- Haskell-Cafe - Haskell.org

I'm still having challenges to get a Haskell GUI to work under Windows 7
by caseyh＠istar.ca 08 Oct '10

08 Oct '10

I'm still having challenges to get a Haskell GUI to work under Windows 7; even after various instructions on the web. e.g. Haskell Platform 2010.2.0.0, wxWidgets-2.9.1, wxHaskell 0.12.1.6 Similar challenges with GTK+ and gtk2hs.

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Lambda-case / lambda-if
by Max Bolingbroke 08 Oct '10

08 Oct '10

Hi Cafe, I implemented the proposed Haskell' feature lambda-case/lambda-if [1] during the Haskell Implementors Workshop yesterday for a bit of fun. The patches are online [2, 3]. The feature is demonstrated in this GHCi session: $ inplace/bin/ghc-stage2 --interactive -XLambdaCase GHCi, version 7.1.20101002: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> (if then "Haskell" else "Cafe") False "Cafe" Prelude> (case of 1 -> "One"; _ -> "Not-one") 1 "One" Prelude> :q Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Max [1] http://hackage.haskell.org/trac/haskell-prime/ticket/41 [2] http://www.omega-prime.co.uk/files/LambdaCase-Testsuite.patch [3] http://www.omega-prime.co.uk/files/LambdaCase-Compiler.patch

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Pronouncing "Curry" and "currying"
by Petr Pudlak 08 Oct '10

08 Oct '10

Hi all, I have a question for native English speakers: What is the correct pronunciation of the name "Curry" (in "Haskell Curry") and the derived verb "currying"? I found on Wikitonary the name is (probably) of Irish orgin, so I suppose that the pronunciation may by nonstandard. Probably the best way to answer would be either a link to some voice sample or written using some standard phonetic alphabet. Thanks a lot, Petr

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Re: [Haskell-cafe] A model theory question
by Alexander Solla 08 Oct '10

08 Oct '10

On Sep 30, 2010, at 1:39 AM, Patrick Browne wrote: > I think my original question can be rephrased as: > Can type classes preserve satisfaction under the the expansion > sentences > (signature/theory morphisms inducing a model morphism). > > According to [1] expansion requires further measures (programming?) > which you demonstrated. But this raises are further question. Does > Haskell’s multiple inheritance represent a model expansion where the > classes in the context of an instance are combined in the new bigger > model? In principle, the answer is yes (I think). But I kept running into walls when I tried. It is almost as if there is "too big" a gap to be filled between compile-time and run-time. At least for the approaches I tried. I have two suggestions though. First, monadism is a great way to approach this problem from a run-time level. Indeed, a monad is an interpreter, which comes with the associated notions of a free algebra/ model and the like. Injecting a new axiom amounts to creating a new monadic action. Monad transformers can do lifting and lowering in a fairly straight forward way. Another suggestion is to check out the OOHaskell paper. I know they use type level forcing to get stuff done, but I guess they used a different cluster of extensions than I tried. http://homepages.cwi.nl/~ralf/OOHaskell/ Sorry for the delay in responding. -Alex

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Is there a more recent paper or article than "Can GUI Programming Be Liberated From The IO Monad
by caseyh＠istar.ca 08 Oct '10

08 Oct '10

Is there a more recent paper or article than "Can GUI Programming Be Liberated From The IO Monad"? :)

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Notes from "Haskell takes over the world" BoF at ICFP
by Don Stewart 08 Oct '10

08 Oct '10

Here are the notes transcribed from the Future of Haskell BoF held after the Haskell Symposium last week. -- Don = Future of Haskell BoF Notes = A "birds of a feather" meeting was held at ICFP, organized by Bryan and Johan. We had 30 (?) people in a room, for 2 hours, discussing how to ensure Haskell succeeds. These are the transcribed notes. == Libraries == * Need to encourage special interest groups (SIGs) to form around particular domain areas in the libraries. AI, BioInf, Networking, etc. * Document the SIG process: what resources they have, how you get results. * Missing SIGs for certain platforms: Mac, CentOS etc. * More tutorials on use of Haskell in particular domains. * Keen for Hackage 2.0 quality review tools: embedded wikis, voting * Hackage analysis tools: reverse indexing of the code base. * There is no PVP tool. * Library fragmentation occuring due to type duplication (String, Text, ...) * Focus on intitiatives for common, core types. HP polish. * What is the "Son of Containers" going to be? * Build reports would help the community a lot. Integrated with Hackage. Haddock pages on hackage should be editable wikis (per-top-level function, dvcs backed? record history, captchas. roll back spam, by default assume good contributions) * Need to register patches to packages with lost maintainers. * Belgium Hackathon coming up. * Improve libraries process: esp. for small contributions. * Hackage maintainance: + maintainer timeout. + hackage package clobbering. + tweaks. hackage-local .cabal fixes. * Unclear what the rules for contributing are. Document them! * In particular, document how to contribute to: * ghc * cabal * core and HP * hackage * dockathon == IDEs == * A new ghci. + scriptable, scion-server? + repls on top of ghc-api + ruby repl + .ghci + documentation + hpaste/ghci. * IDE + scion. contributions. market places. + documentation. + EclipseFP, scion? + NetBeans. * Special Interest Groups + as the vehicle for domain projects. + formal process for forming a group. + a small amount of structure for groups + document process for forming a strike team. * haskell.org + polishing the wiki. + wikibooks. + call for sig. * haskell visual design group? + consistent color themes across haskell.org sites. + consistent haskell branding. * community server mailing lists in poor state * Move community more online ? Community services. + google groups + digests. + reply at the right point in the threads. + serious lists. + ghc users. libraries. + announces. + stackoverflow for new questions? + keep refering to SO. + reddit for news? == GHC == * ghc status + 50% split in room on moving ghc from darcs to git. + bug reports and interaction load with ghc + well documented workflow for lightweight changes + heavy weight process for major work. + bugs, tickets. + Simon Marlow "contributions are going up, and process is working well" * Release schedule. + RC at ICFP. + GHC release page. + HP/GHC release. + stage releases. + more clearly announced that ghc will be a beta prior to GHC HP. * HP, uses GHC installers as is. GHC zip. == Teaching / tutorials == * Tutorials. Documentation. + Writing a blog post that is wrong is a pretty good way to get feedback. + Find a forum to ask the question, so the answer is findable. + Ask on SO. + Clojure and Scala communities have active blog spheres. + they love descriptions of data structures + e.g. high fanout trees. + Aggregating the ephemera. + Archives: Haskell Reddit has all the interesting blog posts. -- Haskell Wikibook. * Cabal and integration with other languages. + Easier integration with C. + Higher barrier to entry on the Mac. + Playing well with other languages?

