- Haskell-Cafe - Haskell.org

Chordify, a new web startup using Haskell
by José Pedro Magalhães 19 Jan '13

19 Jan '13

Hi all, I'd like to introduce Chordify <http://chordify.net/> [1], an online music player that extracts chords from musical sources like Soundcloud, Youtube or your own files, and shows you which chord to play when. Here's an example song: http://chordify.net/chords/passenger-let-her-go-official-video-passengermus… The aim of Chordify is to make state-of-the-art music technology accessible to a broader audience. Behind the scenes, Chordify uses the HarmTrace Haskell package to compute chords from audio. I've been working on this project with a couple of colleagues for a while now, and recently we have made the website public, free to use for everyone. We do not use Haskell for any of the frontend/user interface, but the backend is entirely written in Haskell (and it uses pretty advanced features, such as GADTs and type families [3]). We're particularly interested in user feedback at this stage, so if you're interested in music and could use an automatic chord transcription service, please try Chordify! Cheers, Pedro [1] http://chordify.net/ [2] http://hackage.haskell.org/package/HarmTrace [3] José Pedro Magalhães and W. Bas de Haas. Functional Modelling of Musical Harmony: an Experience Report. In Proceedings of the 16th ACM SIGPLAN International Conference on Functional Programming (ICFP'11), pp. 156–162, ACM, 2011. http://dreixel.net/research/pdf/fmmh.pdf

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resources for learning Hindley-Milner type inference for undergraduate students
by oleg＠okmij.org 19 Jan '13

19 Jan '13

Petr Pudlak wrote: > could somebody recommend me study materials for learning Hindley-Milner > type inference algorithm I could recommend to undergraduate students? Perhaps you might like a two-lecture course for undergraduates, which uses Haskell throughout http://okmij.org/ftp/Haskell/AlgorithmsH.html#teval It explained HM type inference in a slightly different way, strongly emphasizing type inference as a non-standard interpretation. The second main idea was relating polymorphism and `inlining' (copying). Type schemas were then introduced as an optimization, to inlining. It becomes clear why it is unsound to infer a polymorphic type for ref []: expressions of polymorphic types are always inlined (conceptually, at least), which goes against the dynamic semantics of reference cells. The lectures also show how to infer not only the type but also the type environment. This inference helps to make the point that `tests' cannot replace typing. We can type check open expressions (and infer the minimal environment they make sense to use in). We cannot run (that is, dynamically check) open expressions.

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resources for learning Hindley-Milner type inference for undergraduate students
by Petr P 18 Jan '13

18 Jan '13

Dear Haskellers, could somebody recommend me study materials for learning Hindley-Milner type inference algorithm I could recommend to undergraduate students? The original paper is harder to understand, I'm looking for something more didactic. The students are familiar with the lambda calculus, natural deduction and System F. Thanks for help, Petr Pudlak

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ANNOUNCE: MFlow 0.2
by Alberto G. Corona 18 Jan '13

18 Jan '13

Hello. haskellers and, specially, web developpers. This is the second version of MFlow, a deep-first effort in the development of a platform for web applications at the higher level, by including as much haskell magic as possible. It is at the same time experimental and intended to be used in industry. I believe that haskell will not have its chance in the Web if it does not bring unique advantages and challenging paradigms beyond speed and type safety. Deep-first means that the effort is more in the addition higher level features rather than in being complete and bug free, with the conviction that feedback from real usage at the highest level is the best guide for development. Rather than to mimic other platforms in other languages, MFlow is as Haskell'ish as possible, and introduces new approaches like statefulness, event sourcing, back-execution, composable, active, self contained widgets and persistent STM. All of them collaborate to create a high level environment for web programming. This release adds bidings for WAI, blaze-html, stateful AJAX, active widgets, requirements, content management, multilanguage and URLs to pages inside stateful procedures. The package: [http://hackage.haskell.org/package/MFlow The entry in my blog, with the announcement and the philosophy behind http://haskell-web.blogspot.com.es/2013/01/announce-mflow-02.html The same, in other words: http://haskell-web.blogspot.com.es/2012/12/on-spirit-of-mflow-anatomy-of-wi… An online example, with some links that explain the new features. http://81.169.134.95:8081 Stateful, but virtually stateless, thanks to event sourcing http://haskell-web.blogspot.com/2013/01/stateful-but-stateless-at-last-than… Example of use of active widgets http://haskell-web.blogspot.com.es/2013/01/now-example-of-use-of-active-wid… Bug tracker https://github.com/agocorona/MFlow/issues Source repository git clone http://github.com/agocorona/MFlow Feedback is welcomed warmly Alberto.

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ANN: crypto-pubkey: all your public key crypto algorithms belong to us.
by Vincent Hanquez 18 Jan '13

18 Jan '13

Hi Cafe, I've recently released crypto-pubkey [1][2], which provide a comprehensive solution for public key cryptography. Most known RSA modes (PKCS15, OAEP, PSS) are supported, and there's also DSA and ElGamal signature support. Most of the code originally lived in cryptocipher, but have now been made better and safer, and support more modes. I've spend some good chunk of time adding KATs and tests, documentation, and making sure the performance was ahead of other haskell implementations. Enjoy, [1] http://hackage.haskell.org/package/crypto-pubkey-0.1.1 [2] https://github.com/vincenthz/hs-crypto-pubkey -- Vincent

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Feedback on FFI bindings for C++ library
by Nathan Hüsken 18 Jan '13

18 Jan '13

Hey, I would like to write FII bindings in haskell for cocos2d-x (http://www.cocos2d-x.org/) which is a C++ library. Since I have little experience with this, I would like some feedback before I discover that concept is bad half way. In cocos2d there is a base class with much functionality: CCNode Many classes derive from it (i.E. CCLayer) and use the virtual functions of CCNode (i.E. setPosition). How do I map this to haskell? The general Idea: I have a typeclass, in which everything that is derived from CCNode is an instance: class NodeDerived a where toNode :: a -> Node setPosition :: a -> (Double,Double) -> IO () setPosition a pos = setNodePosition (toNode a) pos instane NodeDerived Layer where toNode = layerToNode and layerToNode :: Layer -> Node would be implemented in C++ as: CCNode* layerToNode(CCLayer* l) { return dynamic_cast<CCNode*>(l); } This way I would have only to implement toNode for every class derived from CCNode and get all the functions CCNode defines. What do you think if this Idea? How would you do it? Thanks! Nathan

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TAP 2013: Final Call for Papers
by Achim D. Brucker 18 Jan '13

18 Jan '13

(Apologies if you receive this announcement multiple times.) !LNCS publication is confirmed! **************************************************** *** *** *** TAP 2013 *** *** *** *** Abstract submission: January 25, 2013 *** *** Paper submission: February 1, 2013 *** **************************************************** *** TAP 2013 solicits both full papers and *** *** (industrial) experience/tool papers *** *** in combining proofs and (security) testing *** **************************************************** CALL FOR PAPERS 7th INTERNATIONAL CONFERENCE ON TESTS AND PROOFS (TAP 2013) http://www.spacios.eu/TAP2013 Budapest, Hungary, June 18-19, 2013 The TAP conference is devoted to the synergy of proofs and tests, to the application of techniques from both sides and their combination for the advancement of software quality. Testing and proving seem to be contradictory techniques: once you have proved your program to be correct then additional testing seems pointless; on the other hand, when such a proof in not feasible, then testing the program seems to be the only option. This view has dominated the research community since the dawn of computer science, and has resulted in distinct communities pursuing the seemingly orthogonal research areas. However, the development of both approaches has lead to the discovery of common issues and to the realization of potential synergy. Perhaps, use of model checking in testing was one of the first signs that a counterexample to a proof may be interpreted as a test case. Recent breakthroughs in deductive techniques such as satisfiability modulo theories, abstract interpretation, and interactive theorem proving, have paved the way for new and practically effective methods of powering testing techniques. Moreover, since formal, proof-based verification is costly, testing invariants and background theories can be helpful to detect errors early and to improve cost effectiveness. Summing up, in the past few years an increasing number of research efforts have encountered the need for combining proofs and tests, dropping earlier dogmatic views of incompatibility and taking instead the best of what each of these software engineering domains has to offer. The TAP conference aims to bring together researchers and practitioners working in the converging fields of testing and proving, and will offer a generous allocation of papers, panels and informal discussions. Topics of interest cover theory definitions, tool constructions and experimentations, and include (other topics related to TAP are welcome): - Bridging the gap between concrete and symbolic techniques, e.g. using proof search in satisfiability modulo theories solvers to enhance various testing techniques - Transfer of concepts from testing to proving (e.g., coverage criteria) and from proving to testing - Program proving with the aid of testing techniques - New problematics in automated reasoning emerging from specificities of test generation - Verification and testing techniques combining proofs and tests - Generation of test data, oracles, or preambles by deductive techniques such as: theorem proving, model checking, symbolic execution, constraint logic programming - Model-based testing and verification - Generation of specifications by deduction - Automatic bug finding - Debugging of programs combining static and dynamic analysis - Formal frameworks - Tool descriptions and experience reports - Case studies combining tests and proofs - Domain specific applications of testing and proving to new application domains such as validating security protocols, vulnerability detection of programs, security Important Dates: ================ Abstract submission: January 25, 2013 Paper submission: February 1, 2013 Notification: March 3, 2013 Camera ready version: April 5, 2013 TAP conference: June 17-21, 2013 Program Chairs: =============== Margus Veanes (Microsoft Research, USA) Luca Vigano` (University of Verona, Italy) Program Committee: ================== Paul Ammann Dirk Beyer Achim D. Brucker Robert Clarisò Marco Comini Catherine Dubois Juhan Ernits Gordon Fraser Angelo Gargantini Christoph Gladisch Martin Gogolla Arnaud Gotlieb Wolfgang Grieskamp Reiner Hähnle Bart Jacobs Thiérry Jeron Jacques Julliand Gregory Kapfhammer Nikolai Kosmatov Victor Kuliamin Michael Leuschel Karl Meinke Alexandre Petrenko Holger Schlingloff T.H. Tse Margus Veanes (co-chair) Luca Viganò (co-chair) Burkhart Wolff Fatiha Zaidi Submission: =========== Please submit your papers via easychair https://www.easychair.org/conferences/?conf=tap2013 TAP 2013 will accept two types of papers: - Research papers: full papers with at most 16 pages in LNCS format (pdf), which have to be original, unpublished and not submitted elsewhere. - Short contributions: work in progress, (industrial) experience reports or tool demonstrations, position statements; an extended abstract with at most 6 pages in LNCS format (pdf) is expected. Accepted papers will be published in the Springer LNCS series and will be available at the conference. The contents of previous TAP proceedings is available at: http://www.informatik.uni-trier.de/~ley/db/conf/tap/ -- Dr. Achim D. Brucker, SAP AG, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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DIY vs as-a-Service with MemCachier
by Kazu Yamamoto 17 Jan '13

