- Haskell-Cafe - Haskell.org

on Happstack - "embedded HTML"
by Günther Schmidt 12 Jan '10

12 Jan '10

Hi, I'm just introducing myself to Happstack. I come across an example page where HTML and haskell code is mixed. Is this how happstack produces html, it's haskell code embedded in HTML? Günther

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Tokenizing and Parsec
by Günther Schmidt 12 Jan '10

12 Jan '10

Hi all, I've used Parsec to "tokenize" data from a text file. It was actually quite easy, everything is correctly identified. So now I have a list/stream of self defined "Tokens" and now I'm stuck. Because now I need to write my own parsec-token-parsers to parse this token stream in a context-sensitive way. Uhm, how do I that then? Günther a Token is something like: data Token = ZE String | OPS | OPSShort String | OPSLong String | Other String | ZECd String deriving Show

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hackage build errors
by Keith Sheppard 12 Jan '10

12 Jan '10

Hello Cafe, I noticed on my package's hackage page there is a build failure message: http://hackage.haskell.org/package/txt-sushi I don't know if these are real errors or not (I don't experience them on OS X and it's pure Haskell code) but I did poke around and noticed that some popular packages on hackage also have build failures: http://hackage.haskell.org/package/xmonad http://hackage.haskell.org/package/haddock http://hackage.haskell.org/package/pandoc http://hackage.haskell.org/package/alex http://hackage.haskell.org/package/hlint ... so what should I make of these errors? Are they useful in some way or just a problem with the build environment? (If that's the case I think they should probably be removed since they're confusing for potential users) Best Keith -- keithsheppard.name

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Haskell-iPhone mailing list?
by Tom Tobin 11 Jan '10

11 Jan '10

Is there a mailing list for efforts to get Haskell running on the iPhone? As I've been maintaining an iPhone app for my employer (and nearly foaming at the mouth with hatred for Objective-C), I'd love to see a more sane option in this space. I've seen the wiki page [1], but I'm not aware of any other space for collaboration here. [1] http://www.haskell.org/haskellwiki/IPhone

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ANN: chp-2.0.0, chp-plus-1.0.0
by Neil Brown 11 Jan '10

11 Jan '10

Hi, I've just released version 2.0.0 of the Communicating Haskell Processes (CHP) message-passing concurrency library onto Hackage. The main change from 1.x is that I have split the functionality into two libraries: the core functionality remains in the chp package (now version 2.0.0), while some of the additional capabilities have been moved out to the new chp-plus package (version 1.0.0). This should help in the long run by keeping the packages from becoming too large, and by allowing me to trim the dependencies of the core chp package. I've written more about the changes here, for those who it will affect: http://chplib.wordpress.com/2010/01/11/splitting-chp/ This will require small changes to your build system if you're using CHP and you upgrade to the new versions. The new versions should also fix build troubles with GHC 6.12 -- let me know if you have any troubles installing under 6.10 or 6.12. Hackage links: http://hackage.haskell.org/package/chp-2.0.0 http://hackage.haskell.org/package/chp-plus-1.0.0 Thanks, Neil.

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Update for type-level library (0.2.4)
by Seyed Hosein Attarzadeh Niaki 11 Jan '10

11 Jan '10

Dear Haskell developers, A new version of the type-level library for type-level programming is pushed into the development repository and uploaded to the Hackage database. This is only a minor update to fix the compatibility issues with base 4, GHC>=6.10 and cabal-install (not backward compatible). Bests, Hosein Attarzadeh

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Cabal bug?
by Henk-Jan van Tuyl 11 Jan '10

11 Jan '10

L.S., I had a strange response from Cabal: >cabal upgrade hlint Resolving dependencies... cabal: cannot configure containers-0.3.0.0. It requires base >=4.2 && <6 For the dependency on base >=4.2 && <6 there are these packages: base-4.2.0.0. However none of them are available. base-4.2.0.0 was excluded because of the top level dependency base -any So, base-4.2.0.0 cannot be used, because any version of base is OK? Is this a bug in Cabal? Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Approaches to dependent types (DT)
by pat browne 11 Jan '10

11 Jan '10

Hi, I am trying to understand the concept of dependent types as described in Haskell literature (Hallgren 2001),(Hinze,Jeuring et al. 2006). I am particularly interested in understanding two flavours of dependency mentioned by Hinze et al: 1) Types depending on values called dependent types 2) Types depending on types called parametric and type-indexed types I think that ascending levels of abstraction are important here: values - types - classes (Hallgren 2001). Here is a Haskell example of the use of DT: class Named object name | object -> name where name :: object -> name instance (Eq name, Named object name) => Eq object where object1 == object2 = (name object1) == (name object2) I think that the intended semantics are: 1) Objects have names and can be compared for equality using these names. 2) Two objects are considered equal (identical) if they have the same name. 3) The type of a name depends on the type of the object, which gets expressed as a type dependency (object -> name). I am not sure about the last semantic it might be interpreted as "the named object depends on..". This may indicate a flaw in my understanding of DT. I am aware that dependent types can be used to express constraints on the size of lists and other collections. My understanding of Haskell's functional dependency is that object -> name indicates that fixing the type object should fix the type name (equivalently we could say that type name depends on type object). The class definition seems to show a *type-to-type* dependency (object to name), while the instance definition shows how a name value is used to define equality for objects which could be interpreted as a *value-to-type* dependency (name to object) in the opposite direction to that of the class. My questions are: Question 1: Is my understanding correct? Question 2: What flavour of DT is this does this example exhibit? Best regards, Pat Hallgren, T. (2001). Fun with Functional Dependencies. In the Proceedings of the Joint CS/CE Winter Meeting, Varberg, Sweden, January 2001. Hinze, R., J. Jeuring, et al. (2006). Comparing Approaches to Generic Programming in Haskell. Datatype-generic programming: international spring school, SSDGP 2006.

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mtl and transformers
by Günther Schmidt 11 Jan '10

11 Jan '10

Hi, when I cabal-installed the iteratee package, the transformers package was also installed as a dependency. Now when I run applications that import Control.Monad.Transformers I get this: Could not find module `Control.Monad.Trans': it was found in multiple packages: transformers-0.1.4.0 mtl-1.1.0.2 What's the work around for situations like this? Günther

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Haskell Weekly News: Issue 145 - January 10, 2010
by jfredett＠gmail.com 10 Jan '10

