October 2010 - Haskell-Cafe

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

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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.

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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

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