October 2009 - Haskell-Cafe

Merging classes
by pat browne 16 Oct '09

16 Oct '09

Hi, Following on from my earlier *Merging modules* email, where I tried to simulate ontology merging with Haskell modules; http://article.gmane.org/gmane.comp.lang.haskell.cafe/64943 I *think* that the type of merging that I am attempting is difficult with modules so I gave classes a try. I did not get very far (see below). Perhaps I need a combination of type classes and modules. In fairness this is more of a specification task than a programming task and perhaps I am barking up the wrong tree. Any help appreciated. Regards, Pat ============================================== attempting an ontology merge with type classes For specification of requirement see http://article.gmane.org/gmane.comp.lang.haskell.cafe/64943 ============================================== -- Data types for COMMON data Woman = WomanC data Person = PersonC -- Additional data type for ONTOLOGY1 data Bank = BankC -- Additional data type for ONTOLOGY2 -- but how do I let type Bank have the same name in both classes and -- still be distinct (homonyms) in the merged class -- data Bank = BankC class COMMON a b where class (COMMON a b) => ONTOLOGY1 a b where class (COMMON a b) => ONTOLOGY2 a b where class (ONTOLOGY1 a b, ONTOLOGY2 a b) => MERGEDONTOLOGY a b where -- not really sure how to proceed from here instance COMMON Woman Person where

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How to reliably open and close resources in MonadIO in the presence of exceptions?
by Bas van Dijk 16 Oct '09

16 Oct '09

> import Control.Monad.Trans ( MonadIO, liftIO ) > import Control.Exception ( throwIO, ArithException(DivideByZero), onException ) > import Foreign.Marshal.Alloc ( malloc, free ) > import Foreign.Storable ( poke, peek ) Suppose we define a function which receives a computation in an arbitrary monad that is capable of lifting IO actions and returns a computation in the same monad: > foo :: MonadIO m => m () -> m () In 'foo', I would like to open some resource (in this case: allocate memory) and after performing some lifted IO actions and some arbitrary computations in 'm' I want to close the resource (in this case: freeing the allocated memory). Of course the resource should also be closed when an exception is thrown in some lifted IO computation. If I was performing a computation directly in IO I could use: alloca :: Storable a => (Ptr a -> IO b) -> IO b which allocates the memory, performs some action on it and frees the memory after the action terminates (either normally or via an exception). Unfortunately the type of 'alloca' does not allow it to be used in our arbitrary monad 'm'. So it looks like I'm forced to manually allocate the memory using 'malloc' and finally free it using 'free'. To handle exceptions in lifted IO computations I have to put a (`onException` free ptr) around every lifted IO computation: For example: > foo action = do ptr <- liftIO malloc > liftIO $ putStrLn "aaa" `onException` free ptr > liftIO $ poke ptr (3 :: Int) `onException` free ptr > liftIO $ putStrLn "bbb" `onException` free ptr > action > x <- liftIO $ peek ptr `onException` free ptr > liftIO $ print x `onException` free ptr > liftIO $ putStrLn "ccc" `onException` free ptr > liftIO $ free ptr I find this rather ugly. What's worse is that exceptions, thrown in lifted IO computations in 'action' can't be handled! For example, the following won't free the memory: > bar :: MonadIO m => m () > bar = foo action > where > action = liftIO $ throwIO DivideByZero So my question is: How can I reliably open and close resources in MonadIO? And as a supplementary question: Is it possible to define a generalized alloca: genAlloca :: (MonadIO m, Storable a) => (Ptr a -> m b) -> m b Thanks, Bas

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ANN: Haskell Hackathon in Zurich, Switzerland
by Johan Tibell 16 Oct '09

16 Oct '09

Hi all! We are in the early stages of planning a Haskell hackathon/get together, ZuriHac, to be held this March at the Google office, in Zurich, Switzerland. Right now we're looking at four possible dates (Friday-Sunday): March 5-7, March 12-14, March 19-21, or March 26-28. If you might be interested in attending, please add your name on the wiki page: http://haskell.org/haskellwiki/ZuriHac#Possible_Attendees If you don't have an account on the wiki feel free to just respond to this email. If there are any dates that absolutely don't work for you then please note them on the wiki. I'll come back with a finalized date and more details soon. Hope to see you in Zurich! Johan Tibell (tibbe)

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Why there is not standard Monoid instance for ZipList a?
by Vladimir Reshetnikov 16 Oct '09

16 Oct '09

I find the following instance very convenient: ------------------------------------------------------------------------ import Data.Monoid import Control.Applicative instance Monoid a => Monoid (ZipList a) where mempty = pure mempty mappend = liftA2 mappend ------------------------------------------------------------------------ Any reason why it is not in the standard library? Thanks, Vladimir

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ANN: WFLP 2010 Call for papers
by Pablo Nogueira 16 Oct '09