10 Jan '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100110 Issue 145 - January 10, 2010 --------------------------------------------------------------------------- Welcome to issue 145 of HWN, a newsletter covering developments in the [1]Haskell community. Welcome back Haskellers to the HWN! After a short hiatus we return with a megaedition covering the last three weeks of Haskell news and discussion. Needless to say, lots of new package and event announcements, as well as some really great discussions (the discussion about 'lawless' uninstances of Functors was particularly interesting) over the holiday break. Hopefully everyone had a safe and happy holiday season, and is back now steadily working off the winter-weight (in accordance with resolutions or no) by tapping steadily at the keyboard, producing wonderful new packages for me to place in this, your Haskell Weekly News! Announcements Last CfP for TGC 2010: EXTENDED deadline Jan. 20 2010. Emilio Tuosto [2]announced an extension to the TGC'10 call for papers. SourceGraph-0.6.0.0 and Graphalyze-0.9.0.0. Ivan Lazar Miljenovic [3]announced new releases of the SourceGraph and Graphalyze packages. Streaming Component Combinators 0.4. Mario Blazevic [4]announced version 0.4 of Streaming Component Combinators (SCC). safer-file-handles-0.1. Bas van Dijk [5]announced a new member of the 'monadic regions' family -- safer-file-handles. This package provides safety features on top of System.IO for handling file handles and operations. HOR 2010 1st CALL FOR ABSTRACTS. Frederic Blanqui [6]1f4e gmane.comp.lang.haskell.general/17710 announced the first call for abstracts for the Fifth International Workshop on Higher-Order Rewriting. vty-4.2.1.0. Corey O'Connor [7]announced a new version of the vty package. stm-io-hooks-0.6.0. Peter Robinson [8]announced a new version of stm-io-hooks. This version includes major interface changes, including the elimation of the 'onRetry' combinator. hecc-0.2. Marcel Fourne [9]announced the second release of his elliptic curve cryptography package, hecc. This release includes a license change to the BSD3 license, speed improvements, and new algorithms for point multiplication, as well as other changes. ghc-6.12.1 binary package for OpenSolaris. Michael Lee [10]announced a binary package availability of ghc-6.12 for OpenSolaris. Final Call for Participation: TLDI'10. Andrew Kennedy [11]announced the final call for participation in TLDI'10, the Workshop on Types in Language Design and Implementation. Haskell Web News: Looking back on 2009. Don Stewart [12]announced The Haskell Web News, a monthly summary of the hottest news about the Haskell programming language. hakyll-0.4. Jasper Van der Jeugt [13]announced the release of hakyll, a static site generator library. system-uuid-1.2.0. Jason Dusek [14]announced a new release of system-uuid. New versions of ALL the monadic regions packages. Bas van Dijk [15]announced a new version of all of the monadic regions packages, explanation of what this means is best left to the original post, linked previously. tuntap-0.0.1. John Van Enk [16]announced a new package, extracted from his in-progress VPN project. This package provides access to the TUN/TAP device in Linux. CPython / libpython bindings. John Millikin [17]announced a package providing bindings to the C API for CPython/libpython. Discussion Typed Configuration Files. Sebastian Fischer [18]asked about whether there was an analogue to CmdArgs for config files. Review request for my permutations implementation. CK Kashyap [19]asked for a review of his implementation of a function for finding permutations -- in addition to a review he asked for earlier for code involving monads. The result is a nice pair of threads with introductory material about idiomatic, efficient Haskell code. lawless instances of Functor. Paul Brauner [20]asked about 'lawless' instances (Editor's Note: more so -- 'uninstances', since by definition, it cannot be a functor without satisfying the laws, even if it an instance can be written) of the `Functor` class. This is in an effort to better understand how Functors work in Haskell and in the wider theory. Blog noise [21]Haskell news from the [22]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Neil Mitchell: [23]Using .ghci files to run projects. * Tom Moertel: [24]A formal language for recipes: brain dump. * Mark Jason Dominus: [25]A monad for probability and provenance. * Alex Mason: [26]A small follow up. * Russell O'Connor: [27]Constructive Classical Completeness. * Galois, Inc: [28]GHC Nominated for Programming Language Award. * Neil Brown: [29]CHP vs CML, Forking and Picky Receivers. * Conal Elliott: [30]Is Haskell a purely functional language?. * Conal Elliott: [31]Can functional programming be liberated from the von Neumann paradigm?. * Real-World Haskell: [32]Real World Haskell in Japaneseâå®æŠã§åŠã¶é¢æ°åèšèªããã°ã©ãã³ã°. * Conal Elliott: [33]Garbage collecting the semantics of FRP. * Alex Mason: [34]Why I love Cereal. * Mark Jason Dominus: [35]A short bibliography of probability monads. * Conal Elliott: [36]Communication, curiosity, and personal style. * Epilogue for Epigram: [37]Bidirectional Basics. * Neil Mitchell: [38]Explaining Haskell IO without Monads. * Conal Elliott: [39]Is program proving viable and useful?. * Conal Elliott: [40]Why program with continuous time?. Quotes of the Week * fasta: Haskell is like an efficient version of Python and is used for the same things. * koeien37: there are the languages that everybody complains about, and the languages that nobody use * Pseudonym: Olegs can be warded off by adding a note claiming that it's impossible to implement in the type system. * elly: I leave for five minutes and godel numbering of endofunctors is invokved as a potential proof that the universe is simulated. Thanks, #haskell :P * kmc: a monad is like an invisible train filled with jello traveling backwards in time * conal: "Obviously" is one of those words that is most useful when it is least true * Jafet: Javascript is pretty much a DSL for making your web browser take up more CPU * copumpkin: limpac: in this channel we're all automata with no feelings, except for the occasional feeling of hate. * dmhouse: I'm not sure it's working correctly but it sure is fast... * Cale: We need a bunch of "Will it monad?" video clips, with "It monads!" at the end of each. * merijn: I run on HST (Hacker Standard Time) which is essentially current time zone -3 * ray: [about an anti-FP blog post] i came up with some contrived example, announced that it's impossible, and therefore FP sucks * hiredman: I used to think "dons" was a title, like people who were recognized as being really good at haskell were called dons About the Haskell Weekly News New editions are posted to [41]the Haskell mailing list as well as to [42]the Haskell Sequence and [43]Planet Haskell. [44]RSS is also available, and headlines appear on [45]haskell.org. To help create new editions of this newsletter, please see the information on [46]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [47]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17716 3. http://article.gmane.org/gmane.comp.lang.haskell.general/17714 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17713 5. http://article.gmane.org/gmane.comp.lang.haskell.general/17711 6. http://article.gmane.org/ 7. http://article.gmane.org/gmane.comp.lang.haskell.general/17705 8. http://article.gmane.org/gmane.comp.lang.haskell.general/17704 9. http://article.gmane.org/gmane.comp.lang.haskell.general/17701 10. http://article.gmane.org/gmane.comp.lang.haskell.general/17700 11. http://article.gmane.org/gmane.comp.lang.haskell.general/17699 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17698 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68864 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68857 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68827 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68807 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68388 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/68865 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/68811 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/68711 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://neilmitchell.blogspot.com/2010/01/using-ghci-files-to-run-projects.h… 24. http://blog.moertel.com/articles/2010/01/08/a-formal-language-for-recipes-b… 25. http://blog.plover.com/prog/haskell/probmonad.html 26. http://random.axman6.com/blog/?p=139 27. http://r6.ca/blog/20100107T035902Z.html 28. http://www.galois.com/blog/2010/01/06/ghc-nominated-for-programming-languag… 29. http://chplib.wordpress.com/2010/01/06/chp-vs-cml-forking-and-picky-receive… 30. http://conal.net/blog/posts/is-haskell-a-purely-functional-language/ 31. http://conal.net/blog/posts/can-functional-programming-be-liberated-from-th… 32. http://www.realworldhaskell.org/blog/2010/01/05/real-world-haskell%e2%80%94… 33. http://conal.net/blog/posts/garbage-collecting-the-semantics-of-frp/ 34. http://random.axman6.com/blog/?p=124 35. http://blog.plover.com/prog/haskell/probmonad-refs.html 36. http://conal.net/blog/posts/communication-curiosity-and-personal-style/ 37. http://www.e-pig.org/epilogue/?p=283 38. http://neilmitchell.blogspot.com/2010/01/haskell-io-without-monads.html 39. http://conal.net/blog/posts/is-program-proving-viable-and-useful/ 40. http://conal.net/blog/posts/why-program-with-continuous-time/ 41. http://www.haskell.org/mailman/listinfo/haskell 42. http://sequence.complete.org/ 43. http://planet.haskell.org/ 44. http://sequence.complete.org/node/feed 45. http://haskell.org/ 46. http://haskell.org/haskellwiki/HWN 47. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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ANNOUNCE: SourceGraph-0.6.0.0 and Graphalyze-0.9.0.0
by Ivan Lazar Miljenovic 10 Jan '10

10 Jan '10

I'm pleased to announce the latest releases of SourceGraph [1] and Graphalyze [2]. [1]: http://hackage.haskell.org/package/SourceGraph [2]: http://hackage.haskell.org/package/Graphalyze SourceGraph is a program that performs static code analysis on Haskell projects on the by applying graph theoretic techniques to the project's call graph. Graphalyze is a library for analysing discrete data using graph theory, and as such performs the heavy lifting for SourceGraph. Sample analysis reports generated by SourceGraph are available at http://code.haskell.org/~ivanm/Sample_SourceGraph/SampleReports.html . I will also be demoing SourceGraph at PEPM [3] in Madrid on 19 January. [3]: http://www.program-transformation.org/PEPM10/ Changes since the previous version include: * Now supports "implicitly exported" entities (as requested by Curt Sampson). This includes instantiated class methods from other modules and functions, etc. that start with an underscore (see http://www.haskell.org/ghc/docs/latest/html/users_guide/options-sanity.html for more information). * All inaccessible entities are now reported, not just those that were root nodes in the call graph. * Edges are now colour-coded based upon whether they are part of a clique, cycle or a chain. * Level-based analyses: visualise how deep an entity is from those exported entities. * A re-done TODO that lists in detail what is planned for SourceGraph. * Lots of under-the-hood changes that don't sound as interesting as the above :-( -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Hackage down
by Neil Mitchell 10 Jan '10

10 Jan '10

Hi Hackage is down: http://downforeveryoneorjustme.com/hackage.haskell.org Thanks, Neil

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unexpected behavior from filterM doesFileExist =<< getDirectoryContents
by Thomas Hartman 10 Jan '10

10 Jan '10

Can somebody explain this? > getDirectoryContents inD ["..","#sanity.txt#",".","sanity.txt","etc.txt","patchTagDir.txt","jail.txt","notjail.txt","alldata.txt","allobjs.txt","namesNSizes.txt"] > filterM doesFileExist =<< getDirectoryContents inD > ["sanity.txt"]

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HackageDB Question
by Gregory Crosswhite 09 Jan '10

09 Jan '10

How does HackageDB generate the documentation for uploaded packages? Specifically, if I am using my own build system rather than Cabal, then what do I need to do (e.g., what interface do I need to supply) for HackageDB to correctly generate the documentation? I've glanced through the source code posted at http://darcs.haskell.org/hackage-scripts/, but I am having trouble finding the code which generates the documentation. Thanks! Greg

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Space Efficiency When Sorting a List of Many Lists
by Peter Green 09 Jan '10

