November 2009 - Haskell-Cafe

How to convert records into Haskell structure in Haskell way?
by Magicloud Magiclouds 02 Nov '09

02 Nov '09

Hi, Say I have something like this: [ Record { item = "A1", value = "0" } , Record { item = "B1", value = "13" } , Record { item = "A2", value = "2" } , Record { item = "B2", value = "10" } ] How to convert it into: [ XXInfo { name = "A", value1 = "0", value2 = "2" } , XXInfo { name = "B", value1 = "13", value2 = "10" } ] If XXInfo has a lot of members. And sometimes the original data might be not integrity. -- 竹密岂妨流水过山高哪阻野云飞

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advice for dispatch question
by John Lato 02 Nov '09

02 Nov '09

Hello, I've been thinking about a problem recently, and would like to know if there are any recommendations for a solution. I have two container-like type classes, defined as follows: > import Control.Monad > > type family ElemOf c :: * > type family MonadOf c :: * -> * > > class PureContainer c where > chead :: c -> ElemOf c > > class MonadicContainer c where > mhead :: c -> MonadOf c (ElemOf c) now suppose I have two functions, like this: > pureOp :: (PureContainer c, Monad m) => c -> m () > pureOp c = doSomethingWithElem $ chead c > > monadOp :: (Monad (MonadOf c), MonadicContainer c) => c -> MonadOf c () > monadOp c = mhead c >>= doSomethingWithElem > > -- assume this function does something > doSomethingWithElem :: Monad m => a -> m () > doSomethingWithElem _ = return () the goal is to take the head from either a PureContainer or a MonadicContainer and feed it to the doSomethingWithElem function, which we can assume does something monadic. What I would like to do would be write one function that behaves like this quasi-Haskell: combinedOp :: c -> m () combinedOp | PureContainer c = doSomethingWithElem $ chead c combinedOp | MonadicContainer c = mhead c >>= doSomethingWithElem I have many functions under consideration, and it would be nice to not have to duplicate all of them. So far I've thought of two possible solutions: 1. create a newtype > newtype M s (m :: * -> *) = M{ unM :: s} then I can write > type instance ElemOf (M c m) = ElemOf c > type instance MonadOf (M c m) = m > > instance (Monad m, PureContainer c) => MonadicContainer (M c m) where > mhead = return . chead . unM In this case, I would ignore the PureContainer case of combinedOp and just stick every PureContainer of interest into the newtype as necessary. 2. Use a GADT with two constructors to special-case appropriately. I think this should work, but haven't tried it yet. It seems more complicated, and less transparent, than using a newtype as above. Does anyone have any advice for this situation? I'm defining PureContainer instances anyway, so it seems silly to have to duplicate everything for the monadic case. Likewise the MonadicContainer version is highly desirable for certain types, so I don't want to forgo that either. Thanks, John

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Re: Suggested algorithm to control upper bound of space "leaks"
by Shelby Moore 02 Nov '09

02 Nov '09

http://www.haskell.org/pipermail/cvs-ghc/2009-October/050928.html Shelby Moore wrote: > One possible runtime optimization to provide an upper bound for cache > control, might be to not cache thunks when the runtime computation time > times number of cache hits, is less than the round-trip paging time > multiplied by recent system paging load. > > Is this novel? Is it useful? Clarify and improve: One possible idea for runtime optimization to provide a more deterministic upper bound on paging load for cache control, might be to not cache thunk-wise when for each thunk, the total tree of computation time under the thunk multiplied by the number of cache hits on the thunk, is less than the round-trip paging time of thunks under the tree multiplied by a chosen factor of recent system paging load. Or toggle the test on and off on a chosen threshold of recent system paging load. Obviously "heap size" could be substituted for "paging load", but my goal is to make the optimization resolution independent, i.e. orthogonal to system configuration such as RAM size, speed, virtual media size, algorithm, etc.. The "catch-22" of the idea if any, probably derives from the permutations of computation time from the lazy computation of thunks under the tree? Note I realize this list is probably not the correct place to discuss research. I do not have time to do this research. Any suggestion where I could post or send this idea so people interested in this type of research could see it? Add a feature request to GHC bug tracker? To the general Haskell mailing list?

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Haskell Weekly News: Issue 137 - October 31, 2009
by jfredett＠gmail.com 02 Nov '09

