May 2009 - Haskell-Cafe

swish - semantic web in Haskell
by Vasili I. Galchin 02 May '09

02 May '09

Hello, http://www.ninebynine.org/Software/swish-0.2.1.html .... I am trying to get permission from the author of Swish to move it into Hackage. I will write "cabalize" it get up to current Haskell standards. Before I go to this effort is anybody attempting to do this? Kind regards, Vasili

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The Haskell Reddit is 1 year old
by Don Stewart 01 May '09

01 May '09

Just a reminder, the Haskell Reddit is live and active: http://www.reddit.com/r/haskell/ It is a a place for fast, daily, comprehensive news about what's going on in the Haskell community, combining blogs, mail, irc, ghc's patches, the freaking types@ mailing list! It's all here, with comments. Enjoy. Contribute. -- Don

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ANN: Takusen 0.8.4
by Alistair Bayley 01 May '09

01 May '09

ANN: Takusen 0.8.4 This is mainly a "get it working with ghc-6.10" release. Changes since 0.8.3: - ODBC support: some bug fixes, and basic support for Out-parameters. There is at least one major outstanding bug with Out-parameters, where marshalling a String output bind-variable doesn't work correctly (if it was modified, you still get the String you passed in back, rather than the new String). - Sqlite: bug fix for resource leak: if a command (like rollback or commit) raised an error, the prepared-statement handle was not closed. - cabal: requires cabal 1.6 (or later). With ghc-6.6 on Windows you will need a later cabal than 1.6.0.1, as there is a bug which prevents building (setup configure fails with "setup: fdGetMode: invalid argument (Invalid argument)"). Eric Mertens patch for ODBC on OS x. Greg Leclerq patch for cabal 1.6. Thanks also to Austin Siepp and Brian Callendar for their patches. - ghc-6.10: will build with ghc's 6.10, 6.8, 6.6. Note that Takusen won't work with ghc-6.10.2 out-of-the-box, because the time package is missing. If you install time manually then it should be OK (but this is untested). We use the new extensible exceptions, but we have done so by including it within our own source tree. This will make it easier to use Takusen with older ghc's, and reduces the number of external package dependencies. We have dropped support for 6.4. That's not to say Takusen isn't usable with 6.4, just that we no longer test that configuration. The release bundle: http://hackage.haskell.org/packages/archive/Takusen/0.8.4/Takusen-0.8.4.tar… The latest code: darcs get http://darcs.haskell.org/takusen Docs: http://darcs.haskell.org/takusen/doc/html/index.html If you have cabal-install, then this command should work: cabal install Takusen --flags="sqlite odbc oracle postgres" ... but it doesn't (at least for the version of cabal-install that I have) because the .tar.gz file I uploaded has a bogus checksum. A comprehensive description of API usage can be found in the documentation for module Database.Enumerator (look for the Usage section): http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html Note that the haddock docs won't build with cabal-1.6.0.2 or earlier. This is because our literate-haskell source files need to be properly preprocessed before being passed to haddock. At the time of writing, you'll need the cabal HEAD to build the docs. Future plans: - Output bind-parameters and multiple-result sets for ODBC - support for Blobs and Clobs - FreeTDS backend (Sybase and MS Sql Server)... maybe For those of you unfamiliar with Takusen, here is our HCAR blurb: Takusen is a DBMS access library. Like HSQL and HDBC, we support arbitrary SQL statements (currently strings, extensible to anything that can be converted to a string). Takusen's 'unique-selling-point' is safety and efficiency. We statically ensure all acquired database resources - such as cursors, connection and statement handles - are released, exactly once, at predictable times. Takusen can avoid loading the whole result set in memory, and so can handle queries returning millions of rows in constant space. Takusen also supports automatic marshalling and unmarshalling of results and query parameters. These benefits come from the design of query result processing around a left-fold enumerator. Currently we fully support ODBC, Oracle, Sqlite, and PostgreSQL.

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What to do when cabal dependencies are not your friend.
by Thomas Hartman 01 May '09

01 May '09

I did a little write-up on an annoyance I frequently have when developing patch-tag, a happs application that has a lot of dependencies. Basically, on a virgin linux environment with the same cabal and ghc version as before, a cabal install that had previously worked, didn't work anymore. I assume this is because of changes on hackage that didn't affect my previous version because I had not done cabal update for a while. Luckly I had a working environment on a test box elsewehre so... http://blog.patch-tag.com/2009/05/01/what-to-do-when-cabal-install-works-in… Cheers, thomas.

