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ANN: HacPhi 2012
by Chris Casinghino 02 May '12

02 May '12

Greetings, I am very pleased to officially announce Hac Phi 2012, a Haskell hackathon/get-together to be held August 3-5 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's Hac Phi was a lot of fun, drawing more than 30 Haskellers, and many people have already expressed interest in coming back this year. Come meet your Haskelly comrades-in-arms! I want to stress that everyone is welcome - you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the Hac Phi wiki [2]. This year, we are making a public list of attendees' names and nicks - please let us know if you'd prefer not to be listed when you register. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game - a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hac Phi 2012 is supported by contributions from Amgen and Jane Street. Hope to see you in Philadelphia! - The Hac Phi team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://www.haskell.org/haskellwiki/Hac_%CF%86/Register [2] http://www.haskell.org/haskellwiki/Hac_%CF%86 [3] http://www.haskell.org/haskellwiki/Hac_%CF%86/Talks

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heterogeneous environment
by Ben 02 May '12

02 May '12

dear static typing masters -- while working on an STM-like library, i ran into the issue of trying to create a transaction log for reads / writes of heterogeneous reference types. i don't have a good answer to this problem. the problem is twofold : first, the general heterogeneous collection problem, and second, connecting a reference to it's log. there are some solutions i've tried but each has drawbacks : - store the transaction log inside of the reference itself. essentially, each reference has an IORef (Map ThreadId a) associated to it. this is the approach used by [1]. unfortunately this creates a point of concurrency contention at each reference for each read / write, and a lot of repeated computation (the transaction log is spread out in a lot of pieces.) - use Data.Unique to identify Refs, and use existential quantification or Data.Dynamic to create a heterogenous Map from uid to log. for example, to represent a log of compare-and-swaps we might do something like data Ref a = Ref (IORef a) Unique data OpaqueCAS = forall a . OpaqueCAS { casRef :: Ref a, oldVal :: a, newVal :: a } type CASLog = Map Unique OpaqueCAS logCAS r@(Ref _ uid) o n log = insert uid (OpaqueCAS r o n) log... but what if the transaction wants to perform a second CAS on a reference we've already CAS'd? then we should create a combined OpaqueCAS record which expects the first oldVal and swaps in the second newVal. unfortunately the type checker balks at this, as it can't prove that the type variable 'a from the first record is the same as the 'a in the new record; i suppose there might be some fancy type shenanigans which might solve this... Data.Dynamic works but puts a Typeable constraint on the Ref type, so requires the user to modify their data types, and requires a run-time type check (and while performance isn't paramount now it will become important to me later.) also it doesn't feel right... - tupling and functional representations. a monadic function that does a read on an reference can be thought of as a pure function with an extra argument. a monadic function that does a write can be thought of as a pure function with an extra return value. combining all the reads and writes into a transaction log is a big network / switchboard connecting problem, e.g. creating the extra inputs / outputs to the various functions and then stitching them together. running the monad then just is connecting up the final composed function to the actual input / output references. in a sense this amounts to tupling (or currying) the heterogeneous types. (is this is a kind of "final" representation, in the "finally tagless" sense?) i looked at this there were two problems : 1 - the representation is very difficult to manipulate, at least the way i was trying (using the arrow operations); the switchboard problem is extremely verbose, and 2 - it is hard to reconcile identity and position in the tuples -- the repeated read / write problem again. i also experimented with HList but got stuck on similar issues. it strikes me this is just the problem of keeping an environment for an interpreter of a language with mutable heterogeneous reference types. this must be a problem that has either been solved or else there is a haskell point of view on it i'm not grasping which avoids the need for this data structure. maybe there is a way of writing this as an interpreter or using some existing monad, like the ST monad? best, ben [1] - Frank Huch and Frank Kupke, A High-Level Implementation of Composable Memory Transactions in Concurrent Haskell

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using ResourceT with MVars
by Warren Harris 02 May '12

