January 2007 - Haskell-Cafe

Why does the wiki search facility not work properly?
by Brian Hulley 07 Jan '07

07 Jan '07

Hi, There is a page on the Haskell Wiki titled "Talk:SantaClausProblem" subtitled "Beautiful concurrency". However typing any of the following into the search box *doesn't* lead to the page: Santa talk:santaclausproblem beautiful beautiful concurrency Talk:Santa In fact to get the page you have to type the *exact* full title in the *exact* case. Luckily I could always refer back to SimonPJ's post to the Haskell mailing list to find the link to the page, but I think this situation is really bad because it means that things on the wiki are more or less just being thrown down a gaping void never to be seen again unless you know which catagories to look in from the main page. Another point is that the word "beautiful" does occur on the page but the page is not included in the list of page text matches. Perhaps the site is not being automatically re-indexed frequently enough? There is also a bunch of check boxes at the bottom of the search results page that doesn't seem to do anything either except cause nothing at all to happen when you click search unless the (Main) box is the only box ticked. Anyway I thought I'd just point this out in case anyone knows how to fix it. Thanks, Brian. -- http://www.metamilk.com

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less than helpful GHC error message
by Nicolas Frisby 07 Jan '07

07 Jan '07

Consider the terminal session at the bottom of this message. The extra "xc" in Test.hs was the result of me missing CTRL during an Emacs command (I'm guessing...). Unfortunately this took 30 minutes to find since I had far more than two modules and the error message doesn't point this out. Sure it's a silly mistake to make, but I'm guessing it's pretty common. I'm running GHC 6.6 on a PPC Mac. I know pragmas are wierd things, but could this error message be improved? Thanks, Nick $ head -n 10000 Main.hs Test.hs ==> Main.hs <== module Main where import Test main = test ==> Test.hs <== xc{-# OPTIONS -fglasgow-exts #-} module Test where test = putStrLn "weee" $ ghc --make Main <no location info>: file name does not match module name `Main'

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build error with happy-1.15 on ghc-6.6
by Yang 07 Jan '07

07 Jan '07

per a previous thread, i addressed the 'package lang not found' issue by changing the following lines to: $ grep -rsn '\blang\b' . | grep Makefile ./happy/src/Makefile:27:SRC_HC_OPTS += #-package lang ./happy/tests/Makefile:62:HC_OPTS += -fglasgow-exts # -package lang however i still get a build error, seemingly from LALR.lhs: /usr/local/bin/ghc -H16m -O -cpp -fglasgow-exts -O -c LALR.lhs -o LALR.o -ohi LALR.hi LALR.lhs:626:34: Not in scope: `bounds' i wasn't able to find 'bounds' defined in any other of the src files. thanks in advance for any help.

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Re: [Haskell-cafe] Seeking advice on a style question
by Steve Schafer 07 Jan '07

07 Jan '07

On Tue, 26 Dec 2006 20:21:45 -0800, you wrote: >How would this example look if you named only multiply-used expressions? >I'd like to see it in a more conventional pointful style with nested >expressions. I'm still wondering whether the awkwardness results from your >writing style or is more inherent. Showing the real variable names may also >help also. This is what it looks like "for real": > process :: Item -> MediaKind -> MediaSize -> Language -> SFO > process item mediaKind mediaSize language = > let pagemaster = loadPagemaster item mediaKind mediaSize; > questions = stripUndisplayedQuestions mediaKind $ > appendEndQuestions item pagemaster $ > coalesceParentedQuestions $ > validateQuestionContent $ > loadQuestions item; > (numberedQuestions,questionCategories) = numberQuestions pagemaster questions; > numberedQuestions' = coalesceNAQuestions numberedQuestions; > (bands,sequenceLayouts) = buildLayout mediaKind language numberedQuestions'; > bands' = resolveCrossReferences bands; > groupedBands = groupBands bands'; > pages = paginate item mediaKind mediaSize pagemaster groupedBands; > pages' = combineRows pages; > sfo = pages' sequenceLayouts; > in sfo These are the function signatures: > loadPagemaster :: Item -> MediaKind -> MediaSize -> Pagemaster > loadQuestions :: Item -> [Question] > validateQuestionContent :: [Question] -> [Question] > coalesceParentedQuestions :: [Question] -> [Question] > appendEndQuestions :: Item -> Pagemaster -> [Question] -> [Question] > stripUndisplayedQuestions :: MediaKind -> [Question] -> [Question] > numberQuestions :: Pagemaster -> [Question] -> ([NumberedQuestion],[QuestionCategory]) > coalesceNAQuestions :: [NumberedQuestion] -> [NumberedQuestion] > buildLayout :: MediaKind -> Language -> [NumberedQuestion] -> ([Band],[SequenceLayout]) > resolveCrossReferences :: [Band] -> [Band] > groupBands :: [Band] -> [[Band]] > paginate :: Item -> MediaKind -> MediaSize -> Pagemaster -> [[Band]] -> [Page] > combineRows :: [Page] -> [Page] > createSFO :: [Page] -> [SequenceLayout] -> SFO MediaKind, MediaSize and Language are simple enumerations; everything else is a complex structure. Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/