09 Jan '10

Heinrich, thanks for some great help and food for thought! (More on performance of your solution below) > Thanks for describing the background of this problem in detail! I was > mainly asking because I'm always looking for interesting Haskell > topics > that can be turned into a tutorial of sorts, and this problem makes a > great example. Another interesting problem is starting from a file of single wagers and trying to compress them (i.e. inverse of 'explosion'. I believe this problem is analogous to Set Cover and therefore NP-Complete. Heuristics are the order of the day here. (OT, but might also make an interesting multi-part tutorial question/ topic: A simpler (fairly unrelated problem) is doing exact k-means clustering of a list of n items (i.e. single dimension data) via the k- segmentations of a (sorted by values one is clustering on) list of n items. The number of k-segmentations of an n-list is (n-1) Choose (k-1). One generates all the k-segmentations and each set of k- segments constitutes a candidate clustering solution. One selects the candidate solution which minimises the 'goodness of fit criterion' used in k-means clustering (easily found in the literature). Needless to say, this is only feasible for smallish n and for values of k closer to the LHS and RHS of Pascal's triangle. K-means clustering in more than one dimension is NP-Complete.) > Concerning optimization, the background also reveals some additional > informations, namely > > * The single wager combinations are short lists > * and each list element is small, i.e. `elem` [1..20] > * No duplicate single wagers > * You are free to choose another ordering than lexicographic > >> I need to explode these compressed wagers back to single combinations >> because, I need to (eventually) do set comparisons on collections of >> single combinations. i.e. given File A and File B of compressed >> wagers, >> I need to be able to answer questions like: >> >> (1) How many single combinations are common to File A and File B >> (2) How many single combinations in File A are missing from File B >> .. >> .. >> etc. i.e a few of the common set comparison operations. >> >> And the dumbest, quickest and dirtiest and most idiot-proof way of >> doing >> this as a baseline approach before I start using maps, tries, >> etc... is >> to explode Files A and B into lex sorted single combinations order >> and >> then use diff -y with judicious grepping for '>' and <'' > > Looks good to me, sorted lists are a fine data structure for set > operations. > >> I'm probably going to go with an external sort for starters to keep >> memory usage down. For most use-cases, I can get away with my current >> in-memory sort - just have to beware edge cases. > > Using an out-of-the box external sort library is probably the path of > least resistance, provided the library works as it should. Seems to be some issue with the external-sort package. As it currently exists, it blows up on my [[Int]] data and exhibits more memory usage than vanilla sort! I'm keen to look into fixing this as might be one small way I can give back, but still taking baby steps with Haskell. > However, your problem has a very special structure; you can interleave > explode and sort and turn the algorithm into one that is partially > on-line, which will save you a lot of memory. I've tried to show how > to > do this in my previous post; the resulting code will look something > like > this > > {-# LANGUAGE NoMonomorphismRestriction #-} > import qualified Data.Map > import Control.Arrow (second) > > newtype Map a b = Map { unMap :: [(a,b)] } deriving (Eq, Show) > > instance Functor (Map a) where > fmap f = Map . (map . second) f . unMap > > hylo :: (Map a b -> b) -> (c -> Map a c) -> (c -> b) > hylo f g = f . fmap (hylo f g) . g > > type Row a = [a] > > headsOut :: Map a [Row a] -> [Row a] > headsOut (Map []) = [[]] > headsOut m = [x:row | (x,rows) <- unMap m, row <- rows] > > explode1 :: [Row [a]] -> Map a [Row [a]] > explode1 rows = Map [(x,[row]) | (xs:row) <- rows, x <- xs] > > sort1 :: Ord a => Map a [b] -> Map a [b] > sort1 = Map . Data.Map.toList . Data.Map.fromListWith (++) . unMap > > sortExplode = hylo headsOut (sort1 . explode1) > > example = [[[8,12],[11],[7,13],[10]], [[1,2,3],[1,9],[3,6],[4]]] > test = sortExplode example > > main = interact (unlines . toCSV . sortExplode . fromCSV . lines) > > In other words, sortExplode will "stream" the sorted single wagers > on > demand, unlike the monolithic sort . explode which has to produce > all > single wagers before sorting them. Thank you *very* much for this code! I will try to get my head around it. I understand the broad outline you posted elsewhere, but will take me a while to fully grasp the above as I'm only up to ~p200 in Real World Haskell :). As for performance of your code above on my file of compressed wagers which expands to 3.7M single wagers: (My original version posted here and using vanilla sort) 541 MB total memory in use (5 MB lost due to fragmentation) INIT time 0.00s ( 0.01s elapsed) MUT time 13.98s ( 15.82s elapsed) GC time 8.69s ( 9.64s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 22.67s ( 25.47s elapsed) (Your much improved version) 10 MB total memory in use (1 MB lost due to fragmentation) INIT time 0.00s ( 0.00s elapsed) MUT time 7.61s ( 9.38s elapsed) GC time 3.48s ( 3.58s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 11.08s ( 12.95s elapsed) Very impressive and thanks again! >> However, later on, I'm keen to do everything with set theoretic >> operations and avoid writing large files of single combinations to >> disk. > > Note that there's also the possibility of not expanding the compressed > wagers at all, and perform set operations directly. For instance, it > is > straightforward to intersect two such sets of size n in O(n^2) time. > Since n ~ 5000 , n^2 is about the same ballpark as the exploded single > wager combinations. Not sure I quite understand you here. In my mind, set elements *are* single combinations. It is possible for two quite different-looking files of compressed wagers to contain exactly the same single wager elements. So I'm not sure how to compare without some notion of explosion to single combinations. >> So, I guess the things I'd like to get a feel for are: >> >> (1) Is it realistic to expect to do in-memory set comparisons on sets >> with ~1M elements where each element is a (however-encoded) list of >> (say) 6 integers? I mean realistic in terms of execution time and >> space >> usage, of course. >> >> (2) What would be a good space-efficient encoding for these single >> combinations? I know that there will never be more than 8 integers >> in a >> combination, and none of these values will be < 1 or > 20. So >> perhaps I >> should map them to ByteString library Word8 strings? Presumably >> sorting >> a big list of ByteStrings is going to be faster than sorting a big >> list >> of lists of int? > > The overhead for lists and sets are quite high, it's not uncommon > for 1M > elements to occupy 10M-100M of memory. > > Not to mention that your elements are, in fact, small lists, > introducing > another factor of 10-100. But this can indeed be ameliorated > considerably by representing your elements in a packed format like > ByteStrings or a Word64. I haven't tried packing ByteStrings yet in this context, but *did* get some improvement in external-sort performance when using ByteStrings instead of [[Int]]. Presumably because it somewhat ameliorated the effects of a bug in external-sort where blocks are fully compared rather than just the heads of blocks. I'll be interested to play around with your version more and see if it benefits much from ByteString. It's already very good now in terms of space usage, but perhaps can speed up the I/O (output dominates, of course). I'll be learning from your post for a long time. So thanks again!

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Upgrading Arch Linux Haskell Packages
by Cetin Sert 09 Jan '10

09 Jan '10

Hi, Is there a way to manually upgrade Haskell packages in the aur repository of Arch Linux which were previously contributed automatically by arch-haskell? Or is there a batch upgrade planned for the near future? What is the mailing list for arch Best Regards, Cetin

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Re: [Haskell-cafe] Infix instance headers
by Bas van Dijk 08 Jan '10

08 Jan '10

OK thanks, Bas On Wed, Dec 23, 2009 at 10:50 AM, Simon Peyton-Jones <simonpj(a)microsoft.com> wrote: > Ah I see. It's an oversight. Thank you for pointing that out. > > I'll fix it, but it'll be in the HEAD not 6.12. > > Simon > > | -----Original Message----- > | From: Bas van Dijk [mailto:v.dijk.bas@gmail.com] > | Sent: 22 December 2009 10:52 > | To: Simon Peyton-Jones > | Subject: Re: [Haskell-cafe] Infix instance headers > | > | On Tue, Dec 22, 2009 at 9:53 AM, Simon Peyton-Jones > | <simonpj(a)microsoft.com> wrote: > | > works for me. what version of GHC are you using? > | > | ghc-6.12.1 > | > | Sorry, maybe I wasn't totally clear. The example I posted works but if > | you write the class name in the instance header as infix I get a > | "Malformed instance header" error. > | > | So the following doesn't work: > | > | class (Monad pr, Monad cr) => pr `ParentOf` cr > | > | instance Monad r => r `ParentOf` r > | > | instance ( Monad cr > | , cr `TypeCast2` RegionT resource s pcr > | , pr `ParentOf` pcr > | ) > | => pr `ParentOf` cr > | > | regards, > | > | Bas > >

1 1

Re: ANNOUNCE: Clutterhs 0.1
by Gour 08 Jan '10

08 Jan '10

On Sat, 28 Nov 2009 23:18:00 -0500 >>>>>> "Matt" == Matt Arsenault <arsenm2(a)rpi.edu> wrote: Hi Matt, Matt> This is a very early announcement for an initial release of Matt> bindings for Clutter 1.0, now on Matt> Hackage,http://hackage.haskell.org/package/clutterhs Thanks a lot for this project. I'm just thinking about using Haskell on Moblin OS. ;) Matt> It uses c2hs, and depends on gtk2hs from darcs for the Matt> glib/gobject bindings. Overall, its usage should be similar to Matt> gtk2hs. This is very cool. Matt> This is my first Haskell project, and suggestions on the API, bug Matt> reports, etc. are appreciated. What do you think about binding Moblin's nbtk (now it's called mx) ? Otoh, are you aware of: http://github.com/elliottt/clutter & http://github.com/yav/clutter Sincerely, Gour -- Gour | Hlapicina, Croatia | GPG key: F96FF5F6 ----------------------------------------------------------------

4 7

Testing for statistical properties
by Gregory Crosswhite 08 Jan '10

08 Jan '10

Hey everyone! I have some computations that satisfy statistical properties which I would like to test --- that is, the result of the computation is non-deterministic, but I want to check that it is sampling the distribution that it should be sampling. Is anyone aware of a Haskell library out there that does anything like this? Cheers, Greg

2 2

telling ghc to run several jobs in parallel
by Paul Brauner 08 Jan '10

08 Jan '10

Hi, I have to processors but ghc --make only uses one of them, even if some files could be compiled in parallel. Is there some option similar to the -j one of the make tool ? (I read the manual but didn't find it) Regards, Paul

