January 2009 - Haskell-Cafe

Connect to DBMS - what's the cost
by Belka 04 Jan '09

04 Jan '09

Hello, community people! Is anybody aware, what aproximately is the cost for the acquiring connection to DB (and also disconnecting from it)? I guess that may differ from DBMS to DBMS, so I mostly am interested in PostgreSQL case, but for other DBMS it's also good to know. Actually, since all Haskell libraries, that provide interfaces to DBMS, use foreign functions, - the question should be focused on commonly used C libraries. Thanks in advance! -- View this message in context: http://www.nabble.com/Connect-to-DBMS---what%27s-the-cost-tp21275600p212756… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: about the concatenation on a tree
by Achim Schneider 04 Jan '09

04 Jan '09

"Max cs" <max.cs.2009(a)googlemail.com> wrote: > hi all, not sure if there is someone still working during holiday > like me : ) > > I got a little problem in implementing some operations on tree. > > suppose we have a tree date type defined: > > data Tree a = Leaf a | Branch (Tree a) (Tree a) > > I want to do a concatenation on these tree just like the concat on > list. Anyone has idea on it? or there are some existing > implementation? > The semantics of tree concat vary, depending on what type of tree you want to have. In the simplest case, it's unbalanced, so you can just do concat :: Tree a -> Tree a -> Tree a concat x y = Branch x y if, on the other hand, you want to keep the tree balanced, or sorted, or whatever, things get more involved. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.

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How do we decide on the new logo?
by Fritz Ruehr 03 Jan '09

03 Jan '09

Now that the new year is upon us, I suppose we must decide how to decide* on the new logo for the Haskell site. I'm not sure what the protocol and procedure for voting and tallying should be, but see below for a suggestion. (* a higher-order decision, very appropriate) If you haven't seen the page on the Wiki with proposed new logos, be sure to go there and take a look (but recall that submissions are now closed): <http://haskell.org/haskellwiki/Haskell_logos/New_logo_ideas> I want to thank Don for proposing the contest and everyone who contributed logo designs or modifications. As the designer of the last "official" logo, I think this new crop is terrific, with many suggestions that are more professional-looking and sleeker than the old one: I will have a hard time deciding which one to vote for. Without starting a war on the theory of voting systems, perhaps we should use a system which allows for a certain amount of secondary (etc.) preference to be expressed? (Uh-oh, here come Control.Monad.Voting.HareSTV and Control.Monad.Voting.BordaCount and a hundred other variations, complete with back-tracking and trampolined continuations and ... .) Once we have a winning design, we could perhaps award the designer(s) with a T-shirt or some similar item. We have a small amount of "CafeBucks" (or whatever) accrued in the CafePress account, despite all efforts to avoid profit (rather like avoiding success at all costs). In the past, these funds have been used to purchase courtesy shirts for a few Haskell luminaries, at the discretion of the store proprietor. (Currently I think there are about $60 available.) -- Fritz (Ruehr)

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[ANN] Haskell web server + wiki: salvia-0.0.4 + orchid-0.0.6
by Sebastiaan Visser 03 Jan '09