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Stupid newbie question
by Brian Hurt 06 Jan '07

06 Jan '07

My apologies for wasting bandwidth on what I'm sure is a stupid newbie question. Given: -- Reimplementing the wheel here, I know data Option a = Some a | Empty deriving (Eq,Ord,Show,Read) nth 0 (x:xs) = Some x nth i (x:xs) = if i < 0 then Empty else nth (i-1) xs nth i [] = Empty That: nth 1000000 [1..10000000] returns the right answer, while: nth 1000000 [1..] blows stack. Furthermore, given: makelist i = i : (makelist (i-1)) Why is it that: nth 1000000 (makelist 1) blows stack? What are the rules for being tail recursive in Haskell? I'm an Ocaml programmer, so I'm familiar with this problem, I just don't see the solution. Some help would be appreciated. Thanks in advance, Brian

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MVar style question
by Chad Scherrer 05 Jan '07

05 Jan '07

When using MVars, is there a reason to prefer using MVar (a,b) over (MVar a, MVar b), or vice versa? I'm not sure if this is really a question of style, or if there are practial implications I'm missing one way or another. Thanks! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Composing functions with runST
by Yitzchak Gale 05 Jan '07

05 Jan '07

Can anyone explain the following behavior (GHCi 6.6): Prelude Control.Monad.ST> runST (return 42) 42 Prelude Control.Monad.ST> (runST . return) 42 <interactive>:1:9: Couldn't match expected type `forall s. ST s a' against inferred type `m a1' In the second argument of `(.)', namely `return' In the expression: (runST . return) 42 In the definition of `it': it = (runST . return) 42 Thanks, Yitz

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RE: [Haskell] Re: state of HaXml?
by Simon Marlow 05 Jan '07

05 Jan '07

[ moving to haskell-café... ] Norman Ramsey wrote: > > There seems to be a misunderstanding here: readFile in > itself is not the > > solution. readFile is defined thus: > > > > readFile name = openFile name ReadMode >>= hGetContents > > > and the original code was this: > > > > load fn = do handle <- IO.openFile fn IO.ReadMode > > contents <- IO.hGetContents handle > > IO.hClose handle > > return $ XP.xmlParse fn contents > > > Sure, you can replace the openFile/hGetContents pair by > readFile, but the > > real problem is the presence of the hClose. Removing that > will solve your > > problem (but note that you now have no control over when > the file is > > actually closed). > > Can I just leave it hanging and rely on the garbage collector to > close it in the fullness of time? Yes. The problem I was alluding to arises when you have many lazilly-closed files, and you run into the system's open file limit because the runtime doesn't close them eagerly enough. To be sure of closing the file at the right time, you need to force the entire file to be read (e.g. by forcing the result of the parse), then close the handle. > Because of laziness, I believe there's no point in my writing the > following: > > > load fn = do handle <- IO.openFile fn IO.ReadMode > > contents <- IO.hGetContents handle > > let xml = XP.xmlParse fn contents > > IO.hClose handle > > return xml > > Is that correct? Yes. Cheers, Simon

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Re: [Haskell] Re: state of HaXml?
by dons＠cse.unsw.edu.au 04 Jan '07

04 Jan '07

nr: > > Sure, you can replace the openFile/hGetContents pair by readFile, but the > > real problem is the presence of the hClose. Removing that will solve your > > problem (but note that you now have no control over when the file is > > actually closed). > > Can I just leave it hanging and rely on the garbage collector to close > it in the fullness of time? Yeah, once your program has demanded the entire file, it'll close the Handle. > Because of laziness, I believe there's no point in my writing the > following: > > > load fn = do handle <- IO.openFile fn IO.ReadMode > > contents <- IO.hGetContents handle > > let xml = XP.xmlParse fn contents > > IO.hClose handle > > return xml > > Is that correct? Yep. Its not neccessary in the usual programming cases to explicitly close the handle. IF you start really hammering the filesystem do you start to care about ensuring files are closed (so you don't hang on to too many FDs). Or if you start mutating files on disk. For these situations there are strict readFiles, or Data.ByteString.readFile -- Don