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HTML library with DOM?
by Günther Schmidt 08 Oct '10

08 Oct '10

Hi all, is there an HTML parsing library that creates a DOM from a page? Günther

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CFP MSCS Issue: Dependently Typed Programming
by Conor McBride 07 Oct '10

07 Oct '10

(s:S)->(p:P s)*(s:S)->(p:P s)*(s:S)->(p:P s)*(s:S)->(p:P s)*(s:S)- >(p:P s)* OPEN CALL FOR PAPERS for a Special Issue of MATHEMATICAL STRUCTURES in COMPUTER SCIENCE in association with the workshop DEPENDENTLY TYPED PROGRAMMING 2010 editors: Thorsten Altenkirch (Nottingham), Conor McBride (Strathclyde) 2011 timeline: submission Jan 31; notification May 31; final version June 30 (s:S)->(p:P s)*(s:S)->(p:P s)*(s:S)->(p:P s)*(s:S)->(p:P s)*(s:S)- >(p:P s)* Thorsten Altenkirch and Conor McBride are delighted to invite contributions to a Special Issue of the journal Mathematical Structures in Computer Science (Cambridge University Press), in association with the Workshop on Dependently Typed Programming, which we organised on 9 and 10 July 2010 in Edinburgh, as part of FLoC, associated with LiCS. The workshop had a packed programme of exciting developments, reflecting the strength of work on this topic at this time. More recent workshops and conferences have exhibited a significant contribution from researchers in this area. We are grateful to Editor-in-Chief Giuseppe Longo and to Editor Eugenio Moggi for the opportunity to reflect these welcome developments in the pages of MSCS, and we encourage researchers to consider submitting a paper. submission deadline: January 31 2011 notification: May 31 2011 final versions due: June 30 2011 We invite full journal articles concerning Dependently Typed Programming or related topics, from authors at work in this area. Submissions are particularly welcome from but not limited to contributors to the workshop, and the same journal-standard peer review process will apply in any case. Please feel free to address any enquiries about scope and suitability to the guest editors, Thorsten Altenkirch (University of Nottingham) and Conor McBride (University of Strathclyde). Submissions should usually not exceed 35 pages. Authors should adhere to the guidelines issued by Cambridge University Press for MSCS contributors: http://assets.cambridge.org/MSC/MSC_ifc.pdf. These include directions to the relevant LaTeX resources. We very much look forward to hearing from you. Thorsten and Conor

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EDSL for Makefile
by C K Kashyap 07 Oct '10

07 Oct '10

Hi All, I was thinking about doing an EDSL for Makefile (as an exercise) I put down my line of thought here - http://hpaste.org/40233/haskell_makefile_edsl I'd appreciate some feedback on the approach. Also, I wanted some idea on how(in the current approach) I could make the target name and the dependency available to the action writer - as shown below. r1 = Rule { target = "file1", dependsOn = ["file2"], action = do execute ("gcc -c " ++ dependencyList ++ " -o " ++ target) } -- Regards, Kashyap

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Lazy evaluation from "Why Functional programming matters"
by C K Kashyap 07 Oct '10

07 Oct '10

Hi All, I was going through the paper's "lazy evaluation" section where the square root example is given. It occurred to me that one could implement it in a modular way with just higher order functions (without the need for lazy evaluation that is). function f (within, eps, next, a0){ while(true){ a1=next(a0); if(within(a0,a1,eps)return a0; a0=a1; } } Is this not the case? -- Regards, Kashyap

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Packaging a Gtk2hs based program for Windows
by Dmitry V'yal 07 Oct '10

07 Oct '10

Hello, haskellers. Recently I stumbled upon a problem. It may sound quite off-topic for this list, but, I'm sure, almost every haskell programmer runs into it sooner or later. It sounds: How to make a neat Windows installer for a nice Gtk2hs program I wrote last week? How to solve the problem of dependency on GTK? Should I ask my users to install a GTK package or it would be better to package all the dynamic libraries needed along with my program? Are there any troubles with different character encodings used in Windows? How to change the application icon? And so on.. I understand, there is no ultimate answer, but I hope there is some wisdom to share. I've been a Linux user for more then five years and now I've found myself completely illiterate on how to make my code easily accessible for a nontechnical Windows user. I would appreciate any thoughts and advises Best wishes, Dmitry

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Haskellers.com profiles: advice requested
by Michael Snoyman 07 Oct '10

07 Oct '10

Hi all, After finally getting OpenID 2 support worked out, I've now put up the Haskellers.com website[1]. Not all features are implemented yet, but the basics are in. One of the most important features is going to be the user profiles, and I wanted some community input on the kind of stuff they'd like to see. For now, I collect email address (spam-protected, don't worry), website, number of years of Haskell experience, and a free-form description. I've also added a skills section, but have purposely not added many. I wanted community input on the kinds of skills. Some ideas: * Web programming * Compiler writing * Write a monad tutorial :/ I see two main questions when it comes to selecting this list of skills: * How granular should we get? For web programming, for instance, should we ask about Yesod, Happstack, Snap, etc? * Should we include non-Haskell skills. I can imagine that employers would like to know that people also have experience with Java/C#/etc, but on the other hand we might want to make this Haskell-specific. One recommendation (from my wife actually) is to have two separate lists: a static list of Haskell-specific skills that users simply checkmark off, and then "additional skills" that everyone makes up themselves. Other things to consider for the profile: Twitter/Facebook/GChat/AIM accounts, Hackage username, list of packages authored. I'm also planning on adding a "real Haskeller" feature, which would mean a site admin (they don't exist yet) has verified that you really are a contributing member of the community. I would imagine the threshold for this status would be very low, eg you've written a package or sent email to one of the mailing lists. One last point: for those of you wondering why your profiles aren't showing up on the homepage: you need to verify your email address first. I just added this feature this morning which is why your profiles are not visible. This is simply an anti-spam measure. If it turns out to be insufficient, I might need to add a recaptcha, and if that is still not enough we can make things moderated. But I doubt spam will end up being a real problem. Cheers, Michael [1] http://www.haskellers.com/

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darcs vs. git
by Conrad Parker 07 Oct '10

07 Oct '10

(subject changed for easy filtering of flamebait, removed libraries@) On 7 October 2010 10:45, Jason Dagit <dagit(a)codersbase.com> wrote: > At the risk of starting a darcs vs. git discussion I have some > thoughts about the tension. > > On Wed, Oct 6, 2010 at 12:10 PM, Don Stewart <dons(a)galois.com> wrote: > [snip] >> == GHC == >> >> * ghc status >> + 50% split in room on moving ghc from darcs to git. > > I don't see that tension resolving itself easily. VCS tools are > subject to "network effects". If your friends are using VCS Foo and > you want to work with them, then you adopt Foo too. It's also a part > of the toolchain that people tend to have strong opinions about. > > One possible way to lessen the tension would be good vcs bridge tools. > There have been numerous repo converter projects, some even > supporting synchronization. There is already a git-svn tool. Maybe > there should be a git-darcs (and a darcs-git)? If a git hacker out > there wants to add darcs support to git, I'd certainly be willing to > help them get started. > > Pushing on this a bit more, I'm fairly convinced that darcs and git > are dual to each other in terms of underlying models. As such, I have > some ideas on how to unify them/convert between models. > Unfortunately, actually having something to use is very far off as the > ideas themselves are still immature. > > As far as I can tell, the main reasons to vote for git: > * Some people simply love git and want to use it for every project > * Git is faster and/or more memory efficient for some operations (most? all?) > * Github > > As far as I can tell, the main reasons to vote for darcs: > * GHC already uses it (inertia) > * The windows support appears to be more mature (I admit, this is > somewhat subjective as neither has a spotless record here) > * Key players, such as the Simons, prefer the darcs UI over the git > UI (i.e., some people prefer darcs) > * Darcs 2.x has consistently improved in robustness and efficiency > over the last several years, continues to improve, and incorporates > ideas from git. (there is currently an experimental 'rebase' command > in the development branch of darcs) > * Cherry picking > > Note: I didn't mention feature branches as a reason to prefer one over > the other. Both darcs and git support this. Git uses in-repo > branches and with darcs you can simply do a local lazy get. Some > people prefer one mechanism over the other, but the point is they both > support the workflow. I hope they do :) With darcs, is it possible to refer to various other (local and remote) branch names, eg. to do diffs between branches? What I mean is that with git, I can do: $ git fetch origin $ git diff my-foo origin/random-bar to see changes between my local branch "my-branch-foo" and the remote branch "random-bar" without having to make an explicit checkout of origin/random-bar. When the origin repo contains many branches that other people are working on, this is a pretty neat way of keeping track of those. I'd really like to be able to get an overview of what branches are available (on my local copy and remotely), like with git's various graphing tools; I just use a "git log --graph" alias to get a tree view in the console: http://blog.kfish.org/2010/04/git-lola.html Does something similar exist for darcs? > I would like to see the tension of darcs vs git for GHC reduced. I > think it ultimately amounts to: Contributors need to be able to use > the one they prefer, instead of being forced to use the one GHC devs > use. I agree with that, and I don't mind switching between the two for different projects. Perhaps some standard subcommand aliases would go a long way to making that easier though (log vs. changes etc.) cheers, Conrad.