16 Oct '09

[Apologies for multiple receptions of this message] ******************************************************************** Preliminary Call For Papers 19th International Workshop on Functional and (Constraint) Logic Programming Madrid, Spain, January 17, 2010 http://babel.ls.fi.upm.es/events/wflp2010/ ********* colocated with Principles of Programming Languages POPL 2010 http://www.cse.psu.edu/popl/10/ ******************************************************************** IMPORTANT DATES Abstract Submission: November 9, 2009 Full Paper Submission: November 15, 2009 Acceptance Notification: December 15, 2009 Preliminary Proceedings: January 5, 2010 Workshop: January 17, 2010 SCOPE The aim of the Workshop on Functional and (Constraint) Logic Programming is to bring together researchers interested in functional programming and (constraint) logic programming with special emphasis on the integration of both paradigms and of other declarative programming extensions. It promotes the cross-fertilizing exchange of ideas and experiences among researchers and students from the different communities interested in the foundations, applications, and combinations of high-level declarative programming languages and related areas. The previous WFLP editions are: WFLP 2009 (Brasilia, Brazil), WFLP 2008 (Siena, Italy), WFLP 2007 (Paris, France), WFLP 2006 (Madrid, Spain), WCFLP 2005 (Tallinn, Estonia), WFLP 2004 (Aachen, Germany), WFLP 2003 (Valencia, Spain), WFLP 2002 (Grado, Italy), WFLP 2001 (Kiel, Germany), WFLP 2000 (Benicassim, Spain), WFLP'99 (Grenoble, France), WFLP'98 (Bad Honnef, Germany), WFLP'97 (Schwarzenberg, Germany), WFLP'96 (Marburg, Germany), WFLP'95 (Schwarzenberg, Germany), WFLP'94 (Schwarzenberg, Germany), WFLP'93 (Rattenberg, Germany), and WFLP'92 (Karlsruhe, Germany). LOCATION WFLP 2010 will be held on January 17, 2010 in Madrid, Spain, colocated with the 37th ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages (POPL 2010). WFLP 2010 solicits papers in all areas of functional and (constraint) logic programming, including but not limited to: * Foundations: formal semantics, logic variables, binding and abstract syntax, rewriting and narrowing, unification, constraint solving, dynamics, type theory, meta-theory, effects, etc. * Language Design: security, services, modules, type systems, multi-paradigm languages, concurrency and distribution, objects, libraries, generic programming, interoperability, etc. * Implementation: abstract machines, parallelism, compile-time and run-time optimizations, foreign-language interfaces, memory management, multi-threading, exploiting parallel hardware, etc. * Transformation and Analysis: abstract interpretation, specialization, partial evaluation, program transformation, program calculation, program proof, meta-programming, generative programming, etc. * Software Development: algorithms, data structures, design patterns, components and composition, specification, proof assistants, verification and validation, model checking, debugging, testing, profiling, tracing, etc. * Paradigm Integration: integration of declarative programming with other paradigms or features such as imperative, object-oriented, aspect-oriented, concurrent, real-time programming, event-driven architectures, etc. * Applications: education, industry, commercial uses, domain-specific languages, visual/graphical user interfaces, embedded systems, WWW applications, XML processing, artificial intelligence, knowledge representation and machine learning, deductive databases, advanced programming environments and tools, etc. SUBMISSIONS and PROCEEDINGS Authors are invited to submit papers presenting original unpublished work. Papers must be at most 15 pages long. Exceptionally, authors may surpass the page limit by providing well-marked appendices intended as reviewing aids. Appendices will not appear in the final publication. Submission categories include regular research papers, system descriptions, and short papers describing on-going work (at most 8 pages). Submissions must be formatted in Lecture Notes in Computer Science (LNCS) style. This requirement needs not apply to appendices. Papers should be submitted in pdf or postscript format electronically via the web-based submission site http://www.easychair.org/conferences/?conf=wflp2010 Preliminary proceedings will be available at the workshop. Selected authors will be invited to submit a full version of their papers after the workshop. Contributions accepted for the post-workshop proceedings have usually appeared in journals -- those of the 2009 edition have been published in LNCS, previous editions in ENTCS, etc. PROGRAMME CHAIR Julio Marino (Universidad Politecnica de Madrid, Spain) PROGRAMME COMMITTEE Maria Alpuente (Universidad Politecnica de Valencia, Spain) Sergio Antoy (Portland State University, USA) Rachid Echahed (CNRS-IMAG, France) Santiago Escobar (Universidad Politecnica de Valencia, Spain) Moreno Falaschi (Universita di Siena, Italy) Murdoch Gabbay (Heriot-Watt University, UK) Maria Garcia de la Banda (Monash University, Australia) Victor Gulias (Lambdastream SL, Spain) Michael Hanus (CAU Kiel, Germany) Herbert Kuchen (Univ. of Muenster, Germany) Francisco Lopez-Fraguas (Universidad Complutense de Madrid, Spain) James Lipton (Wesleyan University, USA) Mircea Marin (University of Tsukuba, Japan) Juan Jose Moreno-Navarro (Ministry of Science & Innovation, Spain) Brigitte Pientka (McGill University, Canada)