17 Jan '13

Hello, The following blog post by Joyent is worth reading: http://joyent.com/blog/diy-vs-as-a-service-with-memcachier "We don't really believe in Node.js (Go & Haskell are our choices), so that is a small concern to us, but everyone has their failings." --Kazu

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ANN: hoodle-0.1.1
by Ian-Woo Kim 17 Jan '13

17 Jan '13

Hi, hoodle lovers :-) I am pleased to announce hoodle v0.1.1. In this version, I added the following functionality and made several improvement. * context (popup) menu support : after you select, you will see a context menu for the selected items (make pdf, make svg, cut, copy, delete ...) You can also make your own context menu by scripting $HOME/.hoodle.d/start.hs. I will explain this later in this email. * text input support : text can be entered. currently font cannot be changed yet. * SVG import support : SVG can be imported as vector graphics * JPG import support : in addition to PNG, now JPG can be imported * embedding image : by default, hoodle now embed image in the file. It can be turned off by checking off in option menu. * full screen mode support * color picker support * rendering engine is improved much I have added the following experimental functions. * LaTeX input support (more precisely itex and MathML) : using lasem library ( https://live.gnome.org/Lasem git repository : http://git.gnome.org/cgit/lasem/ ), mathematical expression input is supported. * smooth scrolling Internally, hoodle generates SVG image from text (and latex input), so that it can be viewed exactly the same for any computer and font configuration. Hoodle has a scripting support like xmonad. If you have $HOME/.hoodle.d/start.hs , then the script will be automatically compiled using dyre and you can customize your hoodle. There are not so many hooks available yet, but I will add more and more hooks as time goes. Currently, I use file name suggestion hook (using current date/time) and custom context menu. (using this, I automatically generate png image from selected items and put it on my dropbox folder. This turned out extremely useful.) See my example start.hs : https://gist.github.com/4551914 Installation is always simple :-) after > cabal update you can install hoodle without poppler (pdf rendering support) > cabal install hoodle or with poppler (pdf rendering support) : recommended > cabal install -fpoppler hoodle-render hoodle-core hoodle If you have previous version of hoodle on your system, to uninstall previous version > ghc-pkg unregister coroutine-object --force > ghc-pkg unregister hoodle-types --force > ghc-pkg unregister xournal-types --force then install using cabal as above. Please give it a try. Hope that you will enjoy the new version. best, Ian-Woo Kim p.s. if you are interested in hoodle development and any questions about using hoodle, please subscribe to hoodle google group ( https://groups.google.com/forum/?fromgroups#!forum/hoodle ) . Issuing at http://github.com/wavewave/hoodle is also very welcome. Thanks!

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9.3 - (2 * 4.5) => 0.3000000000000007
by ivan dragolov 16 Jan '13

16 Jan '13

9.3 - (2 * 4.5) => 0.3000000000000007 I expected 0.3 ? -- Иван Драголов dragolov.net GSM: 0888 63 19 46 GSM за SMS: 0878 82 83 93 facebook.com/ivan.dragolov twitter.com/dragolov

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Type hierarchy
by Thiago Negri 16 Jan '13

16 Jan '13

Hello. How do I achieve type hierarchy in Haskell? Suppose we have the following code: foo :: A -> C bar :: B -> C I want something that allow me to say that B is a subtype of A, meaning: 1. I can use a value of type A where a value of type A is needed. 2. I can use a value of type B where a value of type B is needed. 3. I can use a value of type B where a value of type A is needed. 4. I can't use a value of type A where a value of type B is needed. What are my options? I've thought in type classes and data types with an attribute representing the extension. Any other way to do this? Thanks, Thiago.

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ANN: Hakyll 4
by Jasper Van der Jeugt 16 Jan '13

16 Jan '13

After a few weeks of beta status [1], I've now released version 4.0 of the Hakyll static site generator library. I'm really glad with this release, as from what I've found and heard, it makes many things a lot easier. # Main changes - The important `Compiler` type has been changed from `Arrow` to `Monad`: this makes it much easier to write custom compilers, as most Haskellers are more familiar with monads. - The template stays superficially the same, but it has grown much more powerful and flexible underneath. - Early and fast access to metadata makes things such as tags and pagination much easier. - All items (images, css...) can now have metadata associated. Metadata can no longer be manipulated, and this immutability should reduce the number of encountered bugs. - A `check` command has been added. This allows you to check that all internal (or external) links are still alive. # Installation, migration In order to install Hakyll 4, grab it from Hackage: cabal update cabal install hakyll Here are some useful links: - Website: <http://jaspervdj.be/hakyll/> - Tutorial index: <http://jaspervdj.be/hakyll/tutorials.html> - Migration guide: <http://jaspervdj.be/hakyll/tutorials/hakyll-3-to-hakyll4-migration-guide.ht…> All feedback is welcome as always. [1]: https://groups.google.com/d/topic/hakyll/eRKWz8bVB0w/discussion Peace, Jasper

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PhD studentships in Nottingham
by Graham Hutton 16 Jan '13

16 Jan '13

Dear all, The School of Computer Science in Nottingham is advertising a number of PhD studentships. Applicants in the area of the Functional Programming Lab (fp.cs.nott.ac.uk) would be very welcome. If you are interested in applying, please contact a potential academic supervisor in the FP lab prior to submitting your application. Note that I'm not available to supervise new students myself this coming academic year. Graham Hutton University of Nottingham +---------------------------------------------------------------+ Fully-Funded PhD Studentships School of Computer Science University of Nottingham, UK Applications are invited for a number of fully-funded PhD studentships in the School of Computer Science at the University of Nottingham, starting on 1st October 2013. The topics for the studentships are open, but will relate to the research interests of existing members of staff within the School. Applicants should have discussed their interest and obtained the support of a named potential supervisor before applying. The studentships are for three years and include a maintenance grant of 13,726 UK pounds per year and tuition fees for UK students. The positions are open to students of any nationality, but non-UK students will require additional funding to cover tuition fees, which is awarded by the University on a competitive basis. Applicants will normally require a first-class Undergraduate or Masters degree in Computer Science or a related discipline. Informal enquiries may be addressed to Christine Fletcher <Christine.Fletcher(a)nottingham.ac.uk>. To apply, please submit the following items by email to the above address: (1) a brief covering letter that describes your reasons for wishing to pursue a PhD, any ideas you have regarding possible areas or topics, and the name of a potential supervisor; (2) a copy of your CV, including your actual or expected degree class(es), and results of all University examinations; (3) an example of your technical writing, such as a project report or dissertation; (4) contact details for two academic referees. *Closing date for applications:* 8th February 2013 +---------------------------------------------------------------+ -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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How to disable document in .cabal file?
by Magicloud Magiclouds 16 Jan '13

16 Jan '13

Hi, I have enabled document in .cabal/config, so I could get document every time I installed libraries or so. But when I compile my own applications, it also takes time on generating non-exist documents. How to disable it just for this project? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Makefile for a Haskell Project
by xuan bach 15 Jan '13

15 Jan '13

Hi everyone, I'm a newbie in Haskell. I'm wondering that if there is any tool support creating Makefile for Haskell project like Ocamlbuild for Ocaml project? Thank you, Regards. -- *Le Dinh Xuan Bach* *Tel: 01234711869 or +65 86967149 * *Email: pig28789(a)gmail.com School of Information and Communication, * *Hanoi University of Science and Technology --------------------------------------------------------------------- レ。ディン。スアン。バイック電話番号：01234711869 or +65 86967149 メール： pig28789(a)gmail.com *

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hackage builds stopped?
by Alberto G. Corona 15 Jan '13

15 Jan '13

What is the schedule of the hackage builds?. My package has been uploaded since three days ago, and it has not been compiled yet... -- Alberto.

