- Haskell-Cafe - Haskell.org

Re: Add haskell-src as an official machine-readable component of the Haskell standard
by Niklas Broberg 16 Nov '10

16 Nov '10

(Moving to Café) > (Examples of controversies possible in haskell-src: we have the Hs > prefix on constructors everywhere, we can't provide fixity information > (and the haskell-src-exts implementation of this is unsatisfactory in > several important ways) ... What ways? http://trac.haskell.org/haskell-src-exts/ :-) Cheers, /Niklas

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Bracket around every IO computation monad
by Mitar 16 Nov '10

16 Nov '10

Hi! I have a class Neuron which has (among others) two functions: attach and deattach. I would like to make a way to call a list/stack/bunch of attach functions in a way that if any of those fail (by exception), deattach for previously already attached values (called attach on them) are deattached (called deattach on them). I have come up with such way: data Growable where Growable :: Neuron n => LiveNeuron n -> Growable growNeurons :: [IO Growable] -> IO [Growable] growNeurons attaches = growNeurons' attaches [] where growNeurons' [] ls = return ls growNeurons' (a:ats) ls = bracketOnError a (\(Growable l) -> deattach l) (\l -> growNeurons' ats (l:ls)) So I give growNeurons a list of attach actions and it returns a list of attached values ((live)neurons). This works nice, but syntax to use it is ugly: neurons <- growNeurons [ do { a <- attach nerve1; return $ Growable a }, do { a <- attach nerve2; return $ Growable a }, do { a <- attach nerve3; return $ Growable a } ] Types of attach and deattach are (if I simplify): attach :: Nerve n -> IO (LiveNeuron n) deattach :: LiveNeuron n -> IO () Growable is only used so that I can put actions of different type in the list. It seems to me that all this could be wrapped into a monad. So that I would be able to call something like: neurons <- growNeurons' $ do attach nerve1 attach nerve2 attach nerve3 Where I would be allowed to call actions of a special type class which defined also clean-up function (in my case called deattach). And which would be called if there was any exception thrown (and then at the end rethrown). Otherwise, the result would be a list of all computed values. In my case all this in IO monad. So it is possible that evaluation of monad actions would be stacked inside of bracketOnError and in a case of error clean-up functions would be called, otherwise returns a list of results? Mitar

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Updating Haskell Packages through Archlinux becoming A Pain
by Mathew de Detrich 16 Nov '10

16 Nov '10

Well basically, I haven't update my Haskell libraries for some time now (around a month) so when I finally got around to updating my Haskell packages (which are installed through Archlinux's AUR) my GHC installation broke (again). This has been happening for some time now (I have been having this problem for around a year) Basically it occurs when pacman updates a package using an AUR package, which cabal refuses to install because it can break other packages (yet the package still gets installed according to pacman). So this means I have to manually go through /usr/lib/ghc-6.12.3/package.conf.d and manually updating all the package hashes while doing some manual dependency tracking through ghc-check This honestly starting to become annoying, is there any ideas or plans on how to resolve this issue. The end user shouldn't have to be dealing with this, and if installing a package can break the system, it shouldn't be 'semi' installed in the first place

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Annual dutch functional programming day, january 7 2011
by Christiaan Baaij 16 Nov '10

16 Nov '10

The annual Dutch functional programming day will take place soon: Date: Friday, January 7th, 2011 Location: University of Twente, Enschede During the day there will be several presentation related to FP-oriented research results, the use of FP concepts in industry, and the use of FP in education. Of course there is also plenty of opportunity to talk, exchange information, give demo's, and to meet old friends. Most participants are 'advanced' functional programmers; but those who recently developed an interest in all that is FP are of course more than welcome to join. Tradition dictates that at the end of the day we will enjoy a dinner together with those who wish join (at your own costs). You can register for the event by sending an email to me, Christiaan Baaij (c.p.r.baaij(a)utwente.nl) More information can be found at the website: http://caes.ewi.utwente.nl/External/NLFP Please indicate if you want to give presentation, and if you like to attend the dinner afterwards. Please also register if you only want to participate, as we have to know for how many people we should order lunch. There is of course NO participation fee. See you the 7th of January! Christiaan Baaij

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dynamic loading of code on windows
by Arnaud Bailly 16 Nov '10

16 Nov '10

Hello, I recently tried to cabal install plugins on a windows box and it failed with the following error: Resolving dependencies... Downloading plugins-1.5.1.4... Configuring plugins-1.5.1.4... cabal: The package has a './configure' script. This requires a Unix compatibility toolchain such as MinGW+MSYS or Cygwin. cabal: Error: some packages failed to install: plugins-1.5.1.4 failed during the configure step. The exception was: ExitFailure 1 What solution can I use to load dynamically code in a cross-platform way ? Thanks in advance Arnaud