2 1

Splitting a Hackage project (was ANN: Streaming Component Combinators 0.4)
by Mario Blazevic 08 Jan '10

08 Jan '10

I couldn't find any Hackage policy guideline on the appropriate balance between the package list size and package size, so I'll just ask the question. I have released the version 0.4 of my SCC package yesterday. In this particular release, there is at least one module that's nicely self-contained, providing useful functionality of its own, and not expected to evolve much: http://hackage.haskell.org/packages/archive/scc/0.4/doc/html/Control-Concur… My question then is if I should split this module into a separate package or leave it where it is? If I split it out, what should I do about the ParallelizableMonad class in the module? > class Monad m => ParallelizableMonad m where > bindM2 :: (a -> b -> m c) -> m a -> m b -> m c This class and its instances would again be something *usable* both outside the SCC package and outside the Control.Concurrent.Coroutine module. But would they be *useful* and would they be *used*? Should I split the class out into a Control.ParallelizableMonad package? I'm sure I'm not the only one with the same dilemma. It would be nice to have some written rules to follow on what belongs on Hackage as a separate package. > Version 0.4 of Streaming Component Combinators, or SCC for short, has > been released on Hackage. Get it at > > http://hackage.haskell.org/package/scc > > There isn't much new high-level functionality compared to the > previous version, but the implementation has been heavily refactored and > the foundations completely replaced. > > I'm particularly happy to have found a way to drop the ugly reliance > on Data.Dynamic and to encode the required constraints in the type > system instead. The foundation of streaming components in this version > is the new Control.Concurrent.Coroutine module, whose main export is the monad > transformer Coroutine. It can transform any monad into a suspendable, resumable, > trampoline-style-runnable monad. Coroutines can be nested, which was the > requirement for streaming and the main stumbling block for the implementation. > The solution, worth at least 10 milliOlegs according to my estimate, was to > parameterize the Coroutine with a functor that wraps the coroutine suspension, > and to use nested functors for suspension from nested coroutines. The type system > automatically figures out how many wrappings to apply to each suspension > depending on how many intermediate coroutines it suspends. > > In other news is the project's Wiki page at > http://trac.haskell.org/SCC/wiki/. It's still rudimentary, but growing. > > All feedback will be appreciated. > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

1 0

ANN: hakyll-0.4
by Jasper Van der Jeugt 08 Jan '10

08 Jan '10

Hello, I am announcing the release of hakyll-0.4. Hakyll is a static site generator library written Haskell. It is written in a very configurable way and uses an xmonad-like DSL for configuration. Notable changes since the last big release (0.1) include: - CSS compression - Dependency handling (aka. not generate everything again every time) - A simple http server for previewing your site - Speed improvements and bug fixes - Several specialized functions for dealing with dates, tags... - Abstraction of context manipulations - Some tutorials are added and documentation is mostly complete - Example sites were added More information can be found at http://jaspervdj.be/hakyll All feedback is welcome. Kind regards, Jasper Van der Jeugt

1 0

Testing for statistical properties
by Lee Pike 08 Jan '10

08 Jan '10

Greg, > Hey everyone! I have some computations that satisfy statistical > properties which I would like to test --- that is, the result of the > computation is non-deterministic, but I want to check that it is > sampling the distribution that it should be sampling. Is anyone > aware of a Haskell library out there that does anything like this? I don't know of a library, but here are a couple of ideas/pointers: I wanted to do something vaguely similar in the realm of real-time hardware testing. I used QuickCheck (QC) <http://hackage.haskell.org/package/QuickCheck > to do some stochastic testing (in my case, generate test cases from the reals satisfying somewhat complex constraints), and I have a small patch to the QC library to make this possible. I have a short '09 Haskell Symposium paper about it (probably Sec. 5 is the most interesting). The paper and patch are here: <http://www.cs.indiana.edu/~lepike/pub_pages/qc-biphase.html >. One caveat though is that I didn't check that the tests generated by QC satisfied any particular distribution---I was assuming it is roughly uniformly distributed. Also, I didn't investigate how to generate other distributions, but I don't think that'd be too hard. FYI, Bryan O'Sullivan has a statistics library <http://hackage.haskell.org/package/statistics >, which I plan to use myself soon! I'm personally interested in how things turn out for you---please keep me posted. Lee

1 0

ANNOUNCE: system-uuid-1.2.0
by Jason Dusek 08 Jan '10

08 Jan '10

Patched by Antoine S. Latter to interoperate with UUID. http://hackage.haskell.org/package/system-uuid-1.2.0 -- Jason Dusek

1 0

ANNOUNCE: New versions of ALL the monadic regions packages
by Bas van Dijk 08 Jan '10

08 Jan '10

Hello, As I explained in my announcement of 'safer-file-handles', I discovered a serious lack of expressive power in my 'regions' package. I have now solved that problem in the way I envisaged by removing the 'resource' parameter from 'RegionT' and using existential quantification to bring the 'resource' type back at the place I need it but hidden from the outside. Now you're able to open multiple types of resources in a single region which all will be closed automatically on termination of the region. These are the new releases: http://hackage.haskell.org/package/regions-0.2 http://hackage.haskell.org/package/regions-monadsfd-0.2 http://hackage.haskell.org/package/regions-monadstf-0.2.0.1 http://hackage.haskell.org/package/usb-safe-0.5 http://hackage.haskell.org/package/safer-file-handles-0.2 (Note that in the process I shuffled a bit with the API's. That's one of the reasons all these are major releases.) Do you want to see this in action? See 'example.hs' in the following package that demonstrates opening multiple different types of resources in a single region. The example extends the example in the 'usb-safe-examples' package by first opening a USB device (my USB mouse) and a temporary file in the same region. Next the device is configured, an interface is claimed and an alternate is set. Next some bytes are read from an endpoint of the device. Finally these bytes are written to the temporary file. It's fun to see this in action so do try it out: darcs get http://code.haskell.org/~basvandijk/code/usb-safe-and-safer-file-handles-ex… regards and see some of you at FP-NL tomorrow! Bas

4 7

Review request for my permutations implementation
by CK Kashyap 08 Jan '10

08 Jan '10

Hi All, I've written this piece of code to do permutations - perms :: String -> [String] perms []= [] perms (x:[])= [[x]] perms (x:xs)= concat (f [x] (perms xs)) spread :: String -> String -> [String] -- interpolate first string at various positions of second string spread str1 str2 = _spread str1 str2 (length str2) where _spread str1 str2 0= [str1 ++ str2] _spread str1 str2 n= [(take n str2) ++ str1 ++ (drop n str2)] ++ (_spread str1 str2 (n-1)) f xs = map (spread xs) The number of outcomes seem to indicate that correctness of the algo .. however, I'd be very obliged if I could get some feedback on the Haskellness etc of this ... also any performance pointers ... Regards, Kashyap

5 7

darcs 2.4 beta 1 release
by Reinier Lamers 08 Jan '10

08 Jan '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.4 beta 1. darcs 2.4 will contain many improvements and bugfixes compared to darcs 2.3.1. Highlights are the fast index-based diffing which is now used by all darcs commands, and the interactive hunk-splitting in darcs record. This beta is your chance to test-drive these improvements and make darcs even better. If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install --reinstall darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.3.98.1.tar.gz , and build it by hand as explained in the README file. A list of important changes since 2.3.1 is as follows (please let me know if there's something you miss!): * Use fast index-based diffing everywhere (Petr) * Interactive patch splitting (Ganesh) * An 'optimize --upgrade' option to convert to hashed format in-place (Eric) * Hunk matching (Kamil Dworakowski, tat.wright) * Progress reporting is no longer deceptive (Roman Plášil) * A 'remove --recursive' option to remove a directory tree from revision control (Roman Plášil) * A '--remote-darcs' flag for pushing to a host where darcs isn't called darcs * Many miscellaneous Windows improvements (Salvatore, Petr and others) * 'darcs send' now mentions the repository name in the email body (Joachim) * Handle files with boring names in the repository correctly (Petr) * Fix parsing of .authorspellings file (Tomáš Caitt) * Various sane new command-line option names (Florent) * Remove the '--checkpoint' option (Petr) * Use external libraries for all UTF-8 handling (Eric, Reinier) * Use the Haskell zlib package exclusively for compression (Petr) Kind Regards, the darcs release manager, Reinier Lamers

5 6

Explicit garbage collection
by Miguel Mitrofanov 07 Jan '10

07 Jan '10

Is there any kind of "ST" monad that allows to know if some STRef is no longer needed? The problem is, I want to send some data to an external storage over a network and get it back later, but I don't want to send unnecessary data. I've managed to do something like that with weak pointers, System.Mem.performGC and unsafePerformIO, but it seems to me that invoking GC every time is an overkill. Oh, and I'm ready to trade the purity of runST for that, if necessary. Thanks.