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Cannot read a large integer on linux
by Arnaud Bailly 15 Jan '13

15 Jan '13

Hello, I am encountering a strange issue while trying to read a string into an integer. On windows 7 64bit, I have: > read "18780189038289e49" :: Integer =187801890382890000000000000000000000000000000000000000000000000 On linux (64bit, libgmp.so.3.5.2) I have: > read "18780189038289e49" :: Integer *** Exception: Prelude.read: no parse That's weird and I really have no clue what's going on. I use ghci 7.4.2 on linux and 7.4.1 on windows. What am I doing wrong ? Arnaud

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Re: [Haskell-cafe] Advice on type families and non-injectivity?
by Simon Peyton-Jones 14 Jan '13

14 Jan '13

| > > {-# LANGUAGE TypeFamilies #-} | > > | > > type family F a | > > | > > foo :: F a | > > foo = undefined | > > | > > bar :: F a | > > bar = foo There is a real difficulty here with type-checking 'bar'. (And that difficulty is why 'foo' is also rejected.) Namely, when typechecking 'bar', we must instantiate foo with an unknown type, say alpha. So now we must find a type 'alpha' such that F a ~ F alpha Certainly alpha=1 is one solution, but there might be others. For example, suppose type instance F [b] = F b Then alpha=[a] would also be a solution. In this particular case any solution will do, but suppose there was an addition constraint (C alpha) arising from the right hand side, where C is a class. Then if we had instance C [b] where ... then the second solution (alpha=[a]) would work, but not the first. This can get arbitrarily complicated, and GHC's type inference does not "search" various solutions; it follows one path. The solution is to provide a way to fix alpha. For example, foo :: a -> F a is fine. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell- | users-bounces(a)haskell.org] On Behalf Of Richard Eisenberg | Sent: 14 January 2013 03:47 | To: Conal Elliott | Cc: glasgow-haskell-users(a)haskell.org; Haskell Cafe | Subject: Re: Advice on type families and non-injectivity? | | Hi Conal, | | I agree that your initial example is a little puzzling, and I'm glad | that the new ambiguity checker prevents both definitions, not just one. | | However, your initial question seems broader than just this example. I | have run into this problem (wanting injective type functions) several | times myself, and have been successful at finding workarounds. But, I | can't think of any unifying principle or solid advice. If you can post | more information about your problem, perhaps I or others can contribute. | | For what it's worth, the desire for injective type functions has been | entered as ticket #6018 in the GHC Trac, but I see you're already on the | cc: list. I believe Simon PJ has given serious thought to implementing | this feature and may have even put in some very basic code toward this | end. | | Richard | | On Jan 13, 2013, at 2:10 PM, Conal Elliott <conal(a)conal.net> wrote: | | > I sometimes run into trouble with lack of injectivity for type | families. I'm trying to understand what's at the heart of these | difficulties and whether I can avoid them. Also, whether some of the | obstacles could be overcome with simple improvements to GHC. | > | > Here's a simple example: | > | > > {-# LANGUAGE TypeFamilies #-} | > > | > > type family F a | > > | > > foo :: F a | > > foo = undefined | > > | > > bar :: F a | > > bar = foo | > | > The error message: | > | > Couldn't match type `F a' with `F a1' | > NB: `F' is a type function, and may not be injective | > In the expression: foo | > In an equation for `bar': bar = foo | > | > A terser (but perhaps subtler) example producing the same error: | > | > > baz :: F a | > > baz = baz | > | > Replacing `a` with a monotype (e.g., `Bool`) eliminates the error. | > | > Does the difficulty here have to do with trying to *infer* the type | and then compare with the given one? Or is there an issue even with type | *checking* in such cases? | > | > Other insights welcome, as well as suggested work-arounds. | > | > I know about (injective) data families but don't want to lose the | convenience of type synonym families. | > | > Thanks, -- Conal | > | > _______________________________________________ | > Glasgow-haskell-users mailing list | > Glasgow-haskell-users(a)haskell.org | > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users | | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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Advice on type families and non-injectivity?
by Conal Elliott 14 Jan '13

14 Jan '13

I sometimes run into trouble with lack of injectivity for type families. I'm trying to understand what's at the heart of these difficulties and whether I can avoid them. Also, whether some of the obstacles could be overcome with simple improvements to GHC. Here's a simple example: > {-# LANGUAGE TypeFamilies #-} > > type family F a > > foo :: F a > foo = undefined > > bar :: F a > bar = foo The error message: Couldn't match type `F a' with `F a1' NB: `F' is a type function, and may not be injective In the expression: foo In an equation for `bar': bar = foo A terser (but perhaps subtler) example producing the same error: > baz :: F a > baz = baz Replacing `a` with a monotype (e.g., `Bool`) eliminates the error. Does the difficulty here have to do with trying to *infer* the type and then compare with the given one? Or is there an issue even with type *checking* in such cases? Other insights welcome, as well as suggested work-arounds. I know about (injective) data families but don't want to lose the convenience of type synonym families. Thanks, -- Conal

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using/building ghc head?
by Johannes Waldmann 14 Jan '13

14 Jan '13

Hi. I wanted to do some experiments with GHC Head but * I cannot use the snapshot bindist: ./configure --prefix=/opt checking for path to top of build tree... ./configure: line 2138: utils/ghc-pwd/dist-install/build/tmp/ghc-pwd-bindist: No such file or directory * I cannot compile snapshot from source (with ghc-7.6.1) utils/haddock/src/Haddock/Convert.hs:201:42: Not in scope: data constructor `HsBang' this is for ghc-7.7.20121213 (date Dec 21), which is the latest snapshot according to http://www.haskell.org/ghc/dist/current/dist/ what am I missing? - J.W.

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How to properly optimize MArray functions for speed?
by Petr P 14 Jan '13

14 Jan '13

Dear Haskellers, I'm working on a sorting library for MArrays. Speed is important, so I want to optimize it as much as possible. Currently, I simply INLINE the sorting function. This speeds up the code more than 10 times. However this can easily explode code size if the function is used in several places. The only other alternative seems to SPECIALIZE the function for all instances of MArray. This also enlarges the resulting code, but only by a constant factor, not depending on how many times the function is used. The question is, is it possible that new instanes of MArray appear? Or is MArray so special and bound to GHC's internals so that I can be sure that no new instances can be defined by some other module? Thanks for help, Petr Pudlak

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Repa: traverse delayed array multiple times
by Andrey Yankin 14 Jan '13

14 Jan '13

Greetings to all! Currently I'm trying to do physical simulations using finite difference method. To accomplish this I need to repeat some manipulations on 2-dimensional grid several thousands or millions times, you know. Is it possible to do using repa? I wrote this rough sketch that shows what I am into. Apparently, program is severely slow. I think reason is: "Every time an element is requested from a delayed array it is calculated anew, which means that delayed arrays are inefficient when the data is needed multiple times" (http://www.haskell.org/haskellwiki/Numeric_Haskell:_A_Repa_Tutorial) I started diving into Guiding Parallel Array Fusion with Indexed Types<http://www.cse.unsw.edu.au/%7Ebenl/papers/guiding/guiding-Haskell2012-sub.p…>mentioned there. Is it a right place to find any answer to my question? To summarize, I want to apply my function to Array several thousand times as fast as possible (using multi-threading and GPGPU). It seems I need a lot of memory and throughput. Do I have to call iterate only once at a time or in some kind of batches. What are my possibilities? Thanks in advance! Example program: import System.Environment import Data.Array.Repa as Repa h = 50 w = 50 grid = fromListUnboxed (Z :. h :. w) $ take (h * w) [1, 1..] main = do args <- getArgs let n = read $ head args g <- computeP $ accumulate n grid :: IO (Array U DIM2 Int) putStrLn "Ok" accumulate :: Int -> Array U DIM2 Int -> Array D DIM2 Int accumulate n g = repaIterate step n $ delay g step :: (DIM2 -> Int) -> DIM2 -> Int step f (Z :. i :. j) = center + right + left + top + bottom where left = f' j (i - 1) center = f' j i right = f' j (i + 1) top = f' (j + 1) i bottom = f' (j - 1) i f' j i = if j<0 || i<0 || i>=h || j>=w then 0 else f (Z :. j :. i) repaIterate :: ((DIM2 -> Int) -> DIM2 -> Int) -> Int -> Array D DIM2 Int -> Array D DIM2 Int repaIterate f n = (!! n) . iterate (\array -> traverse array id f) Compilation: ghc -O2 -threaded -rtsopts --make repatest.hs time output: ./repatest 3 +RTS -N2 -H 0,02s user 0,02s system 109% cpu 0,033 total ./repatest 4 +RTS -N2 -H 0,09s user 0,02s system 126% cpu 0,089 total ./repatest 5 +RTS -N2 -H 0,30s user 0,02s system 155% cpu 0,200 total ./repatest 6 +RTS -N2 -H 1,16s user 0,06s system 185% cpu 0,663 total ./repatest 7 +RTS -N2 -H 5,63s user 0,24s system 191% cpu 3,071 total ./repatest 8 +RTS -N2 -H 27,72s user 0,89s system 191% cpu 14,960 total ./repatest 8 +RTS -N1 -H 26,23s user 0,19s system 100% cpu 26,388 total

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Where is the "convergence point" between Category Theory and Haskell?
by Alfredo Di Napoli 14 Jan '13

14 Jan '13

Morning Cafe, I'm planning to do a series of write-ups about Category Theory, to publish them on the company's blog I'm currently employed. I'm not a CT expert, but since the best way to learn something is to explain it to others, I want to take a shot :) In my mind I will structure the posts following Awodey's book, but I'm wondering how can I make my posts a little more "real world". I always read about the "Hask category", which seems to be the "bootstrap" of the whole logic behind Haskell. Can you please give my materials/papers/links/blogs to the Hask category and briefly explain me how it relates to Category Theory and Haskell itself? I hope my question is clear enough, in case is not, I'll restate :P Cheers, A.