02 Nov '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091031 Issue 137 - October 31, 2009 --------------------------------------------------------------------------- Welcome to issue 137 of HWN, a newsletter covering developments in the [1]Haskell community. This week brings a new release of xmonad, some cool bindings to allow interaction with GNOME, KDE, and XFCE desktops and a new version of haskell-mode for the lesser of two editors... Some progress has also been made on the new HWN software, though this was mitigated by the fact that I started playing with the Isabelle theorem prover (after reading about Haskabelle) and so now I find myself convinced I should formally prove all of my software, and also that I'm pretty much incapable of getting things done when cool tools present themselves. In fact, I don't think I'll even be able to finish this [2]editori* [3]Exception: The Haskell Weekly News! Announcements xmonad 0.9 is now available! Don Stewart [4]announced a new version of everyone's favorite Tiling Window Manager, xmonad! The changes and improvments are too numerous to mention, see the post for details! WFLP2010 2nd CFP: LNCS + invited speaker + abstract due Nov 9. Pablo Nogueira [5]announced a second call for papers for the Workshop on Functional and Constraint Logic Programming. dbus-core 0.5 and dbus-client 0.1. John Millikin [6]announced pure-Haskell client libraries for using the D-Bus protocol. D-Bus is heavily used for inter-application IPC on Free and open-source desktop platforms, such as Linux, OpenSolaris, and FreeBSD. These libraries allow applications written in Haskell to inter-operate with other components of recent GNOME, KDE, and XFCE desktops. Singapore Functional Programmer Group First Meetup. Kenny Lu [7]announced (with apologies for the late notice) an informal meeting for the Functional Programmer Group in Singapore on 2 Nov 2009. The theme for the first meeting will be 'meet and greet'. GeBoP 1.7. Henk-Jan van Tuyl [8]updated and cabalized GeBoP (the General Boardgames Player) and uploaded it to hackage. GPS package on Hackage. Thomas DuBuisson [9]announced that he fixed up the GPS package to add correct distance calculation and a separate module for exporting KML. GPipe-TextureLoad 1.0.0 and GPipe 1.0.3. Tobias Bexelius [10]announced a new version of GPipe as well as a utility package for loading textures. HoleyMonoid-0.1. Martijn van Steenbergen [11]announced announce the first release of HoleyMonoid, a datatype that helps you build monoids with holes in them. The holes are filled in later using normal function application. GlomeVec, IcoGrid. Jim Snow [12]announced a couple of packages, GlomeVec is a vector library used in Jim's ray-tracer, and IcoGrid is a library for dealing with grids of hexagons and pentagons wrapped on a sphere. attempt-0.0.0. Michael Snoyman [13]announced a new package for exceptions haskell-mode 2.5. Svein Ove Aas [14]announced a new version of haskell-mode for that other 'editor'... Discussion Applicative but not Monad. Yusaku Hashimoto [15]asked about an example of a datatype that was an instance of Applicative, but not an instance of Monad. AND/OR Perceptron. Hector Guilarte [16]asked about how to understand a particular portion of JP Moresmau's [17]Perceptron in Haskell What is the current state of affairs with supercompilation? Eugene Kirpichov [18]asked about supercompilation in Haskell. Blog noise [19]Haskell news from the [20]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Magnus Therning: [21]Playing with sockets in Haskell. * Holumbus: [22]Switched to Git. * Manuel M T Chakravarty: [23]Finally found the ghci bug on Snow Leopard. * Galois, Inc: [24]Tech Talk: Testing First-Order-Logic Axioms in AutoCert. * Brent Yorgey: [25]Collecting Attributes. * Darcs: [26]darcs weekly news #44. * FP Lunch: [27]Parser Combinators are as expressive as possible. * David Amos: [28]Simple groups, the atoms of symmetry. * Xmonad: [29]xmonad 0.9 available now!. * Michael Snoyman: [30]Introduction to attempt error reporting library. * FP-Syd: [31]Sydney FP Group: FP-Syd #20.. Quotes of the Week * hexpuem: [on learning haskell] the best way is to shove a SD card up your nose with learn yourself a haskell good on it * copumpkin: copumpkin: 'YOU ARE SUCH A PULLBACK' copumpkin: 'YOU KNOW WHAT? I FUNCTORED YOUR MOTHER LAST NIGHT' * saml and mauke: saml: 'How can I convert python file to haskell?' mauke: 'By piping it through a programmer' * monochrom: premature generality is the root of OOP * EvilTerran: {-# LANGUAGE FlexibleMorals #-} -- needed for unsafeLaunchMissiles * Warrigal: Hey, the module loaded. I didn't expect that. * lilac: class Monad m where return and Kleisli compose must form a monoid About the Haskell Weekly News New editions are posted to [32]the Haskell mailing list as well as to [33]the Haskell Sequence and [34]Planet Haskell. [35]RSS is also available, and headlines appear on [36]haskell.org. To help create new editions of this newsletter, please see the information on [37]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [38]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://uncyclopedia.wikia.com/wiki/Haskell 3. http://www.reddit.com/r/haskell/comments/9yt9m/haskell_is_an_obscure_and_mo… 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65286 5. http://article.gmane.org/gmane.comp.lang.haskell.general/17596 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65436 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65406 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65384 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65326 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65320 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65311 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65294 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65281 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/65268 15. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/65418 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/65393 17. http://jpmoresmau.blogspot.com/2007/05/perceptron-in-haskell.html 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/65227 19. http://planet.haskell.org/ 20. http://haskell.org/haskellwiki/Blog_articles 21. http://therning.org/magnus/archives/778 22. http://holumbus.fh-wedel.de/blog/?p=25 23. http://justtesting.org/post/227629236 24. http://www.galois.com/blog/2009/10/28/ahn-autocert/ 25. http://byorgey.wordpress.com/2009/10/28/collecting-attributes/ 26. http://blog.darcs.net/2009/10/darcs-weekly-news-44.html 27. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=271 28. http://haskellformaths.blogspot.com/2009/10/simple-groups-atoms-of-symmetry… 29. http://xmonad.wordpress.com/2009/10/26/xmonad-0-9-available-now/ 30. http://blog.snoyman.com/2009/10/25/introduction-to-attempt-error-reporting-… 31. http://www.mega-nerd.com/erikd/Blog/FP-Syd/fp-syd-20.html 32. http://www.haskell.org/mailman/listinfo/haskell 33. http://sequence.complete.org/ 34. http://planet.haskell.org/ 35. http://sequence.complete.org/node/feed 36. http://haskell.org/ 37. http://haskell.org/haskellwiki/HWN 38. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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Hackage still down?
by Magnus Therning 02 Nov '09