02 May '12

I would like to use LevelDB in my code, and the HEAD version of leveldb-haskell now uses runResourceT to manage open db connections and iterators. I would also like to run multiple threads, and coordinate them for transaction-like behavior, i.e. read and writing data without interruption. LevelDB doesn't support transactions itself, so the alternative would seem to be using MVars for mutual exclusion. With the 0.0.3 version of leveldb-haskell, the get and put operations were in the IO monad, which made it trivial to write something like this: withMVar state $ \db -> do maybeValue <- get db rdOpts key put db wrOpts key $ maybe init incr maybeValue Now with the HEAD version, the get and put operations have types: get :: MonadResource m => DB -> ReadOptions -> ByteString -> m (Maybe ByteString) put :: MonadResource m => DB -> WriteOptions -> ByteString -> ByteString -> m () I don't think it makes sense in general to lift the withMVar into MonadResource and use runResourceT to run its body in IO. This would not satisfy the intention behind using ResourceT -- resource allocated inside the withMVar body would be deallocated when the withMVar scope is exited, rather than being managed by the outermost ResourceT. (Although, in this case we aren't allocating any resources, so maybe it's a moot point for my simple example.) I also don't think it makes sense to use takeMVar and putMVar instead of withMVars as this would subvert the exception handling that withMVars provides. And my attempts to define a withMVarR (in the ResourceT IO monad) also seem to require runResourceT: withMVarR :: MVar a -> (a -> ResourceT IO b) -> ResourceT IO b withMVarR m io = do -- mask $ \restore -> do a <- lift $ takeMVar m b <- {-restore-} (io a) `onExceptionR` (lift $ putMVar m a) lift $ putMVar m a return b onExceptionR :: ResourceT IO a -> ResourceT IO b -> ResourceT IO a onExceptionR a b = do runResourceT $ transResourceT (\a -> transResourceT (onException a) b) a Maybe I need to create a "HasMVar" class analogous to HasRef? (http://hackage.haskell.org/packages/archive/conduit/0.0.0/doc/html/Control-…) I'd appreciate any suggestions on how to make these fit together. Warren

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Haskell Hackathon in Munich
by Heinrich Hördegen 02 May '12

02 May '12

Dear all, the Munich Haskell Meeting is glad to announce a local one day Hackathon in Munich. It will take place the 12th of May 2012 from 10am to 6pm. Please checkout the details at: http://www.haskell-munich.de/dates Cordially yours, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Reader monad, refactoring and missing the point all at once
by Eugene Dzhurinsky 01 May '12

01 May '12

Hi all! Last day I was trying to fix idiii library, because it uses utf8 for parsing non-unicode content. I found the functions > -- | Parses one value and returns it as a 'String' > parseString :: CharEncoding -> TagParser String > parseString enc = do > v <- case enc of > 0x01 -> parseUntilWord16Null -- UTF-16 > 0x02 -> parseUntilWord16Null -- UTF-16 BOM > _ -> parseUntilWord8Null -- ISO-8859-1 or UTF-8 > return $ encPack enc v > > encPack :: CharEncoding -> [Token] -> String > encPack 0x00 s = Text.unpack $ decodeASCII $ BS.pack s > encPack 0x01 (0xFF:0xFE:s) = Text.unpack $ decodeUtf16LE $ BS.pack s > encPack 0x01 (0xFE:0xFF:s) = Text.unpack $ decodeUtf16BE $ BS.pack s > encPack 0x02 s = Text.unpack $ decodeUtf16BE $ BS.pack s > encPack _ s = Text.unpack $ decodeUtf8 $ BS.pack s updated the dependency from > import Data.Text.Encoding (decodeASCII, decodeUtf16LE, decodeUtf16BE, decodeUtf8) to > import Data.Text.ICU.Convert and added implementation for decoding functions: > decodeAny :: String -> BS.ByteString -> Text.Text > decodeAny charset src = unsafePerformIO $ ((flip toUnicode) src) `fmap` open charset (Just True) > > decodeASCII :: BS.ByteString -> Text.Text > decodeASCII = decodeAny "latin1" > > decodeUtf16LE = decodeAny "utf-16le" > > decodeUtf16BE = decodeAny "utf-16be" > > decodeUtf8 = decodeAny "utf-8" Now I want to add possibility to specify encoding to yse with decodeASCII. I was thinking of adding Reader monad and providing some sort of charset configuration there - but it will lead up to complicating the code, which uses this parseString function. And this code is used inside Parser of Text.ParserCombinators.Poly.State - so I will need to update all usages of this parser. Another approach might be to use IORef with encoding stored there, but I don't really like this solution. What would be the best way of refactoring of such kind? Thanks! -- Eugene N Dzhurinsky

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Creating a dependency graph (in the hope it may show some structure)
by Bram Neijt 01 May '12

01 May '12

Dear Haskelllers, I've reacently started with Haskell and during the Dutch Hackaton[3], I decided to generate graphs of dependencies. I was thinking about doing this for Java earlier, and that weekend I decided to write the tooling in Haskell. I've written two programs: hs2dot[1] and dotimate[2]. The first generates a dot file (Graphviz) from all .hs files below the current working directory. The second will take one or more dot files and call neato (from Graphviz) multiple times to generate frames of neato output from one to the next. Trying hs2dot and dotimate on two sequencial commits in the cabal/cabal-install/frames directory already gives me a way to complex and full graph. See attached. ==> My question is: What dependencies, if any, would be nice to graph, and/or which dependencies could I filter out to keep this from becoming a mess? Greetings, Bram Neijt [1] https://gist.github.com/bneijt/hs2dot [2] https://gist.github.com/bneijt/dotimate [3] http://www.haskell.org/haskellwiki/DHD_UHac#Utrecht_Hackathon

