January 2009 - Haskell-Cafe

What could be considered "standard Haskell" these days?
by Mauricio 16 Jan '09

16 Jan '09

Hi, I would like to take some time to study Haskell properly, so that I could help others and pay my debt for the many times I had to bother with my syntax questions. And, of course, make better use of the language. My first attempt was to read the syntax description in Haskell 98 report, and that helped a lot. But I've realized that it's far from the language as used today. But just adding all available extensions would not be good, as some are very experimental e others are obsolete. I saw Haskell-prime page and a few features are marked as accepted. That mean I can trust them to be part of the next Haskell standard? So: if someone wants to learn the details of the language, what could be the subset of extensions one should learn and make regular use, and also include in code supposed to be used by others in the long term? Thanks, Maurício

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Re: [Haskell-cafe] Maintaining laziness
by Jan Christiansen 16 Jan '09

16 Jan '09

Hi, > Although it seems to be overkill for a single module - How about a > cabalized version on Hackage and a darcs repository? It would > simplify using and updating it. I am not sure whether this would be a good idea. The original version makes a lot of suggestions which are not satisfiable but it is not at all trivial to decide which are satisfiable and which are not. I have rewritten StrictCheck from scratch to overcome this problem. But the current implementation is not presentable right now and I have no formal prove that the criterion that I am using is correct. > Your StrictCheck is really tough! I'm currently checking my > routines from > http://hackage.haskell.org/packages/archive/utility-ht/0.0.1/doc/ > html/Data-List-HT.html > which I designed especially with laziness in mind - and I found a > lot of laziness breakers using StrictCheck! > > However now I found an example where it probably proposes > something inefficient: > > So what it proposes is essentially that one should first check the > length of the lists without looking at the contents. > [undefined,undefined] can never be a prefix of [undefined]. > However, first constructing the list spine seems to be inefficient. I know about this problem. It is mentioned in the paper about StrictCheck. I haven't worked on this right now. With respect to least-strictness I would say the tool is right. But it is probably not the kind of advise one wants. When I am sure that I can handle the other problem I will take a look at this one. Cheers, Jan

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F# active patterns versus GHC's view
by Peter Verswyvelen 16 Jan '09

16 Jan '09

When I first read about active patterns in F#, I found it really cool idea, since it allows creating fake data constructors that can be used for pattern matching, giving many views to a single piece of data, and allowing backwards compatibility when you completely change or hide a data structure. So for example one could define a Polar pattern and a Rect pattern that give different views of a Complex number, e.g (pseudo code follows) pattern Polar c = (mag c, phase c) pattern Rect c = (real c, imag c) This seems handy: polarMul (Polar m1 p1) (Polar m2 p2) = mkComplexFromPolar (m1*m2) (p1+p2) However, I think it is flawed, since the following case c of Polar _ _ -> "it's polar!" Rect _ _ -> "it's rect!" seems like valid code but does not make any sense. So I think the GHC extension of view patterns is better than the active patterns in F#? A good coding style is to provide constructor functions and hide data constructors. But then one looses the ability to perform pattern matching, which is so cool in Haskell. Would I have to conclude that it would be good coding style to use view patterns as much as possible in Haskell, creating auxiliary data constructors to expose the "public members" of the hidden data constructors?

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about documentation improvements
by Manlio Perillo 16 Jan '09

16 Jan '09

About latest thread on better naming and documentation improvement, why don't organize an interactive session where: 1) normal users tell what's wrong about a package/module documentation 2) the package/module author/maintainer fix the documentation in real time, so that users can review it again Manlio Perillo

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ANNOUNCE: gitit 0.2 release - wiki using HAppS, git, pandoc
by John MacFarlane 16 Jan '09

16 Jan '09

I've uploaded an early version of gitit, a Haskell wiki program, to HackageDB. Gitit uses HAppS as a webserver, git for file storage, pandoc for rendering the (markdown) pages, and highlighting-kate for highlighted source code. Some nice features of gitit: - Pages and uploaded files are stored in a git repository and may be added, deleted, and modified directly using git. - Pages may be organized into subdirectories. - Pandoc's extended version of markdown is used, so you can do tables, footnotes, syntax-highlighted code blocks, and LaTeX math. (And you can you pandoc to convert pages into many other formats.) - Math is rendered using jsMath (which must be installed separately). - Source code files in the repository are automatically rendered with syntax highlighting (plain/text version is also available). You can check it out on my webserver: http://johnmacfarlane.net:5001/ Or try it locally: cabal update cabal install pandoc -fhighlighting cabal install gitit gitit # note: this will create two subdirectories in the working directory # then browse to http://localhost:5001. There's a git repository at http://github.com/jgm/gitit/tree/master. Comments and patches are welcome. John

