- Haskell-Cafe - Haskell.org

cabal-install prompt problem
by Magicloud Magiclouds 08 Nov '12

08 Nov '12

Hi, I am using git version of cabal-install (1.17.0). It was fine until today I got this: 920 $ cabal install yesod Resolving dependencies... cabal: dependencies conflict: ghc-7.6.1 requires an installed instance of directory ==1.2.0.1 however: There is nothing more after the ":". What is happening? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

1 0

Haskell Weekly News: Issue 250
by Daniel Santa Cruz 08 Nov '12

08 Nov '12

Welcome to issue 250 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of October 28 to November 3, 2012. Quotes of the Week * Cale: A list is a bit like a loop which hasn't happened yet. This operation is similar to "how do I make the 5th iteration of my loop do something different from the others?" * broombs: Why are there multiple packages for EitherT? broombs: edwardk: which is the better one? edwardk: broombs: either And today, we have our first quote submited via twitter! Thank you @gersei for sending this one in. If you'd like to submit quotes via twitter, add @HWNQuotes to your tweet. * @kmett:We now generate pretty core but have ugly code. Top Reddit Stories * Cartesian Closed Comic #15: Iteratees Domain: ro-che.info, Score: 75, Comments: 2 On Reddit: [1] http://goo.gl/9NpD2 Original: [2] http://goo.gl/SB7HZ * Are there any, or is there interest in starting, an online Haskell course like the Scala course on Coursera? Domain: class.coursera.org, Score: 69, Comments: 16 On Reddit: [3] http://goo.gl/UzBdF Original: [4] http://goo.gl/1Zzct * Why is this simple text processing program so much slower than in dynamic languages? Domain: honza.ca, Score: 65, Comments: 189 On Reddit: [5] http://goo.gl/jW5ow Original: [6] http://goo.gl/pPzIb * Fast Code Nation: The Bright Side of High-Level Languages Domain: bos.github.com, Score: 59, Comments: 11 On Reddit: [7] http://goo.gl/KmBRt Original: [8] http://goo.gl/jm9s7 * Fay slides Domain: fay-lang.org, Score: 56, Comments: 42 On Reddit: [9] http://goo.gl/0UM4T Original: [10] http://goo.gl/h8iZE * Data.Records: extensible records using the latest GHC features Domain: github.com, Score: 55, Comments: 44 On Reddit: [11] http://goo.gl/64wUj Original: [12] http://goo.gl/XVdB2 * Why Cabal Has Problems Domain: softwaresimply.blogspot.com, Score: 50, Comments: 41 On Reddit: [13] http://goo.gl/ZX1Ia Original: [14] http://goo.gl/TgnOG * The cabal/hackage situation, and what you can do about it Domain: alpmestan.com, Score: 49, Comments: 14 On Reddit: [15] http://goo.gl/jEGIj Original: [16] http://goo.gl/EK2DB * Brian O'Sullivan's favorite bug in GHC Domain: twitter.com, Score: 48, Comments: 12 On Reddit: [17] http://goo.gl/nMjeK Original: [18] http://goo.gl/odPH4 * Haskell for all: "Hello, core!" Domain: haskellforall.com, Score: 45, Comments: 6 On Reddit: [19] http://goo.gl/qXRd2 Original: [20] http://goo.gl/9cBeo * Michael Snoyman: Yesod, AngularJS and Fay Domain: yesodweb.com, Score: 43, Comments: 26 On Reddit: [21] http://goo.gl/8yfVd Original: [22] http://goo.gl/0xtPP * Cellular automata as comonads Domain: blog.sigfpe.com, Score: 41, Comments: 10 On Reddit: [23] http://goo.gl/73nr2 Original: [24] http://goo.gl/vH4TK * pipes-2.5: Faster and slimmer Domain: haskellforall.com, Score: 41, Comments: 56 On Reddit: [25] http://goo.gl/wtUZg Original: [26] http://goo.gl/rKR2D Top StackOverflow Questions * Writing cojoin or cobind for n-dimensional grid type votes: 41, answers: 3 Read on SO: [27] http://goo.gl/qbz7z * Why not be dependently typed? votes: 39, answers: 3 Read on SO: [28] http://goo.gl/1TSVh * Concrete example showing that monads are not closed under composition (with proof)? votes: 32, answers: 