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Announce: Leksah 0.13.1 (a bit experimental)
by Hamish Mackenzie 14 Jan '13

14 Jan '13

This has mostly bug fixes, GHC 7.4.2, GHC 7.6.1 and Gtk3 support. I have not uploaded it to Hackage as it uses Gtk2Hs patches that are not in Hackage. New features include "View->Dark" (OS X only) and "View->Fullscreen". OS X (using Gtk3) ----------------- Choose the version that matches your installed GHC http://leksah.org/packages/leksah-0.13.1.0-ghc-7.0.3.dmg http://leksah.org/packages/leksah-0.13.1.0-ghc-7.0.4.dmg http://leksah.org/packages/leksah-0.13.1.0-ghc-7.4.1.dmg http://leksah.org/packages/leksah-0.13.1.0-ghc-7.4.2.dmg http://leksah.org/packages/leksah-0.13.1.0-ghc-7.6.1.dmg (probable needs OS X 10.7) Windows (still Gtk2) -------------------- Choose the version that matches your installed GHC http://leksah.org/packages/leksah-0.13.1.0-ghc-7.0.3.exe http://leksah.org/packages/leksah-0.13.1.0-ghc-7.0.4.exe http://leksah.org/packages/leksah-0.13.1.0-ghc-7.4.1.exe http://leksah.org/packages/leksah-0.13.1.0-ghc-7.4.2.exe Linux ----- Follow the steps in the .travis.yml file... https://github.com/leksah/leksah/blob/master/.travis.yml Should go something like this... https://travis-ci.org/leksah/leksah

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cabal sdist warns about optimization levels
by Petr P 13 Jan '13

13 Jan '13

Hi all, I'm working on a library for fast sorting of MArrays. The library is primarily about speed, so I added ghc-options: -O2 to the cabal file. Now cabal sdist complains with: 'ghc-options: -O2' is rarely needed. Check that it is giving a real benefit and not just imposing longer compile times on your users. I wonder: (1) Is there a way how to disable the warning? As the main aim of the library is speed, I believe -O2 is appropriate here. And since the code is quite short, I'm quite sure the increased compile time won't be noticeable. (2) Why does cabal complain about it at the first place? I found a reference saying the warning is adequate: https://github.com/haskell/cabal/issues/808 but not saying why. Maybe for complex programs -O2 prolongs compile time too much, but libraries are usually compiled once and used many times, so using -O2 for them seems reasonable in many cases. Thanks for help, Petr Pudlak

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Re: [Haskell-cafe] Advice on type families and non-injectivity?
by Conal Elliott 13 Jan '13

13 Jan '13

Hi Christian, Given "bar :: Bool", I can't see how one could go from "Bool" to "F a = > Bool" and determine "a" uniquely. > The same question applies to "foo :: Bool", right? Yet no error message there. Regards, - Conal On Sun, Jan 13, 2013 at 11:36 AM, Christian Höner zu Siederdissen < choener(a)tbi.univie.ac.at> wrote: > Hi, > > How would you infer "a" from "F a"? Given "bar :: Bool", I can't see how > one could go from "Bool" to "F a = Bool" and determine "a" uniquely. > > My question is not completely retorical, if there is an answer I would > like to know it :-) > > Gruss, > Christian > > > * Conal Elliott <conal(a)conal.net> [13.01.2013 20:13]: > > I sometimes run into trouble with lack of injectivity for type > families. > > I'm trying to understand what's at the heart of these difficulties and > > whether I can avoid them. Also, whether some of the obstacles could be > > overcome with simple improvements to GHC. > > > > Here's a simple example: > > > > > {-# LANGUAGE TypeFamilies #-} > > > > > > type family F a > > > > > > foo :: F a > > > foo = undefined > > > > > > bar :: F a > > > bar = foo > > > > The error message: > > > > Couldn't match type `F a' with `F a1' > > NB: `F' is a type function, and may not be injective > > In the expression: foo > > In an equation for `bar': bar = foo > > > > A terser (but perhaps subtler) example producing the same error: > > > > > baz :: F a > > > baz = baz > > > > Replacing `a` with a monotype (e.g., `Bool`) eliminates the error. > > > > Does the difficulty here have to do with trying to *infer* the type > and > > then compare with the given one? Or is there an issue even with type > > *checking* in such cases? > > > > Other insights welcome, as well as suggested work-arounds. > > > > I know about (injective) data families but don't want to lose the > > convenience of type synonym families. > > > > Thanks, -- Conal > > > _______________________________________________ > > Glasgow-haskell-users mailing list > > Glasgow-haskell-users(a)haskell.org > > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >

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Generating random arguments for a function
by satvik chauhan 13 Jan '13

13 Jan '13

I am trying to use quickcheck to generate random arguments of a given function (assuming all its types have Arbitrary instance and Show instance) along with the evaluation of the function at those arguments. Suppose I have a function add :: Int -> Int -> Int add a b = a+b Then I assume a behavior like > randomEvaluate add (["1","3"],"4") where 1 and 3 are random values generated for `Int` and 4 is `f 1 3`. I have asked this on SO<http://stackoverflow.com/questions/14294802/evaluating-function-at-random-a…> but am not fully satisfied with the answers. -Satvik

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A Meetup Group for MN Haskellers
by Danny Gratzer 12 Jan '13

12 Jan '13

Hello, After seeing the post about Manchurian Haskellers I decided to start one for people in and around the Twin Cities. Here's the meetup page: http://www.meetup.com/HaskellMN/ If any Haskell guru's are lurking around here we'd love to have you give a talk! -- Danny Gratzer

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Seeking feedback for a tutorial paper draft about GADTs
by Anton Dergunov 12 Jan '13

12 Jan '13

Hi Haskellers, I have written a draft of an introductory-level tutorial paper about GADTs in Haskell (for submittion to proceedings of the recent LASER summer school) and I would like to seek initial feedback about its content: what information is probably missing? are there any subtle mistakes? The main idea of this article was to serve as a starting point for learning GADTs (I was missing this kind of information myself some time ago), so I have collected several examples demonstrating common use cases. Here is a link to the paper: https://docs.google.com/open?id=0B9ZIIsDR0B7Hb1VBTFZOZEUzXzA -- Best Regards, Anton Dergunov

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Is http://hackage.haskell.org/packages/hackage.html having a challenging serving up archive.tar?
by KC 12 Jan '13

12 Jan '13

:) -- -- Regards, KC

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Permanent Academic Positions (Computational Logic/Functional Programming) in Dundee Deadline 23 January
by Ekaterina Komendantskaya 11 Jan '13

11 Jan '13

****** Dundee Fellowships ****** http://www.dundee.ac.uk/dundeefellows/ University of Dundee announces several permanent positions, with a 5-year starting period of a Research Fellowship. One of the named priority areas is Computational Logic/ Functional Programming / Verification. For more details, see below or email me: Katya Komendantskaya <katya(a)computing.dundee.ac.uk> School of Computing University of Dundee Dundee Fellowship Permanent position Closing Date for applications: Wednesday 23 January 2013. University of Dundee is advertising a series of permanent academic jobs across the College of Arts & Science and Engineering, named Dundee Fellows. The aim is to bring in a series of excellent academics onto permanent contracts (subject to the completion of 3 year probation) into any research area within the College. These staff will be labelled as Dundee Fellows during their first 5 years, over which time they will receive enhanced staff development and a research focus to their duties, while automatically moving to a regular academic contract (lecturer, senior lecturer, reader or professor) at the end of these 5 years. These will be independent researchers (and not postdocs under supervision). Their teaching load will start low, and will gradually increase over the first 5 years, to phase them into teaching. As the University of Dundee moves towards its 25 year vision of becoming Scotland’s leading University, we are actively investing in the next generation of our academic community. Our Dundee Fellows scheme offers exciting opportunities to outstanding, high potential, early career researchers of any age to establish themselves at the University of Dundee and develop towards making a full and excellent contribution to research, teaching and leadership in their chosen field. Full details are available here: http://www.dundee.ac.uk/dundeefellows/ ******************************************

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Example programs with ample use of deepseq?
by Joachim Breitner 11 Jan '13

11 Jan '13

Dear Haskellers, I’m wondering if the use of deepseq to avoid unwanted lazyness might be a too large hammer in some use cases. Therefore, I’m looking for real world programs with ample use of deepseq, and ideally easy ways to test performance (so preferably no GUI applications). I’ll try to find out, by runtime observerations, which of the calls ot deepseq could be replaced by id, seq, or „shallow seqs“ that, for example, calls seq on the elements of a tuple. Thanks, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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[announce] second Haskell meetup in Paris
by lucas di cioccio 11 Jan '13

11 Jan '13

Hi, I am pleased to annouce that I'm bootstrapping a Haskell-enthusiastics group in Paris. We recently scheduled our second Haskell meetup in Paris. This meetup will take place next Monday (Monday, January 14, 2013). AF83 ( http://af83.com/ ) has been kind enough to offer us a room for our meeting. You don't have to, but it is recommended to, register for the meetup on http://meetup.haskell-paris.fr/ or by sending me an email. The website for our group is http://haskell-paris.fr/ (in French). You should pay it a click not only because it's written in Haskell but also to glance at our logo, propose a talk, or read the digest of past meetup(s). Regards, --Lucas DiCioccio

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Cabal bug? repeat "--reinstall"
by Niklas Hambüchen 11 Jan '13

11 Jan '13

I get the following: $ cabal install --only-dependencies --reinstall Resolving dependencies... All the requested packages are already installed: Use --reinstall if you want to reinstall anyway. Can somebody confirm that they see the same?