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Thread priority?
by Christopher Lane Hinson 01 May '09

01 May '09

Is there any interest or movement in developing thread priority or any other realtime support in Haskell? Right now, if I have tasks that need to be responsive in real time, even if the realtime needs are very soft, it seems that the only option is to try to ensure that at least one hardware thread is kept clear of any other activity. To be very useful to me, thread priority would not need to come with very strict guarantees, but there would need to be a way to make sure that `par` sparks and DPH inherit the priority of the sparking thread. In part I ask because I'm working on a small library to support a degree of cooperative task prioritization. Friendly, --Lane

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Generating random enums
by michael rice 01 May '09

01 May '09

I'm using the code below to generate random days of the week [Monday..Sunday]. Is there a better/shorter way to do this? Michael ============== [michael@localhost ~]$ ghci dow GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( dow.hs, interpreted ) Ok, modules loaded: Main. *Main> random (mkStdGen 100) :: (DayOfWeek, StdGen) Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. (Friday,4041414 40692) *Main> random (mkStdGen 123) :: (DayOfWeek, StdGen) (Tuesday,4961736 40692) *Main> ============== import System.Random data DayOfWeek = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday deriving (Show, Read, Eq, Enum, Ord, Bounded) instance Random DayOfWeek where randomR (a,b) g = case (randomIvalInteger (toInteger (dow2Int a), toInteger (dow2Int b)) g) of (x, g) -> (int2Dow x, g) where dow2Int Monday = 0 dow2Int Tuesday = 1 dow2Int Wednesday = 2 dow2Int Thursday = 3 dow2Int Friday = 4 dow2Int Saturday = 5 dow2Int Sunday = 6 int2Dow 0 = Monday int2Dow 1 = Tuesday int2Dow 2 = Wednesday int2Dow 3 = Thursday int2Dow 4 = Friday int2Dow 5 = Saturday int2Dow 6 = Sunday random g = randomR (minBound,maxBound) g randomIvalInteger :: (RandomGen g, Num a) => (Integer, Integer) -> g -> (a, g) randomIvalInteger (l,h) rng | l > h = randomIvalInteger (h,l) rng | otherwise = case (f n 1 rng) of (v, rng') -> (fromInteger (l + v `mod` k), rng') where k = h - l + 1 b = 2147483561 n = iLogBase b k f 0 acc g = (acc, g) f n acc g = let (x,g') = next g in f (n-1) (fromIntegral x + acc * b) g' iLogBase :: Integer -> Integer -> Integer iLogBase b i = if i < b then 1 else 1 + iLogBase b (i `div` b)

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Re: [Haskell-cafe] chr/ord?
by michael rice 01 May '09

01 May '09

Thank you. I assumed when I saw this: "we could use Maybe's mplus to define a function, parent s = (mother s) `mplus` (father s)" that mplus was *already* present in Maybe. Michael --- On Fri, 5/1/09, Neil Mitchell <ndmitchell(a)gmail.com> wrote: From: Neil Mitchell <ndmitchell(a)gmail.com> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org>, "Anton van Straaten" <anton(a)appsolutions.com> Date: Friday, May 1, 2009, 6:32 AM Hi > But I get this when I try to use it: > > sheep.hs:30:22: Not in scope: `mplus' > [michael@localhost ~]$ You need to import Control.Monad. You can find answers to these sorts of questions with Hoogle: http://haskell.org/hoogle/?hoogle=mplus Thanks Neil

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Re: [Haskell-cafe] chr/ord?
by michael rice 01 May '09