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Re: [Haskell-cafe] Re: Lambda-case / lambda-if
by Evan Laforge 07 Oct '10

07 Oct '10

> I would also very much like to have multi-argument pattern matching, but in > > \case a b -> ... > ... > > it sure suggests to me that `a` should be applied to `b` before casing. I feel like sugar is designed to make a couple of specific uses nicer. Being as general and orthogonal as possible is the job of the primitives. So I don't mind too much if sugar is a little ad-hoc. Single argument \case addresses the monadic case problem. Does someone have some examples of nice expressions that you need a multi argument case-lambda for? I don't mind writing 'case (a, b) of ...' very much. Seems like you could get the same effect with 'curry': (\case { a b -> ... ; a b -> ...}) x y can be written (curry $ \case { (a, b) -> ... }) x y Not as pretty, but still point-free.

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pointers for EDSL design
by John Lato 06 Oct '10

06 Oct '10

Hello, I'm working on a prototype edsl (my first one), and I was wondering if there are any resources that discuss pros and cons of various implementation issues? I'm trying to decide what should be included in the edsl vs. re-using the meta language implementations (e.g. let-binding, lambdas). Most of the examples I've found are for full DSL's, not EDSL's, so it's not clear what the best approach is. The LLVM interface is sort of close to what I intend, except it creates a very imperative style whereas I'm aiming for something more functional. A little background: I decided on a dsl because I intend to make heavy use of Haskell functions from Data.List and Control.Monad. If I made a full DSL I would need to re-implement much of that functionality, so I thought it would be more sensible to use an edsl. Any advice or references would be very much appreciated. Best, John

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Re: [Haskell-cafe] pointers for EDSL design
by John Lato 06 Oct '10

06 Oct '10

Hi Stephen, > From: Stephen Tetley <stephen.tetley(a)gmail.com> > > Hi John > > For the user level stuff, I don't think CSound really has "functions" > - either for the score or orchestra. The score I think is just a list > of /notes/ with many, many parameters and the orchestra is a graph > description saying how the UGens are connected. This is good news - I believe Pan, Feldspar, Lava etc. generate > functions or procedures in the output code which means they have to > involve the complicated techniques for embedding lambdas and functions > in the EDSL. If they didn't, there would be massive code blow up. > However because CSound is more or less "straight line" code - i.e. > lines are interpreted sequentially, there are no procedures or > functions to define and call - generating it should be much simpler. > Yes, exactly. I'm not interested in anything nearly as sophisticated, so those aren't great examples for me. > Andy Gill's Dot package on Hackage has a crafty, but simple technique > to allow you to reference graph nodes and link them within a monad and > output as "foreign" code - here dot files. Something similar might be > satisfactory for orchestra files. > The orchestra graph is basically the issue I'm looking at (ignoring the score for now). My first implementation used an Orch monad very similar to the one used in Andy Gill's dotgen. It worked and the implementation was very straightforward, however I wanted to see if it was possible to create a non-monadic interface. That is, change my classes from class GenM repr a where sigGenM :: a -> repr (ASig repr) to class Gen repr a where sigGen :: repr a -> repr (ASig repr) This is in tagless-final style (which really is slick BTW); that's why everything is represented through type classes. The second version is really the one I want to use, although it was more work to implement. For the Csound interpreter, I needed a naming mechanism like TH's Q monad, along with some other machinery. So here's a very simple expression: t1 = let v = sigGen (cnst 1) in outs v v which is what led to my question. I'm binding the sigGen to 'v' to introduce sharing at the meta-level. Would it be better to introduce support for this in the dsl? Anyway, here are a few simple test expressions to provide the flavor of what I'm doing: -- additive synthesis, 20 partials of constant amplitude t6 = let so = sum . zipWith (oscil (cnst 1000)) [ cnst (110*f) | f <- [4..]] (replicate 20 1) in outs so so -- stacked frequency modulation using 4 oscillators t8 = let stack = foldr ($) (csig 40) (replicate 4 \fq -> oscil (cnst 1000) fq 1) in outs stack stack The edsl provides the functions "oscil", "cnst", "csig", and "outs", but most of the magic happens in the csound interpreter. Cheers, John

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Haskell Weekly News: Issue 153 - October 06, 2010
by Daniel Santa Cruz 06 Oct '10