3 12

Example of sending headers/cookies with Network.HTTP.simpleHTTP or Network.Browser?
by Matt Brown 07 Jan '10

07 Jan '10

Hi all, Does anyone know of a good/simple example of sending cookies and other headers in an HTTP request via simpleHTTP or Browser? I know what the cookies should be (i.e. I don't want them to be sent by the server). More specifically, I'm looking for an analogue to curl's -b and -H flags. thanks, -matt

1 0

Distinct types in a list
by rodrigo.bonifacio 07 Jan '10

07 Jan '10

Hi all, I have a family of parsers that return either (Success t) or (Fail), using the following data type: > data ParserResult a = Success a | Fail String > deriving (Read, Show, Eq, Ord) > > isSuccess (Success _) = True > isSuccess (Fail _) = False > ... I want to add the results of different parsers to a list. Such as: > m1 = parseFirstModel file1 -- it returns a ParserResult of t1 > m2 = parseSecondModel file2 -- it returns a ParserResult of t2 > ps = [m1, m2] In such a way that I could write something like: > if and (map isSuccess ps) > then process m1 m2 > else ... Actually, in the real program I have to check more than two input models. However, since Lists do only hold elements of a same type, I couldn't proceed in this way. Which improvements to the ParserResult data type should I wrote in order to proceed as I want to. Best regards, Rodrigo.

6 5

pcre-light install fails with undefined reference to _impure_ptr
by Patrick Caldon 07 Jan '10

07 Jan '10

Not seen this one before. I'm trying to install pcre-light with WinXP 64, Cygwin 1.7.1, Cabal 0.8.0, and ghc 6.12.1. Trying to reinstall "base" with cabal fails as well with an internal error (see below), but I'm assuming that's quasi-intentional/unsupported feature or something. Any ideas? My ultimate goal is to install ghc-core to help track down some performance problems I'm having. Thanks, Patrick. patc@rose ~/rp4/loansim $ cabal install pcre-light --extra-lib-dirs=C:\\cygwin\\lib --extra-include-dirs=C:\\cygwin\\usr\\include --reinstall Resolving dependencies... Configuring pcre-light-0.3.1... Preprocessing library pcre-light-0.3.1... In file included from C:/cygwin/usr/include/pcre.h:90, from Text\Regex\PCRE\Light\Base.hsc:103: C:/cygwin/usr/include/stdlib.h:104: warning: `warning' attribute directive ignored C:/cygwin/usr/include/stdlib.h:109: warning: `warning' attribute directive ignored dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o:Base_hsc_make.c:(.text+0x47): undefined reference to `_impure_ptr' dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o:Base_hsc_make.c:(.text+0x7b): undefined reference to `_impure_ptr' dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o:Base_hsc_make.c:(.text+0x93): undefined reference to `_impure_ptr' dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o:Base_hsc_make.c:(.text+0xc7): undefined reference to `_impure_ptr' dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o:Base_hsc_make.c:(.text+0xf3): undefined reference to `_impure_ptr' dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o:Base_hsc_make.c:(.text+0x127): more undefined references to `_impure_ptr' follow collect2: ld returned 1 exit status linking dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o failed command was: C:\ghc\ghc-6.12.1\mingw\bin\gcc.exe -LC:\cygwin\lib -lpcre -LC:\ghc\ghc-6.12.1\bytestring-0.9.1.5 -LC:\ghc\ghc-6.12.1\base-3.0.3.2 -LC:\ghc\ghc-6.12.1\syb-0.1.0.2 -LC:\ghc\ghc-6.12.1\base-4.2.0.0 -lwsock32 -luser32 -lshell32 -LC:\ghc\ghc-6.12.1\integer-gmp-0.2.0.0 -LC:\ghc\ghc-6.12.1\ghc-prim-0.2.0.0 -LC:\ghc\ghc-6.12.1 -LC:\ghc\ghc-6.12.1/gcc-lib -lm -lwsock32 -LC:\ghc\ghc-6.12.1 dist\build\Text\Regex\PCRE\Light\Base_hsc_make.o -o dist\build\Text\Regex\PCRE\Light\Base_hsc_make.exe cabal.exe: Error: some packages failed to install: pcre-light-0.3.1 failed during the building phase. The exception was: exit: ExitFailure 1 patc@rose ~/rp4/loansim $ cabal --version cabal-install version 0.8.0 using version 1.8.0.2 of the Cabal library patc@rose ~/rp4/loansim $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 patc@rose ~/rp4/loansim $ cabal install base --reinstall -p Resolving dependencies... cabal.exe: internal error: impossible patc@rose ~/rp4/loansim $

3 5

Re: Why doesn't laziness save the day here?
by oleg＠okmij.org 07 Jan '10

07 Jan '10

Dale Jordan wrote: > The motivation for iterateR is to be able to have the ultimate > consumer determine how many random values to force, but still have a > single random generator used throughout the computation. > > My intuition tells me that since the infinite list is produced in > finite batches, ... > > Can anyone explain why this is looping or point out a better way to > generate an arbitrary-length random list while still being able to > reuse the generator? The others have already pointed out the problem with the imperative solution, which used the mutation of the global state with the new random seed. Imperative approach is indeed often a problem. There is a simple solution however. In fact, your message already described it. The key phrase is ``the infinite list is produced in finite batches.'' We merely need to define a list-like data structure that matches our intuitions. The complete code follows. All three run functions, run1 through run3, produce a finite result (shown in the comments after the definition). module RList where -- I don't have Mersenne Twistor installed; so I'll use the stdGen... import System.Random import Control.Monad.State data RList m a = RList [a] -- known finite prefix [m [a]] -- a stream of producing actions pullR :: Monad m => RList m a -> m (RList m a) pullR (RList p (x:xs)) = x >>= \p' -> return $ RList (p++p') xs headR :: Monad m => RList m a -> m a headR (RList (x:_) _) = return x headR x = pullR x >>= headR tailR :: Monad m => RList m a -> m (RList m a) tailR (RList (_:xs) ms) = return $ RList xs ms tailR x = pullR x >>= tailR -- appendR doesn't have to have the monadic type. We go for uniformity with -- the headR appendR :: Monad m => RList m a -> RList m a -> m (RList m a) appendR (RList p1 ms1) (RList p2 ms2) = return $ RList p1 (ms1 ++ (return p2):ms2) takeR :: Monad m => Int -> RList m a -> m (RList m a) takeR 0 l = return l takeR n (RList p ms) | length p >= n = return $ RList (take n p) [] takeR n l = pullR l >>= takeR n -- quite inefficient, but short -- Other list library functions can be implemented in terms of head, tail -- the evaluator, so to speak. It is possibly strict, use it at the -- very end toList :: (Functor m, Monad m) => RList m a -> m [a] toList (RList p []) = return p toList (RList p ms) = pullR (RList [] ms) >>= fmap (p ++) . toList -- Dale Jordan's library, slightly re-written -- Specialized iterator for running actions in Rand monad, to thread -- the generator. The idea is that the action returns a finite list -- of random values and iterateR lazily creates an infinite list of -- values. -- Again, the monadic type is unncessary; given for the sake of -- uniformity iterateR :: Monad m => m [a] -> m (RList m a) iterateR act = return $ RList [] (repeat act) type Rand r a = State r a -- A simple example of a finite action something :: (RandomGen g) => Int -> Int -> Rand g [Int] something n m = sequence . replicate n . State $ randomR (m,m+9) run1 = evalState (toList =<< takeR 10 =<< (iterateR (something 2 0))) $ mkStdGen 42 -- [1,1,7,4,6,1,8,1,8,5] run2 = evalState (toList =<< takeR 10 =<< (iterateR (something 2 0) >> iterateR (something 3 10))) $ mkStdGen 42 -- [11,11,17,14,16,11,18,11,18,15] run3 = evalState (toList =<< (takeR 10 =<< (iterateR (something 2 0) >>= headR >>= iterateR . something 3))) $ mkStdGen 42 -- [8,5,7,2,9,2,9,6,6,10]

2 1

dph and ghc-6.12 ?
by Johannes Waldmann 07 Jan '10

07 Jan '10

Dear all, I'd love to use Data Parallel Haskell (for now, just as a demonstration, in a lecture). I want to convince my students (and myself) that this thing "really works". What would be an easy-to-understand, but still impressive enough benchmark? (to be run on 8 core amd64)? What is the current status of dph with ghc-6.12? Is there some up-to-date tutorial? Do we still need separate modules/compilation/linking? (... and where can I download the CUDA backend :-) best regards, J.W.

1 0

ANNOUNCE: safer-file-handles-0.1
by Bas van Dijk 07 Jan '10

07 Jan '10

Hello, I'm happy to announce another member in the 'monadic regions' family: safer-file-handles: http://hackage.haskell.org/package/safer-file-handles-0.1 The package uses my 'regions' and 'explicit-iomodes' packages to add two safety features on top of the regular System.IO file handles and operations: * Regional file handles. Files must be opened in a region. When the region terminates all opened files are automatically closed. The main advantage of regions is that the handles to the opened files can not be returned from the region which ensures no I/O with closed files is possible. * Explicit IOModes. The regional file handles are parameterized by the IOMode in which they were opened. All operations on handles explicitly specify the needed IOMode. This way it is impossible to read from a write-only handle or write to a read-only handle for example. The primary technique used in this package is called "Lightweight monadic regions" which was invented by Oleg Kiselyov and Chung-chieh Shan. See: http://okmij.org/ftp/Haskell/regions.html#light-weight This technique is implemented in the 'regions' package which is re-exported from 'safer-file-handles'. See the 'safer-file-handles-examples' package for examples how to use this package: darcs get http://code.haskell.org/~basvandijk/code/safer-file-handles-examples regards, Bas P.S. While I wrote this message I realized a serious lack of expressive power of my regions package. Namely that I can't write a program that opens two different types of resources in the _same_ region. If for example I want to combine my usb-safe package and this package the following won't type-check: openBoth ∷ Device → FilePath → IO () openBoth usbDevice filePath = runTopRegion $ do h1 ← open usbDevice ∷ RegionT Device s IO (RegionalHandle Device (RegionT Device s IO)) h2 ← openFile filePath ReadMode ∷ RegionT File s IO (RegionalHandle File (RegionT File s IO)) return () The reason, as can be seen from the types of the monadic actions, is that the monad types differ between the two actions and bind ((>>=) :: Monad m => m a -> (a -> m b) -> m b) obviously requires them to be the same. The main problem thus, is that I parameterized the region with the type of resources. Note that I'm working on a solution where I remove the resource type from RegionT. So instead of: newtype RegionT resource s (pr ∷ * → *) α = RegionT { unRegionT ∷ ReaderT (IORef [Opened resource]) pr α } I will have: newtype RegionT s (pr ∷ * → *) α = RegionT { unRegionT ∷ ReaderT (IORef [SomeOpenedResource]) pr α } data SomeOpenedResource = ∀ resource. Resource resource ⇒ Some (Opened resource) Hopefully I will be able to release this fix before FP-NL tomorrow.