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[ANNOUNCE] Darcs 2.2.0.
by Petr Rockai 16 Jan '09

16 Jan '09

Hi all! I am happy to announce general availability of darcs 2.2.0. Getting the release ------------------- For this release, we have decided to provide two flavours, depending on the build system used: 1) The source tarball, <http://www.darcs.net/darcs-2.2.0.tar.gz>, which can be built using the traditional autoconf-based system. This is the fully supported version. After downloading and unpacking, you can issue: $ ./configure $ make and possibly # make install More detailed instructions inside the tarball (file README). Please note that we had at least one report of build failure, with quickcheck-related message. The currently best workaround, if this happens to you, is to use the cabal version of the package instead, see below. 2) Cabalised source. You can either download a tarball from <http://hackage.haskell.org/packages/archive/darcs/2.2.0/darcs-2.2.0.tar.gz> and build manually (see the build instructions in README inside the tarball), or, alternatively, you can use cabal-install to obtain a copy: $ cabal update $ cabal install darcs This will give you a darcs binary in ~/.cabal/bin -- you should probably add that to your PATH. In addition to source tarballs, we expect binary packages for various UNIX platforms will be available in due time. For Windows users, Salvatore Insalaco has prepared a binary build, available from <http://homepage.mac.com/kirby81_it/darcs/darcs-2.2.0-win1.zip>. You just need to unpack the directory somewhere and add it to your path (if you like). Moreover, an experimental TortoiseDarcs release for darcs 2 has been made available by Kari Hoijarvi <hoijarvi-at-seas-dot-wustl-dot-edu> and is looking for home. It can be found at <http://datafed.net/darcs> (unfortunately, at the time of this writing, the site seemed unreachable... If you can help with hosting, please mail Kari.) What's New ---------- The summary of changes since version 2.1.2 (released last November) follows: * In interactive record, it is now possible to get a list of currently selected hunks (command 'l'). (Christian Kellermann) * It is now possible to specify --in-reply-to when using darcs send, to generate correct references in the mail header. (Pavel Shramov) * New repositories with --no-pristine-tree can no longer be created. This only has effect on legacy darcs-1 repositories. * Improvements in Windows support. (Salvatore Insalaco) * Performance improvements in `darcs repair` and robustness improvements in `darcs check`. (Petr Ročkai) * Extensive manual and online help improvements. (Trent W. Buck) * Support for GHC 6.10. * Overhaul of the make-based build system. (Trent W. Buck) * Cabal is now supported as a build method for darcs. (Duncan Coutts, Petr Ročkai, Gwern Branwen) * First stab at libdarcs -- when building through Cabal, all of darcs implementation is now exposed in modules. No API guarantees whatsoever. (Eric Kow) * Additions to Haddock documentation of the existing darcs modules for improved development experience. * Improvements in the testing infrastructure. (Christian Kellermann, Gwern Branwen) * Low-level optimisations in filesystem code. (Ganesh Sittampalam) * Numerous major and minor bug fixes, refactorings and cleanups by David Roundy, Eric Kow, Jason Dagit, Dmitry Kurochkin, Thorkil Naur, Salvatore Insalaco, Christian Kellerman, Florent Becker, Duncan Coutts, Reinier Lamers, Ganesh Sittampalam, Petr Ročkai. And a summary of issues that have been fixed in darcs since version 2.1.2 (compiled by Thorkil Naur): 525 amend-record => darcs patches show duplicate additions 971 darcs check fails (case sensitivity on filenames) 1006 darcs check and repair do not look for adds 1043 pull => mergeAfterConflicting failed in geteff (2.0.2+) 1101 darcs send --cc recipient not included in success message 1117 Whatsnew should warn on non-recorded files 1144 Add darcs send --in-reply-to or --header "In-Reply-To: <x(a)y.z>" 1165 get should print last gotten tag 1196 Asking for changes in /. of directory that doesn't exist gives changes in entire repo 1198 Reproducible "mergeConflictingNons failed in geteff with ix" 1199 Backup files darcs added after external merge 1223 sporadic init.sh test failure (2.1.1rc2+472) 1238 wish: darcs help setpref should list all prefs 1247 make TAGS is broken 1249 2.1.2 (+ 342 patches) local drive detection on Windows error 1272 amend-record not the same as unrecord + record 1273 renameFile: does not exist (No such file or directory) 1223 sporadic init.sh test failure (2.1.1rc2+472) I would like to thank all contributors for making this release possible. Future ------ The next release will be 2.2.1, fixing low-risk issues found in 2.2.0, or those that have been excluded for 2.2.0 due to freeze. This release will appear in two or three weeks time, depending on circumstances. The next major release will be 2.3, due in June or July this year. The focus of this release will be new features and further work on performance. Moreover, we expect that it will use Cabal as its default build system and will make first steps towards sustainable libdarcs API. Yours, Petr. -- Peter Rockai | me()mornfall!net | prockai()redhat!com http://blog.mornfall.net | http://web.mornfall.net "In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton on the subject of C program indentation

