- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] hsql won't install due to system.time
by John Lato 22 Jan '10

22 Jan '10

Hello, Could this be a global/user install issue? That is, if the old-time package is installed per-user, and you're trying to install a package globally, the user-installed packages all show up as hidden, because they can't be dependencies of a global install. This shows up frequently because global installs are the default when doing "runghc Setup.hs install", but per-user installs are the default for cabal install. If you use both of these commands, you'll run into this sooner or later. You can run "ghc-pkg list" to get a listing of what's installed where. If "old-time" is only in the local database, you should reinstall it into the global database (or install everything globally). Cheers, John > From: Brian Denheyer <briand(a)aracnet.com> > > Database/HaskellDB/HSQL.hs:25:7: > Could not find module `System.Time': > it is a member of the hidden package `old-time-1.0.0.3' > it is a member of the hidden package `old-time-1.0.0.2' > Use -v to see a list of the files searched for. > > old-time-1.0.0.2 is installed (via cabal). > > Suggestions on how to fix this or which haskell db (for sqlite3) package > will actually install greatly appreciated. > > I get a lot of failures in hackages which involve that dreaded "hidden > package" message. Anybody care to enlighten us mere mortals as to why > it seems to occur so frequently (at least for me). > > Brian >

2 1

problems installing ghc-6.12.1-x86_64
by Andrew Polonsky 22 Jan '10

22 Jan '10

Dear all, Help!! I am trying to build 6.12 on a fresh 64-bit Ubuntu, with no GHC installed yet. The compiler itself builds happily, but when I try to "make install" the Haskell platform, I eventually get the error Setup: GLUT-2.1.1.2: dependency "OpenGL-2.2.1.1-b638210d1627880d532e3b035b53bcd2" doesn't exist (use --force to override) I can't even install cabal, because it requires having the ghc6 package, which is not installed. And bootstrapping cabal-install requires parsec and network, which also require ghc6. When I finally install ghc6 (why should I have to anyway?), it still thinks I am on 6.12, and ./bootstrap.sh can't find parsec with that number. Is there anyway to break out of this non-terminating reduction? Is YI solvable? :'"( Andrew

2 2

ANNOUNCE: Haskell XML Toolbox Version 8.5.0
by Uwe Schmidt 22 Jan '10

22 Jan '10

Haskell XML Toolbox 8.5.0 I would like to announce a new version of the Haskell XML Toolbox. The main change in this release is the separation of the XPath module and the XSLT module into separate packages hxt-xpath and hxt-xslt. So the base package becomes a bit smaller. Installation shouldn't become more complex because of the easy install with cabal and hackage. A second major change concerns the XPath module. The internal data structures, especially the XPath node-sets, have been refactored for runtime efficiency. So even complex XPath expressions with ambiguous paths to single nodes run in acceptable time. The source repositories have been converted to the git version control system. There is a single git repository for all HXT packages at "http://git.fh-wedel.de/repos/hxt.git" More information and also downloads can be found at http://www.fh-wedel.de/~si/HXmlToolbox/index.html There's also a copy of all packages on Hackage. Please email comments, bugs, etc. to hxmltoolbox(a)fh-wedel.de or si(a)fh-wedel.de Uwe -- University of Applied Sciences, Wedel, Germany http://www.fh-wedel.de/~si/index.html

1 0

ssh ports for monk and nun?
by Alistair Bayley 22 Jan '10

22 Jan '10

Trying to get ssh working via putty from behind my company firewall. Had some success in the past with sourceforge because they had ssh daemons listening on ports 80 and 443, to aid prisoners like myself. Does anyone know if the monk (darcs.haskell.org) and nun (code.haskell.org) servers accept ssh on ports other than 22? Thanks, Alistair

5 7

haddock - 'could not find link destination"
by Andrew U. Frank 22 Jan '10

22 Jan '10

i try to use haddock for documentation, but have no success. no output is produced. i assume i make some stupid mistake (not having used haddock before) help is appreciated! thank you andrew ------------------------------- i get the following error: $ haddock Hadtest2.hs Warning: Main: could not find link destinations for: GHC.Types.Int GHC.Base.String Hadtest2.hs is a simplistic test program with some parts copied from the haddock manual: module Main (square, this) where -- | a constant this :: String this = "this" -- |The 'square' function squares an integer. square :: Int -> Int square x = x * x main = do putStrLn $ "test " ++ show (square 4) return () --

3 2

Cannot rebuild GHC 6.12.1 due to Cabal type conflict
by James Britt 22 Jan '10

22 Jan '10

Hey all. I recently wanted to rebuild GHC 6.12.1 from source (from a downloaded tarball). After the first install from that source I went and updated Cabal (from a tarball grabbed from Hackage). When I now try to do a rebuild of GHC, I get this error compiler/main/Packages.lhs:233:63: Couldn't match expected type `InstalledPackageInfo_ String' against inferred type `Cabal-1.8.0.2:Distribution.InstalledPackageInfo.InstalledPackageInfo_ m' Expected type: [InstalledPackageInfo_ String] Inferred type: [Cabal-1.8.0.2:Distribution.InstalledPackageInfo.InstalledPackageInfo_ m] In the second argument of `map', namely `conf' In the first argument of `return', namely `(map installedPackageInfoToPackageConfig conf)' make[1]: *** [compiler/stage1/build/Packages.o] Error 1 make[1]: *** Waiting for unfinished jobs.... make: *** [all] Error 2 (also here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=16380#a16380) I'm running the build like this: make distclean sh boot ./configure --enable-shared make -j 2 but (I'm guessing) it's looking at the already-installed Cabal libs and not liking what it's getting. Any ideas on how to get past this? Thanks, James Britt -- www.jamesbritt.com - Playing with Better Toys www.ruby-doc.org - Ruby Help & Documentation www.rubystuff.com - The Ruby Store for Ruby Stuff www.neurogami.com - Smart application development

1 0

Haskell on Ideone!
by Sphere Research Labs 22 Jan '10

22 Jan '10

Hi, test Haskell on ideone.com, see the example: http://ideone.com/bbloQ9r0 regards, Ideone Team

3 2

Haskell Platform
by Rafael Gustavo da Cunha Pereira Pinto 21 Jan '10

21 Jan '10

Hi folks, How are the efforts on Haskell Platform for GHC 6.12.1 going? I'm considering joining those brave heroes. Where can I apply? Rafael Gustavo da Cunha Pereira Pinto

4 3

state of High Level GUI libs?
by Günther Schmidt 21 Jan '10

21 Jan '10

Hello, what is the current state of the higher GUI libs with reference to usability? Günther

1 0

Extracting all pruned sub trees
by Tom Hawkins 21 Jan '10

21 Jan '10

I'm looking for an elegant way to generate a list of all pruned trees where each pruned tree has one of its leaves removed. Something like this: data Leaf = ... data Tree = Leaf Leaf | Branch [Tree] prunedSubTrees :: Tree -> [(Leaf, Tree)] -- [(the leaf removed, the pruned tree)] Any suggestions?