1 0

Problem with cabal install zlib
by Ozgur Akgun 07 Jan '10

07 Jan '10

Hi, When I run cabal install zlib or cabal upgrade zlib I get the following error: Resolving dependencies... Configuring zlib-0.5.2.0... Preprocessing library zlib-0.5.2.0... Only one output file may be specified cabal: Error: some packages failed to install: zlib-0.5.2.0 failed during the building phase. The exception was: exit: ExitFailure 1 I have zlib-0.5.0.0 installed now. The OS is Snow Leopard. I couldn't find anything related to this error, that's why I'm asking community's opinion. Best, -- Ozgur Akgun

6 15

ANNOUNCE: tuntap-0.0.1
by John Van Enk 07 Jan '10

07 Jan '10

Hi All, As some of you have requested, I've extracted the code from my (under work) VPN which talks to the TUN/TAP device under Linux (/dev/net/tun). Some things to note: 1) I've only tested this under Linux. 2) The help-win.c file under cbits HAS allowed similar code to talk to the TUN/TAP driver used by the OpenVPN project, but I haven't implemented this functionality yet. 3) The help-bsd.c file under cbits HAS allowed Mac/BSD flavors to talk to a TUN/TAP driver, but I've never tested this and haven't implemented this functionality yet. I don't have a whole lot there right now, but keep a lookout for more releases with some examples and better docs (as I get around to it). For a very verbose example, you can look at how I use the Network.TUNTAP module in this project: http://github.com/sw17ch/Scurry, specifically http://github.com/sw17ch/Scurry/blob/master/src/Scurry/TAPTask.hs. Hackage: http://hackage.haskell.org/package/tuntap GitHub: http://github.com/sw17ch/tuntap John Van Enk PS: Thanks to Matthew Isleb and Job Vranish for their help on the C code used.

1 0

Value classes
by Mads Lindstrøm 07 Jan '10

07 Jan '10

Hi A function inc5 inc5 :: Float -> Float inc5 x = x + 5 can only be a member of a type class A, if we make all functions from Float -> Float members of type class A. Thus, I assume, that _type_ class is named type class, as membership is decided by types. However, it makes no sense to say that all functions from Float -> Float are invertible or continuous. We would want to specifically say that inc5 is continuous, not all Float -> Float functions. Thus, we could have another abstraction, lets call it _value_ class, where we could say that an individual value (function) could be member. We could say something like: class Invertible (f :: a -> a) where invert :: f -> (a -> a) instance Invertible inc5 where invert _ = \x -> x - 5 In many cases this would be too specific. We would like to say, that applying the first argument to `+` returns an invertible function. Something like: instance Invertible (`+` x) where invert (x +) = \y -> y - x We would properly also like to say, that composing two invertible functions results in another invertible function. I guess there are many more examples. This idea, of value classes, do not feel all that novel. Somebody has properly thought about it before, but gave it a different name. If anybody has links to some papers it would be much appreciated. If anybody has some thoughts of the desirability of value class it would also be much appreciated. /Mads Lindstrøm

3 2

Significant slow-down in parallel code?
by Jamie Morgenstern 06 Jan '10

06 Jan '10

Hello; I have a piece of code in which I employ the `par` construct to add some implicit parallelism to a theorem prover. However, when running the *same* code with +RTS -N1 +RTS -N5 +RTS -N10 I see a huge slowdown (a factor of 50 with 5 processes and a factor of 100 for 10 on an 8-core machine). Very little time is being spent using the garbage collector. Any suggestions? Thanks, -Jamie

2 3

Hackage down
by Daniel Fischer 06 Jan '10

06 Jan '10

Just a heads up, hackage is down, doesn't respond to ping even, cabal update says cabal: <socket: 3>: resource vanished

2 2

Reminder: ZuriHac - Haskell hackathon in Zurich, March 19-21
by Johan Tibell 06 Jan '10

06 Jan '10

Hi all! This is to remind you that we're organizing ZuriHac, a Haskell hackathon/get-together, to be held March 19-21 at the Google Office in Zurich, Switzerland. Lots of people have already registered but we still have space for more! If you plan on coming, please register [1] so we can make sure there's seating for everyone. Registration, travel, lodging and many other details will soon be available on the ZuriHac wiki [2]. If you have any questions don't hesitate to drop Christophe or me an email (or email addresses can be found on the wiki.) WHEN Friday March 19 2:30pm to 6:30pm Saturday March 20 10am to 6pm Sunday March 21 10am to 6pm WHERE We will be in the TechTalk area of the Google Office at Brandschenkestrasse 110. Please see the wiki [3] for directions. ORGANIZERS Johan Tibell Christophe Poucet Hope to see you in Zurich! - The ZuriHac team [1] http://haskell.org/haskellwiki/ZuriHac/Register [2] http://haskell.org/haskellwiki/ZuriHac [3] http://haskell.org/haskellwiki/ZuriHac#Getting_to_the_Google_Office

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Seven ways to store 16 bytes
by Mark Lentczner 06 Jan '10

06 Jan '10

In preparing the speed ups in uuid-0.1.2, I investigated various ways to store 16 bytes of data in a Haskell object. Surprisingly, storing as 4 Word32 values in a standard data type worked best for that application. I've extracted out the testing work for that and put it here: http://bitbucket.org/mtnviewmark/haskell-playground/src/tip/16bytes/ There you can find the code that tests storing 16 bytes in various ways: > import qualified Data.Array as A > import qualified Data.Array.Unboxed as U > import qualified Data.Array.Vector as V > import qualified Data.ByteString as B > > data InBytes = BY !Word8 !Word8 !Word8 !Word8 !Word8 !Word8 !Word8 !Word8 > !Word8 !Word8 !Word8 !Word8 !Word8 !Word8 !Word8 !Word8 > deriving (Eq, Ord) > > data InWords = WO !Word32 !Word32 !Word32 !Word32 > deriving (Eq, Ord) > > data InList = LI [Word8] deriving (Eq, Ord) > data InByteString = BS B.ByteString deriving (Eq, Ord) > data InArray = AR (A.Array Int Word8) deriving (Eq, Ord) > data InUArray = UA (U.UArray Int Word8) deriving (Eq, Ord) > data InVector = VE (V.UArr Word8) deriving (Eq) Depending on operations will be needed most, different storage methods do best. Enjoy! - Mark Mark Lentczner (MtnViewMark) http://www.ozonehouse.com/mark/

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GHC 6.12.1 FreeBSD binaries
by Michael Snoyman 05 Jan '10

05 Jan '10

Does anyone know what the status is of the FreeBSD binaries for the most recent GHC release? In particular, I'm looking or FreeBSD 7.2 binaries. I tried building from source, but it says I don't have iconv library. I'm not enough of a FreeBSD guy to track this one down. Thanks, Michael

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lawless instances of Functor
by Paul Brauner 05 Jan '10

05 Jan '10

Hi, I'm trying to get a deep feeling of Functors (and then pointed Functors, Applicative Functors, etc.). To this end, I try to find lawless instances of Functor that satisfy one law but not the other. I've found one instance that satisfies fmap (f.g) = fmap f . fmap g but not fmap id = id: data Foo a = A | B instance Functor Foo where fmap f A = B fmap f B = B -- violates law 1 fmap id A = B -- respects law 2 fmap (f . g) A = (fmap f . fmap g) A = B fmap (f . g) B = (fmap f . fmap g) B = B But I can't come up with an example that satifies law 1 and not law 2. I'm beginning to think this isn't possible but I didn't read anything saying so, neither do I manage to prove it. I'm sure someone knows :) Paul

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Re: About Atom
by Tom Hawkins 05 Jan '10