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Quasiquoter invocation no longer requires/allows a leading dollar sign.
by Michael Snoyman 16 Nov '10

16 Nov '10

Sorry, maybe I missed the memo on this one, but I just noticed this change to quasi-quotation syntax on the GHC 7 upgrade page[1]. So if GHC 6.12 *requires* the dollar sign, and GHC 7 *rejects* the dollar sign, is there any way to write code that will run on both? Is there a reason we can't keep the dollar sign as supported syntax for a release or two to make for a cleaner migration? Michael [1] http://haskell.org/haskellwiki/Upgrading_packages/Updating_to_GHC_7#Quasiqu…

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Error installing hp2any-graph on Snow Leopard
by Mark Spezzano 16 Nov '10

16 Nov '10

Hi, I get the following error when doing a 'cabal install hp2any-graph' (the profiling tool) Does anyone know why? (I'm running Snow Leopard and I've tried using other flags suggested...but with no success). GLUT is already installed via MacPorts and via cabal. Resolving dependencies... Configuring hp2any-graph-0.5.2... cabal: Missing dependency on a foreign library: * Missing C library: glut This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: hp2any-graph-0.5.2 failed during the configure step. The exception was: ExitFailure 1 Thanks in advance, Mark

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Ralf Laemmel's riddle on surviving without the monad transformation library
by C K Kashyap 16 Nov '10

16 Nov '10

Hi, Can someone provide me the solution to the following riddle that Ralf asked in his lecture at http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Lmmel-Advance… Riddle: define a custom made monad (only involving (->) and Either String) to survive without the monad transformation library. -- Regards, Kashyap

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Is there any way to prevent a computation from being shared?
by Petr Prokhorenkov 16 Nov '10

16 Nov '10

I've came upon very a strange situation with memory consumption. The smallest test case I've been able to come up with is the following: import Data.List wtf d = head . dropWhile (< 10^100) . map (*d) $ enumFrom 2 main = do print $ wtf 1 print $ wtf 2 -- Everything is ok without this line Expected result is that program runs in constant space. What really happening is that the program consumes memory until killed. If second call to wtf is removed, memory usage stays constant while the program is working. The problem is in that list returned from enumFrom is being saved between calls to wtf. Is there any way to overcome this? -- Regards, Petr

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Error handling with safer-file-handling
by Florian Weimer 15 Nov '10

15 Nov '10

How am I supposed to write an exception handle for an invocation of System.IO.SaferFileHandles.openFile? Currently, I have got this: case req of Open path -> do handle <-openFile (asAbsPath path) ReadMode liftIO $ forcePut result Success run handle Follwing safer-file-handling-examples, I rewrote it to: case req of Open path -> do (do runRegionT $ do handle <-openFile (asAbsPath path) ReadMode liftIO $ forcePut result Success run handle) `catch` \(e :: IOException) -> do liftIO $ forcePut result $ Failure $ show e (I would rather like to apply the handler to the openFile invocation only, but the above matches the example code more closely.) But this gives me a nice error message: Handles.hs:48:17: Could not deduce (Control.Monad.CatchIO.MonadCatchIO (RegionT s IO)) from the context () arising from a use of `catch' at Handles.hs:(48,17)-(54,50) Possible fix: add (Control.Monad.CatchIO.MonadCatchIO (RegionT s IO)) to the context of the polymorphic type `forall s. RegionT s pr α' or add an instance declaration for (Control.Monad.CatchIO.MonadCatchIO (RegionT s IO)) In the expression: do { (do { runRegionT $ do { handle <- openFile (asAbsPath path) ReadMode; .... } }) } `catch` \ (e :: IOException) -> do { liftIO $ forcePut result $ Failure $ show e } In a case alternative: Open path -> do { (do { runRegionT $ do { ... } }) } `catch` \ (e :: IOException) -> do { liftIO $ forcePut result $ Failure $ show e } In the expression: case req of { Open path -> do { (do { runRegionT $ ... }) } `catch` \ (e :: IOException) -> do { ... } Close -> return () _ -> liftIO $ forcePut result $ Failure "handle not open" } safer-file-handling-examples results in similar error messages, after applying the s/runTopRegion/runRegionT/ fix. This is with GHC 6.12.1, with ScopedTypeVariables enabled.

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