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Parallel cabal install ignores order
by Niklas Hambüchen 11 Jan '13

11 Jan '13

I have "jobs: 4" enabled in my cabal config to get parallel installs with cabal-install >= 1.16. When I do cabal install --reinstall --force-reinstalls world to reinstall all packages (e.g. to enable profiling or shared), it fails after some point, complaining that the required dependency package is not compiled with the needed flag (for profiling / -dyn). It looks like for reinstalls, parallel cabal-install simply ignores the necessary order. Can somebody confirm this or tell me if this is known? If not, where should it be reported? Thanks Niklas

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 113, Issue 11
by Yuriy Solodkyy 10 Jan '13

10 Jan '13

OK. It tells me the same error message in popup, but all the projections are running after that. I'll make sure that it does not say any strange errors on empty DB in new version. Does it start projections for you as well (even if it says error)? On Tue, Jan 8, 2013 at 1:00 PM, <haskell-cafe-request(a)haskell.org> wrote: > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > Today's Topics: > > 1. Example programs with ample use of deepseq? (Joachim Breitner) > 2. Re: Example programs with ample use of deepseq? (Edward Z. Yang) > 3. Re: Example programs with ample use of deepseq? (Joachim Breitner) > 4. Re: Example programs with ample use of deepseq? (Johan Tibell) > 5. Re: Announce: Leksah 0.13.1 (a bit experimental) (Peter Simons) > 6. Re: Announce: Leksah 0.13.1 (a bit experimental) > (Hamish Mackenzie) > > > ---------- Forwarded message ---------- > From: Joachim Breitner <mail(a)joachim-breitner.de> > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Cc: > Date: Mon, 07 Jan 2013 13:06:35 +0100 > Subject: [Haskell-cafe] Example programs with ample use of deepseq? > Dear Haskellers, > > I’m wondering if the use of deepseq to avoid unwanted lazyness might be > a too large hammer in some use cases. Therefore, I’m looking for real > world programs with ample use of deepseq, and ideally easy ways to test > performance (so preferably no GUI applications). > > I’ll try to find out, by runtime observerations, which of the calls ot > deepseq could be replaced by id, seq, or „shallow seqs“ that, for > example, calls seq on the elements of a tuple. > > Thanks, > Joachim > > -- > Joachim "nomeata" Breitner > mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C > xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ > > > > ---------- Forwarded message ---------- > From: "Edward Z. Yang" <ezyang(a)MIT.EDU> > To: Joachim Breitner <mail(a)joachim-breitner.de> > Cc: Haskell Cafe <haskell-cafe(a)haskell.org> > Date: Mon, 07 Jan 2013 04:20:43 -0800 > Subject: Re: [Haskell-cafe] Example programs with ample use of deepseq? > There are two senses in which deepseq can be overkill: > > 1. The structure was already strict, and deepseq just forces another > no-op traversal of the entire structure. This hypothetically affects > seq too, although seq is quite cheap so it's not a problem. > > 2. deepseq evaluates too much, when it was actually sufficient only to > force parts of the structure, e.g. the spine of a list. This is less > common for the common use-cases of deepseq; e.g. if I want to force pending > exceptions I am usually interested in all exceptions in a (finite) data > structure; a space leak may be due to an errant closure---if I don't > know which it is, deepseq will force all of them, ditto with work in > parallel programs. Certainly there will be cases where you will want snip > evaluation at some point, but that is somewhat difficult to encode > as a typeclass, since the criterion varies from structure to structure. > (Though, perhaps, this structure would be useful: > > data Indirection a = Indirection a > class DeepSeq Indirection > rnf _ = () > ) > > Cheers, > Edward > > Excerpts from Joachim Breitner's message of Mon Jan 07 04:06:35 -0800 2013: > > Dear Haskellers, > > > > I’m wondering if the use of deepseq to avoid unwanted lazyness might be > > a too large hammer in some use cases. Therefore, I’m looking for real > > world programs with ample use of deepseq, and ideally easy ways to test > > performance (so preferably no GUI applications). > > > > I’ll try to find out, by runtime observerations, which of the calls ot > > deepseq could be replaced by id, seq, or „shallow seqs“ that, for > > example, calls seq on the elements of a tuple. > > > > Thanks, > > Joachim > > > > > > > ---------- Forwarded message ---------- > From: Joachim Breitner <mail(a)joachim-breitner.de> > To: haskell-cafe(a)haskell.org > Cc: > Date: Mon, 07 Jan 2013 16:59:45 +0100 > Subject: Re: [Haskell-cafe] Example programs with ample use of deepseq? > Hi, > Am Montag, den 07.01.2013, 13:06 +0100 schrieb Joachim Breitner: > > I’m wondering if the use of deepseq to avoid unwanted lazyness might be > > a too large hammer in some use cases. Therefore, I’m looking for real > > world programs with ample use of deepseq, and ideally easy ways to test > > performance (so preferably no GUI applications). > > surprisingly, deepseq is not used as much as I thought. > http://packdeps.haskellers.com/reverse/deepseq lists a lot of packages, > but (after grepping through some of the code) most just define NFData > instances and/or use it in tests, but rarely in the „real“ code. For > some reason I expected it to be in more widespread use. > > But therefore I am even more interested in non-hackaged applications > that I can be allowed to stud – in return I might be able to tell you > way to speed up your application. > > Greetings, > Joachim > > -- > Joachim "nomeata" Breitner > mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C > xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ > > > > ---------- Forwarded message ---------- > From: Johan Tibell <johan.tibell(a)gmail.com> > To: Joachim Breitner <mail(a)joachim-breitner.de> > Cc: Haskell Cafe <haskell-cafe(a)haskell.org> > Date: Mon, 7 Jan 2013 08:12:31 -0800 > Subject: Re: [Haskell-cafe] Example programs with ample use of deepseq? > On Mon, Jan 7, 2013 at 4:06 AM, Joachim Breitner > <mail(a)joachim-breitner.de> wrote: > > I’m wondering if the use of deepseq to avoid unwanted lazyness might be > > a too large hammer in some use cases. Therefore, I’m looking for real > > world programs with ample use of deepseq, and ideally easy ways to test > > performance (so preferably no GUI applications). > > I never use deepseq, except when setting up benchmark data where it's > a convenient way to make sure that the data is evaluated before the > benchmark is run. > > When removing space leaks you want to avoid creating the thunks in the > first place, not remove them after the fact. Consider a leak caused by > a list of N thunks. Even if you deepseq that list to eventually remove > those thunks, you won't lower your peak memory usage if the list was > materialized at some point. > > In addition, by not creating the thunks in the first place you avoid > some allocation costs. > > -- Johan > > > > > ---------- Forwarded message ---------- > From: Peter Simons <simons(a)cryp.to> > To: haskell-cafe(a)haskell.org > Cc: gtk2hs-devel(a)lists.sourceforge.net > Date: Mon, 07 Jan 2013 19:25:22 +0100 > Subject: Re: [Haskell-cafe] Announce: Leksah 0.13.1 (a bit experimental) > Hi Hamish, > > would it be possible to get an update for process-leksah that works with > recent versions of the 'filepath' package? I cannot build leksah-server > with GCC 7.4.2 because of this issue. > > Take care, > Peter > > > > > > ---------- Forwarded message ---------- > From: Hamish Mackenzie <hamish.k.mackenzie(a)gmail.com> > To: Peter Simons <simons(a)cryp.to> > Cc: gtk2hs-devel(a)lists.sourceforge.net, haskell-cafe(a)haskell.org > Date: Tue, 8 Jan 2013 14:05:08 +1300 > Subject: Re: [Haskell-cafe] Announce: Leksah 0.13.1 (a bit experimental) > Features in process-leksah have been merged into process. For > newer versions of GHC leksah-server just depends on process. > > If it is trying to install process-leksah then something else > has probably gone wrong. > > Check "ghc-pkg list" for old versions of leksah. Make sure > you have the latest versions of ltk, leksah and leksah-server > from github. (if you use cabal-meta they will be in > the "leksah/vendor" subdirectory). > > Here are the steps for installing from scratch... > https://github.com/leksah/leksah/blob/master/.travis.yml > > Here is what it should look like when it installs... > https://travis-ci.org/leksah/leksah > > On 8 Jan 2013, at 07:25, Peter Simons <simons(a)cryp.to> wrote: > > > Hi Hamish, > > > > would it be possible to get an update for process-leksah that works with > > recent versions of the 'filepath' package? I cannot build leksah-server > > with GCC 7.4.2 because of this issue. > > > > Take care, > > Peter > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Yuriy Solodkyy (y.solodkyy(a)gmail.com)

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First Los Angeles Haskell User Group Meetup
by Phil Freeman 10 Jan '13

10 Jan '13

Hello Haskell Cafe, I'd like to announce that I've created a meetup group for any Haskell users in the LA area. The first meeting will be a meet-and-greet session, held next Tuesday, 15th January 2013, at Wurstkuche in the Downtown LA Arts District. After that, the goal is to organise presentations and discussions on Haskell-related topics roughly once a month. Interested parties can register for the group and RSVP here: http://www.meetup.com/Los-Angeles-Haskell-User-Group/ Thanks, Phil Freeman.