05 Jan '10

On Tue, Jan 5, 2010 at 7:05 PM, CK Kashyap <ck_kashyap(a)yahoo.com> wrote: > Hi Tom, > Happy new year :) > I was wondering if I could use Atom for the purpose of an x86 operating system generator? Hi Kashyap, Ironically Atom was intended to eliminate the need for operating systems -- at least on small embedded projects. But yes, maybe it could be use develop an OS. (I've never written one, so I'm speaking way out of my comfort zone.) > I've explored Minix and wanted to get rid of all the noise introduced by C and focus on the logic. > I am sorry, I have not really explored Atom sufficiently but I am just too eager to know :) ... I have installed Atom using cabal - I was just going through Example.hs ... One thing I noticed is that the logic to calculate gcd was not clear from the implementation (Haskell side that is) ... Perhaps I am not reading it with the right perspective - can you elaborate it? > Also, why exactly did you name the whole thing Atom? Because an Atom design is composed of a bunch of small atomic operations. Take the GCD example: -- A rule to modify A. If a > b then set a = a - b. atom "a_minus_b" $ do cond $ value a >. value b a <== value a - value b -- A rule to modify B. If b > a then set b = b - a. atom "b_minus_a" $ do cond $ value b >. value a b <== value b - value a These independent rules periodically wake up and check their guard conditions (cond). If the guard condition is true, it performs the action atomically, in isolation from everything else in the system. In the case of the GCD example, the algorithm converges when a == b, at which point both the "a_minus_b" and "b_minus_a" rules are disabled. -Tom > Regards, > Kashyap > > > > >

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Re: [darcs-users] Iteratees, streams, and mmap
by Jason Dagit 05 Jan '10

05 Jan '10

Hi Heinrich, On Sat, Dec 12, 2009 at 2:42 AM, Heinrich Apfelmus < apfelmus(a)quantentunnel.de> wrote: > Jason Dagit wrote: > > My next experiment will be to find ways to express "take this operation > and > > apply it to a stream without letting the stream leak". One implication is > > that gzReadFilePS should not be used outside of a core set of modules > which > > have been auideted to be resource concious. Another implication is that > we > > need to be really careful about wether or not we allow returning of > > sequences of patches. Possibly, we need several foldl-like functions > that > > open the stream internally. For example, to process the pending maybe we > > should have: > > withPending :: (a -> Patch -> a) -> IO a > > > > And withPending would start the streaming and make sure that the stream > > cannot be visible as a data dependency outside of withPending. > > Just a small comment on a potential flaw in this scheme and the > observation that even the rank-2 type trick from the ST s monad > wouldn't help. > I would say it does help, but it doesn't make it perfect. > > > Namely, withPending does not guarantee that the stream does not leak, > it only makes it more natural/convenient to formulate one's code so that > it doesn't leak. In particular, using (:) as argument pretty much > defeats the whole purpose: > Right. And the iteratee library points out that your iteratees have to be well-behaved (I think there they say "bounded"). I'm well aware of this issue and thanks for pointing it out for others who are reading along. > > withPending (flip (:)) > > Fortunately, the type system can ensure that the patches don't leak. > > withPending :: (forall s. a -> Patch s -> a) -> IO a > > Now, a may not mention s and the type checker will reject flip (:) > as argument. See also > > Oleg Kiselyov, Chung-chieh Shan. > Lightweight Monadic Regions. > http://www.cs.rutgers.edu/~ccshan/capability/region-io.pdf<http://www.cs.rutgers.edu/%7Eccshan/capability/region-io.pdf> > > for an elaboration of this technique. > I'm still on the fence as to whether this style of writing it will add value greater than the complexity it brings. I am certainly considering it :) The darcs source does other things that are also fairly complex. > > > However, the line between leaking and not leaking is very thin here. As > soon as we are given for example a function > > name :: Patch s -> String > > that discards the s , its results can "leak", in the sense that we > could now build a list of names > > foo :: IO [String] > foo = withPending . flip $ (:) . name > > Even worse, any type a that doesn't have O(1) space usage will "leak" > > bar :: IO [()] > bar = withPending . flip $ const (() :) > > In other words, exporting only a foldl' -like interface does not really > prevent us from writing functions that have O(n) instead of O(1) space > usage. But trying to rectify that with the forall s trick is a doomed > idea, too. > I realize it's not perfect, but the problem we have now is that it's too easy to write things that have dismal space usage. If we can't force proper space usage, how can we make it more natural to have bounded space? Or at least a good approximation. It seems that: * foldl'-style helps * rank-n can help * no approach I've seen *forces* the behavior we want * existing code and bug reports demonstrate we need to improve the situation I'm open to suggestions on how to ensure the code has the space behavior I want. Lazy IO* and streams of patches is more compositional and natural to Haskell programmers, but it seems that it's too hard to ensure the code has reasonable space usage. At least where the darcs source is concerned. Therefore, I think the status quo demonstrates that in the darcs source it's worth experimenting with alternatives to lazy io and streams. In other words, the human effort to make the code behave how we want is currently too high and that's the issue I want to address. I don't know how we could make it impossible to have space leaks, although that would be interesting. (*) Note: Lazy IO itself is used in very few places in darcs these days because it has lead to serious bugs. These days me point is more about big streams getting retained. Finding where and why has proven difficult. Thanks, Jason

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64-bit Bloom filters?
by Ketil Malde 05 Jan '10

05 Jan '10

Hi, I've previously used Bloom filters on 32-bit Linux with some success. However, after upgrading to 64 bit, my Bloom filter applications crash or misbehave in random ways. So: is anybody successfully using Bloom filters on 64 bit computers? Although I'm not clear on why it would cause crashes (SEGV, infinite looping, etc), my prime suspect is the hashing function used. This is from C and returns a uint32, but it is imported to return a CInt, which I suspect is 64 bits. I'll look further into it, but I thought I'd check if anybody else has the same problem, and especially, a solution :-) -k -- If I haven't seen further, it is by standing in the footprints of giants

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Using Haskell to write dbus server
by Maciej Piechotka 05 Jan '10

05 Jan '10

How to write (is it possible) a dbus server in Haskell? Regards

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[Very long] (CHP?) Compressing, MD5 and big files
by Maciej Piechotka 05 Jan '10