03 Jan '09

Happy new year, you all! I'm pleased to announce three new packages on Hackage: * salvia-0.0.4: A lightweight modular web server framework. * orchid-0.0.6: A(nother) Wiki written in Haskell, currently using Darcs as a versioning back-end and Salvia as the application server. Orchid ships as a library that can be installed as a server module for the Salvia framework. * orchid-demo-0.0.4: A simple demo application using Salvia and Orchid to serve an example darcs repository. An online demo of the wiki can be found at [1]. The front-end is a typical Ajax application and heavily relies on the availability of JavaScript. A more plain (REST like) web interface to system can be found at [2]. You can signup, login and edit/create some pages. Feel free to play around, it's just a demo. Orchid has an abstract notion of a versioning back-end, theoretically allowing for multiple versioning systems. Currently only a Darcs back- end is available, but it shouldn't be too much work to implement a liaison for Git, Subversion, Mercurial etc. The Wiki uses its own experimental document system with printers to HTML, LaTeX, PDF via LaTeX and some others in the making. The system makes us of the UUAGC [3] attribute grammar system, but is not required in order to build the package. The document system is plug-in based allowing easy addition of new structures. There are currently plug-ins for inline LaTeX formulae, HSColour'ed Haskell code snippets and a table of contents. Some others in the making. Be aware, this is an early release to allow people the play around with the packages and enable me to see how this all behaves in the wild. The tools are not yet finished and I cannot guarantee any form of stability. But, in my experience, they seem to `just work'. Most of the time. I had some trouble getting all dependencies right on systems other than my own. Using `cabal install' seems to behave differently from `./ Setup install' in the package directory, but this may vary from system to system. It is known to work at least on Macosx and Linux with GHC 6.8, 6.10 or even 6.11. Pdflatex is needed for PDF generation, dvipng for inline formulas. Have fun and best wishes for this new year! -- Sebastiaan [1] http://funct.org/wiki/ [2] http://funct.org/wiki/data/Index.html [3] http://www.cs.uu.nl/wiki/HUT/AttributeGrammarSystem

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Haskell Weekly News: Issue 99 - January 3, 2009
by Brent Yorgey 03 Jan '09