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ANN: bitwise - fast multi-dimensional unboxed bit packed Bool arrays
by Claude Heiland-Allen 01 May '12

01 May '12

Hi all, I'm pleased to announce bitwise-0.1: http://hackage.haskell.org/package/bitwise --8<-- excerpt from the hackage page Unboxed multidimensional bit packed Bool arrays with fast aggregate operations based on lifting Bool operations to bitwise operations. There are many other bit packed structures out there, but none met all of these requirements: 1. unboxed bit packed Bool array, 2. multi-dimensional indexing, 3. fast (de)serialization, or interoperable with foreign code, 4. fast aggregate operations (fold, map, zip). --8<-- end excerpt bitwise has been tested in hugs -98, ghc-7.0.4, ghc-7.4.1. Bool data structures comparison (features numbered as excerpt above): package features additional notes 1 2 3 4 array Y Y n n bitarray Y n n ? based on array bitstream n n ? ? based on vector bitstring Y n ? ? based on bytestring bit-vector n n Y ? based on vector bitwise Y Y Y Y repa n Y Y ? based on vector uvector Y n ? ? deprecated vector n n Y ? uses a Word8 for each Bool Known shortcomings of this first release of bitwise: * missing common instances (Eq, Ord, Read, Show, ...); * missing additional functions (counting, searching, ...); * missing documentation on ByteString format: the bits are taken from the array and packed into Word8s, with the first bit (in array order) becoming the least significant bit of the first Word8; the last Word8 is padded with 0 at the most-significant-bit end; * PBM reading is not compliant to the official specification; * misleading benchmark (bitwise zipWith is fast, but there may be a faster way to zipWith UArray ix Bool than the one I used). What I'm using bitwise for at the moment: A building block for fractal imaging: an image is made up of cells, each cell can be in one of 4 states (represented with 2 bits): * cell is 100% interior to a set of points in the plane * cell is 100% exterior * partially interior and partially exterior (ie, on the boundary) * unknown with recursive subdivision of cells into images taking place only when the cell is on the boundary of the set being imaged. Thanks, Claude

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Re: [Haskell-cafe] Darcs home page updated
by Paul R 01 May '12

01 May '12

On Mon, 30 Apr 2012 12:27:57 -0700, Simon Michael <simon(a)joyful.com> said: Simon> With 2.8 released, I felt Darcs deserves better presentation. After Simon> surveying other VCS sites I worked on an update to our home page Simon> layout and content over the last few days, with review and input from Simon> #darcs, and it went live last night. It's far from perfect but I hope Simon> it's a good step forward. Thanks for the input, and have at it: Simon> http://darcs.net Hi, that looks good, thanks. Here are some ideas : - The purple color of titles is not very attractive, changing it to something more saturated would make it look fresher. - the purple color rounded box is not very attractive either :) - there is a video right on the first page, good ! Unfortunatly, it is not showing darcs but its cousin. Also, the title is "why do we continue to develop ... [camp]", which isn't the most positive approach one can expect on a start page - the compact tutorial is great, I love it. It deserves more poish. For inspiration, one can have a look at this website for example : http://gembundler.com/ - underlined links looks a bit 90's. You can add a simple css rule to choose an other color and remove the underline (though it is good practice to set it on mouseover) cheers, -- Paul

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ANN: HandsomeSoup-0.3: CSS selectors for HXT
by aditya bhargava 30 Apr '12

30 Apr '12

*Homepage:* http://egonschiele.github.com/HandsomeSoup *On Hackage:* http://hackage.haskell.org/package/HandsomeSoup *Blurb:* HandsomeSoup is the library I wish I had when I started parsing HTML in Haskell. It is built on top of HXT and adds a few functions that make is easier to work with HTML. Most importantly, it adds CSS selectors to HXT. The goal of HandsomeSoup is to be a complete CSS2 parser for HXT (it is very close to this right now). -- adit.io

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Darcs home page updated
by Simon Michael 30 Apr '12

30 Apr '12

With 2.8 released, I felt Darcs deserves better presentation. After surveying other VCS sites I worked on an update to our home page layout and content over the last few days, with review and input from #darcs, and it went live last night. It's far from perfect but I hope it's a good step forward. Thanks for the input, and have at it: http://darcs.net -Simon

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