05 Jan '10

I have following problem: I'd like to operate on big files so I'd prefere to operate on 'stream' instead of whole file at a time to avoid keeping too much in memory. I need to calculate MD5 and compress file. I tried to use something like that but I'm afraid that I'd need to patch zlib package as it results in deadlock: > {-# LANGUAGE GADTs #-} > import Codec.Compression.GZip > import Control.Applicative > import Control.Concurrent.CHP > import qualified Control.Concurrent.CHP.Common as CHP > import Control.Concurrent.CHP.Enroll > import Control.Concurrent.CHP.Utils > import Control.Monad.State.Strict > import Data.Digest.Pure.MD5 > import Data.Maybe > import qualified Data.ByteString.Char8 as BS > import qualified Data.ByteString.Lazy.Char8 as LBS > import qualified Data.ByteString.Lazy.Internal as LBS > import System.Environment > import System.IO > import System.IO.Unsafe > > > calculateMD5 :: (ReadableChannel r, > Poisonable (r (Maybe BS.ByteString)), > WriteableChannel w, > Poisonable (w MD5Digest)) > => r (Maybe BS.ByteString) > -> w MD5Digest > -> CHP () > calculateMD5 in_ out = evalStateT (forever loop) md5InitialContext > `onPoisonRethrow` (poison in_ >> poison out) > where loop = liftCHP (readChannel in_) >>= > calc' > calc' Nothing = gets md5Finalize >>= > liftCHP . > writeChannel out >> > put md5InitialContext > calc' (Just b) = modify (flip md5Update > $ LBS.fromChunks [b]) Calculate MD5 hash of input stream. Nothing indicates EOF. > unsafeInterleaveCHP :: CHP a -> CHP a > unsafeInterleaveCHP = fromJust <.> liftIO <=< > unsafeInterleaveIO <.> embedCHP Helper function. It is suppose to move the execution in time - just as unsafeInterleaveIO. I belive that the main problem lives here. Especially that Maybe.fromJust: Nothing is the error. > chan2List :: (ReadableChannel r, Poisonable (r a)) > => r a -> CHP [a] > chan2List in_ = unsafeInterleaveCHP ((liftM2 (:) (readChannel in_) > (chan2List in_)) > `onPoisonTrap` return []) Changes channel to lazy read list. > chanMaybe2List :: (ReadableChannel r, > Poisonable (r (Maybe a))) > => r (Maybe a) > -> CHP [[a]] > chanMaybe2List in_ = splitByMaybe <$> chan2List > where splitByMaybe [] = [] > splitByMaybe (Nothing:xs) = > []:splitByMaybe xs > splitByMaybe (Just v :[]) = [[v]] > splitByMaybe (Just v :xs) = > let (y:ys) = splitByMaybe xs > in (v:y):ys Reads lazyly from channel o list of list > compressCHP :: (ReadableChannel r, > Poisonable (r (Maybe BS.ByteString)), > WriteableChannel w, > Poisonable (w (Maybe BS.ByteString))) > => r (Maybe BS.ByteString) > -> w (Maybe BS.ByteString) > -> CHP () > compressCHP in_ out = toOut >>= mapM_ sendBS > where in_' :: CHP [LBS.ByteString] > in_' = fmap LBS.fromChunks <$> > chanMaybe2List in_ > toOut :: CHP [LBS.ByteString] > toOut = fmap compress <$> in_' > sendBS :: LBS.ByteString -> CHP () > sendBS LBS.Empty = writeChannel out > Nothing > sendBS (LBS.Chunk c r) = writeChannel out > (Just c) > >> sendBS r Compress process > readFromFile :: (ReadableChannel r, > Poisonable (r String), > WriteableChannel w, > Poisonable (w (Maybe BS.ByteString))) > => r String > -> w (Maybe BS.ByteString) > -> CHP () > readFromFile file data_ = > forever (do path <- readChannel file > hnd <- liftIO $ openFile path ReadMode > let copy = liftIO (BS.hGet hnd LBS.defaultChunkSize) >>= > writeChannel data_ . Just > copy `onPoisonRethrow` liftIO (hClose hnd) > writeChannel data_ Nothing > liftIO $ hClose hnd) > `onPoisonRethrow` (poison file >> poison data_) Process reading from file > writeToFile :: (ReadableChannel r, > Poisonable (r String), > ReadableChannel r', > Poisonable (r' (Maybe BS.ByteString))) > => r String > -> r' (Maybe BS.ByteString) > -> CHP () > writeToFile file data_ = > forever (do path <- readChannel file > hnd <- liftIO $ openFile path WriteMode > let writeUntilNothing = readChannel data_ >>= > writeUntilNothing' > writeUntilNothing' Nothing = return () > writeUntilNothing' (Just v) = liftIO (BS.hPutStr > hnd v) >> > writeUntilNothing > writeUntilNothing `onPoisonFinally` liftIO (hClose hnd)) > `onPoisonRethrow` (poison file >> poison data_) Process writing to file > getFiles :: (WriteableChannel w, Poisonable (w String)) > => w String -> CHP () > getFiles out = mapM_ (writeChannel out) ["test1", "test2"] >> > poison (out) Sample files. Each contains "Test1\n" > pipeline1 :: CHP () > pipeline1 = do md5sum <- oneToOneChannel' $ chanLabel "MD5" > runParallel_ [(getFiles ->|^ > ("File", readFromFile) ->|^ > ("Data", calculateMD5)) > (writer md5sum), > forever $ readChannel (reader md5sum) >>= > liftIO . print] First pipeline. Output: fa029a7f2a3ca5a03fe682d3b77c7f0d fa029a7f2a3ca5a03fe682d3b77c7f0d < File."test1", Data.Just "Test1\n", Data.Nothing, MD5.fa029a7f2a3ca5a03fe682d3b77c7f0d, File."test2", Data.Just "Test1\n", Data.Nothing, MD5.fa029a7f2a3ca5a03fe682d3b77c7f0d > > pipeline2 :: CHP () > pipeline2 = enrolling $ do > file <- oneToManyChannel' $ chanLabel "File" > fileMD5 <- oneToOneChannel' $ chanLabel "File MD5" > data_ <- oneToOneChannel' $ chanLabel "Data" > md5 <- oneToOneChannel' $ chanLabel "MD5" > md5BS <- oneToOneChannel' $ chanLabel "MD5 ByteString" > fileMD5' <- Enroll (reader file) > fileData <- Enroll (reader file) > liftCHP $ runParallel_ [getFiles (writer file), > (forever $ readChannel fileMD5' >>= > writeChannel (writer fileMD5) . > (++".md5")) > `onPoisonRethrow` > (poison fileMD5' >> > poison (writer fileMD5)), > readFromFile fileData (writer data_), > calculateMD5 (reader data_) (writer md5), > (forever $ do v <- readChannel (reader md5) > let v' = Just $ BS.pack $ show v > writeChannel (writer md5BS) v' > writeChannel (writer md5BS) > Nothing) > `onPoisonRethrow` > (poison (writer md5BS) >> > poison (reader md5)), > writeToFile (reader fileMD5) (reader md5BS)] Correct pipeline (testing EnrollingT): < _b4, File MD5."test1.md5", Data.Just "Test1\n", Data.Nothing, MD5.fa029a7f2a3ca5a03fe682d3b77c7f0d, _b4, MD5 ByteString.Just "fa029a7f2a3ca5a03fe682d3b77c7f0d", Data.Just "Test1\n", Data.Nothing, MD5 ByteString.Nothing, MD5.fa029a7f2a3ca5a03fe682d3b77c7f0d, File MD5."test2.md5", MD5 ByteString.Just "fa029a7f2a3ca5a03fe682d3b77c7f0d", MD5 ByteString.Nothing > % cat test1.md5 fa029a7f2a3ca5a03fe682d3b77c7f0d% > pipeline3 :: CHP () > pipeline3 = enrolling $ do > file <- oneToManyChannel' $ chanLabel "File" > fileGZ <- oneToOneChannel' $ chanLabel "File GZ" > data_ <- oneToManyChannel' $ chanLabel "Data" > compressed <- oneToManyChannel' $ chanLabel "Data Compressed" > md5 <- oneToOneChannel' $ chanLabel "MD5" > md5Compressed <- oneToOneChannel' $ chanLabel "MD5 Compressed" > fileGZ' <- Enroll (reader file) > fileData <- Enroll (reader file) > dataMD5 <- Enroll (reader data_) > dataCompress <- Enroll (reader data_) > compressedFile <- Enroll (reader compressed) > compressedMD5 <- Enroll (reader compressed) > liftCHP $ runParallel_ [getFiles (writer file), > (forever $ readChannel fileGZ' >>= > writeChannel (writer fileGZ) . > (++".gz")) > `onPoisonRethrow` > (poison fileGZ' >> poison (writer fileGZ)), > readFromFile fileData (writer data_), > calculateMD5 dataMD5 (writer md5), > compressCHP dataCompress > (writer compressed), > writeToFile (reader fileGZ) compressedFile, > calculateMD5 compressedMD5 > (writer md5Compressed), > forever $ readChannel dataMD5 >>= > liftIO . print >> > readChannel compressedMD5 >>= > liftIO . print] Problems: (CHP) Thread terminated with: thread blocked indefinitely in an STM transaction < _b3, _b4, File GZ."test1.gz" > > onPoisonFinally :: CHP a -> CHP () -> CHP a > onPoisonFinally m b = (m `onPoisonRethrow` b) <* b > Utility function (used for closing handles) > (<.>) :: Functor f => (b -> c) -> (a -> f b) -> a -> f c > f <.> g = fmap f . g <.> is for <$> as . to $. > instance MonadCHP m => MonadCHP (StateT s m) where > liftCHP = lift . liftCHP Missing instance for strict monad > (->|^) :: Show b > => (Chanout b -> CHP ()) -> (String, Chanin b -> c -> CHP ()) > -> (c -> CHP ()) > (->|^) p (l, q) x = do c <- oneToOneChannel' $ chanLabel l > runParallel_ [p (writer c), q (reader c) x] 'Missing' helper function > data EnrollingT a where > Lift :: CHP a -> EnrollingT a > Enroll :: (Enrollable b z) => b z -> EnrollingT (Enrolled b z) > > enrolling :: EnrollingT a -> CHP a > enrolling (Lift v) = v > enrolling (Enroll b) = enroll b return > > instance Monad EnrollingT where > (Lift m) >>= f = Lift $ m >>= enrolling . f > (Enroll b) >>= f = Lift $ enroll b (enrolling . f) > return = Lift . return > instance MonadIO EnrollingT where > liftIO = Lift . liftIO > instance MonadCHP EnrollingT where > liftCHP = Lift Helper monad for enrolling (I know T should stand for transforming but then I realize problems). Thanks in advance

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Book reviews for the Journal of Functional Programming
by S.J.Thompson 05 Jan '10

05 Jan '10

If you would be interested in reviewing books for the journal of functional programming you can find a list of currently available books at http://www.cs.kent.ac.uk/people/staff/sjt/JFP/ Please let me know if you would be interested in taking on any of these reviewing assignments. Kind regards, Simon Thompson Book Reviews Editor Journal of Functional Programming

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Why doesn't laziness save the day here?
by Dale Jordan 05 Jan '10

05 Jan '10

Kind and Generous Haskellers: I am ensnared in a briar patch of infinite lists and Random gnerators trying to use laziness to advantage. Here's my code: ----------------- 8< -------------------- import Control.Applicative import Control.Monad import Control.Monad.Random import Control.Monad.State import System.Random.Mersenne.Pure64 -- Specialized iterator for running actions in Rand monad, to thread -- the generator. The idea is that the action returns a finite list -- of random values and iterateR lazily creates an infinite list of -- values. iterateR act = do gen <- get let (as,gen') = runRand act gen put $! gen' (as ++) <$> iterateR act -- A simple example of a finite action something :: (RandomGen g) => Int -> Int -> Rand g [Int] something n m = sequence . replicate n $ getRandomR (m,m+9) run1 = evalState (take 10 <$> (iterateR (something 2 0))) $ pureMT 42 run2 = evalState (take 10 <$> (iterateR (something 2 0) >> iterateR (something 3 10))) $ pureMT 42 run3 = evalState (take 10 <$> (iterateR (something 2 0) >>= iterateR . (something 3 . head))) $ pureMT 42 ------------------- >8 ---------------------- Evaluating run1 works fine (ghci 10.3): *Main> run1 [1,9,5,3,6,9,1,5,1,8] Evaluating run2 or run3 loops and quickly exhausts the heap. (Using Control.Monad.State.Strict causes stack overflow, though) The motivation for iterateR is to be able to have the ultimate consumer determine how many random values to force, but still have a single random generator used throughout the computation. My intuition tells me that since the infinite list is produced in finite batches, the generator shouldn't be tangled up in an infinite list such as produced by MonadRandom's getRandoms, but I only have a pink belt in Haskell-fu. Can anyone explain why this is looping or point out a better way to generate an arbitrary-length random list while still being able to reuse the generator? (I'd rather not use split since this generator doesn't support it and its of dubious soundness.) Dale Jordan

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