03 Jan '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090103 Issue 99 - January 03, 2009 --------------------------------------------------------------------------- Welcome to issue 99 of HWN, a newsletter covering developments in the [1]Haskell community. Happy new year to all! May 2009 be a year full of joy, family, friends, professional success, much Haskell hacking, and a minimal number of rabid weasels. Just in case. Announcements #haskell IRC channel reaches 600 users. Don Stewart [2]announced that 7 years after its inception, under the guiding hand of Shae Erisson (aka shapr), the [3]#haskell IRC channel on freenode has reached 600 concurrent users! citeproc-hs-0.2. andrea rossato [4]announced the release of [5]citeproc-hs-0.2, a Haskell implementation of the Citation Style Language, which adds a Bibtex like citation and bibliographic formatting and generation facility to [6]pandoc. This version adds support for citation collapsing, a wrapper around [7]hs-bibutils, and some [8]API documentation. hs-bibutils-0.1. andrea rossato [9]announced the first release of [10]hs-bibutils, Haskell bindings to Chris Putnam's [11]bibutils. Bibutils is a library and a set of bibliographic utilities to interconvert between various bibliography database formats using a common MODS-format XML intermediate. Haskell koans. Gwern Branwen [12]issued an RFK (Request for [13]Koans), following the success of his CFH (Call for [14]Haiku). [ANN] Haskell web server + wiki: salvia-0.0.4 + orchid-0.0.6. Sebastiaan Visser [15]announced the release of three new packages: [16]salvia, a lightweight modular web server framework; [17]orchid, a(nother) wiki written in Haskell, using Darcs as a versioning back-end and Salvia as the application server; and [18]orchid-demo, a simple demo application using Salvia and Orchid to serve an example darcs repository. You can play around with an [19]online demo. gitit-0.4.1, recaptcha-0.1. John MacFarlane [20]announced the release of [21]gitit-0.4.1, a wiki program that stores pages in a git repository. This release adds support for (optional) captchas, using the reCAPTCHA service. The reCAPTCHA code has been packaged as a separate library on Hackage, [22]recaptcha. monte-carlo-0.2, gsl-random-0.2.3. Patrick Perry [23]announced the release of a new version of the [24]monte-carlo package. The new version includes a more general type class, MonadMC, which allows all the functions to work in both MC and MCT monads; functions to sample from discrete distributions, and functions to sample subsets. There is also a [25]quick tutorial. Reading group for Programming Collective Intelligence. Creighton Hogg [26]announced that he would like to start a small group for the O'Reilly book [27]Programming Collective Intelligence, to work through translating some of the examples to Haskell. Email Creighton if you are interested in participating. Maintaining laziness. Henning Thielemann [28]announced that he has written a [29]tutorial on how to make functions lazy and how to test whether they are actually lazy. Request for feedback: Understanding Haskell Monads. Ertugrul Soeylemez [30]requested feedback on a new [31]monad tutorial. Discussion How do we decide on the new logo?. Fritz Ruehr began a [32]discussion of how to go about choosing a winner of the [33]Great 2009 Haskell Logo Contest. Weigh in if you care! Jobs Two Positions as Associate Professor in Software Engineering at Chalmers University. Koen Claessen [34]announced the availability of [35]two positions as Associate Professor at Chalmers University in Gothenburg, Sweden, within the division of Software Engineering and Technology at the department of Computer Science and Engineering. The application deadline is January 12, 2009. Blog noise [36]Haskell news from the [37]blogosphere. * GHC / OpenSPARC Project: [38]Bootstrapping. * Dan Piponi (sigfpe): [39]Rewriting Monadic Expressions with Template Haskell. * GHC / OpenSPARC Project: [40]Fighting dependencies. * GHC / OpenSPARC Project: [41]A new year and a new project. * Alson Kemp: [42]2009: The Year Of Hackage. * Patrick Perry: [43]Monte Carlo Poker Odds. * Joachim Breitner: [44]Handling explicit and implicit recursion in Haskell data. * Luke Palmer: [45]Domain Convergence. * Eric Kow (kowey): [46]riot is almost a Haskell mail client. * John Goerzen (CosmicRay): [47]Real World Haskell update. * Alson Kemp: [48]A Plea For "cabal install". * Alson Kemp: [49]Cyptol on Slashdot. Quotes of the Week * lilac: <bohdan> how do I see the number of reductions required to calculate something? <lilac> bohdan: the usual method is to ask Cale to reduce it by hand :) * conal: If it's purely functional, how do you *do* anything? You don't ;-) * ddarius: The opposite ends of CS meet in the Haskell world. * EvilTerran: forcedYet :: a -> Bool; forcedYet x = x `seq` True -- :P * bmh: I dream in folds. One day I'll dream in monads. * sclv: dreaming is a monad. About the Haskell Weekly News New editions are posted to [50]the Haskell mailing list as well as to [51]the Haskell Sequence and [52]Planet Haskell. [53]RSS is also available, and headlines appear on [54]haskell.org. To help create new editions of this newsletter, please see the information on [55]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [56]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49824 3. http://haskell.org/haskellwiki/IRC_channel 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16709 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/citeproc-hs 6. http://johnmacfarlane.net/pandoc/ 7. http://code.haskell.org/hs-bibutils/ 8. http://code.haskell.org/citeproc-hs/docs/ 9. http://article.gmane.org/gmane.comp.lang.haskell.general/16708 10. http://code.haskell.org/hs-bibutils/ 11. http://www.scripps.edu/~cdputnam/software/bibutils/ 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49774 13. http://haskell.org/haskellwiki/Koans 14. http://haskell.org/haskellwiki/Haiku 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49759 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/salvia 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/orchid 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/orchid-demo 19. http://funct.org/wiki/] 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49747 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gitit 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/recaptcha 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49739 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monte-carlo 25. http://quantile95.com/2008/12/31/monte-carlo-poker-odds/ 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49716 27. http://tinyurl.com/7x4rga 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49655 29. http://www.haskell.org/haskellwiki/Maintaining_laziness 30. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49546 31. http://ertes.de/articles/monads.html 32. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/49833 33. http://haskell.org/haskellwiki/Haskell_logos/New_logo_ideas 34. http://article.gmane.org/gmane.comp.lang.haskell.general/16712 35. http://www.chalmers.se/cse/EN/news/vacancies/positions/two-positions-as 36. http://planet.haskell.org/ 37. http://haskell.org/haskellwiki/Blog_articles 38. http://ghcsparc.blogspot.com/2009/01/building.html 39. http://sigfpe.blogspot.com/2009/01/rewriting-monadic-expressions-with.html 40. http://ghcsparc.blogspot.com/2009/01/in-in-morning.html 41. http://ghcsparc.blogspot.com/2008/12/alright.html 42. http://www.alsonkemp.com/haskell/2009-the-year-of-hackage/ 43. http://quantile95.com/2008/12/31/monte-carlo-poker-odds/ 44. https://www.joachim-breitner.de/blog/archives/316-Handling-explicit-and-imp… 45. http://lukepalmer.wordpress.com/2008/12/31/domain-convergence/ 46. http://koweycode.blogspot.com/2008/12/riot-is-almost-haskell-mail-client.ht… 47. http://changelog.complete.org/archives/861-real-world-haskell-update-2 48. http://www.alsonkemp.com/haskell/a-plea-for-cabal-install/ 49. http://www.alsonkemp.com/haskell/cyptol-on-slashdot/ 50. http://www.haskell.org/mailman/listinfo/haskell 51. http://sequence.complete.org/ 52. http://planet.haskell.org/ 53. http://sequence.complete.org/node/feed 54. http://haskell.org/ 55. http://haskell.org/haskellwiki/HWN 56. http://code.haskell.org/~byorgey/code/hwn/