02 Nov '09

It seems to have been down for at least the better part of today. Is it something worth worrying about? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Experiments with defunctionalization, church-encoding and CPS
by Eugene Kirpichov 02 Nov '09

02 Nov '09

I took a toy problem - find the first node satisfying a predicate in a binary tree, started with a naive Maybe-based implementation - and experimented with 3 ways of changing the program: - Church-encode the Maybe - Convert the program into CPS - Defunctionalize the Church-encoded or CPS-transformed program http://hpaste.org/fastcgi/hpaste.fcgi/view?id=10686 The link points to code, a benchmark and conclusion. Conclusion: - Haskell implements Maybe well enough that it is not possible to do better - Defunctionalization and consequent optimization yields same performance as the one with Maybe - Non-simplified CPS and Church-encoded representations do bad -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Using HaskellDB on Tables that contain same column names
by R. Emre Başar 02 Nov '09

02 Nov '09

Hi all, I'm trying to create a HaskellDB application for keeping the inventory of a set of devices. My database includes some tables which have some columns with same names. Here is an example: Server: id, name, vendor (fk), model(fk), system_id (always null) Vendor: id, name Model: id, name I'm trying to join those three tables and create a result that contains the names of the server, vendor and model. I use the projection: project (Server.name << s!Server.name # Vendor.name << v!Vendor.name # Model.name << m!Model.name # Server.system_id << s!Server.system_id) When I execute the query, the result contains just the name of the servers repeated three times. When I check the generated SQL, I see that it generates not what I expected. Am I missing something here? If you need, I can put the full source & generated SQL to somewhere. -- R. Emre Başar İstanbul Bilgi University Department of Computer Science

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AND/OR Perceptron
by Hector Guilarte 02 Nov '09

02 Nov '09

Hi cafe, I'm trying to implement a Perceptron in Haskell and I found one in: http://jpmoresmau.blogspot.com/2007/05/perceptron-in-haskell.html (Thanks JP Moresmau) but there is one line I don't understand, I was wondering if someone could explain it to me. I know the theory behind a perceptron, my question is more about the Haskell syntax in that line I don't understand. epoch :: [([Float],Float)] -> -- ^ Test Cases and Expected Values for each test case [Float] -> -- ^ weights ([Float],Float) -- ^ New weights, delta epoch allInputs weights= let f w (inputs,expected) = step inputs w expected -- I don't understand this line newWeights = foldl f weights allInputs -- Neither this one delta = (foldl (+) 0 (map abs (zipWith (-) newWeights weights))) / (fromIntegral $ length weights) in (newWeights,delta) What is f and what is w? I really don't get it, Is like it is defining a function f which calls step unziping the input, taking one of the elements from the fst and it's corresponding snd and invoking step with that, along with w (which seems to be a list according to step's signature but I don't know where it comes from), and then applying fold to the weights and all the Inputs using that f function... But I don't get it! Maybe if someone could rewrite that redefining f as an separate function and calling fold with that function I'll get it. The input for epoch would be something like this: epoch [([0,0],0),([0,1],0),([1,0],0),([1,1],1)] [-0,413,0.135] and the output for that examples is: ([0.0,412.9],3.333537e-2) Thanks a lot, Hector Guilarte

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hackage is down.
by Thomas Hartman 02 Nov '09

02 Nov '09

http://hackage.haskell.org

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Bulding a library for C users on OS X
by Chris Eidhof 02 Nov '09

02 Nov '09

Hey all, I'm trying to call a Haskell function from C, on OS X. There's an excellent post [1] by Tomáš Janoušek that explains how to do this on Linux. However, on OS X, it's different. First of all, it looks like the -no-hs-main flag is ignored, because I get the following error: > ghc -O2 --make -no-hs-main -optl '-shared' -optc '- DMODULE=Test' -o Test.so Test.hs module_init.c > [1 of 1] Compiling Main ( Test.hs, Test.o ) > > Test.hs:1:0: The function `main' is not defined in module `Main' Second, I learned [2] that I have to pass in different flags on OS X, but unfortunately, GHC still wants me to have main. I'm probably doing something wrong here, is there anyone who can give me some pointers? (no pun intended). I'm running GHC 6.10.2 on Leopard. Thanks, -chris [1]: http://blog.haskell.cz/pivnik/building-a-shared-library-in-haskell/ [2]: http://www.mail-archive.com/haskell-cafe@haskell.org/msg51303.html

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