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Re: Relational Algebra
by oleg＠okmij.org 16 Oct '09

16 Oct '09

> In hindsight it occurred to me that the algorithm can be abstractly > expressed in terms of relational algebra for which I need an EDSL. > > One concrete implementation could then be an interpreter / compiler to > SQL, if I choose to use a database backend, and another implementation > could be a interpretor to in-memory data structures (maps, whathaveyou), > or a debugger to see if I have got the abstract algorithm right. Perhaps the CoddFish project (what a great name!) may be relevant here: http://wiki.di.uminho.pt/twiki/bin/view/Research/PURe/CoddFish Perhaps it is the first project that systematically exploited the connection between functional dependencies in type classes and functional dependencies among database attributes.

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Very evil
by Andrew Coppin 16 Oct '09

16 Oct '09

{-# LANGUAGE EmptyDataDecls #-} module Main (main) where import Unsafe.Coerce data Anything newtype Key x = Key Int deriving Eq type Dict = [(Key Anything, Anything)] put :: Key x -> x -> Dict -> Dict put k' v' = raw (unsafeCoerce k') (unsafeCoerce v') where raw k0 v0 [] = [(k0,v0)] raw k0 v0 ((k,v):zs) | k == k0 = (k0 ,v0) : zs | otherwise = (k ,v ) : raw k0 v0 zs get :: Key x -> Dict -> Maybe x get k' zs = unsafeCoerce (raw (unsafeCoerce k') zs) where raw k0 [] = Nothing raw k0 ((k,v):zs) | k == k0 = Just v | otherwise = raw k0 zs main = do let k1 = Key 1 :: Key Int let k2 = Key 2 :: Key Double let k3 = Key 3 :: Key String let k4 = Key 4 :: Key Bool let d0 = [] let d1 = put k1 123 d0 let d2 = put k2 123 d1 let d3 = put k3 "123" d2 let d4 = put k4 True d3 print (get k1 d4) print (get k2 d4) print (get k3 d4) print (get k4 d4) Unsafe coerce, anyone? This particular example appears to run without incident, but the GHC docs suggest that this is very... well, unsafe. (In particular, the docs claim this will fall over on function types.) I presume there's some less-evil way of doing this?

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Fwd: [BostonHaskell] Next meeting: October 21st at MIT (32G-882)
by Ravi Nanavati 15 Oct '09

15 Oct '09

---------- Forwarded message ---------- From: Ravi Nanavati <ravi_n(a)alum.mit.edu> Date: Thu, Oct 15, 2009 at 11:07 PM Subject: [BostonHaskell] Next meeting: October 21st at MIT (32G-882) To: bostonhaskell(a)googlegroups.com I'm pleased to announce the October meeting of the Boston Area Haskell Users' Group. Based on the feedback from the meeting polls and the constraints of our speakers, the October meeting has been scheduled for Wednesday, October 21st from 7pm - 9pm. Like the past three meetings, it will be held in the MIT CSAIL Reading Room (32-G882, i.e. a room on the 8th floor of the Gates Tower of the MIT's Stata Center at 32 Vassar St in Cambridge, MA). Based on the topic poll (and because he volunteered), our first speaker will be Brian Sniffen with an introduction to Parsec. Following his presentation will be our traditional break (with the now-traditional baked goods). Our second speaker will be Edward Kmett, who will be picking up the part of his monoids and parsing presentation that we didn't get to in August or September (third time's the charm!): "A Parallel Parsing Trifecta: Iteratees, Parsec, and Monoids". Edward's slides from August are here: http://comonad.com/reader/wp-content/uploads/2009/08/IntroductionToMonoids.… I'd recommend looking at them if you weren't at the August meeting (or want a refresher on the material) since the two parts of his presentation were designed to go concurrently. There's also a preview of the presentation as part of this post on Edward's blog: http://comonad.com/reader/2009/iteratees-parsec-and-monoid/ The October attendance poll is at: http://doodle.com/u7762nfxpghsrn2r As always, responding to this poll will help with two things: 1. Getting an idea of what fraction of the Boston-area Haskell community can and can't attend this meeting (to help with future scheduling). 2. Giving me an estimated count of attendees, since I have talked my wife into baking some more goodies and bringing drinks again. Helping with BostonHaskell (besides volunteering to speak at future meetings): 1. Sponsorship of refreshments is still being eagerly solicited. 2. Please let me know if you're willing to arrive early (6:30pm) to help me move refreshments from my car. Based on past experience, I could use 3-4 volunteers. 3. Please let me know if you're willing to arrive early (6:30pm) to stand by the door and let people into the 8th floor. I'm guessing 1 or 2 volunteers is enough here. 4. If someone is willing to edit the HaskellWiki page, post to reddit or otherwise publicize the meeting please just go ahead and do that (especially as I'm sending this out a bit later than intended). 5. I'm also still having trouble embedding this poll in a page on our Google Group, so if anyone has any ideas about that, please feel free to try (any group member can edit pages) and/or offer other suggestions. If you have any questions about the meeting please send them to the BostonHaskell mailing list: bostonhaskell(a)googlegroups.com or contact me directly. I look forward to seeing many Boston-area Haskellers at the October meeting! Thank you, - Ravi Nanavati --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "BostonHaskell" group. To post to this group, send email to bostonhaskell(a)googlegroups.com To unsubscribe from this group, send email to bostonhaskell+unsubscribe(a)googlegroups.com For more options, visit this group at http://groups.google.com/group/bostonhaskell?hl=en -~----------~----~----~----~------~----~------~--~---

