March 2011 - Beginners

HDBC - outside a transaction
by Petr Novotnik 08 Mar '11

08 Mar '11

Hello all, while working through `Real World Haskell' Chapter 21 I hit the following problem: Using sqlite3, enabling foreign keys with "PRAGMA foreign_keys = ON;" must be done outside a transaction. However, HDBC doesn't seem to provide a possibility to submit such a statement to the database outside of a transaction. The only workaround I've found looks like this: > openDatabase :: String -> IO Connection > openDatabase name = do > conn <- connectSqlite3 name > run conn "COMMIT;" [] -- manually close HDBC's underlyingTX > run conn "PRAGMA foreign_keys = ON;" [] -- execute outside of TX > run conn "BEGIN;" [] -- open a new TX for HDBC again > prepareDatabase conn > return conn Am I missing something in HDBC's API or is this the only way to do it? Many thanks in advance, Pete.

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Monads
by Patrick Lynch 07 Mar '11

07 Mar '11

In Learn You a Haskell for Great Good!, the author Miran Lipovaca indicates that to understand Monads you need to know Functors... So, I noticed the following: class Functor f where fmap::(a->b)->f a->f b instance Functor [] where fmap = map map::(a->b)->[a]->[b] if [] is substituted for f in the class definition of Functor the following is obtained class Functor [] where fmap::(a->b)->[]a->[]b my questions are: 1. is this substitution ok? 2. is []a = [a]? 3. is []b = [b]? if 2. and 3. are valid then the following is obtained: fmap::(a->b)->[a]->[b] which is the same as map type and therefore: fmap = map. QED. Can you please answer questions 2 and 3 above? Thank you

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GHC compilation without using module Main
by Andrew Pennebaker 06 Mar '11

06 Mar '11

Is there a way to compile a Haskell script with a different module name than Main? #!/usr/bin/env runhaskell module ScriptedMain where meaningOfLife :: Int meaningOfLife = 42 main :: IO () main = putStrLn $ "Main: The meaning of life is " ++ show meaningOfLife $ ghc -o scriptedmain scriptedmain.hs Undefined symbols: "_ZCMain_main_closure", referenced from: _ZCMain_main_closure$non_lazy_ptr in libHSrtsmain.a(Main.o) (maybe you meant: _ZCMain_main_closure$non_lazy_ptr) "___stginit_ZCMain", referenced from: ___stginit_ZCMain$non_lazy_ptr in libHSrtsmain.a(Main.o) (maybe you meant: ___stginit_ZCMain$non_lazy_ptr) ld: symbol(s) not found collect2: ld returned 1 exit status Cheers, Andrew Pennebaker www.yellosoft.us

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cabal multi-library setup
by Cristian Petrescu Prahova 06 Mar '11

06 Mar '11

Hi, I have a small project consisting of a library and an application. I cabalized both. However, whenever I tweak the library I cabal install it, then move to the app directory, then reconfigure, then rebuild. This is annoying repetitive shell typing. Is there a way to configure my setup such that cabal build app automatically rebuilds (and reinstalls?) the library dependency? I'd like something like: # builds the library and runs the tests cabal make lib # builds the app and the library dependency, then runs the app cabal make app Thank you, Cristian

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Windows 7 time function
by Sayth Renshaw 05 Mar '11

05 Mar '11

I was completing the 5 step quickstart guide http://www.haskell.org/haskellwiki/Haskell_in_5_steps When running the program though haskell appears to be not able to access time on windows 7. Would this be normal? C:\Users\RenshawFamily\Documents\Scripts\Haskell>time ./A +RTS -N2 The system cannot accept the time entered. Enter the new time: C:\Users\RenshawFamily\Documents\Scripts\Haskell>time A.exe +RTS -N2 The system cannot accept the time entered. Enter the new time: 14.13 A required privilege is not held by the client.

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Pure function with timeout
by Alex Rozenshteyn 04 Mar '11

04 Mar '11

I read http://www.haskell.org/haskellwiki/Timing_out_computations I thought I saw an issue and tested it: http://hpaste.org/44512/timed_iteration IMO, the timeout function should return the last iterated value when it is signaled, and not the next one after the signal. Is there a convention for this behavior? -- Alex R

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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 02 Mar '11

02 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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