01 May '09

Understood. Thanks. A little further on in the tutorial they define a "parent" function. The mplus operator is used to combine monadic values from separate computations into a single monadic value. Within the context of our sheep-cloning example, we could use Maybe's mplus to define a function, parent s = (mother s) `mplus` (father s), which would return a parent if there is one, and Nothing is the sheep has no parents at all. For a sheep with both parents, the function would return one or the other, depending on the exact definition of mplus in the Maybe monad. But I get this when I try to use it: sheep.hs:30:22: Not in scope: `mplus' [michael@localhost ~]$ And here's the sheep.hs file, attempting to use "parent" near the bottom ========= {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: <Mon Nov 10 11:59:14 2003> License: GPL -} {- DESCRIPTION Example 1 - Our first monad Usage: Compile the code and execute the resulting program. It will print Dolly's maternal grandfather. -} -- everything you need to know about sheep data Sheep = Sheep {name::String, mother::Maybe Sheep, father::Maybe Sheep} -- we show sheep by name instance Show Sheep where show s = show (name s) -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x parent s = (mother s) `mplus` (father s) -- now we can use `comb` to build complicated sequences maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = (Just s) `comb` mother `comb` father fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandmother s = (Just s) `comb` father `comb` mother `comb` mother mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father -- this builds our sheep family tree breedSheep :: Sheep breedSheep = let adam = Sheep "Adam" Nothing Nothing eve = Sheep "Eve" Nothing Nothing uranus = Sheep "Uranus" Nothing Nothing gaea = Sheep "Gaea" Nothing Nothing kronos = Sheep "Kronos" (Just gaea) (Just uranus) holly = Sheep "Holly" (Just eve) (Just adam) roger = Sheep "Roger" (Just eve) (Just kronos) molly = Sheep "Molly" (Just holly) (Just roger) in Sheep "Dolly" (Just molly) Nothing -- print Dolly's maternal grandfather main :: IO () main = let dolly = breedSheep in do print (parent dolly) -- END OF FILE Michael --- On Wed, 4/29/09, Anton van Straaten <anton(a)appsolutions.com> wrote: From: Anton van Straaten <anton(a)appsolutions.com> Subject: Re: [Haskell-cafe] chr/ord? To: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 29, 2009, 12:33 PM michael rice wrote: > Since I'm trying to learn Monads, let's look at this as a teaching moment. The example code (see below), which I pulled off YAMT (Yet Another Monad Tutorial ;-)), is the source of my 'comb' function. > > I understand the code as it now stands, and I understand that the Prelude (>>=) would replace the 'comb'. Adding whatever statements are needed, how would you "specialize" the (>>=) to Maybe and solve this particular problem. Saying that "comb is just (>>=) specialized to Maybe" just means that you can define comb like this: comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb = (>>=) Which also of course means that you can typically use (>>=) instead of comb. Although in some cases, being more specific about the type can be useful. You can do this sort of specialization for any polymorphic function, e.g.: -- id is predefined in Haskell, definition given as example id :: a -> a id x = x intID :: Int -> Int intId = id In that case, the compiler basically specializes the function for you, providing a version of it that's specific to Ints. However, (>>=) is defined by the Monad type class, and as it happens there's also already a definition for it that's specific to the Maybe type. You can see GHC's source for it here: http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Maybe.html#… Not surprisingly, that definition is essentially identical to the definition of comb: (Just x) >>= k = k x Nothing >>= _ = Nothing So defining "comb = (>>=)" just uses that definition. Anton _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Few Alex questions
by Dimitry Golubovsky 01 May '09

01 May '09

Hi, Q1: I am trying to create an Alex equivalent of the following flex file: http://code.haskell.org/yc2js/es-idl/lexer.ll (lexer for Web IDL which is a dialect of OMG IDL) I haven't been able to find Alex equivalent for the following line: PoundSign ^{WhiteSpace}*# that is any amount of whitespace at the beginning of line followed by '#' I defined a @WhiteSpace macro some place earlier, but Alex gives me parser error on the line I created: @PoundSign = ^@WhiteSpace*# in the position 28 that is position of '@', next to caret, that is, caret is not recognized by Alex. Can beginning of line (caret) be recognized by Alex? Q2: The original line says: MultiLineComment \/\*(([^*])|(\*[^/]))*\*\/ basically same as comments in C or Java. I had to prefix star and slash even within square brackets with backslashes to make it compile. So I got: @MultiLineComment = \/\*(([^\*])|(\*[^\/]))*\*\/ Is this correct understanding that if we want to match any character except for an asterisk, then Alex would like to see [^\*] rather than [^*]? And [^\/] rather than [^/]? Or would it be better to use a hex code for the asterisk and slash? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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ANNOUNCE: graphviz-2009.5.1
by Ivan Lazar Miljenovic 01 May '09

01 May '09

I'd like to announce version 2009.5.1[1] of the graphviz[2] library, less than a week since Matthew Sackman passed maintainership onto me[3]. The graphviz library provides a Haskell interface to the GraphViz[4] program. [1] http://hackage.haskell.org/packages/archive/graphviz/2009.5.1/graphviz-2009… [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphviz [3] http://www.mail-archive.com/haskell@haskell.org/msg21994.html [4] http://graphviz.org Major changes with this release are: * Supports polyparse >= 1.1 (the previous version didn't work with 1.3). * Requires base == 4.* (i.e. GHC 6.10.*) due to the new exception handling. * Includes functions from my Graphalyze [5] library for actually running the various GraphViz commands, etc. * Slight API breakages: - dirCommand, undirCommand and commandFor no longer return String, but GraphvizCommand - Data.GraphViz.ParserCombinators is no longer exported by the Cabal file, since I see no need for anyone to require acccess to this. If this is a problem for you, please let me know and I'll re-export it. * A lot more Haddock documentation. * A darcs repository is now available [6]. Patches are welcome! [5] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Graphalyze [6] http://code.haskell.org/graphviz Plans for the future: * Add support for Data.Graph-style graphs (maybe even making it generic such that custom graph types can be used). * Allow the user to choose whether the graph is directed or undirected. * Include all attributes offered by the Dot language. * Remove (or at least minimise) usage of extensions to increase portability. Oh, and to pre-empt Don, this is already available in the Gentoo-Haskell overlay ;-) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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