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Re: [Haskell-cafe] How to check object's identity?
by Xie Hanjian 03 Jan '09

03 Jan '09

Thanks guys :-) * Nicolas Pouillard <nicolas.pouillard(a)gmail.com> [2009-01-03 16:39:59 +0100]: > Excerpts from Xie Hanjian's message of Sat Jan 03 16:28:30 +0100 2009: > > Hi, > > > > I tried this in ghci: > > >Prelude> 1:2:[] == 1:2:[] > > True > > > > Does this mean (:) return the same object on same input, or > > (==) is not for identity checking? If the later is true, how > > can I check two object is the *same* object? > > Here (==) is the structural equality. There is generally no exposed function > to check for identity of values. This would expose too much of the memory > model and the compilation process to give a specification to such a function. > > -- > Nicolas Pouillard -- jan=callcc{|jan|jan};jan.call(jan)

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Re: Updating doubly linked lists
by oleg＠okmij.org 03 Jan '09

03 Jan '09

Stephan Guenther wrote: > Is it possible to change a particular node of the doubly linked list? > That is to say, that would like to have a function: > update :: DList a -> a -> DList a > where > update node newValue > returns a list where only the value at the node which is passed in is > set to the new Value and all other values are the same. All this of > course in a pure way, that is without using (M/T/TM)Vars or IORefs. It is possible to do all of this, and more: - no rebuilding of the whole list on updates to the list - the update operation takes constant time (for lists longer than 32 elements on 32-bit platform) - both cyclic and terminated lists can be handled, uniformly - no monads used or mentioned - let alone no IORef, STRef, TVars, etc. The algorithm is essentially imperative (and so permits identity checking and in-place `updates') but implemented purely functionally. No destructive updates are ever used. Therefore, all the changes can be undone and re-done, and the code is MT-safe. The code is easily generalizable to 2D. Here are the tests > testl = fromList [1..5] > testl_s = takeDL 11 testl *FL> testl_s [5,1,2,3,4,5,1,2,3,4,5] > testl1 = update (-1) testl > testl1_s = takeDL 11 testl1 *FL> testl1_s [-1,1,2,3,4,-1,1,2,3,4,-1] > testl2 = update (-2) . move_right' . move_right' $ testl1 > testl2_s = takeDL 11 testl2 *FL> testl2_s [-2,3,4,-1,1,-2,3,4,-1,1,-2] > -- Old testl is still available > testl3 = update (-2) . move_right' . move_right' $ testl > testl3_s = takeDL 11 testl3 *FL> testl3_s [-2,3,4,5,1,-2,3,4,5,1,-2] It is not for nothing Haskell is called the best imperative language. One can implement imperative algorithms just as they are -- purely functionally, without any monads or other categorical notions. module FL where import qualified Data.IntMap as IM -- Representation of the double-linked list type Ref = Int -- positive, we shall treat 0 specially data Node a = Node{node_val :: a, node_left :: Ref, node_right :: Ref} data DList a = DList{dl_counter :: Ref, -- to generate new Refs dl_current :: Ref, -- current node dl_mem :: IM.IntMap (Node a)} -- main `memory' -- Operations on the DList a empty :: DList a empty = DList{dl_counter = 1, dl_current = 0, dl_mem = IM.empty} -- In a well-formed list, dl_current must point to a valid node -- All operations below preserve well-formedness well_formed :: DList a -> Bool well_formed dl | IM.null (dl_mem dl) = dl_current dl == 0 well_formed dl = IM.member (dl_current dl) (dl_mem dl) is_empty :: DList a -> Bool is_empty dl = IM.null (dl_mem dl) -- auxiliary function get_curr_node :: DList a -> Node a get_curr_node DList{dl_current=curr,dl_mem=mem} = maybe (error "not well-formed") id $ IM.lookup curr mem -- The insert operation below makes a cyclic list -- The other operations don't care -- Insert to the right of the current element, if any -- Return the DL where the inserted node is the current one insert_right :: a -> DList a -> DList a insert_right x dl | is_empty dl = let ref = dl_counter dl -- the following makes the list cyclic node = Node{node_val = x, node_left = ref, node_right = ref} in DList{dl_counter = succ ref, dl_current = ref, dl_mem = IM.insert ref node (dl_mem dl)} insert_right x dl@DList{dl_counter = ref, dl_current = curr, dl_mem = mem} = DList{dl_counter = succ ref, dl_current = ref, dl_mem = IM.insert ref new_node $ IM.insert curr curr_node' mem} where curr_node = get_curr_node dl curr_node'= curr_node{node_right = ref} new_node = Node{node_val = x, node_left = curr, node_right = node_right curr_node} get_curr :: DList a -> a get_curr = node_val . get_curr_node move_right :: DList a -> Maybe (DList a) move_right dl = if next == 0 then Nothing else Just (dl{dl_current=next}) where next = node_right $ get_curr_node dl -- If no right, just stay inplace move_right' :: DList a -> DList a move_right' dl = maybe dl id $ move_right dl fromList :: [a] -> DList a fromList = foldl (flip insert_right) FL.empty takeDL :: Int -> DList a -> [a] takeDL 0 _ = [] takeDL n dl | is_empty dl = [] takeDL n dl = get_curr dl : (maybe [] (takeDL (pred n)) $ move_right dl) -- Update the current node update :: a -> DList a -> DList a update x dl@(DList{dl_current = curr, dl_mem = mem}) = dl{dl_mem = IM.insert curr (curr_node{node_val = x}) mem} where curr_node = get_curr_node dl testl = fromList [1..5] testl_s = takeDL 11 testl testl1 = update (-1) testl testl1_s = takeDL 11 testl1 testl2 = update (-2) . move_right' . move_right' $ testl1 testl2_s = takeDL 11 testl2 -- Old testl is still available testl3 = update (-2) . move_right' . move_right' $ testl testl3_s = takeDL 11 testl3

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What are side effects in Haskell?
by Hans van Thiel 03 Jan '09

03 Jan '09

Hello All, I just saw somewhere that one of the purposes of monads is to capture side effects. I understand what a side effect is in C, for example. Say you want to switch the contents of two variables. Then you need a third temporary variable to store an intermediate result. If this is global, then it will be changed by the operation. But what is a side effect in an fp language like Haskell? As a follow up, I understand why an IO action like getLine or getChar is not a function; its results can be different for different calls. But why not have something like getChar c or getLine str? The types of c or str are pretty clear, aren't they? Just something that's been puzzling me for some time now...thanks. Regards, Hans van Thiel