2 2

Parsers (Parsec and Iteratee-based Parsers)
by Günther Schmidt 21 Jan '10

21 Jan '10

Hi, are there any examples how to build parsers using the library in Oleg's iteratee package? I've been using parsec for almost all my parsing needs, in fact it was parsec that got me started with Haskell. I'd like to try to build a few parsers based on the left-fold-enumerator thingy and compare this approach to parsec. Any good tips how to get started? Günther

9 10

HTML - based GUIs - follow up
by Günther Schmidt 21 Jan '10

21 Jan '10

Hello, in my first post on this subject from 01-10-10 I asked for opinions on using HTML to create the gui part of a desktop app instead of using Gtkhs or WxHaskell. The responses were encouraging. It occurred to me that I might now even need to run an actual webserver (happs for instance) but that I could use either Webkit or a com interface to Internet Explorer to control the browser. When it opens, register event sinks and so on. Still that involves a lot of boilerplate code. I have seen examples of *formlets* and I'm impressed, the wiring is certainly taken care of. My question is: do formlets only work server based or is it also possible to use formlet sans happs? Günther

3 2

GUI: Custom widgets - wxHaskell or gtk2hs ?
by Günther Schmidt 21 Jan '10

21 Jan '10

Hi, I need to create a custom widget and I wonder which of the above gui-toolkits makes that easier. Günther

1 0

Please help me debug my arrow
by Paul Johnson 20 Jan '10

20 Jan '10

Hi, I'm trying to write an arrow for a real-time stream processor. I'm basing it on the SP type in Hughes paper on Generalising Monads to Arrows (http://www.cs.chalmers.se/~rjmh/Papers/arrows.pdf) section 6. I've extended this with a notion of time by making each step a function of time. But I can't get the compose operator to work. The arrow itself is defined in http://haskell.pastebin.com/m49944f64 with the (.) function highlighted. Some simple tests are in http://haskell.pastebin.com/m6d90f27 with the problematic call highlighted. When run it produces an infinite list of puts, which causes the SimulateRTSP interpreter function to diverge. But I've run the expansions by hand, and they seem to work (see the test case file at the bottom). I'm going nuts looking at this. Can anyone see what I'm doing wrong? Thanks, Paul.

1 1

Poor man's generic programming
by Henning Thielemann 20 Jan '10

20 Jan '10

Is any of the existing Generics packages able to work without compiler extensions, that is Haskell 98? I mean, it is ok if the developer of the generic parts of a library needs compiler extensions or extra tools, but the user who calls 'cabal install' shall not need them and the developer, who does not touch generic definitions, should not need them as well. I think this could be done with pragmas in the following way: class C a where f :: a -> [Int] {-# GENERICDEFAULT f {| Unit |} Unit = [] #-} {-# GENERATEDINSTANCEBEGIN Automatically generated - Do not edit! #-} instance C () where f () = [] {-# GENERATEDINSTANCEEND Automatically generated - Do not edit! #-} Now, by running a tool, the part between the GENERATEDINSTANCE pragmas can generated or updated. Everything outside the pragmas can be Haskell 98. It does not only avoid extensions of the building compiler, but also extensions of a programmer's brain. Because for programmers it is often easier to understand a set of instances and see the common patterns, than to understand generic code that builds the instances.

3 7

darcs 2.4 beta 2 release
by Reinier Lamers 20 Jan '10

20 Jan '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.4 beta 2. darcs 2.4 will contain many improvements and bugfixes compared to darcs 2.3.1. Highlights are the fast index-based diffing which is now used by all darcs commands, and the experimental interactive hunk editing. This beta is your chance to test-drive these improvements and make darcs even better. The easiest way to install darcs is using the Haskell Platform [1]. If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install --reinstall darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.3.98.2.tar.gz , and build it by hand as explained in the README file. Interactive hunk editing ------------------------ The interactive hunk editing feature is included in this beta. However, be aware that we are not entirely sure that the user interface in this version is ready yet. You're welcome to try it out, but be prepared for a learning curve that's somewhat steeper than usual for darcs. To try out interactive hunk editing, press 'e' when you are prompted with a hunk patch by 'darcs record'. You will then be shown an editor screen in which you can edit the contents of the hunk as darcs should consider them to have been before and after the hunk patch you edit. You can edit the state you want to record between the last two "===" lines. You can find more information about the hunk editing feature on http://wiki.darcs.net/HunkEditor . Reporting bugs -------------- If you have an issue with darcs 2.4 beta 2, you can report it via the web on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net. What's New ---------- A list of important changes since 2.3.1 is as follows (please let me know if there's something you miss!): * Use fast index-based diffing everywhere (Petr) * Interactive patch splitting (Ganesh) * An 'optimize --upgrade' option to convert to hashed format in-place (Eric) * Hunk matching (Kamil Dworakowski, tat.wright) * Progress reporting is no longer deceptive (Roman) * A 'remove --recursive' option to remove a directory tree from revision control (Roman) * 'show files' accepts arguments to show a subset of tracked files (Luca) * A '--remote-darcs' flag for pushing to a host where darcs isn't called darcs * Many miscellaneous Windows improvements (Salvatore, Petr and others) * 'darcs send' now mentions the repository name in the email body (Joachim) * Handle files with boring names in the repository correctly (Petr) * Fix parsing of .authorspellings file (Tomáš) * Various sane new command-line option names (Florent) * Remove the '--checkpoint' option (Petr) * Use external libraries for all UTF-8 handling (Eric, Reinier) * Use the Haskell zlib package exclusively for compression (Petr) A list of issues resolved since 2.3.1: * 183: do not sort changes --summary output * 223: add --remote-darcs flag to specify name of remote darcs executable * 291: provide (basic) interactive patch splitting * 540: darcs remove --recursive * 835: 'show files' with arguments * 1122: get --complete should not offer to create a lazy repository * 1216: list Match section in ToC * 1224: refuse to convert a repo that's already in darcs-2 format * 1300: logfile deleted on unsucessful record * 1308: push should warn about unpulled patches *before* patch-selection * 1336: sane error message on --last "" (empty string to numbers parser) * 1362: mention repo name in mail send body * 1377: getProgname for local darcs instances * 1392: use parsec to parse .authorspelling * 1424: darcs get wrongly reports "using lazy repository" if you ctrl-c old-fashioned get * 1447: different online help for send/apply --cc * 1488: fix crash in whatsnew when invoked in non-tracked directory * 1548: show contents requires at least one argument * 1554: allow opt-out of -threaded (fix ARM builds) * 1563: official thank-you page * 1578: don't put newlines in the Haskeline prompts * 1583: on darcs get, suggest upgrading source repo to hashed * 1584: provide optimize --upgrade command * 1588: add --skip-conflicts option * 1594: define PREPROCHTML in makefile * 1620: make amend leave a log file when it should * 1636: hunk matching * 1643: optimize --upgrade should do optimize * 1652: suggest cabal update before cabal install * 1659: make restrictBoring take recorded state into account * 1677: create correct hashes for empty directories in index * 1681: preserve log on amend failure * 1709: fix short version of progress reporting * 1712: correctly report number of patches to pull * 1720: fix cabal haddock problem * 1731: fix performance regression in check and repair Kind Regards, the darcs release manager, Reinier Lamers [1]: You can download the Haskell platform from http://hackage.haskell.org/platform/