4 Read on SO: [29] http://goo.gl/sys7F * Monad Transformers vs Passing parameters to functions votes: 24, answers: 1 Read on SO: [30] http://goo.gl/UpU57 * Why is there no IO transformer in Haskell? votes: 16, answers: 2 Read on SO: [31] http://goo.gl/04Xc7 * Is it possible to make GHC optimize (deforest) generic functions such as catamorphisms? votes: 16, answers: 1 Read on SO: [32] http://goo.gl/f4ZlU * Is it possible to use irrefutable tilde (`~`) patterns in lambda expressions? votes: 14, answers: 1 Read on SO: [33] http://goo.gl/BmGY0 * Can Scala's Cake Pattern be implemented in Haskell? votes: 13, answers: 2 Read on SO: [34] http://goo.gl/QiMfg * Confused by the meaning of the 'Alternative' type class and its relationship to other type classes votes: 13, answers: 5 Read on SO: [35] http://goo.gl/RXNuV * How to add fields that only cache something to ADT? votes: 13, answers: 0 Read on SO: [36] http://goo.gl/FjLlX * Type algebra and Knuth's up arrow notation votes: 13, answers: 1 Read on SO: [37] http://goo.gl/kyYIn Until next time, Daniel Santa Cruz References 1. http://ro-che.info/ccc/15.html 2. http://www.reddit.com/r/haskell/comments/11t7ru/cartesian_closed_comic_15_i… 3. https://class.coursera.org/course/progfun 4. http://www.reddit.com/r/haskell/comments/12865b/are_there_any_or_is_there_i… 5. http://honza.ca/2012/10/haskell-strings 6. http://www.reddit.com/r/haskell/comments/120h6i/why_is_this_simple_text_pro… 7. http://bos.github.com/reaktor-dev-day-2012/reaktor-talk-slides.html 8. http://www.reddit.com/r/haskell/comments/11sl47/fast_code_nation_the_bright… 9. http://fay-lang.org/fay-talk/ 10. http://www.reddit.com/r/haskell/comments/11yrpi/fay_slides/ 11. https://github.com/jonsterling/Data.Records 12. http://www.reddit.com/r/haskell/comments/12835b/datarecords_extensible_reco… 13. http://softwaresimply.blogspot.com/2012/11/why-cabal-has-problems.html 14. http://www.reddit.com/r/haskell/comments/12hbzb/why_cabal_has_problems/ 15. http://alpmestan.com/posts/2012-11-02-cabal-hackage-what-you-can-do-about-i… 16. http://www.reddit.com/r/haskell/comments/12ijs7/the_cabalhackage_situation_… 17. https://twitter.com/bos31337/status/116372971509121025 18. http://www.reddit.com/r/haskell/comments/127emo/brian_osullivans_favorite_b… 19. http://www.haskellforall.com/2012/10/hello-core.html 20. http://www.reddit.com/r/haskell/comments/11vuzz/haskell_for_all_hello_core/ 21. http://www.yesodweb.com/blog/2012/10/yesod-fay-js 22. http://www.reddit.com/r/haskell/comments/12ct6p/michael_snoyman_yesod_angul… 23. http://blog.sigfpe.com/2006/12/evaluating-cellular-automata-is.html 24. http://www.reddit.com/r/haskell/comments/11o6nu/cellular_automata_as_comona… 25. http://www.haskellforall.com/2012/10/pipes-25-faster-and-slimmer.html 26. http://www.reddit.com/r/haskell/comments/12gikh/pipes25_faster_and_slimmer/ 27. http://stackoverflow.com/questions/12963733/writing-cojoin-or-cobind-for-n-… 28. http://stackoverflow.com/questions/12961651/why-not-be-dependently-typed 29. http://stackoverflow.com/questions/13034229/concrete-example-showing-that-m… 30. http://stackoverflow.com/questions/12968351/monad-transformers-vs-passing-p… 31. http://stackoverflow.com/questions/13056663/why-is-there-no-io-transformer-… 32. http://stackoverflow.com/questions/13099203/is-it-possible-to-make-ghc-opti… 33. http://stackoverflow.com/questions/13185632/is-it-possible-to-use-irrefutab… 34. http://stackoverflow.com/questions/12947176/can-scalas-cake-pattern-be-impl… 35. http://stackoverflow.com/questions/13080606/confused-by-the-meaning-of-the-… 36. http://stackoverflow.com/questions/13098448/how-to-add-fields-that-only-cac… 37. http://stackoverflow.com/questions/13170803/type-algebra-and-knuths-up-arro…