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Re: [Haskell-cafe] Don't make 'show*' functions
by Donn Cave 02 Jan '09

02 Jan '09

Quoth brian <brianchina60221(a)gmail.com>: | On Fri, Dec 26, 2008 at 1:55 PM, Jeff Heard <jefferson.r.heard(a)gmail.com> wrote: |> Off the top of my head, I would say that the traditional meaning of |> Show could be changed to Serial, where serial encompasses both Read |> and Show -- possibly we could find a more efficient read function, |> several have been proposed. Then a separate class could be made for |> HumanReadable (or Loggable) where the point would bet that we write |> something that can be read by humans without conforming to a |> particular grammar that Haskell could read back in. | | +1. 'Show' sounds like it's for what Java 'toString' and Ruby 'to_s', | etc., do, and that's not right. To be fair, this seems to be a natural ambiguity with string representation. Python kind of blundered into it around 2.0, when someone decided to "fix" the __repr__ function for floating point to display the best decimal value, where it had until then used a rounded value. So users were confounded to see for example that 1.0 - 0.1 is 0.90000000000000002, instead of 0.9, but that does make some sense if __repr__ is to be used for string marshalling. Python has two string representation functions, __str__ and __repr__. The documentation of these two functions is a long standing problem that will probably never be resolved. If as proposed above there had also been a third function that is strictly about marshalling, that might have made it a little easier to focus on the difference betweeen str and repr, where unfortunately the documentation has confused the issue with ambiguous notions like friendliness. For me, "str" is about conversion to string data, where a data type contains or represents string data. Conversion implies loss. For example, an HTML or RFC822 header would convert to its "wire" data. But that's at odds with expectations for numeric types, for example - I'd make str(65) = 'A', but of course it's actually '65'. "repr" portrays the value as a Python object - what you would see for example from the interactive interpreter. Since Python objects are commonly "in the IO monad", there are all kinds of cases where the repr value can't fully marshal the object. Donn

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Parsec : Problems with operator precedence
by Erik de Castro Lopo 02 Jan '09

02 Jan '09

Hi all, I'm using Text.ParserCombinators.Parsec.Expr to parse expressions for a Javascript like language. This language has C-like logical operators ('&&' and '||') and bitwise operators ('&' and '|'). Furthermore, the language definition states that bitwise operators have a higher precedence than the logical ones. I therefore have the following (trimmed): import qualified Text.ParserCombinators.Parsec.Expr as E opTable :: [[ E.Operator Char st Expression ]] opTable = [ -- Operators listed from highest precedence to lowest precedence. {- snip, snip -} [ binaryOp "&" BinOpBinAnd E.AssocLeft ], [ binaryOp "^" BinOpBinXor E.AssocLeft ], [ binaryOp "|" BinOpBinOr E.AssocLeft ], [ binaryOp "&&" BinOpLogAnd E.AssocLeft ], [ binaryOp "||" BinOpLogOr E.AssocLeft ] ] binaryOp :: String -> (SourcePos -> a -> a -> a) -> E.Assoc -> E.Operator Char st a binaryOp name con assoc = E.Infix (reservedOp name >> getPosition >>= return . con) assoc but I still get the following parse error: unexpected "|" expecting end of "|" or term on the line: if (name == null || value == null) If I change the above from a logical to a bitwise OR, the parser accepts it quite happily. Any clues as to what I'm doing wrong here? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- BSD: A psychoactive drug, popular in the 80s, probably developed at UC Berkeley or thereabouts. Similar in many ways to the prescription-only medication called "System V", but infinitely more useful. (Or, at least, more fun.) The full chemical name is "Berkeley Standard Distribution".

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