1 0

Having a look at XMonad window manager
by michael rice 20 Jan '10

20 Jan '10

I downloaded XMonad from the Fedora 12 repository and would like to see it in action. What must I do to get it working from the Gnome desktop environment? Michael

7 14

Conditional code for Haddock
by Roel van Dijk 20 Jan '10

20 Jan '10

I want to generate documentation for the following code snippet: > import qualified Data.ByteString as BS ( ByteString, length ) > > -- | Foo 'BS.empty' equals 0 > foo ∷ BS.ByteString → Int > foo = BS.length Because I do not explicitly import BS.empty haddock will not generate a nice link. I can fix it by adding: > import qualified Data.ByteString as BS ( empty ) But now the compiler rightly warns me: > Warning: Imported from `Data.ByteString' but not used: `BS.empty' There is a ticket related to this problem [1], but I do not think making haddock 'magically' find BS.empty is the right answer. What I would like to do is: > #ifdef __DOC__ > import qualified Data.ByteString as BS ( empty ) > #endif Because I use additional symbols in my comments it feels natural to also have special imports for them. I know that cabal used to define the __HADDOCK__ macro. Is it possible to manually define a preprocessor macro when haddock is run? Perhaps an equivalent to ghc-options: haddock-options? Ideally I want only have to add the following to my .cabal file: > haddock-options: -D__DOC__ Regards, Roel van Dijk 1 - http://trac.haskell.org/haddock/ticket/78

3 5

formatTime doesn't work with local time ??
by Brian Denheyer 20 Jan '10

20 Jan '10

My error: Couldn't match expected type `T.UTCTime' against inferred type `T.LocalTime' In the second argument of `formatTime', namely `date' In the expression: formatTime "%Y/%m/%e %k:%M:%S" date But the docs have LocalTime listed as one of the FormatTime instances: Instances FormatTime Day FormatTime UTCTime FormatTime TimeZone FormatTime TimeOfDay FormatTime ZonedTime FormatTime LocalTime ? Brian

2 2

Non-ASCII characters in .cabal files break Haddock?
by John Millikin 20 Jan '10

20 Jan '10

I recently uploaded a package to Hackage[1], which contains a non-ASCII character in the .cabal file. That file is encoded with UTF-8, and it displays fine in Vim, Firefox, and even the Hackage package landing page. However, Haddock is failing to build it[2] -- apparently it can't handle UTF-8 encoded text properly. In documentation sections, Haddock supports special syntax for escaping non-ASCII. Is there any equivalent syntax for Cabal files? [1] http://hackage.haskell.org/package/natural-sort' [2] http://hackage.haskell.org/packages/archive/natural-sort/0.1/logs/failure/g…

3 2

cabal install glfw
by Ozgur Akgun 20 Jan '10

20 Jan '10

Dear Cafe and Paul, I am constantly having problems with cabal install in Snow Leopard. Some I solve, some I cannot unfortunately. When I run sudo cabal install glfw -v2 in Snow Leopard, I get the following. glfw/lib/macosx/macosx_enable.c:1:0: error: bad value (apple) for -march= switch glfw/lib/macosx/macosx_enable.c:1:0: error: bad value (apple) for -mtune= switch cabal: Error: some packages failed to install: GLFW-0.4.1 failed during the building phase. The exception was: exit: ExitFailure 1 Regards, -- Ozgur Akgun

2 2

ANNOUNCE: Cardinality-0.1
by Andrey Sisoyev 20 Jan '10

20 Jan '10

Hi everybody! I developed a new package that you may find handy, when transforming containers. With this package we now have unified ways to make safe transforms "from a -> Maybe (to a)" and "from -> Maybe to" between simple containers like [], Maybe, Identity, Map. Test set that returns all True: ((sContTransT []) :: Maybe (Maybe Int)) == Just Nothing ((sContTrans ()) :: Maybe (Maybe Int)) == Just Nothing ((sContTransT [1]) :: (Maybe (Maybe Int))) == Just (Just 1) ((sContTransT [1, 2]) :: (Maybe (Maybe Int))) == Nothing ((sContTransT $ Just "Hello") :: (Maybe (Identity String))) == Just (Identity "Hello") ((sContTransT ["Hello"]) :: Maybe (Identity String)) == Just (Identity "Hello") ((sContTransT (EmptySet :: EmptySet String)) :: (Maybe [String])) == Just [] ((sContTransT "Hello") :: Maybe (EmptySet Char)) == Nothing ((sContTransT ("key", "elem")) :: Maybe (Map String String)) == Just (singleton "key" "elem") ((sContTrans [("key1", "elem1"), ("key2", "elem2")]) :: Maybe (Map String String)) == Just (fromList [("key1", "elem1"), ("key2", "elem2")]) ((sContTrans (EmptySet :: EmptySet (String, String))) :: Maybe (Map String String)) == Just empty ((sContTrans []) :: Maybe ()) == Just () ((sContTrans (NEL 'H' "i!")) :: Maybe String) == Just "Hi!" -- Data.NeverEmptyList ((sContTrans ()) :: Maybe String) == Just "" ((sContTrans "") :: Maybe (Identity Char)) == Nothing ((sContTrans "Hi!") :: Maybe ()) == Nothing Package is to be found here: http://hackage.haskell.org/package/Cardinality It dances around cardinality - a measure of of the number of elements of the set (container); and cardinality constraint of the container. Cardinality constraints: ** (): 0 elements ** Identity a: 1 element ** (Maybe a): 0..1 elements ** [a]: 0..Infinity elements ** Map k e: 0..Infinity elements ** (a,a,a): 3 elements I also introduced 2 containers (which I found strange not to be in the Prelude): ** data EmptySet a = EmptySet -- (0 elements) ** data NeverEmptyList a = NEL a [a] -- (1..Infinity elements) The package provides all sorts of transformation instances between this types (except for tuples, 3ples, 4ples... I didn't make all transforms fo them - only "list -> (N)ple" and opposites) I tried also to deal with transforms like "container a -> a", "a -> container a" using OverlappingInstances. But failed to approve them - probably due to the lack of experience with this extension. I found them introducing too much unwanted complexity into the reasoning about types. It was the last straw to me, when GHC started to proving me, that unity is not less general then any type variable "a", and demanding tribute of IncoherentInstances. ----------------------------------- Any feedback is appreciated. Best regards, Andrey. -- View this message in context: http://old.nabble.com/ANNOUNCE%3A-Cardinality-0.1-tp27239964p27239964.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