1 0

Compiling with hsc2hs and -Wextra
by Niklas Hambüchen 08 Nov '12

08 Nov '12

Hi, when I compile my C binding using hsc2hs with the -Wextra enabled in my cabal file: cc-options: -std=c99 -Wall -Wextra -Werror This breaks the compilation started by hsc2hs: MyHsc.hsc: In function ‘main’: MyHsc.hsc:16:15: error: unused parameter ‘argc’ [-Werror=unused-parameter] MyHsc.hsc:16:27: error: unused parameter ‘argv’ [-Werror=unused-parameter] cc1: all warnings being treated as errors compiling dist/build/MyHsc_hsc_make.c failed (exit code 1) I would really like to compile my code with -Wextra -Werror though. Can't we just (void) argc; (void) argv; the arguments in the generated code, like described in [1]? Thanks Niklas [1] http://stackoverflow.com/questions/3599160/unused-parameter-warnings-in-c-c…

1 0

Where is the documentation on exception types kept?
by timothyhobbs＠seznam.cz 07 Nov '12

07 Nov '12

Hello, I am trying to catch an "thread blocked on MVar indefinitely exception." Of course I can use ::SomeException as explained in http://hackage.haskell.org/ packages/archive/base/latest/doc/html/Control-Exception.html#g:3 but there is no explanation as to how to find the more case specific exceptions. http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control- Concurrent-MVar.html#t:MVar does not discuss exceptions at all! Indeed, I don't know of any haddock pages that include descriptions of exception types :( :O Thanks, Timothy

3 4

Cross platform method for getting the /actual/ contents of a directory
by timothyhobbs＠seznam.cz 07 Nov '12

07 Nov '12

Hello, I'm writing a function to find out if a given directory is empty. I came up with the obvious: do{ contents <- getDirectoryContents "/home/timothy/works/current/anonGraph/ empty/" ; let realContents = filter (\file -> case file of "." -> False ; ".." -> False ; _ -> True) contents in return $ case realContents of [] -> True ; _ -> False} And this works fine on linux, but I wonder. On other systems, is "." and ". ." allowed as file names? Couldn't a windows user actually end up with a file name named "." and this method would fail? Thank you Timothy

2 1

[ANNOUNCE] hashable-generics
by Clark Gaebel 07 Nov '12

07 Nov '12

Hi everybody! I have just released a handy package on Hackage that will interest you if you've ever used unordered-containers with a custom type. In order to do such a thing, you'd need to define an instance of Hashable. This process could easily be automated. And so I did. {-# LANGUAGE DeriveGeneric #-} module ThisIsPrettyNeat where import Data.Hashable.Generic import GHC.Generics data MyCoolType a = MCT0 | MCT1 (Either Int a) | MCT2 (MyCoolType a) (MyCoolType a) deriving Generic instance Hashable a => Hashable MyCoolType where hashWithSalt s x = gHashWithSalt s x {-# INLINEABLE hashWithSalt #-} and voila. You have a very performant instance of Hashable, with minimal boilerplate, no template haskell, and no subtle bugs. If you want to play with it, here it is: http://hackage.haskell.org/package/hashable-generics-1.1.6 Have fun! - Clark Oh yeah, and if anyone wants to help me figure out why it's 1.3x slower than my hand-rolled instances, that would be really helpful.

4 16

ANNOUNCE: rdf4h
by Rob Stewart 07 Nov '12

07 Nov '12

Hi, I am pleased to announce the recent release of rdf4h 1.1.0 . The Resource Description Framework (RDF) is a language for representing information about resources in the World Wide Web, and is a family of W3C specifications. RDF is a standardized machine readable data model that provides a common framework for expressing information to be exchanged between applications without loss of meaning. It is becoming a popular data-modelling language, thanks to the growing adoption of semantic web principles for publishing linked open data. This `rdf4h' is a library for working with RDF in Haskell. It includes parsers and serializers for the N-Triples and Turtle formats. It provides abilities such as RDF graph comparison, querying for triples containing a particular subject, predicate, or object, or selecting triples that satisfy an arbitrary predicate function. The library is on hackage: http://hackage.haskell.org/package/rdf4h Examples are on GitHub: https://github.com/robstewart57/rdf4h Contributions very welcome! -- Rob Stewart