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ANN: Reverse Dependencies in Hackage (demo)
by Roel van Dijk 15 Oct '09

15 Oct '09

Hello, I have implemented reverse dependencies in Hackage. You can play with the demo here: http://bifunctor.homelinux.net/~roel/hackage I already send a message to Haskell-Cafe, but I made a few changes and would like some feedback. You can sort the tables of reverse dependencies by clicking on the column headers. Large tables (base or the total overview) might take some time. Sorting is done client-side with a bit of JavaScript. I added a link to an overview of the reverse dependencies of every package in hackage. It is sorted by the total number of rev. deps by default, so the most dependent-on packages (within hackage) are at the top. In the future such a list might be expanded with other information, like the download count. Things I would like feedback on: - Rev. deps overview for all packages; is it useful? - If you look at the reverse dependencies for a package like digest you'll see a list of packages that depend on digest. Next to the names of the packages that depend on digest are their respective reverse dependencies. The idea is that you can see which are the most "important" (for some value of important) packages that depend on digest. Is that useful? - Is this the best way to present the information? Currently I am working on a patch for the new hackage-server. Regards, Roel

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Parsec bug, or...?
by Uwe Hollerbach 15 Oct '09

15 Oct '09

a brain fart? Hi, cafe, I've been playing a little bit with a small command processor, and I decided it'd be nice to allow the user to not have to enter a complete command, but to recognize a unique prefix of it. So I started with the list of allowed commands, used filter and isPrefixOf, and was happy. But then I increased the complexity a little bit and it got hairier, so I decided to rewrite the parser for this bit in parsec. The function I came up with is parsePrefixOf n str = string (take n str) >> opts (drop n str) >> return str where opts [] = return () opts (c:cs) = optional (char c >> opts cs) which I call as parseFoo = parsePrefixOf 1 "foo" and it recognizes all of "f", "fo", and "foo" as "foo". OK so far, this also seems to work fine. But during the course of writing this, I made a stupid mistake at one point, and the result of that seemed odd. Consider the following program. It's stupid because the required prefix of "frito" is only 2 characters, which isn't enough to actually distinguish this from the next one, "fromage". (And if I change that to 2 to 3 characters, everything works fine.) So here's the complete program module Main where import Prelude import System import Text.ParserCombinators.Parsec as TPCP myPrefixOf n str = string (take n str) >> opts (drop n str) >> return str where opts [] = return () opts (c:cs) = optional (char c >> opts cs) myTest = myPrefixOf 1 "banana" <|> myPrefixOf 1 "chocolate" <|> TPCP.try (myPrefixOf 2 "frito") <|> myPrefixOf 3 "fromage" myBig = spaces >> myTest >>= (\g -> spaces >> eof >> return g) parseTry input = case parse myBig "test" input of Left err -> return (show err) Right val -> return ("success: '" ++ val ++ "'") main = getArgs >>= (\a -> parseTry (a !! 0)) >>= putStrLn If I compile this, say as program "opry", and run it as shown below, I expect the results I get for all but the last one: % ./opry b success: 'banana' % ./opry c success: 'chocolate' % ./opry fr success: 'frito' % ./opry fri success: 'frito' % ./opry fro "test" (line 1, column 3): unexpected "o" expecting "i", white space or end of input Sooo... why do I get that last one? My expectation was that parsec would try the string "fro" with the parser for "frito", it would fail, having consumed 2 characters, but then the TPCP.try which is wrapped around all of that should restore everything, and then the final parser for "fromage" should succeed. The same reasoning seems to me to apply if I specify 3 characters as the required initial portion for "frito", and if I do that it does succeed as I expect. So is this a bug in parsec, or a bug in my brain? thanks... Uwe

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