342949/ My basic problem is I'm not quite sure how to define Command. When a user calls the script they can either call add, remove, update, or query, which all have different type signatures, which is why I need Command so it'll pass through haskell's type checking. If I get rid of query, A definition of { data Command m = Save m | Rem m } seemed to work, from what I can tell, but query screws it up, so I'm trying to define exactly what Save, Rem, etc. should be, which I've taken directly from the MongoDB API, but it just keeps throwing errors, as seen here: http://paste.pocoo.org/show/342924/ Also, I'm very new to haskell and programming in general, so sorry for the possibly messy code / noobish question, but anyone have any ideas here? :P

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