1 0

Serializing with alignment
by Ben Franksen 07 Nov '12

07 Nov '12

Hi Everyone I want to implement a binary protocol that, unfortunately, has some alignment restrictions. In order to fulfill these, I need access to the "current offset" in the bytestring. The binary package does provide a function bytesRead :: Get Int64 but only for the Get monad; there is no equivalent for the Put monad. So my first question: is there a serialization library that offers something like bytesWritten :: PutM Int64 Failing that, would you think adding it to binary is a reasonable feature request? I have taken a cursory look at the implementation and it looks like this is not a matter of simply adding a missing function, but would probably need an addition to internal data structures. I could also try and wrap the PutM from binary with a StateT transformer and count the bytes myself. I will probably have to use at least a ReaderT wrapper anyway, since I have to pass the byte order as a parameter (byte order gets negotiated between client and server, it is not fixed). I was really hoping that there is some library that has built-in support for stateful serialization (alignment, byte-order, etc). Any pointers, hints, etc are much appreciated. Cheers -- Ben Franksen () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

2 2

Looking for a Full-time Haskell Programmer
by Doug Beardsley 06 Nov '12

06 Nov '12

I've posted this before but did not get a whole lot of responses, so here it is again: Karamaan Group, a principal investment firm based in Manhattan is looking for an outstanding software developer to develop tools for financial analysis and knowledge management. Candidates should have at least a bachelor's degree in computer science, math, or a related field and a good sense of coding style. This is a high-impact, high-visibility job opportunity where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. Karamaan Group is a small hedge fund, so the next hire will have a very direct influence on our software development and the firm as a whole. We are a growth oriented firm that values people who take a craftsman's pride in their work. We like to take a contrarian view of things, value experimenting, and use often overlooked technologies. We are looking for a candidate who shows strong analytical, organizational, and communication skills. We have a legacy Java code base that may need occasional maintenance, but for the past few years, most of our new development has been done with Haskell (with bits and pieces of a few other languages here and there). The ideal candidate would have at least some experience with Java and the related technologies used in our existing code, but since Haskell is our language of choice, we're looking for strong Haskell skills. We have a knowledge management system built on Snap, so experience with Snap or Haskell web development in general would help. Unlike most finance companies, we don't require previous experience in finance or business, but knowledge in those areas is definitely a plus, although candidates should already be able to work legally in the U.S. and be willing to move to New York City. Please send your CV and cover letter to recruitment at karamaan.com.

1 0

Problems updating cabal-install on Mac OS
by Harald Bögeholz 06 Nov '12

06 Nov '12

Hi Cafe, I just installed the brand new Haskell Platform 2012.4.0.0 for Mac OS X, 32 bit, after removing everything left over from the old one (I hope). Next I started cabal which said to me: > Downloading the latest package list from hackage.haskell.org > Note: there is a new version of cabal-install available. > To upgrade, run: cabal install cabal-install So next I did cabal install cabal-install which started working > Resolving dependencies... > Downloading Cabal-1.16.0.2... and finished with > Installing executable(s) in > /Users/bo/Library/Haskell/ghc-7.4.2/lib/cabal-install-1.16.0.1/bin > Updating documentation index /Users/bo/Library/Haskell/doc/index.html When I then tried cabal update it still said > Downloading the latest package list from hackage.haskell.org > Note: there is a new version of cabal-install available. > To upgrade, run: cabal install cabal-install I thought I just had done that. What could have gone wrong? > Haralds-iMac:~ bo$ which cabal > /Users/bo/Library/Haskell/bin/cabal > Haralds-iMac:~ bo$ ls -l `which cabal` > lrwxr-xr-x 1 bo staff 49 6 Nov 18:15 /Users/bo/Library/Haskell/bin/cabal -> ../ghc-7.4.2/lib/cabal-install-1.16.0.1/bin/cabal Now that I am writing this up I am noticing that the last thing that seems to be installed has version number 1.16.0.1, while it started out with 1.16.0.2. So on a second thought I am attaching the full log of the install. Am I doing anything wrong? Harald -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

2 3