1 0

Fwd: [BostonHaskell] Next meeting: January 21st at MIT (**NEW ROOM** 32G-449)
by Ravi Nanavati 20 Jan '10

20 Jan '10

---------- Forwarded message ---------- From: Ravi Nanavati <ravi_n(a)alum.mit.edu> Date: Wed, Jan 20, 2010 at 12:13 AM Subject: Next meeting: January 21st at MIT (**NEW ROOM** 32G-449) To: bostonhaskell(a)googlegroups.com I'm pleased to announce the January meeting of the Boston Area Haskell Users' Group. The meeting has been scheduled for Thursday, January 21st from 7pm to 9pm. Like recent meetings, it will be at MIT. However, it will be in the Patil/Kiva conference room, NOT the CSAIL reading room. That room is 32G-449 (i.e. a room on the 4th floor of the Gates Tower of MIT's Stata Center at 32 Vassar St in Cambridge, MA). My apologies for the late notice - I've been busy recently. This month, Paul Chiusano and Runar Bjarnason will be presenting "Scala for Haskell Programmers". Scala is a statically-typed functional language that targets the JVM and it has a type system that, in its own way, is as unique as Haskell's. This presentation will take up the bulk of the meeting, but if anyone wants to give a Lightning Talk, there is room for that. The January attendance poll is at: http://doodle.com/xnwcec4w4hbh6y65 As always, responding to this poll will help with two things: 1. Getting an idea of what fraction of the Boston-area Haskell community can and can't attend this meeting (to help with future scheduling). 2. Giving me an estimated count of attendees, since I have talked my wife into baking some more goodies and bringing drinks again. By the way, if someone is willing to edit the HaskellWiki page, post to reddit or otherwise publicize the meeting please just go ahead and do that (especially as I'm sending this out on the later side). If you have any questions about the meeting please send them to them BostonHaskell mailing list: bostonhaskell(a)googlegroups.com or contact me directly. I look forward to seeing many Boston-area Haskellers at the January meeting! Thank you, - Ravi Nanavati

1 0

Re: Atom bug(?)
by Tom Hawkins 19 Jan '10

19 Jan '10

On Tue, Jan 19, 2010 at 10:42 PM, Lee Pike <leepike(a)gmail.com> wrote: > Tom, > > Is this a bug? The following program compiles, but the rule is scheduled at > period 1 (rather than 0). I wouldn't have thought to have an assignment > outside of an atom until another engineer here wrote it. In any event, I > would have expected that the rule should have period 3. > > Lee > > example :: Atom () > example = period 3 $ do > > a <- word32 "a" 0 > > a <== 21 The top-most rule is implied with the compile command. This makes it appear as if the assignment doesn't belong to a rule, when if fact it belongs to the implicit top level rule. The top level rule always has a period of 1. Period constraints are only applied to sub-rules, not the current rule. This prevents ambiguous situations, such as: atom "something" $ do period 1 (a <== 1) -- Does 'something' execute every 1 or ... period 2 (b <== 2) -- 2 cycles? So in your example, 'period 3' has no sub rules that it applies to. -Tom

2 1

Parsers for Text Adventures
by Mark Spezzano 19 Jan '10

19 Jan '10

Hi, I am writing a Text Adventure game in Haskell (like Zork) I have all of the basic parser stuff written as described in Hutton's Programming in Haskell and his associated papers. (I'm trying to avoid using 3rd party libraries, so that I can learn this myself) Everything that I have works (so far...) except for the following problem: I want to define a grammar using a series of Verbs like this: data Verb = Go | Get | Jump | Climb | Give etc, etc deriving (Show, Read) and then have my parser "get" one of these Verb tokens if possible; otherwise it should do something (?) else like give an error message stating "I don't know that command" Now, Hutton gives examples of parsing strings into string whereas I want to parse Strings into my Verbs So, if the user types "get sword" then it will tokenise "get" as type Verb's data constructor Get and perhaps "sword" into a Noun called Sword My parser is defined like this: newtype Parser a = Parser (String -> [(a, String)]) So I CAN give it a Verb type but this is where I run into a problem.... I've written a Parser called keyword keyword :: Parser Verb keyword = do x <- many1 letter return (read x) (read this as "take-at-least-one-alphabetic-letter-and-convert-to-a-Verb-type") which DOES work provided that the user types in one of my Verbs. If they don't, well, the whole thing fails with an Exception and halts processing, returning to GHCi prompt. Question: Am I going about this the right way? I want to put together lots of "data" types like Verb and Noun etc so that I can build a kind of "BNF grammar". Question: If I am going about this the right way then what do I about the "read x" bit failing when the user stops typing in a recognised keyword. I could catch the exception, but typing an incorrect sentence is just a typo, not really appropriate for an exception, I shouldn't think. If it IS appropriate to do this in Haskell, then how do I catch this exception and continue processing. I thought that exceptions should be for exceptional circumstances, and it would seem that I might be misusing them in this context. Thanks Mark Spezzano

9 10

ANNOUNCE: Functional Programming Bibliography
by James Russell 19 Jan '10

19 Jan '10

I am pleased to announce the Functional Programming Bibliography at http://www.catamorphism.net/ The functional programming bibliography was created in the hope that it will be a useful resource to the functional programming community. The site is still in an early stage of development, and is pretty raw, and incomplete in a number of ways. Keyword categorization, in particular, is still fairly spotty. It currently contains in excess of 1500 references, heavily slanted toward Haskell-related topics, and contains links to publicly available versions of many papers, as well as links to gated versions of some papers. I am eager for suggestions as to how the site could be made more useful. Regards, James Russell

4 6

#haskell stopped being logged on tunes.org
by Eugene Kirpichov 19 Jan '10

19 Jan '10

Hello. The #haskell channel stopped being logged because now it accepts only someone who has IDENTIFY'd. For the last 3 days essentially nothing has been recorded. I am not even sure whether this should be fixed on the tunes.org site or on #haskell's, but something has to be done, and I plea those with sufficient power for that to do it :) -- Eugene Kirpichov Web IR developer, market.yandex.ru