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Re: [Haskell-cafe] some ideas for Haskell', from Python
by Manlio Perillo 16 Jan '09

16 Jan '09

Artyom Shalkhakov ha scritto: > Hi Manlio, > Hi Artyom. Note that it seems you have sent this message only to me; I'm sending the reply to both you and the mailing list. > 2009/1/14 Manlio Perillo <manlio_perillo(a)libero.it>: >> 2) In Python it is possible to import modules inside a function. >> >> In Haskell something like: >> >> joinPath' root name = >> joinPath [root, name] >> importing System.FilePath (joinPath) > > I guess you're talking about first-class modules, the ones that > might be passed to functions as arguments, returned from functions > as a result, loaded at run-time, etc. No, I was not going that far :). I simply was proposing a method to keep imported definitions local to a function. By the way, here is a strange (for me) problem I hit, and made me think about this extension. Suppose a file foo.hs defines: import Control.Monad import System.Posix.Files as PF isDirectory :: FilePath -> IO Bool isDirectory path = PF.isDirectory `liftM` PF.getFileStatus path If I try to load the file from ghci I get: Prelude> :l foo.hs [1 of 1] Compiling Main ( foo.hs, interpreted ) Ok, modules loaded: Main. *Main> isDirectory "/var" <interactive>:1:0: Ambiguous occurrence `isDirectory' It could refer to either `Main.isDirectory', defined at foo.hs:6:0 or `PF.isDirectory', imported from System.Posix.Files at foo.hs:2:0-30 > [...] Manlio Perillo

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Names in Haskell (Was: [Haskell-cafe] Comments from OCaml Hacker Brian Hurt)
by roconnor＠theorem.ca 16 Jan '09

16 Jan '09

What I don't understand is why Monoid and Monad are objectionable, while Hash, Vector, Boolean, and Integer are (presumably) not objectionable. They all appear equally technical to me. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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3 applications of "indexed composition" as a language design principle
by Greg Meredith 16 Jan '09

16 Jan '09

Haskellians, i've found a way to generalize the LogicT transformer and calculated it's application to three fairly interesting examples. The approach -- with some sample codes as scala+lift web apps -- is described here<http://biosimilarity.blogspot.com/2009/01/3-applications-of-indexed-composi…>. Comments welcome. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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Type errors, would extensions help?
by Mauricio 16 Jan '09

16 Jan '09

Hi, I have this problem trying to define a function inside a do expression. I tried this small code to help me check. This works well: --- import Data.Ratio ; main = do { printNumber <- let { print :: (Num n,Show n) => n -> IO () ; print n = do { putStrLn $ show n} } in return print ; print (1%5) ; print 5.0 } --- However, just removing 'Num n' gives: Ambiguous type variable `n' in the constraint: `Show n' arising from a use of `print' at teste.hs:7:16-20 Why is it ambiguous? Since I don't use numeric functions, 'Num n,Show n' doesn't seem more specific. Besides that, the real problem I have is a similar function declared inside a 'let' as col :: (WidgetClass w) => w->IO Int col wid = do {... that, after beeing used with w beeing Table (and instance WidgetClass Table do exist) refuses beeing used with DrawingArea (also with instance WidgetClass DrawingArea), but I wasn't able to simulate that in a small program, and don't know if it's related to the case I'm showing. Thanks for your kindness, Maurício

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