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Lens, with ability to create a data-field when it's missing
by Takayuki Muranushi 10 Jan '13

10 Jan '13

Dear All, I really am enjoying the lens (it is tough to learn, though :) Recently, I've been trying to implement a dynamic object --- a collection of methods that you can update at run-time --- using lens [1][2][3]. After several attempts, I think I have located the center of my problem: Lens with the ability "to create a data-field when it's missing." My question: is it possible? There are many excellent examples of partial lenses in the lens library, such as '_head' from Data.List.Lens 'dynamic' from Data.Dynamic.Lens . To me, they seem to share the following set of common behavior (allow me to use inaccurate terms...) - set l a s : if the field is present, insert 'a'; otherwise, do nothing. - over l f s : if the field is present, map 'f' over it. otherwise, do nothing. - preview l s : if the field is present, return 'Just a'. otherwise, return 'Nothing' . Instead, I want the following set of behaviors: - set l a s : if the field is present, insert 'a'; otherwise, *create the field* and insert 'a'. - over l f s : if the field is present, map 'f' over it. otherwise, do nothing. - preview l s : if the field is present, return 'Just a'. otherwise, return 'Nothing' . For example, the current behavior of _head is >>> [] & _head .~ 1 [] I want to implement a variant _head' , such that >>> [] & _head' .~ 1 [1] Let us call this behavior 'Member' for the moment. I have a implementation of 'Member' [4][5] but they are unsatisfactory. [4] involves runtime errors and breaking of lens laws. [5] shows that we can "create a field" only if we treat Dynamic explicitly out of lens context. I don't know how 'Member' will fit in the lens hierarchy http://i.imgur.com/FgfVW.png or even if it fits at all. Member is weaker than Getter and probably is a Fold since you have only partial getting (^?) for Member. Member is weaker than Prism since you cannot construct a whole Object from just one Member. On the other hand, setter side of Member requires (s -> b -> t) interface of a Lens, because the setter need to update the whole Object when the field under concern is missing. That said, let me put this question in another way; " We can construct a (Simple Lens s a) from (getter :: s -> a) and (setter :: s -> a -> s), and we can construct a (Getter s a) from (getter :: s -> a). Then why we cannot construct a (Simple Setter s a) from (setter :: s -> a -> s) ? " Does Member deserve a new node in The Lens Hierarchy tree? Or can we implement it by combinations of existing prisms, folds etc? Or does it fall outside of the lens framework? I appreciate any comments. Takayuki [1] https://github.com/nushio3/practice/tree/master/duck [2] http://d.hatena.ne.jp/nushio/20121226#p2 (Japanese) [3] http://hackage.haskell.org/packages/archive/dynamic-object/0.1.0.1/doc/html… [4] https://github.com/nushio3/practice/blob/master/lens/newfield.hs [5] https://github.com/nushio3/practice/blob/master/lens/object-4.hs -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Haskell Weekly News: Issue 254
by Daniel Santa Cruz 10 Jan '13

