- Haskell-Cafe - Haskell.org

C++ Parser?
by Christopher Brown 01 Feb '12

01 Feb '12

Hi, I have stumbled across language-c on hackage and I was wondering if anyone is aware if there exists a full C++ parser written in Haskell? Many thanks, Chris.

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network-conduit failing to close sockets
by Ertugrul Söylemez 01 Feb '12

01 Feb '12

Hello there, I have tried to implement a simple echo server using the network-conduit library version 0.2.1. This is the code: module Main where import Data.Conduit import Data.Conduit.Network main :: IO () main = runTCPServer (ServerSettings 4000 Nothing) ($$) It works, but at some point it dies with too many open files, even though I never open two connections simultaneously: % ./echo-server echo-server: accept: resource exhausted (Too many open files) Apparently it fails to close the sockets properly. This is the client side code I have used for testing in GHCi: > :m Network Control.Monad System.IO > replicateM_ 512 (connectTo "127.0.0.1" (PortNumber 4000) >>= hClose) I'm running the action once, then wait a few seconds to give the server a chance to close the handles. Then I run it again, causing the server program to die with the above mentioned error message. Am I doing something wrong or is this a bug in network-conduit? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Memory usage with TVar?
by Johan Brinch 01 Feb '12

01 Feb '12

Here's the example program: https://gist.github.com/1cbe113d2c79e2fc9d2b When I run the program (which maintains a list inside an STM TVar), I get the following statistics: > ./Test +RTS -s 176,041,728 bytes allocated in the heap 386,794,976 bytes copied during GC 69,180,224 bytes maximum residency (7 sample(s)) 42,651,080 bytes maximum slop 179 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 336 colls, 0 par 0.44s 0.44s 0.0013s 0.0033s Gen 1 7 colls, 0 par 0.39s 0.40s 0.0570s 0.1968s INIT time 0.00s ( 0.00s elapsed) MUT time 0.23s ( 0.23s elapsed) GC time 0.83s ( 0.84s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 1.06s ( 1.07s elapsed) %GC time 77.9% (78.3% elapsed) Alloc rate 749,153,093 bytes per MUT second Productivity 22.1% of total user, 22.0% of total elapsed How come this program uses 179 MB of memory? I'm on a 64-bit machine where 2'000'000 integers uses 32 MB. Where does the overhead come from? -- Johan Brinch

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Serialize instance for System.Posix.Files.FileStatus
by Johan Brinch 01 Feb '12

01 Feb '12

Are there plans on implementing an efficient Serialize instance of the FileStatus type? I'm aware that it's just a wrapper around a C stat structure, and I reckon it can be done more efficiently than copying from C to Haskell for every value. -- Johan Brinch

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[ANN] Crypto-API 0.9 Release
by Thomas DuBuisson 01 Feb '12

01 Feb '12

Crypto-API is a generic interface for cryptographic operations, defining classes for hashes, ciphers, and random number generation while also providing convenience functions such as block cipher modes and padding. Maintainers of hash and cipher implementations are encouraged to add instances for the classes defined in Crypto.Classes. Crypto users are similarly encouraged to use the interfaces defined in the Classes module. Release 0.9 Changes: * Crypto.Classes now exports 'Data.Serialize.encode' * AsymCipher now has proper fundeps * cpolysArr is no longer one big line Cheers, Thomas

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Stuck on HXT basics
by Pēteris Paikens 01 Feb '12