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Haskell Weekly News: January 02, 2007
by dons＠cse.unsw.edu.au 04 Jan '07

04 Jan '07

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20070102 Issue 55 - January 02, 2007 --------------------------------------------------------------------------- Welcome to issue 55 of HWN, a weekly newsletter covering developments in the Haskell community. This week brings a new release of vty and HsColour, and some interesting discussion over the holiday break. Announcements hscolour-1.6. Malcolm Wallace [1]announced HsColour, a popular syntax-highlighter for Haskell code. It can generate ANSI terminal colour codes, HTML, and CSS, and can insert hyperlink anchors for function definitions (useful in conjunction with [2]Haddock). [3]HsColour-1.6 is now available. The major addition is a new LaTeX output mode. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14677 2. http://haskell.org/haddock 3. http://www.cs.york.ac.uk/fp/darcs/hscolour Dimensional: Statically checked physical dimensions. Bj�rn Buckwalter [4]announced version 0.1 of [5]Dimensional, a module for statically checked physical dimensions. The module facilitates calculations with physical quantities while statically preventing e.g. addition of quantities with differing physical dimensions. 4. http://article.gmane.org/gmane.comp.lang.haskell.general/14691 5. http://code.google.com/p/dimensional/ vty 2.0. Stefan O'Rear [6]announced a new major version of [7]vty. Differences from 1.0 include: vty now uses a record type for attributes, instead of bitfields in an Int; vty now supports setting background colors; you can now explicitly specify 'default' colors; vty now supports Unicode characters on output, automatically setting and resetting UTF-8 mode. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/14703 7. http://members.cox.net/stefanor/vty 'Lambda Revolution' tshirts. Paul Johnson [8]announced the creation of a new Haskell tshirt, on the theme of 'The Lambda Revolution'. Tshirts are available from [9]CafePress, and the designs are freely available. 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17916 9. http://www.cafepress.com/l_revolution Discussion Beautiful concurrency. Simon Peyton-Jones [10]mentioned that he's been writing a chapter on concurrency and transactional memory for a new book, 'Beautiful code'. [11]A first draft is available and Simon welcomes constructive suggestions for improvement. The book is aimed at a general audience of programmers, not Haskell geeks, so tries to explain everything necessary. If you are not a Haskell expert, your input would be particularly valuable. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14681 11. http://haskell.org/haskellwiki/Talk:SantaClausProblem Limits to implicit parallelism in functional applications. John DeTreville [12]announced a short paper about how much implicit parallelism there might be in ordinary functional applications. 12. http://article.gmane.org/gmane.comp.lang.haskell.general/14699 Inlining higher order functions. Norman Ramsey [13]asked about fine grained control for inlining in higher order functions. 13. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/11467 Red-black trees as a nested datatype. Jim Apple [14]described how to implement red-black trees as a nested datatype. 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17957 SYB for XML: deserialization and collections. Alexander Jacobson [15]asked about approaches to simplifying boilerplate in HAppS associated with XML serialization and state deserialization. 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17922 Flattening a lisp-style tree. pphetra [16]asked about flattening heterogeneous lists (or trees) in Haskell. 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17976/focus=17976 Jobs Functional programming at Jane Street Capital. Yaron Minsky [17]announced that [18]Jane Street Capital is again looking to hire some top-notch functional programmers. Of particular note is that Jane Street Europe Ltd. now has an office in London, and we are particularly interested in hiring someone for that office with strong systems administration skills in addition to experience with functional programming languages. The ideal candidate has: a commitment to the practical, experience with functional programming languages (such as Haskell). Applicants should also have experience with UNIX and a deep understanding of computers and technology and a strong mathematical background. 