10 Jan '13

Welcome to issue 254 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of December 02, 2012 to January 05, 2013. As some of you might have noticed, there has been an interruption in the scheduled transmission of the newsletter. I have, over the past few weeks, completed a move to NYC! It has been wonderful to meet many in the community at the Haskell NYC meetups. Here is to a great year for our cherised community. Let's get things started out the right way, with a litte "Lambda Style" :-) [1] http://goo.gl/nQYdp Quotes of the Week * GuySteele: Some people prefer not to commingle the functional, lambda-calculus part of a language with the parts that do side effects. It seems they believe in the separation of Church and state. * edwardk: type level ieee floats are a crime against nature. i had to implement them in c++ for template meta programming once. never ever again * acowley: (regarding Proxy) Haven't you ever wanted to put something in a box and never think about it? elliott: children * merijn: xenocons: I spend the entire week grading FP homework, I think it's impossible for you to surprise me :p merijn: That's confusing :< * andr00: Today's software engineering word is "farpotshket." This is a Yiddish word meaning, "broken, because someone tried to fix it." * dmwit: I know, I know. "0, 1, many". But then I also write most of my programs in a language whose type system has 4 levels. Four! Is that even a number?? * aristid: a dubious motivation IMHO edwardk: i've had worse =P * ziarkaen: I'm thinking I can kill two birds with one stone by learning Haskell while producing a website, but all the birds are still alive at the moment. * ion: monad.bind = function (func, args) { return func.apply(undefined, [value].concat(Array.prototype.slice.apply(args || []))); }; is idiomatic JavaScript? * beaky: if stacks are recursion, what are queues? Hafydd: Procrastination. * elliott: I think lens is the first library I've seen that promises letting you write abstract totally-inefficient-looking compositional code and getting reliably good compilation which has actually delivered on that promise. * elliott: I didn't say half the things I said. * shachaf: The two-letter and three-letter identifiers are for edwardk's libraries. The one-letter identifiers are for his actual code. elliott: edwardk has non-library code? * neutrino: in many ways, Haskell is like this primodial soup out of which other languages end up springing * mm_freak: bruce schneier is an endofunctor that turns all automorphisms into identities * roconnor: where are all the category theoriest? why don't they already have all the answers for us? edwardk: roconnor: this is the point in your career where you look around for the cavalry and realize that you're it ;) * cmccann: multi-letter type variables is an experimental new extension * elliott: I can't remember the last time I wrote any Haskell without an evil grin. * neutrino: when you're told you have to use javascript, the viable alternative always remains in seppuku. and it's much more honorable. * shachaf: chrisdone: Please make Author not mandatory. [on hpaste] donri: shachaf: just write "not shachaf" and no one will ever know it's you * tac: If you're not part of the solution, you're part of the precipitate * cmccann: some people blame themselves, some people blame the language, but the people who really know what they're doing blame shachaf. Top Reddit Stories * The end of an era, and the dawn of a new one Domain: haskell.org, Score: 148, Comments: 32 On Reddit: [2] http://goo.gl/fiyVt Original: [3] http://goo.gl/dlxsF * Simon Peyton-Jones invests in FP Complete Domain: fpcomplete.com, Score: 92, Comments: 15 On Reddit: [4] http://goo.gl/nPTk1 Original: [5] http://goo.gl/wzfpP * 24 Days of Hackage - Thank YOU! Domain: ocharles.org.uk, Score: 88, Comments: 12 On Reddit: [6] http://goo.gl/2yodj Original: [7] http://goo.gl/hSsH6 * Heist 0.10...now with a >3000x performance improvement Domain: snapframework.com, Score: 81, Comments: 38 On Reddit: [8] http://goo.gl/p7R7u Original: [9] http://goo.gl/znugn * 24 Days of Hackage: EKG Domain: ocharles.org.uk, Score: 68, Comments: 15 On Reddit: [10] http://goo.gl/ltPb1 Original: [11] http://goo.gl/UeYOA * pipes-3.0: Simpler, unified API Domain: haskellforall.com, Score: 64, Comments: 71 On Reddit: [12] http://goo.gl/ZSVtT Original: [13] http://goo.gl/6Yt01 * Hoodle : Pen note-taking program written in haskell Domain: youtube.com, Score: 64, Comments: 21 On Reddit: [14] http://goo.gl/aJ9G9 Original: [15] http://goo.gl/GsZ85 * 24 Days of Hackage: lens Domain: ocharles.org.uk, Score: 62, Comments: 46 On Reddit: [16] http://goo.gl/71HbV Original: [17] http://goo.gl/L78Hp * Cartesian Closed Comic #19: Facebook Domain: ro-che.info, Score: 61, Comments: 8 On Reddit: [18] http://goo.gl/LKqzz Original: [19] http://goo.gl/tSrjJ * Announcing: Snap Framework v0.10 Domain: snapframework.com, Score: 61, Comments: 11 On Reddit: [20] http://goo.gl/TibIw Original: [21] http://goo.gl/Nm98e * Surprises of the Haskell module system (part 1) Domain: ro-che.info, Score: 57, Comments: 11 On Reddit: [22] http://goo.gl/tOiiK Original: [23] http://goo.gl/vLEZP * 24 Days of Hackage: configurator Domain: ocharles.org.uk, Score: 53, Comments: 7 On Reddit: [24] http://goo.gl/jFKt2 Original: [25] http://goo.gl/nu2zj * Fay :: Haskell -> JavaScript Domain: fay-lang.org, Score: 52, Comments: 14 On Reddit: [26] http://goo.gl/wAH7C Original: [27] http://goo.gl/uitoD * Announcing: Clay, a CSS preprocessor as a Haskell EDSL. Domain: sebastiaanvisser.github.com, Score: 52, Comments: 53 On Reddit: [28] http://goo.gl/1ej1n Original: [29] http://goo.gl/bWfU2 * 24 Days of Hackage: base Domain: ocharles.org.uk, Score: 51, Comments: 10 On Reddit: [30] http://goo.gl/e5gLp Original: [31] http://goo.gl/7MY8o * Haskell for all: The Continuation Monad Domain: haskellforall.com, Score: 51, Comments: 42 On Reddit: [32] http://goo.gl/YKDOo Original: [33] http://goo.gl/zJ42x * ANNNOUNCE: lens 3.7 released Domain: haskell.org, Score: 50, Comments: 9 On Reddit: [34] http://goo.gl/FecK0 Original: [35] http://goo.gl/n4tR0 * 24 Days of Hackage: optparse-applicative Domain: ocharles.org.uk, Score: 50, Comments: 21 On Reddit: [36] http://goo.gl/JyFVh Original: [37] http://goo.gl/PP4CX * 24 Days of Hackage: snap Domain: ocharles.org.uk, Score: 50, Comments: 35 On Reddit: [38] http://goo.gl/9Wp3q Original: [39] http://goo.gl/WlHlU * What doesn't Haskell do well? Domain: self.haskell, Score: 47, Comments: 153 On Reddit: [40] http://goo.gl/VRJMn Original: [41] http://goo.gl/VRJMn * 24 Days of Hackage: criterion Domain: ocharles.org.uk, Score: 46, Comments: 10 On Reddit: [42] http://goo.gl/wYrdm Original: [43] http://goo.gl/rYT9C * Some specific thoughts on why using Haskell makes you a better programmer. Domain: dubhrosa.blogspot.co.uk, Score: 46, Comments: 30 On Reddit: [44] http://goo.gl/dN1Zd Original: [45] http://goo.gl/DDNuQ * Growing Haskell Platform Domain: mtnviewmark.wordpress.com, Score: 43, Comments: 16 On Reddit: [46] http://goo.gl/jNefn Original: [47] http://goo.gl/a5udB * 24 Days of Hackage: text Domain: ocharles.org.uk, Score: 43, Comments: 21 On Reddit: [48] http://goo.gl/3VmmH Original: [49] http://goo.gl/13MOc Top StackOverflow Questions * What's the absurd function in Data.Void useful for? votes: 34, answers: 5 Read on SO: [50] http://goo.gl/ny4gh * Benefit of avoiding multiple list traversals votes: 27, answers: 4 Read on SO: [51] http://goo.gl/Z6DNg * Images in Haddock documentation votes: 22, answers: 2 Read on SO: [52] http://goo.gl/7tFgE * Idiomatic option pricing and risk using Repa parallel arrays votes: 22, answers: 1 Read on SO: [53] http://goo.gl/Ih9cn * How much memory does a thunk use? votes: 21, answers: 2 Read on SO: [54] http://goo.gl/VkdIX * Can I constrain a type family? votes: 20, answers: 2 Read on SO: [55] http://goo.gl/0kBEU * How to write platform-independent code in Haskell (ghc) votes: 18, answers: 1 Read on SO: [56] http://goo.gl/13bEB * Play a wav file with Haskell votes: 17, answers: 3 Read on SO: [57] http://goo.gl/sKpgf * Why are difference lists more efficient than regular concatenation? votes: 16, answers: 2 Read on SO: [58] http://goo.gl/GCVpe * GUI tools that are actively developed and well documented for Haskell votes: 15, answers: 2 Read on SO: [59] http://goo.gl/M2L0l * What are the adjoint functor pairs corresponding to common monads in Haskell? votes: 15, answers: 2 Read on SO: [60] http://goo.gl/UnUik Until next time, +Daniel Santa Cruz https://plus.google.com/105107667630152149014/about References 1. http://www.youtube.com/watch?v=Ci48kqp11F8 2. http://www.haskell.org/pipermail/haskell-cafe/2012-December/105041.html 3. http://www.reddit.com/r/haskell/comments/14bvtz/the_end_of_an_era_and_the_d… 4. http://fpcomplete.com/why-im-investing-in-fp-complete/ 5. http://www.reddit.com/r/haskell/comments/15wc7s/simon_peytonjones_invests_i… 6. http://ocharles.org.uk/blog/posts/2012-12-24-24-days-of-hackage.html 7. http://www.reddit.com/r/haskell/comments/15e0nz/24_days_of_hackage_thank_yo… 8. http://snapframework.com/blog/2012/12/9/heist-0.10-released 9. http://www.reddit.com/r/haskell/comments/14kjct/heist_010now_with_a_3000x_p… 10. http://ocharles.org.uk/blog/posts/2012-12-11-24-day-of-hackage-ekg.html 11. http://www.reddit.com/r/haskell/comments/14ovdp/24_days_of_hackage_ekg/ 12. http://www.haskellforall.com/2012/12/pipes-30-simpler-unified-api.html 13. http://www.reddit.com/r/haskell/comments/14qe8b/pipes30_simpler_unified_api/ 14. http://www.youtube.com/watch?v=Z2wzpyxsVSU 15. http://www.reddit.com/r/haskell/comments/14seiv/hoodle_pen_notetaking_progr… 16. http://ocharles.org.uk/blog/posts/2012-12-09-24-days-of-hackage-lens.html 17. http://www.reddit.com/r/haskell/comments/14kbbi/24_days_of_hackage_lens/ 18. http://ro-che.info/ccc/19.html 19. http://www.reddit.com/r/haskell/comments/145cra/cartesian_closed_comic_19_f… 20. http://snapframework.com/blog/2012/12/10/snap-0.10-released 21. http://www.reddit.com/r/haskell/comments/14lrlr/announcing_snap_framework_v… 22. http://ro-che.info/articles/2012-12-25-haskell-module-system-p1.html 23. http://www.reddit.com/r/haskell/comments/15fnrx/surprises_of_the_haskell_mo… 24. http://ocharles.org.uk/blog/posts/2012-12-21-24-days-of-hackage-configurato… 25. http://www.reddit.com/r/haskell/comments/158svo/24_days_of_hackage_configur… 26. http://fay-lang.org/ 27. http://www.reddit.com/r/haskell/comments/14dxru/fay_haskell_javascript/ 28. http://sebastiaanvisser.github.com/clay/ 29. http://www.reddit.com/r/haskell/comments/15aoif/announcing_clay_a_css_prepr… 30. http://ocharles.org.uk/blog/posts/2012-12-23-24-days-of-hackage-base.html 31. http://www.reddit.com/r/haskell/comments/15c0jm/24_days_of_hackage_base/ 32. http://www.haskellforall.com/2012/12/the-continuation-monad.html 33. http://www.reddit.com/r/haskell/comments/15ogxe/haskell_for_all_the_continu… 34. http://www.haskell.org/pipermail/haskell/2012-December/023598.html 35. http://www.reddit.com/r/haskell/comments/14fw7l/annnounce_lens_37_released/ 36. http://ocharles.org.uk/blog/posts/2012-12-17-24-days-of-hackage-optparse-ap… 37. http://www.reddit.com/r/haskell/comments/150iem/24_days_of_hackage_optparse… 38. http://ocharles.org.uk/blog/posts/2012-12-19-24-days-of-hackage-snap.html 39. http://www.reddit.com/r/haskell/comments/154i72/24_days_of_hackage_snap/ 40. file://localhost/r/haskell/comments/14nj0r/what_doesnt_haskell_do_well/ 41. http://www.reddit.com/r/haskell/comments/14nj0r/what_doesnt_haskell_do_well/ 42. http://ocharles.org.uk/blog/posts/2012-12-18-24-days-of-hackage-criterion.h… 43. http://www.reddit.com/r/haskell/comments/152yv3/24_days_of_hackage_criterio… 44. http://dubhrosa.blogspot.co.uk/2012/12/lessons-learning-haskell.html 45. http://www.reddit.com/r/haskell/comments/15ch1k/some_specific_thoughts_on_w… 46. http://mtnviewmark.wordpress.com/2012/12/06/growing/ 47. http://www.reddit.com/r/haskell/comments/14fmw8/growing_haskell_platform/ 48. http://ocharles.org.uk/blog/posts/2012-12-12-24-days-of-hackage-text.html 49. http://www.reddit.com/r/haskell/comments/14quxz/24_days_of_hackage_text/ 50. http://stackoverflow.com/questions/14131856/whats-the-absurd-function-in-da… 51. http://stackoverflow.com/questions/13686926/benefit-of-avoiding-multiple-li… 52. http://stackoverflow.com/questions/14017112/images-in-haddock-documentation 53. http://stackoverflow.com/questions/14082158/idiomatic-option-pricing-and-ri… 54. http://stackoverflow.com/questions/13982863/how-much-memory-does-a-thunk-use 55. http://stackoverflow.com/questions/14133121/can-i-constrain-a-type-family 56. http://stackoverflow.com/questions/13799199/how-to-write-platform-independe… 57. http://stackoverflow.com/questions/14005592/play-a-wav-file-with-haskell 58. http://stackoverflow.com/questions/13879260/why-are-difference-lists-more-e… 59. http://stackoverflow.com/questions/13887213/gui-tools-that-are-actively-dev… 60. http://stackoverflow.com/questions/13937289/what-are-the-adjoint-functor-pa…

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haskell.cs.yale.edu error
by Greg Fitzgerald 10 Jan '13

10 Jan '13

I was hoping to check on the status of Yampa, but: http://haskell.cs.yale.edu/ [an error occurred while processing this directive] You don't have permission to access the requested directory. There is either no index document or the directory is read-protected. [an error occurred while processing this directive] Do you know who maintains this website? Thanks, Greg