01 Feb '12

I'm quite stuck on getting HXT working to do my xml parsing, can you help me to get started? Even in http://www.haskell.org/haskellwiki/HXT#Getting_started:_Hello_world_examples the basic filter example that's given there in section 6: import Text.XML.HXT.Core import Text.XML.HXT.DOM.XmlTreeFilter selectAllText :: ArrowXml a => a XmlTree XmlTree selectAllText = deep isXText fails with the following error message. What could I be doing wrong? Couldn't match expected type `Data.Tree.NTree.TypeDefs.NTree XNode' with actual type `hxt-8.5.4:Data.Tree.NTree.TypeDefs.NTree hxt-8.5.4:Text.XML.HXT.DOM.TypeDefs.XNode' Expected type: a (Data.Tree.NTree.TypeDefs.NTree XNode) XmlTree Actual type: Text.XML.HXT.DOM.XmlTreeTypes.XmlFilter In the first argument of `deep', namely `isXText' In the expression: deep isXText On the other hand, perhaps you can suggest something other than HXT - my needs would be taking "heavy" XML's (a lot of little-neccessary structure, but correctly validated), extracting text tokens from it and outputting them with some light tagsoup annotation. Regards, Peteris

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Haskell User Group Hamburg - Meeting on 9th Feb.
by Andreas Baldeau 31 Jan '12

31 Jan '12

Hello cafe, in Hamburg this month a new Haskell User Group has formed. We are trying to establish a monthly meetup with hopefully interesting talks. For the start we are having our first regular meetup on the 9th of February, 19:00 in the betahaus hamburg. On this day I will give a talk about datastructures in Haskell and give some introduction into amortized O(1) queues and Tries. So if you are interested to come have a look at http://www.doodle.com/da9tr6zeynmcq3z6 (german). Further announcements will be made via Twitter, so you may want to follow https://twitter.com/hug_hh. Andreas Baldeau

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help with safecopy + acid-state
by Johannes Waldmann 31 Jan '12

31 Jan '12

Dear all, I can't quite get safecopy to work with acid-state: old version of code : data T = T Foo $(deriveSafeCopy 0 'base ''T) new version : data T_orig = T_orig Foo $(deriveSafeCopy 0 'base ''T_orig) data T = T Foo Bar $(deriveSafeCopy 0 'extension ''T) instance Migrate T where type MigrateFrom T = T_Orig ... but when my (new) application reads the state from disk (written by the old application) I get "Could not parse saved checkpoint due to the following error: Failed reading: Duplicate version tags: [0,0]" When I change the version number in deriveSafeCopy, I get "Failed reading: safecopy: Map: Cannot find getter associated with this version number: Version {unVersion = 2}" I don't even know where the "2" comes from. Any hints appreciated - J.W.

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Re: [Haskell-cafe] [Haskell] ANNOUNCE multiarg - parser combinators for command line parsing
by Henning Thielemann 31 Jan '12

31 Jan '12

On Sun, 29 Jan 2012, Simon Meier wrote: > I'm currently using Neil Mitchell's cmdargs package [1]. How does your > package compare to that? Last time I checked cmdargs it was not referential transparent. Is multiarg better in this respect?

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Monad-control rant
by Mikhail Vorozhtsov 31 Jan '12