2 1

ZuriHac registration deadline on February 14, 2010
by Johan Tibell 19 Jan '10

19 Jan '10

Hi all, Due to budget and security constraints, we need to know the final number of attendees a bit in advance. We've therefore set a registration deadline on February 14, 2010. To register please follow the instructions on the registration page: http://www.haskell.org/haskellwiki/ZuriHac/Register Cheers, Johan & Christophe

1 0

parallel matrix multiply (dph, par/pseq)
by Johannes Waldmann 19 Jan '10

19 Jan '10

Hello. How can I multiply matrices (of Doubles) with dph (-0.4.0)? (ghc-6.12.1) - I was trying type Vector = [:Double:] type Matrix = [:Vector:] times :: Matrix -> Matrix -> Matrix times a b = mapP ( \ row -> mapP ( \ col -> sumP ( zipWithP (*) row col ) ) ( transposeP b ) ) a but there is no such thing as transposeP. When I try any kind of index manipulations, the compiler invariably tells me that it does not want to build [: :] - lists of indices (e.g., there is no enumFromToP) (I guess because I'm using Data.Array.Parallel.Prelude.Double) Puzzled - J.W. PS: what's the recommended way to multiply matrices (better modelled as Array (Int,Int) Double or [[Double]] ?) with the par/pseq approach (if this is recommended at all)? As I said earlier, I just want to have some nice and easy benchmarks for demonstration in a lecture (to be run on 2, 4, or 8 cores). Of course if they work, I'd use them in real life as well...

3 3

Declarative binary protocols
by Antoine Latter 19 Jan '10

19 Jan '10

Cafe, We have some fantastic tools for binary parsing in packages like binary and cereal (and presumably attoparsec, which I've not used). But they don't quite scratch an itch I have when writing implementations of binary communication protocols. A good example of my problem is in my implementation of the memcached binary wire protocol: http://hackage.haskell.org/package/starling What I've tried to do is divide up the library into a declarative protocol description and an imperative machine to sit on a handle and link together a server response with the source request. In the declarative core all of the types which come off of the wire have an associated Data.Binary.Get action - but this isn't quite good enough. Data.Binary works on ByteStrings, but I have a handle. I don't want to use hGetContents because I have trouble working out when lazy IO is and is not correct. I can't use hGet because I don't know how much to get until I'm in the middle of the Get action. How do other folks solve this issue? What I've done is broken down and included (getResponse :: Handle -> IO Response) in my core protocol description module, which gets a fixed-length header from which we can figure out how much else to get to form the complete response. But it would be nice to have something cleaner. One thing I've thought of is generalizing Data.Binary.Get to operate over either ByteStrings or Handles. But then I would be doing reads from the handle every couple of bytes. To avoid that I could extend the monad to include declarations of how many bytes the parser will require, which may be declared mutliple times throughout the parser. This felt a bit weird to me, though. Thanks, Antoine Declarative core module: http://hackage.haskell.org/packages/archive/starling/0.1.1/doc/html/Network… IO-centric module: http://hackage.haskell.org/packages/archive/starling/0.1.1/doc/html/Network…

2 2

sorting and merging in haskell
by jjoshua 19 Jan '10

19 Jan '10

So I defined a type called term which is a coefficient and degree of x. int a int b. I want to write a method/methods in haskell to sort them and combine ones with same degree. I think I need to combine somehow, and then use sortby method? I'm just learning haskell, but I now java and c# pretty well and I'm having a hard term converting to functional thinking. its of type [(Int a Int b)] and if b1< b2 put tuple 1 before tuple2, else if = combine by adding a1 to a2, else place 2 before 1. I know how to do it logically, just not the syntax I guess. Thanks so much guys! -- View this message in context: http://old.nabble.com/sorting-and-merging-in-haskell-tp27222668p27222668.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

ANN: afv-0.0.3
by Tom Hawkins 19 Jan '10

19 Jan '10

This release of AFV adds counter example generation for both concrete bounded violations or for inconclusive results when the induction fails to converge. I also put Linux and Windows binaries here: http://tomahawkins.org/. http://hackage.haskell.org/package/afv -Tom

1 0

Windows emulator for testing purposes
by Gregory Crosswhite 19 Jan '10

19 Jan '10

Hey everyone! I have a Linux machine and a OSX machine, and I am developing packages that I would like to upload to Hackage one day. Do you have any recommendations for a free/open source solution that would let me emulate a Windows environment for testing purposes? Cheers, Greg