06 Oct '10

Welcome to issue 153 of the HWN, a newsletter covering developments in the [1]Haskell community. It sure has been a while since we last saw one of these. Hopefully the dry spell for the newsletter is now a thing of the past. I sure hope you enjoy it, and give plenty of feedback. As you can see, there are some changes to the contents of the letter. The first, and major change, is the use of Reddit and StackOverflow as aggregators to interesting links around the net. The number of links on each sections is rather arbitrary, I'd be interested in hearing your thoughts about them. This current issue is missing an "Announcements" section, as I did not have any to put there this time around. Hopefully that will be solved as soon as you start sending them in. I decided that rather than keep on waiting to have all the right pieces in perfectly working order, the priority was to get the newsletter back in circulation as soon as possible. Tweaking can always be done along the way. Some of the sections that I would like to add are: a Hackage report, interesting conversations from the different mailing lists, general community announcements, job opportunities, and the occasional interview with some of Haskell's celebrities. A major event last week were the meetings in Baltimore. Don Stewart collected a list of some of [2]the papers that were presented at the Haskell Symposium. You can find [3]videos from the Haskell Implementors' Workshop, posted by Simon Marlow. From the looks of things, seems like we sure need more than a 767 to fit all Haskellers! Ok. So, what was hot last week? Top Reddit Stories * ANNOUNCE: GHC 7.0.1 Release Candidate 1 -- From (haskell.org) scored 79 with 39 comments. Read on [4]reddit or the [5]original post. * GHC Blog: Let generalisation in GHC 7.0 -- From (hackage.haskell.org) scored 64 with 3 comments. Read on [6]reddit or the [7]original post. * Simon Peyton Jones : GHC 7 Status Update : Video -- From (vimeo.com) scored 51 with 15 comments. Read on [8]reddit or the [9]original post. * Haskell at Google: for the hard problems in the server management team -- From (k1024.org) scored 47 with 42 comments. Read on [10]reddit or the [11]original post. * Enumerators Tutorial Part 1: Iteratee -- From (docs.yesodweb.com) scored 30 with 2 comments. Read on [12]reddit or the [13]original post. * Introducing the programming language eff, monad transformer stacks for everyone -- From (math.andrej.com) scored 27 with 0 comments. Read on [14]reddit or the [15]original post. * Help me find something interesting... -- From (self.haskell), scored 22 with 10 comments. Read on [16]reddit or the [17]original post. * Papers from the Haskell Symposium 2010 -- From (self.haskell), scored 22 with 0 comments. Read on [18]reddit or the [19]original post. * Nikola: array computations embedded in Haskell that compiles to GPUs via CUDA -- From (eecs.harvard.edu) scored 20 with 4 comments. Read on [20]reddit or the [21]original post. * ICFP 2010 - Tribute to Robin Milner (video) -- From (vimeo.com) scored 17 with 2 comments. Read on [22]reddit or the [23]original post. Top StackOverflow Questions * haskell regex substitution (votes: 8, answers: 1) [24]read * Why am I getting "Non-exhaustive patterns in function" when I invoke my Haskell substring function? (votes: 6, answers: 1) [25]read * Why am I getting this warning from GHCi? (votes: 6, answers: 2) [26]read * In Haskell, why non-exhaustive patterns are not compile-time errors? (votes: 5, answers: 2) [27]read * Represent sequence of tetrahedral numbers in Haskell (votes: 5, answers: 2) [28]read Quotes of the Week * anonymous: Why is haskell-platform 138MB? Does it include monad of the entire world? * conal: everything is matter, and using matter you can implement cucumbers, therefore everything is a cucumber. About the Haskell Weekly News New editions are posted to [29]the Haskell mailing list as well as to [30]the Haskell Sequence and [31]Planet Haskell. [32]RSS is also available, and headlines appear on [33]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz * at * gmail * dot * com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack design to get things started again. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://www.reddit.com/r/haskell/comments/dkzn4/papers_from_the_haskell_symp… 3. http://vimeo.com/user2191865/videos/sort:date 4. http://www.reddit.com/r/haskell/comments/dj5wu/announce_ghc_701_release_can… 5. http://www.haskell.org/pipermail/glasgow-haskell-users/2010-September/01924… 6. http://www.reddit.com/r/haskell/comments/dkxwp/ghc_blog_let_generalisation_… 7. http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7 8. http://www.reddit.com/r/haskell/comments/dlsks/simon_peyton_jones_ghc_7_sta… 9. http://www.vimeo.com/15467880 10. http://www.reddit.com/r/haskell/comments/dkpdj/haskell_at_google_for_the_ha… 11. http://k1024.org/~iusty/papers/icfp10-haskell-reagent.pdf 12. http://www.reddit.com/r/haskell/comments/dldlh/enumerators_tutorial_part_1_… 13. http://docs.yesodweb.com/blog/enumerators-tutorial-part-1/ 14. http://www.reddit.com/r/haskell/comments/djx2a/introducing_the_programming_… 15. http://math.andrej.com/2010/09/27/programming-with-effects-ii-introducing-e… 16. http://www.reddit.com/r/haskell/comments/dka2o/help_me_find_something_inter… 17. file://localhost/r/haskell/comments/dka2o/help_me_find_something_interesting/ 18. http://www.reddit.com/r/haskell/comments/dkzn4/papers_from_the_haskell_symp… 19. file://localhost/r/haskell/comments/dkzn4/papers_from_the_haskell_symposium_2010/ 20. http://www.reddit.com/r/haskell/comments/dl3iv/nikola_array_computations_em… 21. http://www.eecs.harvard.edu/~mainland/projects/nikola/ 22. http://www.reddit.com/r/haskell/comments/djnn6/icfp_2010_tribute_to_robin_m… 23. http://www.vimeo.com/15325077 24. http://stackoverflow.com/questions/3847475/haskell-regex-substitution 25. http://stackoverflow.com/questions/3799359/why-am-i-getting-non-exhaustive-… 26. http://stackoverflow.com/questions/3833351/why-am-i-getting-this-warning-fr… 27. http://stackoverflow.com/questions/3804484/in-haskell-why-non-exhaustive-pa… 28. http://stackoverflow.com/questions/3818558/represent-sequence-of-tetrahedra… 29. http://www.haskell.org/mailman/listinfo/haskell 30. http://sequence.complete.org/ 31. http://planet.haskell.org/ 32. http://sequence.complete.org/node/feed 33. http://haskell.org/

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Re: [Haskell-cafe] Re: Haskell web development entries on the Wiki
by Christopher Done 06 Oct '10

06 Oct '10

On 6 October 2010 12:47, Henning Thielemann <thunderbird(a)henning-thielemann.de> wrote: > I for instance use http-shed and mohws all the time. They do what they shall > do for me. I maintain mohws Please move the ones you use and maintain to the active list!

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Big Arrays
by Henry Laxen 06 Oct '10

06 Oct '10

Dear Group, I am trying to create a (relatively) big array, but it seems I cannot make anything larger than 2^30 or so. Here is the code: import Data.Word import Data.Array.Unboxed import Control.Monad.ST import Data.Array.ST import Control.Exception import Prelude hiding (catch) t1 :: Word64 -> UArray Word64 Bool t1 size = runSTUArray (do a <- newArray (0,size) True :: ST s (STUArray s Word64 Bool) writeArray a 0 False return a) catchArrayException x = do let err = show (x :: SomeException) putStrLn $ "Exception [" ++ err ++ "]" return () main = do let a1 = t1 (2^30) a2 = t1 (2^31) a3 = t1 (2^32) catch (print $ (a1!0,a1!1)) catchArrayException catch (print $ (a2!0,a2!1)) catchArrayException catch (print $ (a3!0,a3!1)) catchArrayException This results in: *Main GOA> main (False,True) (Exception [Negative range size] (False,Exception [Error in array index; 1 not in range [0..1)] It looks like array ranges can only be Ints, and not Int64 or Word64 types. Any pointers would be appreciated. Best wishes, Henry Laxen

7 11

hClose: invalid argument (Invalid or incomplete multibyte or wide character)
by Johannes Waldmann 06 Oct '10

06 Oct '10

I have this program main = writeFile "check.out" "ü" that's u-umlaut, and the source file is utf-8-encoded and ghc-6.12.3 compiles it without problems but when running, I get hClose: invalid argument (Invalid or incomplete multibyte or wide character) (debian 5.0.6, kernel 2.6.18-6-686. I know that's somewhat dated. Would upgrading help?) Regards - J.W.

3 5

Re: Re[2]: [Haskell-cafe] Lambda-case / lambda-if
by Luke Palmer 06 Oct '10

06 Oct '10

On Mon, Oct 4, 2010 at 9:04 PM, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > With respect to "datatype destructing" functions, the Prelude has: > > maybe :: b -> (a -> b) -> Maybe a -> b > either :: (a -> c) -> (b -> c) -> Either a b -> c > > which suggests the following signatures for the analogues for Bool and list > types: > > bool :: a -> a -> Bool -> a > list :: b -> (a -> [a] -> b) -> [a] -> b This suggestion is not so clear to me. Maybe and Either are both non-recursive, so the Church and Scott encodings coincide. You've written the Scott encoding of list. The Church encoding should look familiar: list :: b -> (a -> b -> b) -> [a] -> b Intuitively, a Scott encoding peels off one layer of datatype, whereas a Church encoding flattens down a whole recursive structure. Church encodings are more powerful -- you can do more without requiring a fixed point operator. Just to be clear, I am not arguing anything other than "maybe" and "either" don't readily generalize to "list" because of list's recursiveness. Luke