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HaskellDB-HDBC-PostgreSQL installation problem
by Johannes.Reiher 10 Jan '13

10 Jan '13

Hello Community, I have a problem installing the HaskellDB-HDBC-PostgreSQL package with cabal. I'm using Haskell Platform in Windows and I tried to install this package, but there was the error "could not find pq library". So I tried to install libpq wich includes pq but there was another error: "this package needs a unix installation". Is there any way to install pq on Windows or get the HaskellDB-HDBC-PostgreSQL in another way? Thanks for your advice Johannes Reiher

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Undo records
by Casey Basichis 09 Jan '13

09 Jan '13

Hi, I am still getting a hang of Haskell. Sorry if the answer is obvious. What sorts of packages and approaches should I be looking at if I was looking to store something like an Undo stack into a database. Each table would refer to a function. Each records input and outputs would specify both a table ID and record ID. The records would also have a data and a Process ID to associate all functions to a specific process and give them an order. No records are ever deleted. Rolling something back is instead a process of recreating a new, modified graph by taking the old graph from the database. I should note that while I can generate some of the boiler parts from template haskell in advance I'm ultimately using a stage 1 compiler with no GHCI o template haskell. Thanks, Casey -- Casey James Basichis Composer - Cartoon Network http://www.caseyjamesbasichis.com caseybasichis(a)gmail.com 310.387.7540

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Seeking feedback for a tutorial paper draft about GADTs
by Anton Dergunov 08 Jan '13

08 Jan '13

Hi Haskellers, I have written a draft of an introductory-level tutorial paper about GADTs in Haskell (for submittion to proceedings of the recent LASER summer school) and I would like to seek initial feedback about its content: what information is probably missing? are there any subtle mistakes? The main idea of this article was to serve as a starting point for learning GADTs (I was missing this kind of information myself some time ago), so I have collected several examples demonstrating common use cases. -- Best Regards, Anton Dergunov

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Difference Lists versus Accumulators
by Edsko de Vries 08 Jan '13

08 Jan '13

Hey all, The connection between difference lists and accumulators is probably well known, but I only recently realized it myself and a quick Google search didn't find turn up any page where this was explicitly stated, so I thought this observation might be useful to some. Every beginner Haskell student learns that this definition of "reverse" has O(n^2) complexity: reverse [] = [] reverse (x : xs) = reverse xs ++ [x] But what happens when we return a difference list instead, replacing [] with id, (++) with (.) and [x] with (x :)? reverse' [] = id reverse' (x : xs) = reverse xs . (x :) This definition of reverse' has type reverse' :: [a] -> [a] -> [a] Let's inline the second argument: reverse' [] acc = acc reverse' (x : xs) acc = reverse' xs (x : acc) And we have recovered the standard definition using an accumulator! I thought that was cute :) And may help understanding why difference lists are useful. Edsko

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What happened to first-class modules?
by Ismael Figueroa Palet 08 Jan '13

08 Jan '13

I just read this page http://www.haskell.org/haskellwiki/First-class_module. It seems there was not much no ongoing work on this topic... does somebody know what happened to first-class modules? what are the actual research papers about this topic? Thanks -- Ismael

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help diagnosing space leak with IORef/STRef, just incrementing a million times.
by Thomas Hartman 07 Jan '13

07 Jan '13

I have a space leak in a function that increments a number inside IORef or STRef (either lazy or strict). Is this the kind of problem that can be diagnosed and fixed using the approach described in RWH, with space profiling? Otherwise what's the right approach to diagnose and fix? I wrote a non-list-using, and tail call-using, iterateMonadic funcation that does the same thing as replicateM, in case the problem was due to iterateM using a list structure, but iterateMonadic and replicateM had same memory leak. I can run this problem with +RTS -K100M -RTS but this aditional memory shouldn't be needed. {-# LANGUAGE RankNTypes, BangPatterns #-} import Data.List import Control.Monad.State.Strict import Data.Array.MArray import Data.Array.IO import Data.Array.ST import Control.Monad.ST import Control.Applicative import Data.IORef import Data.STRef.Strict import Data.STRef import GHC.ST import Control.Applicative ((<$>)) main = do -- putStrLn "ioIters, iterateMonadic: " -- print =<< ioIters iters iterateMonadic -- putStrLn "ioIters, replicateM_: " -- print =<< ioIters iters replicateM_ -- putStrLn "lazySTIters, iterateMonadic: " -- print $ lazySTIters iters iterateMonadic -- putStrLn "lazySTIters, replicateM_: " -- print $ lazySTIters iters replicateM_ putStrLn "strictSTIters, iterateMonadic: " print $ strictSTIters iters iterateMonadic -- putStrLn "strictSTIters, replicateM_: " -- print $ strictSTIters iters replicateM_ type Iter = (Monad m) => Int -> m () -> m () iterateMonadic :: (Monad m) => Int -> m a -> m a iterateMonadic n mx = do let loop 0 acc = acc loop n acc = loop (n-1) (acc >> mx ) loop n mx iters = 10^7 ioIters :: Int -> Iter -> IO Int ioIters numIters iteratorM = do tmp <- newIORef 0 iteratorM numIters ( modifyIORef tmp (+1) ) readIORef tmp lazySTIters :: Int -> Iter -> Int lazySTIters numIters iteratorM = runST $ do tmp <- Data.STRef.newSTRef 0 iteratorM numIters ( Data.STRef.modifySTRef tmp (+1) ) Data.STRef.readSTRef tmp strictSTIters :: Int -> Iter -> Int strictSTIters numIters iteratorM = runST $ do tmp <- Data.STRef.Strict.newSTRef 0 iteratorM numIters ( Data.STRef.Strict.modifySTRef tmp incr' ) Data.STRef.Strict.readSTRef tmp incr' (!x) = x + 1

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help diagnosing space leak with IORef/STRef, just incrementing a million times.
by Thomas Bereknyei 07 Jan '13

07 Jan '13

I have had issues like this with modifySTRef before. Try to make a strict version modifySTRef' If memory serves, something like this worked for me. modifySTRef' r f = do a <- readSTRef r writeSTRef r $! f a

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Weird problem: Couldn't read cabal file "hashable/1.2.0.0/hashable.cabal"
by Eugene Kirpichov 07 Jan '13

07 Jan '13

Hi, A friend of mine is trying to install my "timeplotters" (see email signature). He's getting a weird error: one of the tools installs fine but the other one fails with the error "Couldn't read cabal file "hashable/ 1.2.0.0/hashable.cabal""; curiously, so does running "cabal install cabal-install". Even more curiously, hashable 1.2.0.0 does not appear anywhere in the list of dependencies, even indirect. Look: $ cabal -v3 install cabal-install searching for ghc in path. found ghc at /usr/bin/ghc ("/usr/bin/ghc",["--numeric-version"]) /usr/bin/ghc is version 7.0.3 looking for tool "ghc-pkg" near compiler in /usr/bin found ghc-pkg in /usr/bin/ghc-pkg ("/usr/bin/ghc-pkg",["--version"]) /usr/bin/ghc-pkg is version 7.0.3 ("/usr/bin/ghc",["--supported-languages"]) ("/usr/bin/ghc",["--info"]) Reading installed packages... ("/usr/bin/ghc-pkg",["dump","--global","-v2"]) ("/usr/bin/ghc-pkg",["dump","--user","-v2"]) ("/usr/bin/ghc",["--print-libdir"]) Reading available packages... Resolving dependencies... cabal: Couldn't read cabal file "hashable/1.2.0.0/hashable.cabal" He's using haskell platform on Ubuntu 12.04. $ ghc —version The Glorious Glasgow Haskell Compilation System, version 7.0.3 $ cabal —version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library He *did* try erasing ~/.cabal, ~/.ghc and /usr/lib/ghc and then reinstalling haskell-platform package - it didn't change anything. He doesn't remember ever having Haskell installed before, but that can't be completely ruled out. I just tried installing a fresh Ubuntu 12.04 onto a VM and I did not have this problem that he has. What additional information can I ask him for? Maybe somebody already encountered this problem? (Actually, I have a whole strace cabal install cabal-install, but it's 33Mb - I can upload it somewhere if it could be helpful). -- Eugene Kirpichov http://www.linkedin.com/in/eugenekirpichov http://jkff.info/software/timeplotters - my performance visualization tools

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ANN: New version of pretty-show, now with Html support
by Iavor Diatchki 06 Jan '13

06 Jan '13

Hello, I've uploaded a new version of the pretty-show package (1.5) to Hackage, which provides some useful tools for inspecting Haskell data-structures. I find it quite useful while debugging. The package provides a binary and a library: the binary, named `ppsh` can take as an input a Haskell value in standard `Show` notation, and can: * Either render it to a more human readable version by adding extra white-space and newlines, thus ensuring that the result is still a valid Haskell value, or * It can render the value as an interactive Html page, which is new in this version. The library provides similar functionality but in a programatically. Also, it has some support for working with non-Showable values, by using the `PrettyVal` class, which may be derived generically. As an example, I am attaching a picture of a part of a data-structure generated with the `language-c-quote` library, by using the standard `Show` instance. I hope you find this useful, -Iavor

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ln, e
by Christopher Howard 06 Jan '13

06 Jan '13

Hi. Are natural log and Euler's constant defined somewhere in base, or a convenience math module somewhere? I'm having trouble finding them with hayoo or system documentation. -- frigidcode.com

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