8 9

FFI, C/C++ and undefined references
by DNM 19 Jan '10

19 Jan '10

Note: I'm relatively new to Haskell, and my knowledge of C and C++ is basically pretty minimal -- I can read, modify and compile C/C++ programs (usually). I'm trying to interface with some C++ code by writing a little bit of C code that uses that C++ code, and I'm getting "undefined reference" errors when I try to 'ghc --make' a client application to test it. Actually, I'm modifying Nitin Madnani's (freely available) Python SRILM toolkit wrapper code. (SRILM, by the bye, is a C++-based toolkit for training and using statistical n-gram language models. I was surprised that no-one has tried to do this yet -- or at least not that they have shared with the rest of us.) Anyhow, I've verified that my modification of Madnani's C code works by compiling it and running it through a SWIG interface in Madnani's Python code, so I'm pretty confident the C client of SRILM is solid. The culprit is either my Haskell FFI code or the client of that code. Without cooking up a microcosm of my problem with little Foo's and Bar's, I'll just give my actual C, header file and Haskell code (or at least the relevant bits), and then the error. ------------- srilm.h ---------------- #ifdef __cplusplus extern "C" { #else typedef struct Ngram Ngram; /* dummy type to stand in for class */ #endif Ngram* bldLM(int order, const char* filename); void deleteLM(Ngram* ngram); float getSeqProb(Ngram* ngram, const char* ngramstr, unsigned order, unsigned length); #ifdef __cplusplus } #endif ----------------------------------------- ------------- srilm.c ---------------- // Initialize and read in the ngram model Ngram* bldLM(int order, const char* filename) { ... } ... // Delete the ngram model void deleteLM(Ngram* ngram) { delete srilm_vocab; delete ngram; } ... // Get the ngram probability of the given string, given n-gram order 'order' and string length // 'length'. float getSeqProb(Ngram* ngram, const char* ngramstr, unsigned order, unsigned length) { ...} ----------------------------- Next, the Haskell FFI specs and code that marshals data between Haskell and C. ---------------- LM.hs ---------------------- {-# INCLUDE "srilm.h" #-} {-# LANGUAGE ForeignFunctionInterface, EmptyDataDecls #-} ... module decl's, imports, etc. {- | A dummy placeholder for SRILM n-gram model thingies. -} data Ngram data NGModel = NGModel {ng :: !(ForeignPtr Ngram)} foreign import ccall "srilm.h bldLM" c_blm :: CInt -> CString -> Ptr Ngram foreign import ccall "srilm.h deleteLM" c_dlm :: FunPtr ((Ptr Ngram) -> IO ()) foreign import ccall "srilm.h getSeqProb" c_ngramProb :: Ptr Ngram -> CString -> CUInt -> CUInt -> CFloat {- | Given an n-gram model, an Int representing the n-gram order and a list of strings (word sequence), compute the n-gram probability of the sequence. -} scoreSequence :: NGModel -> Int -> [String] -> Float scoreSequence ngram order seq = unsafePerformIO $ do stringSeq <- newCString (unwords seq) let sc = c_ngramProb (unsafeForeignPtrToPtr $ ng ngram) stringSeq (fromIntegral order) (fromIntegral $ length seq) return (realToFrac sc) ... buildLM :: Int -> String -> NGModel buildLM order fname = NGModel $ unsafePerformIO $ do cFName <- newCString fname let ng = c_blm (fromIntegral order) cFName return $ unsafePerformIO $ newForeignPtr c_dlm ng -------------------------------------------- Now, I've defined a simple app that tries to use this: ------------------- Main.hs ------------------------- module Main where import SRILM.LM(scoreSequence, buildLM) main :: IO () main = do let lm = buildLM 5 "eng.kn.5g.lm" putStrLn $ show $ scoreSequence lm 5 ["the", "prime", "minister", "gave", "a", "speech", "."] ----------------------------------------------------------- But when I try to compile it (after having successfully compiled the C code with g++), I get: $ ghc --make Main.hs Linking Main ... LM.o: In function `r18k_info': (.text+0x122): undefined reference to `bldLM' LM.o: In function `r18m_info': (.text+0x14e): undefined reference to `deleteLM' LM.o: In function `r18o_info': (.text+0x28b): undefined reference to `getSeqProb' collect2: ld returned 1 exit status Any ideas? Note that I'm not confident that everything on the Haskell side is correct, but it seems that ghc can't find my C client of SRILM. As I said, I've compiled this code using g++, and it works when I interface with it through Python. Sorry for the long-windedness, but I figured I'd err on the side of TMI so that I don't have to keep posting more and more code snippets and error messages. Any help is greatly appreciated. (And I'd be happy to share my interface to SRILM to anyone who's interested, once I get it working -- and I get permission from Nitin Madnani to distribute a modified version of his code.) Thanks, Dennis -- View this message in context: http://old.nabble.com/FFI%2C-C-C%2B%2B-and-undefined-references-tp27139612p… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

9 24

Broken registration link on hackage trac
by Andy Gimblett 18 Jan '10

18 Jan '10

Hi all, I want to register an account on hackage's trac instance, but the "register an account" link on the start page: http://hackage.haskell.org/trac/hackage/wiki/WikiStart is broken. I'm guessing someone here knows what it should be and has registered already and thus can fix it. :-) Cheers, -Andy -- Andy Gimblett http://gimbo.org.uk/

3 2

Mini-announce: A few package updates
by Andrew Coppin 18 Jan '10

18 Jan '10

In case somebody else on this planet besides me is using these... I've just updated my handful of packages on Hackage so they should now build with GHC 6.12.1. (In most cases, just tweaks to the package dependancies.) I have also changed the documentation for AC-EasyRaster-GTK, to hopefully make it clearer that all the event-related stuff is ONLY necessary if you want to display your images in a GTK window. Oh, and I discovered that I fail basic linear algebra; the AC-Vector package had a bug in vcross, which is fixed in 1.2.2 (but exists in all prior versions). AC-Vector gets a minor version bump because I renamed some stuff; nothing really "exciting" is different. PS. Apparently AC-VanillaArray causes GHC 6.12.1 to panic during compilation... I'll have to investigate that. Still, it does play with GHC.Prim, so that's probably got something to do with it.

1 0

Newbie question about Parsec
by david fries 18 Jan '10

18 Jan '10

Hey everybody I've been playing around with Parsec a little bit lately. I like it a lot, but now I've hit a bit of a challenge. Suppose I have to parse a variable length string representing a time interval. Depending on how many fields there are, the time is either interpreted as seconds, minutes and seconds or hours, minutes and seconds. For example: "... 31 ..." would be parsed as 31 seconds. "... 05:31 ..." would be parsed as 5 minutes and 31 seconds. "... 01:05:31 ..." would be parsed as 1 hour, 5 minutes and 31 seconds. I've come up with the following solution using optionMaybe to deal with the problem: data ElapsedTime = ElapsedTime { hours :: Int, minutes :: Int, seconds :: Int } deriving (Show, Eq, Ord) p_elapsed_time :: CharParser () ElapsedTime p_elapsed_time = toElapsedTime <$> (optionMaybe p_Int) <*> (optionMaybe (char ':' *> p_Int)) <*> (optionMaybe (char ':' *> p_Int <* skipSpaces)) where toElapsedTime Nothing Nothing Nothing = ElapsedTime 0 0 0 toElapsedTime (Just s) Nothing Nothing = ElapsedTime 0 0 s toElapsedTime (Just m) (Just s) Nothing = ElapsedTime 0 m s toElapsedTime (Just h) (Just m) (Just s) = ElapsedTime h m s Where p_Int simply parses a sequence of digits as an Int and skipSpaces does just that. This works correctly, but it also feels kinda clumsy. For one the compiler rightly complains about non-exhaustive pattern matches in the definition of the toElapsedTime function, although I believe that's negligible in that particular case. Is there a better i.e. more elegant way to tackle such a problem? regards, david

2 1

unexpected behavior / bug in try . System.FilePath.Find.findWithHandler
by Thomas Hartman 18 Jan '10

