January 2010 - Haskell-Cafe

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

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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

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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

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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.

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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.

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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/

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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

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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

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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

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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…

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