17. http://article.gmane.org/gmane.comp.lang.haskell.general/14683 18. http://www.janestcapital.com/tech.html Blog noise [19]Haskell news from the blogosphere. * [20]Secret Santas in Haskell III: Lather, Rinse, Repeat 1 * [21]More Haskell in Java 7 or 8? * [22]Generalized vs. dynamic interfaces * [23]Type classes in Java * [24]Static vs dynamic typing: do what thou wilt * [25]Hoogle 4 progress * [26]Hoogle Progress, HsMan features * [27]The York Haskell Compiler: 1000 patches! * [28]Evaluating cellular automata is co-monadic * [29]Tying Knots Generically * [30]Rails -> Ruby -> Haskell * [31]A Sermon on Programming Languages * [32]Thoughtful revision control * [33]readFile' * [34]Code unraveller * [35]The Haskell Meta Tutorial * [36]Monads * [37]My Haskell Experience * [38]Communities * [39]Darcs: The source code management system of the future? * [40]On strong type systems * [41]On Programming Languages and Productivity 19. http://planet.haskell.org/ 20. http://mult.ifario.us/articles/2006/12/22/secret-santas-in-haskell-iii-coll… 21. http://blogs.msdn.com/ralflammel/archive/2006/12/21/more-haskell-in-java-7-… 22. http://blogs.msdn.com/ralflammel/archive/2006/12/23/generalized-vs-dynamic-… 23. http://syntaxfree.wordpress.com/2006/12/23/et-tu-brute/ 24. http://syntaxfree.wordpress.com/2006/12/24/static-vs-dynamic-typing-do-what… 25. http://neilmitchell.blogspot.com/2006/12/hoogle-4-progress.html 26. http://neilmitchell.blogspot.com/2006/12/hoogle-progress-hsman-features.html 27. http://yhc06.blogspot.com/2006/12/1000-patches.html 28. http://sigfpe.blogspot.com/2006/12/evaluating-cellular-automata-is.html 29. http://sigfpe.blogspot.com/2006/12/tying-knots-generically.html 30. http://rubynations.wordpress.com/2006/12/28/evolutionrails-ruby-haskell/ 31. http://www.dailyspeculations.com/wordpress/?p=101 32. http://koweycode.blogspot.com/2006/12/distributed-chiming-in.html 33. http://koweynlg.blogspot.com/2006/12/meh.html 34. http://koweycode.blogspot.com/2006/12/unraveller.html 35. http://koweycode.blogspot.com/2006/12/haskell-metatutorial.html 36. http://hierodule.livejournal.com/69052.html 37. http://kawagner.blogspot.com/2006/12/my-haskell-experience.html 38. http://gravityboy.livejournal.com/31930.html 39. http://www.iovene.com/content/view/93/1/ 40. http://blog.tmorris.net/strong-type-systems/ 41. http://www.neilmix.com/2007/01/01/on-programming-languages-and-productivity/ Quotes of the Week * chessguy: [in regards to #haskell] man, it's amazing the difference between what happens when someone asks for help here, and what happens when they ask for help in another language channel * cjeris: It's amazing what some languages do to make thinking impossible, seemingly justified by the assumption that no one thinks anyway, so it's more important to make non-thinking programming as easy as possible. * edwinb: I've just walked past a poster advertising a gig by a band called 'The Awkward Squad'. I assume this means they provide output, play concurrently, and people take exception to them. * kfish: Apparently @pl also doubles as the command for producing an unintelligible flip-stream * Binkley: [Monads as clothes] using unsafePerformIO is kind of like going naked in public, might be safe in some contexts, but you really don't want to know what happens if you do it in a really bad one * quazimodo: I know why you guys are so ready to deal with me and put up with noob questions ... you program so fast compared to other language users that you have time to mess around? * iulus: IO, IO, it's off to bind we go ... * dons: all your imperative are belong to us * Logan Capaldo: I like constructing things with type errors. It lets me read the error and try and figure out what I'm really trying to do * glguy: In true Haskell form, after I realized what I was actually doing... all my functions melted down to about 2 lines each * jcreigh: I've found learning Haskell makes me feel vastly inferior to Haskell coders. ('Oh,', they say, 'That's just a fold over the hyper-monad fluxbox list. Here's the one-line replacement for your entire program.') * Saizan: [New Year's Fun] Yesterday I was so drunk I was trying to typecheck the people at the party... 'What's your monad!?' Code Watch Wed Dec 27 17:03:48 PST 2006. Manuel M T Chakravarty. [42]Parse and desugar equational constraints. With -findexed-types, equational constraints can appear in contexts wherever class predicates are allowed. The two argument types need to be boxed and rank 0. 42. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18551 About the Haskell Weekly News Each week, new editions are posted to [43]the Haskell mailing list as well as to [44]the Haskell Sequence and [45]Planet Haskell. [46]RSS is also available, and headlines appear on [47]haskell.org. Headlines are available as [48]PDF. The Haskell Weekly News is also [49]available in Spanish translation. To help create new editions of this newsletter, please see the [50]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [51]http://www.cse.unsw.edu.au/~dons/code/hwn 43. http://www.haskell.org/mailman/listinfo/haskell 44. http://sequence.complete.org/ 45. http://planet.haskell.org/ 46. http://sequence.complete.org/node/feed 47. http://haskell.org/ 48. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20070102.pdf 49. http://haskell.org/haskellwiki/HWN/es 50. http://haskell.org/haskellwiki/HWN 51. http://www.cse.unsw.edu.au/~dons/code/hwn

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