3 2

Haskell Platform: failed to parse output of 'ghc-pkg dump'
by Tom Hawkins 06 Oct '10

06 Oct '10

I'm having trouble installing Haskell Platform on Windows. After the install, I run "cabal update", which appears to work: 00-index.tar.gz is deposited in C:/Documents and Settings/<user>/Application Data/cabal/packages/hackage.haskell.org. However, when I try to "cabal install" anything, I get: cabal.exe: failed to parse output of 'ghc-pkg dump' "ghc-pkg dump --global" appears to run just fine. Ideas? Suggestions? -Tom

1 1

ByteString missing rewrite RULES (zipWith' f a = pack . zipWith f a)
by Thomas DuBuisson 05 Oct '10

05 Oct '10

All, (I notice ByteString still isn't under l(a)h.o ownership, which is good because this way I can avoid the bureaucracy and e-mail the maintainers directly) The following is a Data.ByteString comment for the (non-exported) function zipWith' -- -- | (...) Rewrite rules -- are used to automatically covert zipWith into zipWith' when a pack is -- performed on the result of zipWith. -- This implies there should be a rule: {-# RULES "ByteString specialise zipWith'" forall (f :: Word8 -> Word8 -> Word8) p q . zipWith' f p q = pack (zipWith f p q) #-} But no such rule exists in the ByteString source (the inverse rule using 'unpack' does exist). 1) Is this an omission? Can we fix it? It's a rather important rule for crypto-api. 2) Can we export zipWith' so people can be explicit? If not, can we get the comment about the rule placed somewhere so it will make its way to the generated Haddock documentation for general users? 3) Very different issue: Could .Lazy export hGetN or have defaultChunkSize configurable by a CPP/compile time macro? If not, perhaps we could make "chunkOverhead = max 16 (2 * sizeOf (undefined ::Int))" so it will be the same on 64 and 32 bit systems (a 128 bit boundary, nice and fast for most modern cipher algorithms, sadly asking for it to match hash block sizes is a bit much). Cheers, Thomas

2 3

darcs hacking sprint 15-17 Oct, Orleans
by Eric Kow 05 Oct '10

05 Oct '10

Hi everybody, Just a quick reminder that the fifth Darcs hacking sprint is taking place in just ten days (Orleans, France, 15-17 Oct): http://wiki.darcs.net/Sprints/2010-10 It's still possible to attend, but you should contact Florent Becker (CC'ed) as soon as possible to confirm there are still spaces. Hope to see you there! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

1 0

Suggestions for improvement
by N. Raghavendra 05 Oct '10

05 Oct '10

I am reading the book `The Haskell Road to Math, Logic, ...". One of the exercises in the first chapter asks for a function that maps a string "abcd" to "abbcccdddd" and "bang!" to "baannngggg!!!!!". Since such a function f fixes the empty word, and maps wa to f(w)a^(length(w)+1) for any word w and any letter a, I came up with the following solution: -- Map "abcd" to "abbcccdddd" and "bang!" to "baannngggg!!!!!". blowup :: String -> String blowup [] = [] blowup x = blowup (allButLast x) ++ lastToTheLength x -- Map "abcd" to "abc". allButLast :: String -> String allButLast [] = [] allButLast [x] = [] allButLast (x : xs) = x : allButLast xs -- Map "abcd" to d^4 = "dddd". lastToTheLength :: String -> String lastToTheLength [] = [] lastToTheLength [x] = [x] lastToTheLength (_ : xs) = lastToTheLength xs ++ [last xs] One question I have is whether I can eliminate points in the above definition of blowup, and write something like blowup = (++) . (blowup . allButLast, lastToTheLength) thinking of (++) as a function String x String -> String. Also, I can't figure out whether it is possible to get a shorter solution using fold. I have tried Hlint on my file, but it gave no suggestions. I am sure there are better ways, and would like some pointers and any general suggestions for improvement. Thanks and regards, Raghavendra. -- N. Raghavendra <raghu(a)mri.ernet.in> | http://www.retrotexts.net/ Harish-Chandra Research Institute | http://www.mri.ernet.in/ See message headers for contact and OpenPGP information.

6 14

Network.HTTP, BasicAuth+Headers
by Tony Morris 05 Oct '10

05 Oct '10

I'm trying to send a simple request using Network.HTTP and getting a bit lost in all the possibilities. I've experimented with both Network.HTTP and Network.Browser but have been unable to come up with something specific to my needs, which are quite simple. I'd like to send a request equivalent to the following curl: curl --basic -u "user:pass" -H "Accept: application/xml" -H "Content-type: application/xml" "https://host/path" It seems with Network.Browser I cannot send headers, but with Network.HTTP I cannot see how to send the BasicAuth information. Any tips are appreciated. -- Tony Morris http://tmorris.net/

1 0

CCC MyType
by Lafras Uys 05 Oct '10

05 Oct '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 List, I'm trying to wrap my head around Haskell and Category Theory---very new to both and in keeping with character, I've jumped into the deep-end. The categories and category-extras packages have helped to illustrate some concepts. However, I remain confused and hope someone can point me in the right direction. The ultimate goal is to construct my own instance of a CCC. Here is the class definition for CCC from the categories package: > class ( Cartesian (<=) > , Symmetric (<=) (Product (<=)) > , Monoidal (<=) (Product (<=)) > ) => CCC (<=) where > type Exp (<=) :: * -> * -> * > apply :: (Product (<=) (Exp (<=) a b) a) <= b > curry :: ((Product (<=) a b) <= c) -> a <= Exp (<=) b c > uncurry :: (a <= (Exp (<=) b c)) -> (Product (<=>) a b <= c) I have the following: > {-# LANGUAGE TypeFamilies, > MultiParamTypeClasses, > UndecidableInstances #-} > import Control.Category.Braided > import Control.Category.Associative > import Control.Category.Cartesian > import Control.Category.Monoidal > import Control.Category.Cartesian.Closed > data MyType a b = MyType {f::(a -> b)} > data SomeType a b = SomeType {g::(a,b)} > instance (Symmetric MyType (Product MyType), > Monoidal MyType (Product MyType), > PreCartesian MyType) => CCC MyType where > type Exp MyType = SomeType It type checks in ghci and gives the appropriate warnings that apply, curry and uncurry have not been defined---I can't see how to define these. Is it actually possible with the two data types I have? I would really appreciate some help. Lafras -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkyaGWYACgkQKUpCd+bV+kqgWgCfY7B7pUttB0xfeOAN1V3NDqRL fgQAniK/EJsV9jS7XWxmxElCVD6AW0as =l0x/ -----END PGP SIGNATURE-----

2 5

Haskell Platform, Hackage and Cabal : The 2nd Year : Status Report
by Don Stewart 04 Oct '10

04 Oct '10

The Haskell Implementors Workshop was held in Baltimore, today. Duncan Coutts from Well-Typed and I presented a status report on the Haskell distribution infrastructure: Hackage , Haskell Platform and Cabal. The slides are here: http://donsbot.wordpress.com/2010/10/01/hackage-cabal-and-the-haskell-platf…

9 10

Template Haskell: hiding declarations
by Jonas Almström Duregård 04 Oct '10

04 Oct '10

Hi Café, I'm doing some code generation with Template Haskell that results in few hundred top level declaration, of which only 10 or so should actually be exposed to the user (the rest are only used by generated code). Since I cant splice stuff into the module header (i.e. into the export list), I cant think of a good way to hide the internal declarations. One way would be to put all the declarations into one or several where-clauses, but this doesn't work very well when several functions share the hidden declarations. Also I might want to hide a few data types. Does anyone have a clever solution? Has anyone had a similar problem (maybe TH needs to be extended with support for hidden declarations)? /J