31 Jan '12

On 11/12/2011 07:34 AM, Bas van Dijk wrote: >> Are you going to release a new version of monad-control right away > > Not just yet. I've split `monad-control` into two packages: > > * `monad-control`: just exports `Control.Monad.Trans.Control`. This part is finished. > * `lifted-base`: wraps all modules of the `base` package which export `IO` computations and provides > lifted version instead. For example we have `Control.Exception.Lifted`, `Control.Concurrent.Lifted`, etc. > > As you can imagine the latter is a lot of boring work. Fortunately it's easy to do so will probably > not take a lot of time. BTW if by any chance you want to help out, that will be much appreciated! > > The repos can be found [here](https://github.com/basvandijk/lifted-base) Maybe I should elaborate on why I stopped using monad-control and rolled out my own version of lifted Control.Exception in monad-abort-fd package. I'm CC-ing the Cafe just in case someone else might be interested in the matter of IO lifting. Imagine we have a monad for multiprogramming with shared state: -- ContT with a little twist. Both the constructor and runAIO -- are NOT exported. newtype AIO s α = AIO { runAIO ∷ ∀ β . (α → IO (Trace s β)) → IO (Trace s β) } runAIOs ∷ MonadBase IO μ ⇒ s -- The initial state → [AIO s α] -- The batch of programs to run. -- If one program exits normally (without using -- aioExit) or throws an exception, the whole batch -- is terminated. → μ (s, Maybe (Attempt α)) -- The final state and the result. -- "Nothing" means deadlock or that -- all the programs exited with -- aioExit. runAIOs = liftBase $ mask_ $ ... bloody evaluation ... data Trace s α where -- Finish the program (without finishing the batch). TraceExit ∷ Trace s α -- Lift a pure value. TraceDone ∷ α → Trace s α -- A primitive to execute and the continuation. TraceCont ∷ Prim s α → (α → IO (Trace s β)) → Trace s β -- Primitive operations data Prim s α where -- State retrieval/modification PrimGet ∷ Prim s s PrimSet ∷ s → Prim s () -- Scheduling point. The program is suspended until -- the specified event occurs. PrimEv ∷ Event e ⇒ e → Prim s (EventResult e) -- Scheduling point. The program is suspended until the state -- satisfies the predicate. PrimCond ∷ (s → Bool) → Prim s () -- Run computation guarded with finalizer. PrimFin ∷ IO (Trace s α) → (Maybe α → AIO s β) → Prim s (α, β) -- Run computation guarded with exception handler. PrimHand ∷ IO (Trace s α) → (SomeException → AIO s α) → Prim s α aioExit ∷ AIO s α aioExit = AIO $ const $ return TraceExit aioAfter ∷ (s → Bool) → AIO s () aioAfter cond = AIO $ return . TraceCont (PrimCond cond) aioAwait ∷ Event e ⇒ e → AIO s (EventResult e) aioAwait e = AIO $ return . TraceCont (PrimEv e) runAIOs slices the programs at scheduling points and enqueues the individual pieces for execution, taking care of saving/restoring the context (finalizers and exception handlers). The Functor/Applicative/Monad/MonadBase/etc instances are pretty trivial: instance Functor (AIO s) where fmap f (AIO g) = AIO $ \c → g (c . f) instance Applicative (AIO s) where pure a = AIO ($ a) (<*>) = ap instance Monad (AIO s) where return = pure AIO g >>= f = AIO $ \c → g (\a → runAIO (f a) c) instance MonadBase IO (AIO s) where liftBase io = AIO (io >>=) instance MonadState s (AIO s) where get = AIO $ return . TraceCont PrimGet put s = AIO $ return . TraceCont (PrimSet s) instance MonadAbort SomeException (AIO s) where abort = liftBase . throwIO trace ∷ AIO s α → IO (Trace s α) trace (AIO g) = g (return . TraceDone) instance MonadRecover SomeException (AIO s) where recover m h = AIO $ return . TraceCont (PrimHand (trace m) h) instance MonadFinally (AIO s) where finally' m f = AIO $ return . TraceCont (PrimFin (trace m) f) -- finally m = fmap fst . finally' m . const -- No async exceptions in AIO instance MonadMask () (AIO s) where getMaskingState = return () setMaskingState = const id Now we have a problem: we can throw/catch exceptions and install finalizers in AIO, but we can't use Control.Exception.Lifted because we can't declare a MonadBaseControl instance for our "ContT with limited interface". So now, instead of trying to wrap IO control operations uniformly, I just reimplement them in terms of the classes I mentioned above (MonadAbort+MonadRecover+MonadFinally+MonadMask), for example: bracket ∷ (MonadFinally μ, MonadMask m μ) ⇒ μ α → (α → μ β) → (α → μ γ) → μ γ bracket acq release m = mask $ \restore → do a ← acq finally (restore $ m a) (release a) It requires more typing (more classes => more instances), but it works for a broader class of monads and I get proper side affects in finalizers for a bonus. The code is on Hackage (monad-abort-fd) and on the GitHub (https://github.com/mvv/monad-abort-fd/blob/master/src/Control/Monad/Excepti…)

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