18 Jan '10

say you want to execute a find function, but abort the computation if it hits any snags, such as an unreadable directory (eg chmod -R a-r dir). Currently try . System.FilePath.Find.findWithHandler will return an exception wrapped in Right, which seems Wrong. For sure it will just get ignored if wrapped in an ErrorT computation and I suspect this could lead to other glitchy/unexpected behavior when used in sysadmin scripts. You do get the expected exception wrapped in Left for certain errors (such as a missing directory), which adds to the confusion. Probable fix: you get the expected behavior if you remove unsafeInterleaveIO from findWithHandler. Does this seem like a reasonable thing to suggest to libraries@haskell? Hacky fix: I haven't actually done this, but I suppose you could trap stdout or stderr, since the exception does get printed although it's not reflected in the result type of the computation. demo of problem: import qualified System.FilePath.Find as F import System.FilePath.Find import System.IO.Error abort path err = fail $ path ++ " , " ++ (show err) prot = "/home/thartman/protected" -- a protected (unreadable) dir {- want result => Left user error (/home/thartman/protected , user error (/home/thartman/protected , /home/thartman/protected: getDirectoryContents: permission denied (Permission denied))) but get result Right *** Exception: user error (/home/thartman/protected , /home/thartman/protected: getDirectoryContents: permission denied (Permission denied)) which means, for example, you would miss the error if you wrapped this in an ErrorT there is probably a workaround from reading stderr, since it does get printed out, but this is awkward compared to just catching the error the right way and certainly feels like unexpected behavior Possible fix: you get the expected behavior if you remove unsafeInterleaveIO from System.FilePath.Find.findWithHandlers don't know why this fixes the problem, I was just messing around I don't know if this breaks anything else, but I was unable to find any breakage when I did a simple test on an unprotected directory What is the gain from unsafeInterleaveIO, and can it be nixed? -} tprot = try . findWithHandler abort (return True) (return True) $ prot tunprot = return . either (error . show) (length) =<< ( try $ findWithHandler abort (return True) (return True) unprot ) unprot = "/home/thartman/unprot" -- Need somewhere to put your code? http://patch-tag.com Want to build a webapp? http://happstack.com

2 1

ghc "make install" question
by Johannes Waldmann 18 Jan '10

18 Jan '10

Hi. How can I get "make install" from a ghc (snapshot) source dir to install binaries $prefix/bin/ghc{,i,-pkg}-$version but *not* overwrite the un-versioned symlinks? Thanks - J.W.

1 0

ANN: atom-0.1.3
by Tom Hawkins 18 Jan '10

18 Jan '10

This release of Atom slightly changes the semantics of assertions and coverage. Assertion and coverage are now checked between the execution of every rule, instead of only when the rules containing assertions are fired. They are still subject to parental guard conditions, but not period or phase constraints. This means... period 20 $ atom "checkSomeStuff" $ do cond ok assert "A" a assert "B" b cover "C" c ... A, B, and C are checked all the time "ok" is true, not just every 20th cycle. Checking between every rule execution obviously impacts simulation time, but the increased testing rigor is worth it. I also added 'linear' to Common, which does linear interpolation and extrapolation on a line given two points. (I found I was replicating this function everywhere for sensor calibrations, control limits, etc.) http://hackage.haskell.org/package/atom

2 5

A numpy equivalent for Haskell
by yairchu＠gmail.com 18 Jan '10

18 Jan '10

Hi Cafe, I've created a numpy equivalent for Haskell. (Numpy is a python library for multi-dimensional arrays and operations on them) Code at http://github.com/yairchu/numkell (not yet on hackage because it needs better names) A numkell array is a pair of a function from integer inputs and a range for its inputs (size). This allows for easy memoizing into in-memory arrays, and additionally, numkell arrays also support useful operations like numpy's newaxis and folding axes away. As the "Array" name was already taken, numkell's array is currently called "Funk" (name suggestions very appreciated). An example: Given an bunch of vectors as a 2d array, compute the distance between each pair of vectors {-# LANGUAGE DeriveDataTypeable, GeneralizedNewtypeDeriving, TypeOperators #-} import Data.HList import Data.NumKell import Data.Typeable newtype PersonIdx = PersonIdx Int deriving (Enum, Eq, Integral, Num, Ord, Real, Show, Typeable) newtype FeatureIdx = FeatureIdx Int deriving (Enum, Eq, Integral, Num, Ord, Real, Show, Typeable) let personProps = fFromList [[5,3,2],[4,8,1],[2,6,9],[5,3,0]] :: Funk (HJust PersonIdx :*: HJust FeatureIdx :*: HNil) Double > personProps FeatureIdx 0 1 2 PersonIdx + - - - 0 | 5.0 3.0 2.0 1 | 4.0 8.0 1.0 2 | 2.0 6.0 9.0 3 | 5.0 3.0 0.0 > sumAxes (fmap (** 2) (liftF2 (-) (personProps !/ (SNewAxis .*. HNil)) (personProps !/ (SAll .*. SNewAxis .*. HNil)))) (TFalse .*. TFalse .*. TTrue .*. HNil) PersonIdx 0 1 2 3 PersonIdx + - - - - 0 | 0.0 27.0 67.0 4.0 1 | 27.0 0.0 72.0 27.0 2 | 67.0 72.0 0.0 99.0 3 | 4.0 27.0 99.0 0.0 In Python the last line looks shorter: >>> ((personProps[newaxis] - personProps[:,newAxis]) ** 2).sum(2) Mostly due to Python's slicing syntax sugar. Still, numkell has one large benefit over numpy (apart from being for Haskell): With numpy this example creates a temporary 3d array in memory. In numkell the array is not allocated in memory unless "fMemo" is called. If anyone has comments, suggestions, naming suggestions, complaints, etc, I would very much like to hear. cheers, Yair

3 2

Takusen - reading from one db inserting into another
by Günther Schmidt 18 Jan '10

18 Jan '10

Hi, I know how to use Takusen with a *single* connection, but I cannot figure out how to read from one database while inserting to another. Could someone please explain to me how I can do that? Günther

3 3

Visualizing function application
by Martijn van Steenbergen 18 Jan '10

18 Jan '10

Dear café, I am deeply impressed with Vacuum[1][2], Ubigraph[3] and especially their combination[4]. I can trivially and beautifully visualize the ASTs that my parser produces. I can visualize zippers of the ASTs and confirm that sharing is optimal. Ubigraph is also able to animate graph *mutations*, as shown by the various demos on Ubigraph's website. How cool would it be if we could tell vacuum-ubigraph: here, show this tree, and then show how the tree changes when we apply this function on it. We could see how [1,2,3] is transformed into a ring when cycle is applied on it, or we could see how a list is consumed node by node when a foldr is applied. I have no idea how difficult this is or how to begin, so I thought I'd throw the idea out here. Perhaps it is appealing enough that someone picks it up and implements it. :-) Martijn. [1] http://hackage.haskell.org/package/vacuum [2] http://www.youtube.com/watch?v=oujaqo9GAmA [3] http://ubietylab.net/ubigraph/content/Demos/index.html [4] http://hackage.haskell.org/package/vacuum-ubigraph

2 1

Parse error
by Andrew Coppin 18 Jan '10

18 Jan '10

Is there a specific reason why GHC consistently refuses to accept the following perfectly reasonable code snippet? main = do putStrLn "Line 1" putStrLn "Line 2" let xs = do x <- [1..10] y <- [1..10] return (x+y) print xs No matter which way I rearrange this, it *insists* that there's a parse error. This is very frustrating, given that it's utterly clear what I want...