3 2

Problem with monad transformer stack
by Michael Vanier 04 Oct '10

04 Oct '10

I'm having a problem with a simple monad transformer stack that has me stumped. Here's the sample code: {-# LANGUAGE GeneralizedNewtypeDeriving #-} import Control.Monad.Error import Control.Monad.State import Data.Typeable data SomeError = Error1 | Error2 | ErrorFail deriving (Eq, Show, Typeable) data MyData a = MyData [a] instance Error SomeError where noMsg = ErrorFail {- This works: -} {- newtype StateError e s a = StateError ((StateT s (Either e) a)) deriving (Monad, MonadState s, MonadError e, Typeable) type MyMonad a = StateError SomeError (MyData a) a -} {- This doesn't work: -} newtype MyMonad a = MyMonad ((StateT (MyData a) (Either SomeError) a)) deriving (Monad, MonadState (MyData a), MonadError SomeError, Typeable) ---------- Basically, the more abstracted (commented-out) version works, but the more specific one gives this error: Weird.hs:33:12: Can't make a derived instance of `Monad MyMonad' (even with cunning newtype deriving): cannot eta-reduce the representation type enough In the newtype declaration for `MyMonad' Weird.hs:34:12: Cannot eta-reduce to an instance of form instance (...) => MonadState (MyData a) MyMonad In the newtype declaration for `MyMonad' Weird.hs:35:12: Can't make a derived instance of `MonadError SomeError MyMonad' (even with cunning newtype deriving): cannot eta-reduce the representation type enough In the newtype declaration for `MyMonad' These error messages mean nothing to me. What's going on? Can the more specific code be made to work? This is with ghc 6.12.3. Thanks, Mike

3 4

A parsec question
by Ben Franksen 04 Oct '10

04 Oct '10

I have a question about Parsec. The following program > import Control.Applicative ((*>),(<*)) > import Text.Parsec > import Text.Parsec.Char > block p = char '{' *> p <* char '}' > parser = block (many digit) > main = parseTest parser "{123a}" gives the output parse error at (line 1, column 5): unexpected "a" expecting "}" Note the last line mentions only '}'. I would rather like to see expecting "}" or digit since the parser could very well accept another digit here. (1) What is the reason for this behaviour? (2) Is there another combinator that behaves as I would like? (3) Otherwise, how do I write one myself? BTW, I am using parsec-3.1.0 and ghc-6.12.3. Cheers Ben

5 10

Polyvariadic functions operating with a monoid
by Kevin Jardine 04 Oct '10

04 Oct '10

I had a situation where I had some related types that all had toString functions. Of course in Haskell, lists all have to be composed of values of exactly the same type, so instead of passing around lists of values with these related types, I created a polyvariadic function polyToString so that I could write: (polyToString value1 value2 value3 ... valueN) which would then become a list of strings: [toString value1, toString value2, ... , toString valueN] I finally figured out how to do this, but it was a bit harder to figure this out than I expected, and I was wondering if it might be possible to create a small utility library to help other developers do this. It seems to me that in the general case, we would be dealing with a Monoid rather than a list of strings. We could have a toMonoid function and then return polyToMonoid value1 value2 ... valueN = (toMonoid value1) `mappend` (toMonoid value2) 'mappend' ... (toMonoid valueN) So anyone who wanted to convert a bunch of values of different types to a Monoid could easily pass them around using polyToMonoid so long as they defined the appropriate toMonoid function. Basically, a generalised list. So I tried writing the following code but GHC said it had undecidable instances. Has this ever been done successfully? class Monoidable a where toMonoid :: Monoid r => a -> r polyToMonoid :: (Monoidable a, Monoid r) => a -> r polyToMonoid k = polyToMonoid' k mempty class PolyVariadic p where polyToMonoid' :: (Monoidable a, Monoid r) => a -> r -> p instance Monoid r => PolyVariadic r where polyToMonoid' k ss = (toMonoid k) `mappend` ss instance (Monoidable a, Monoid r) => PolyVariadic (a -> r) where polyToMonoid' k ss = (\a -> polyToMonoid' k (toMonoid a) `mappend` ss)

2 3

Non-existing types in existential quantification?
by Henning Thielemann 03 Oct '10

03 Oct '10

Given the following code, that is accepted by GHC: > data Exist = forall a. Exist a > > exist :: Exist > exist = Exist undefined What type has the 'undefined' ? So far I assumed that at runtime all objects have a concrete type. This seems not to be true. I can also write > data ExistList = forall a. ExistList [a] > > exist :: ExistList > exist = ExistList [] where I do not need an 'undefined'.

5 6

Haskell web development entries on the Wiki
by Christopher Done 03 Oct '10

03 Oct '10

Decided to move this to a separate thread. I went ahead and refactored the wiki entries: http://www.haskell.org/haskellwiki/Web Now we have the following Web/ sections: Servers Frameworks Interfaces to frameworks Databases and Persistence Libraries Testing and Verification Content Management and Forums and Discussion Literature (research, talks and blogs) Existing Haskell web applications Ongoing projects and ideas And Web/Libraries has the following sections: URLs and Routing Templating Sessions and Authentication CGI Forms JavaScript and AJAX CSS I will add all these pages to the Web category, but not yet, I want to get rid of this page: http://www.haskell.org/haskellwiki/?title=Applications_and_libraries/Web_pr… I've nearly finished moving all of it to organised sections in this new hierarchy. The advantage of doing this is that I have to manually check each thing listed and I discover that quite a few are dead links and no longer maintained. Ideally this page http://www.haskell.org/haskellwiki/Web/Frameworks would have a list of all actually active web projects, and those would be listed at the top, or perhaps there should be another page Web/Frameworks/Active or whatnot. Anyone care to help take this page apart Applications and libraries/Web programming and move it to reasonable pages? Thoughts?

3 13

I still cannot seem to get a GUI working under Windows.
by caseyh＠istar.ca 03 Oct '10

03 Oct '10

I still cannot seem to get a GUI working under Windows. For Haskell GUI's is Ubuntu easier to setup. If so, we're losing people if Haskell GUI's are so hard to get working under Windows.

22 28

Re: [Haskell-cafe] Coding conventions for Haskell?
by Andrew Coppin 03 Oct '10

03 Oct '10

On 29/09/2010 02:18 PM, Henning Thielemann wrote: > Andrew Coppin wrote: > >> Tastes do indeed vary. To me, both of these are incorrect, and the >> correct way is >> >> data Foo a b = >> Foo a | >> Bar b | >> Foobar a b >> deriving (Eq, Ord) > > The truth is: Given the separator style of constructor definition, > there is no correct way to format those declarations. :-) The correct > way would be to allow terminator style. Well, yes, there is that. (And this isn't the only place in the syntax where it applies either. Tried editing export lists lately? Or Cabal module lists?)

2 3

Inverse of HaskellDB
by Jonathan Geddes 03 Oct '10

03 Oct '10

Cafe, HaskellDB takes a database schema and produces Haskell data structures (plus some other query-related stuff for its EDSL query language). What I'm looking for is the inverse of this functionality. I want to create tables based on a Haskell data structure with a few simple rules. These rules include: if a field is not of the form `Maybe a' then it can't be nullable in the database. If a field is not a primitive (in the database) then it is actually stored in another table and a reference id is stored in the table. Tables are produced recursively, unless they already exist, etc. The HaskellDB approach is great for interfacing with existing tables, but in my case I already have data structures and now I would like a quick way to create tables to persist them. Does such a thing exist? If not, would you find it useful? I may take this up as a side project if it does not already exist and others would find it useful. Thanks, --Jonathan

10 17

The Haskell Implementors Workshop
by Don Stewart 02 Oct '10

02 Oct '10

Slides and videos from today's Haskell Implementors Workshop, in Baltimore. http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2010#Programme Enjoy. -- Don

1 0

Haskell list on Twitter?
by Magnus Therning 02 Oct '10

02 Oct '10

Now that there are lists/groups on Twitter (http://mashable.com/2009/11/02/twitter-lists-guide/) maybe one should be created one for Haskellers? I would do it, but it seems I can't be on my own list :-) So unless I've missed something I'd have to create a dummy user just for this purpose. Hence I'm asking first :-) Cheers, M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

5 9

Coding conventions for Haskell?
by Petr Pudlak 01 Oct '10

01 Oct '10

Hi, sometimes I have doubts how to structure my Haskell code - where to break lines, how much to indent, how to name functions and variables etc. Are there any suggested/recommended coding conventions? I searched a bit and I found a few articles and discussions: - Good Haskell coding standards from Stackoverflow http://stackoverflow.com/questions/1983047/good-haskell-coding-standards - Programming guidelines from Haskellwiki http://www.haskell.org/haskellwiki/Programming_guidelines - How to read Haskell from Haskellwiki (this is actually about reading, not composing code, but still worth reading) http://www.haskell.org/haskellwiki/How_to_read_Haskell - Good Haskell Style http://urchin.earth.li/~ian/style/haskell.html but they usually address only a few specifics. I was looking for something more complete and comprehensive. Perhaps something like Java has [1]. Any suggestions? Best regards, Petr [1] http://www.oracle.com/technetwork/java/codeconv-138413.html

25 47

record update
by Jonathan Geddes 01 Oct '10

01 Oct '10

I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway: I find that most of the record updates I read and write take the form >someUpdate :: MyRecord -> MyRecord >someUpdate myRecord = myRecord > { field1 = f $ field1 myRecord > , field2 = g $ field2 myRecord > , field3 = h $ filed3 myRecord > } I find myself wishing I could write something more like >someUpdate :: MyRecord -> MyRecord >someUpdate myRecord = myRecord > { field1 => f > , field2 => g > , field3 => h > } with equivalent semantics. Here => reads "is transformed by". Operator = could still be used for assignment as in current record updates. The best part about such an extension, in my opinion, is that it would open the door for anonymous lambda record updates. Something like: >someUpdate :: MyRecord -> MyRecord >someUpdate = \{field1 => f, field2 => g, field3 => h} again, with the same semantics. This becomes possible because you no longer need to refer to the record within the {} part of the update. This would be useful, for example, in the State monad. We could write: >someStateTransform :: State MyRecord () >someStateTransform = do > modify $ \{field1 => (++"!")} > ... where currently we see code like >someStateTransform :: State MyRecord () >someStateTransform = do > modify $ \record->record{field1 = (++"!") $ field1 record} > ... which repeats the record name 3 times and the field name twice. The repetition just feels out of place next to all the other terse, readable Haskell code in the program. So what do my fellow haskellers think? Is this idea worth writing up a proposal for? Alternatively, can you offer me some advice on writing code in Haskell 2010 that avoids the ugly, repetitive style of record update? --Jonathan

9 13

RE: [Haskell-cafe] Candlestick charts
by Vivian McPhail 01 Oct '10

01 Oct '10

Hi >Message-ID: <AANLkTinROzNiqF4+yKxOfHo0FB=3B_7tfduX=vVbggYC(a)mail.gmail.com> > >Hi - > >What are the libraries to use in Haskell to generate a stock >candlestick chart like >http://stockcharts.com/h-sc/ui?s=SPY&p=D&b=5&g=5&id=p05007254056<http://stockcharts.com/h-sc/ui?s=SPY&p=D&b=5&g=5&id=p05007254056> > >I will use Finance-Quote-Yahoo to get the quote data from Yahoo. > >thanks for all your help. The plot 0.1.1 package supports candle and whisker charts. http://hackage.haskell.org/plot Vivian

3 2

ANN: contstuff: CPS-based monad transformers
by Ertugrul Soeylemez 01 Oct '10

01 Oct '10

Hello fellow Haskellers, (after failing (again) to send to the haskell list, I'm posting this here. I've contacted the mailing list owner about this.) I'm announcing the 0.4.0 release of the 'contstuff' monad transformer library, an alternative to libraries like 'mtl', 'transformers' and 'monadLib': http://hackage.haskell.org/package/contstuff The difference is that contstuff makes heavy use of continuation passing style both to increase performance and to make certain control flow patterns more accessible, including abortion, accumulation of multiple results, etc. On the wiki there is a quickstart introduction to those of you, who are familiar with monad transformers in general: http://haskell.org/haskellwiki/Contstuff Feedback, positive as well as negative, is highly appreciated. Thank you. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

2 2

ANNOUNCE: atomo 0.1, a simple, adventurous programming language
by Alex Suraci 01 Oct '10

01 Oct '10

Over the past few months I've been designing and implementing a new language called Atomo. The idea is to build up a very expressive language from as few parts as possible, akin to Io or Scheme. But first, the fundamentals: prototype-based object-orientation with multiple dispatch and pattern-matching. Still here? Great! Being object-oriented and dynamic, I expect a few boos and hisses, but if you want to dive in, it's a lot of fun. :) An immense amount of progress has been made in a short amount of time. It already sports a powerful documentation system (similar to Racket's Scribble), and a package manager is in the works (it's functional, but it's local-only). For a while I had atomo-lang.org serving up a site powered by Atomo itself, with Snap underneath via Atomo's Haskell interface. It's down at the moment (it was wrestling with darcsden over my VPS's resources), but the docs are always available: Documentation: http://atomo-lang.org/docs/ (work-in-progress) Repository: http://darcsden.com/alex/atomo Examples: http://darcsden.com/alex/atomo/browse/examples I recommend the generators example if you want some mind-bending, or html.atomo for a nice EDSL, or web.(atomo|hs) for an example of Haskell interop. On Hackage: http://hackage.haskell.org/package/atomo Installation: `cabal install atomo` should "just work", though I haven't worked out the appropriate version numbers for all of its dependencies yet. Hackage shows a build failure, but that seems to be caused by Haddock; I still haven't figured that out. If you want to get involved, you can find me on freenode in #atomo (and #haskell); my nick is alexsuraci. Or, you can fork the repository on darcsden. I'd just like to put this out there to see if anyone else is interested in playing with this. -- Alex

1 0

Monad instance for partially applied type constructor?
by DavidA 01 Oct '10

01 Oct '10

Hi, I have the following code: {-# LANGUAGE TypeSynonymInstances #-} data Vect k b = V [(k,b)] -- vector space over field k with basis b -- for example, V [(5, E 1), (7, E 2)] would represent the vector 5 e1 + 7 e2 data Monomial v = M [(v,Int)] -- monomials over variables v -- for example, M [(A,3), (B,5)] would represent the monomial a^3 b^5 type Poly k v = Vect k (Monomial v) -- multivariate polynomials over field k and variables v instance Monad (Poly k) where return v = V [(1, M [(v,1)])] p >>= f = ... -- variable substitution So my thinking is: 1. The Monad type class is for one parameter type constructors (eg [], IO) 2. Poly is a two-parameter type constructor 3. So Poly k is a one-parameter type constructor 4. What I'm trying to express, that polynomials over field k are a monad, is true. However, GHC 6.12.2 complains: Type synonym `Poly' should have 2 arguments, but has been given 1 In the instance declaration for `Monad (Poly k)' What is going wrong?