9 12

xmonad cannot find its library after upgrading GHC
by zaxis 18 Jan '10

18 Jan '10

After `pacman -Syu`, the ghc is upgraded to 6.12.1 %ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 However, the xmonad still stay as it is: %pacman -Q|grep -i xmonad xmonad 0.9-2.1 xmonad-contrib 0.9-1.2 %pacman -Ql xmonad|more ...... xmonad /usr/lib/ghc-6.10.4/ xmonad /usr/lib/ghc-6.10.4/site-local/ xmonad /usr/lib/ghc-6.10.4/site-local/xmonad-0.9/ xmonad /usr/lib/ghc-6.10.4/site-local/xmonad-0.9/HSxmonad-0.9.o xmonad /usr/lib/ghc-6.10.4/site-local/xmonad-0.9/XMonad.hi xmonad /usr/lib/ghc-6.10.4/site-local/xmonad-0.9/XMonad/ ..... So it will fail when running `xmonad --recompile` as xmonad cannot find its library in /usr/lib/ghc-6.12.1. Do i need to re-install xmonad ? Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/xmonad-cannot-find-its-library-after-upgrading-GHC-tp… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

Re: [Haskell-cafe] Re: Arrow instance of Transducer (Was: [Haskell] ANN: Streaming Component Combinators 0.4)
by Mario Blažević 17 Jan '10

17 Jan '10

On Sun 17/01/10 4:47 PM , Twan van Laarhoven twanvl(a)gmail.com sent: > The Arrow class is too big, it includes too many things and it should be > split up. This is yet another example. You can get some of the benefits of > standard classes by making Transducer an instance of Control.Category.Category, for > which you only need to implement (.) and id. Yes, perhaps I'll do that. Is there any relatively accepted package that defines PointedCategory?

1 0

Re: Arrow instance of Transducer (Was: [Haskell] ANN: Streaming Component Combinators 0.4)
by Mario Blažević 17 Jan '10

17 Jan '10

>>> Stupid question: Is it related to Arrows? >> >> Not really. You might say it's more general than arrows, because >> the streaming components are not restricted to a single input and >> single output type. On the other hand, they are all specific to stream >> processing, much like Fudgets and Yampa which are arrow-based. > > Arrows use tuple values for multiple inputs and outputs. Actually I'm > using arrows this way for signal processing. > >> I suppose the Transducer data type [1] could be made an Arrow >> instance, which would let me rename the 'connect' combinator [2] to >> (>>>). I'll have a look at Yampa to see if I can harvest more >> combinator names, thank you! After some investigation, I've concluded that Transducer cannot be made an instance of Arrow in any way that's both practical and general. The reason is that a Transducer converts a finite stream to another finite stream of arbitrary length. If we ignore effects, it's like a function of type [a] -> [b]. When trying to define an instance of Arrow for > newtype Transducer a b = Transducer ([a] -> [b]) the arr and (>>>) methods are trivial: > instance Arrow Transducer where > arr f = Transducer (map f) > Transducer t1 >>> Transducer t2 = Transducer (t2 . t1) but there is no satisfactory definition for the method first. A sensible definition, IMO, would have to give rise to a pairwise (***), so if we have two transducers t1 and t2 which respectively convert [a,b] to [c,d] and [1,2] to [3,4], t1 *** t2 would have to map [(a,1), (b,2)] to [(c,3), (d,4)]. There is no general way to define such a method first for the Transducer data type above. One can try appending the right-hand sides of the input to a queue and adding them to the outputs of the argument transducer: > first (Transducer t) = Transducer (uncurry (zip . t) . unzip) but this works only for injective transducers that map each new input item to exactly one output item. If the input stream has finite length, the input and output stream must be of equal length. In his paper "Generalising Monads to Arrows" from 1998, John Hughes defines a similar Arrow instance for stream processors operating on infinite streams which theoretically works for non-injective transforms, but leaks infinite space and can't be adopted to finite stream processors. HXT defines a different kind of an Arrow instance similar to the list monad instance. This approach can combine non-injective transforms, but in it the t1 *** t2 example above would map the input [(a,1), (b,2)] to [(c,3), (c,4), (d,3), (d,4)]. While this is a simple and lawful Arrow instance, I don't find it useful for stream processing. Yampa and other Functional Reactive Programming libraries associate a unique timestamp to each stream item. Method first can use this timestamp to associate input and output items even if the transducer leaves out some output items, and if the time stamps are ordered in each stream I guess it could also handle extra output items unrelated to any input. I'm not certain to what extent the FRP transducers (i.e., signal processors) are allowed to do this. To summarize: while I'd love the benefits of satisifying an Arrow instance, the loss of generality seems too high for me. SCC transducers work on finite streams of any values, and their output stream need not have any resemblance to the input stream. I may define an Arrow-conformant subclass of Transducer in a future version of SCC, perhaps operating on streams with timestamped values. >> >> [1] >> http://hackage.haskell.org/packages/archive/scc/0.4/doc/html/Control-Concur… >> >> [2] >> http://hackage.haskell.org/packages/archive/scc/0.4/doc/html/Control-Concur… > > In whatever way it is related to arrows, maybe you can mention it on the > project page. I'll copy this message there.

2 1

Type Inference for Overloading without Restrictions
by Peter Verswyvelen 17 Jan '10

17 Jan '10

A while ago, someone provided me a link to the paper "Type Inference for Overloading without Restrictions" http://www.dcc.ufmg.br/~camarao/ct-flops99.ps.gz Although I don't understand everything in this paper, I wander what people's opinions are about this regarding a future Haskell language revision or extension? Would a feature like this be preferable over typeclasses? Would it be practical to implement? Are people working on this? Thanks, Peter Verswyvelen

2 1

ANN: afv-0.0.2
by Tom Hawkins 17 Jan '10

17 Jan '10

AFV is an infinite state model checker for simple, iterative C programs. This release adds some new name checks, a few minor bug fixes, basic support for functions, and a little stronger type checking. Though most of the C language is still not supported, it can verify a lot of interesting behavior -- including one of my bugs that pulverized about $40K of hydraulics hardware last year. :-) http://hackage.haskell.org/package/afv/

1 0

cabal install vacuum-cairo
by Ozgur Akgun 17 Jan '10

17 Jan '10

Cafe, I've been trying to install vacuum-cairo using cabal but I couldn't have it installed because of the missing packages cairo, svg and gtkcairo. What should I do to install vacuum-cairo? Thanks for any help in advance, -- Ozgur Akgun

2 1