8 12

Spell-checking lhs files
by Petr Pudlak 30 Sep '10

30 Sep '10

Hi, recently I was spell-checking a literate Haskell source file (within the vim editor). The spell checker correctly ignored LaTeX commands, but I had to manually skip over code blocks inside |...|. This was really annoying and I couldn't figure out how to configure the spell checker to skip over code. Does anybody have a solution for that? Thanks a lot, Petr

1 0

Fourth Ghent Functional Programming Group Meeting on Thursday, Oct 7th: Program, Final Details and BelHac Commercial
by Jeroen Janssen 30 Sep '10

30 Sep '10

(apologies if you receive multiple copies) Dear All, We are pleased to announce that the fourth Ghent Functional Programming Group (GhentFPG) meeting will take place on Thursday, October 7th, at 19h in the Technicum building of Ghent University (Sint-Pietersnieuwstraat 41, 9000 Gent). As before, you should enter through the electronic sliding doors on the left. The doors will be locked, but there will be a sign with a number that you can call to get in. The final program for our fourth meeting is as follows: 1. Stijn Timbermont - Mapping Interpreters onto Runtime Support When constructing an interpreter, it is hard to achieve a concise and abstract definition of the language semantics and at the same time have sufficient control over how the underlying runtime support is implemented. We illustrate with a number of examples that gaining more control over runtime support implementation forces us to adopt a more machine-like interpreter structure. This is problematic for reuse and evolution of interpreters. We present a solution to this problem that automates this restructuring of interpreters. The solution is based on three cornerstones: the theory of defunctionalized interpreters, effect-driven transformations and generic programming. It allows us to start from a concise and abstract definition of the language features and automatically transform it into a more machine-like form that allows for a customized runtime support implementation to be integrated. 2. Tom Schrijvers - Dictionaries: Eager or Lazy Type Class Witnesses? Type classes are Haskell’s acclaimed solution to ad-hoc overloading. This talk gives an introductory overview of type classes and their runtime witnesses, dictionaries. It asks the questions whether dictionaries should abide by Haskell’s default lazy evaluation strategy. Conceptually, a type class is a type-level predicate: a type is an instance of a type class iff it provides an implementation for overloaded functions. For instance, `Eq a’ declares that type `a’ implements a function `(==) :: a → a → Bool’ for checking equality. Type classes are used as constraints on type variables, in so-called constrained polymorphic functions. E.g. `sort :: Ord a => [a] → [a]’ sorts a list with any type of elements `a’ that are an instance of the Ord type class, i.e. provide implementations for comparison. Witnesses for type class constraints are necessary to select the appropriate implementation for the overloaded functions at runtime. For instance, if `sort’ is called with Int elements, the Int comparison must be used, versus say Float comparison for Float elements. Two forms of witnesses have been considered in the literature, runtime type representations and so-called dictionaries, of which the latter are the most most commonly implementation, e.g., in GHC . Haskell implementations treat dictionaries just like all other data, as lazy values that may potentially consists of non-terminating computations. This way part of the type checker’s work, who has made sure that the dictionaries do exist, is simply forgotten. Is this really necessary? 3. Dominique Devriese - Grammar Combinators - A new model for shallow parser DSL's Parser combinator libraries are a well-known alternative to parser generators in the functional programming community. However, their power is currently fundamentally limited by their representation of recursion in context-free grammars. We present the grammar-combinators Haskell library which removes these limitations, and brings some other advantages (semantic value family polymorphism, natural concepts). In this talk, I will briefly explain the motivation for the library, and then present our approach and some of its most important aspects in a live demonstration and finish with a discussion. Last, note that GhentFPG is organizing the first Belgian Haskell Hackathon, called BelHac, on 5-7 November 2010. The fifth GhentFPG meeting will take place at BelHac and will focus on talks concerning the use of functional programming in industry. More info will be provided soon. As usual, if you want get all the latest info on GhentFPG, you can subscribe to our google group (http://groups.google.com/group/ghent-fpg) or follow us on twitter (@ghentfpg). Hope to see you at both of these events! The GhentFPG Organizing committee.

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Distribution needs
by Joachim Breitner 30 Sep '10

30 Sep '10

Hi, on planet.debian.org, there is some ill-tempered discussion about the seemingly bad relationship between the Ruby community and Debian maintainers. The following blog post summarizes the issues quite well and calmly: http://gwolf.org/blog/ruby-dissonance-debian-again With the Haskell community, luckily the relationship is much better. But since the technical situation is similar, i.e. a custom distribution channel via hackage/cabal-install, the possibility to install multiple versions of a package at once, I thought this might still be an interesting read. I hope this improves the understanding of a distribution’s needs and why we don’t like multiple versions of one package or a splitting in too many single small packages. But let me stress that I’m very happy with the interaction of the communities, Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Problem with a sample from RWH
by C K Kashyap 30 Sep '10

30 Sep '10

Hi, I was going over the Error Handling chapter in RWH and tried out this sample - Prelude> :m Control.Exception Prelude Control.Exception> let x=5 `div` 0 Prelude Control.Exception> let y=5 `div` 1 Prelude Control.Exception> handle (\_ -> putStrLn "Text") (print x) <interactive>:1:0: Ambiguous type variable `e' in the constraint: `Exception e' arising from a use of `handle' at <interactive>:1:0-39 Probable fix: add a type signature that fixes these type variable(s) Prelude Control.Exception> Could someone please tell me what the problem is and how to resolve it? -- Regards, Kashyap

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Parsec question (new user): unexpected end of input
by Peter Schmitz 30 Sep '10

30 Sep '10

I am a new Parsec user, and having some trouble with a relatively simple parser. The grammar I want to parse contains tags (not html) marked by angle brackets (e.g., "<some tag>"), with arbitrary text (no angle brackets allowed) optionally in between tags. Tags may not nest, but the input must begin and end with a tag. Whitespace may occur anywhere (beginning/end of input, inside/between tags, etc.), and is optional. I think my problem may be a lack of using "try", but I'm not sure where. At runtime I get: Error parsing file: "...\sampleTaggedContent.txt" (line 4, column 1): unexpected end of input expecting "<" The input was: <tag1>stuff<tag 2> more stuff < tag 3 > even more <lastTag> The code is below. (I'm using Parsec-2.1.0.1.) I don't really want to return anything meaningful yet; just parse okay. Any advice about the error (or how to simplify or improve the code) would be appreciated. Thanks much, -- Peter > -- Parsers: > taggedContent = do > optionalWhiteSpace > aTag > many tagOrContent > aTag > eof > return "Parse complete." > > tagOrContent = aTag <|> someContent <?> "tagOrContent" > > aTag = do > tagBegin > xs <- many (noneOf [tagEndChar]) > tagEnd > optionalWhiteSpace > return () > > someContent = do > manyTill anyChar tagBegin > return () > > optionalWhiteSpace = spaces -- i.e., any of " \v\f\t\r\n" > tagBegin = char tagBeginChar > tagEnd = char tagEndChar > > -- Etc: > tagBeginChar = '<' > tagEndChar = '>' --------

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