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Efficiency/Evaluation Question
by Christopher Howard 16 Jun '13

16 Jun '13

I'm working through some beginner-level "keyboard problems" I found at users.csc.calpoly.edu. One problem is the Saddle Points problem: quote: -------- Write a program to search for the "saddle points" in a 5 by 5 array of integers. A saddle point is a cell whose value is greater than or equal to any in its row, and less than or equal to any in its column. There may be more than one saddle point in the array. Print out the coordinates of any saddle points your program finds. Print out "No saddle points" if there are none. -------- Let's say I use a simple list grid like so: code: -------- array = Grid 5 [ [1,5,3,6,4] , [8,2,6,3,8] , [3,8,7,2,9] , [0,3,7,1,2] , [7,2,7,4,5] ] data Grid = Grid Int [[Int]] -------- And let's say I take a brute force approach, moving through each cell, checking to see if it is the greatest in its row and the least in its column. And say I have functions like so for getting rows and columns: code: -------- row (Grid s l) n = if (n >= s) then [] else l !! n col g@(Grid s l) n = if (n >= s) then [] else col_ g n 0 where col_ (Grid s l) n i = if (i >= s) then [] else (head l !! n) : col_ (Grid s (tail l)) n (i + 1) -------- My question: With the way Haskell works (thunks, lazy evaluation, and all that mystery), is it actually worth the trouble of /precalculating/ the maximum row values and minimum column values, to compare cell values against? Or will, for example, something like (smallest_list_value (col array 1)) definitely only evaluate once? -- frigidcode.com

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Improved ghc-pkg cache warnings
by Andrew Pennebaker 16 Jun '13

16 Jun '13

When ghc-pkg observes your cache is out of date, it displays a helpful warning, recommending "ghc-pkg recache". However, sometimes running this command does not fix the problem, because it targets the wrong cache. For out of date global caches, "ghc-pkg --global recache" successfully clears the warning. For out of date user caches, "ghc-pkg --user recache" clears the warning. In the future, could ghc-pkg display the command more specific to the problem? -- Cheers, Andrew Pennebaker www.yellosoft.us

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Haskell Platform 2013.2.0.0 64bit.pkg
by Richard A. O'Keefe 15 Jun '13

15 Jun '13

Today I cleared out everything, using uninstall-hs and rm -rf ~/.cabal ~/Library/Haskell I downloaded Haskell Platform 2013.2.0.0 64bit.pkg and installed it. I was unsuccessful in installing the packages I wanted using cabal install, which suggested running ghc-pkg check. So I cleared out everything again and reinstalled the HP. In the admin account, ghc-pkg check says Warning: haddock-interfaces: /Library/Haskell/ghc-7.6.3/lib/haskell-platform-2013.2.0.0/ doc/html/haskell-platform.haddock doesn't exist or isn't a file Warning: haddock-html: /Library/Haskell/ghc-7.6.3/lib/haskell-platform-2013.2.0.0/ doc/html doesn't exist or isn't a directory

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Testing invasive proposals with Hackager
by Niklas Hambüchen 14 Jun '13

14 Jun '13

In many discussions we make guesses about how much code proposals like Functor => Monad would break. You can use https://github.com/dterei/Hackager to build all of Hackage (preferably in a VM). Of course many packages have external dependencies, so I'd like to share the following list of packages to save you some time. (These are the names of packages suited for a Ubuntu 13.04 VM, and with these installed, 2800 packages are successfully built and 1700 failing, so it's quite good coverage. If you find more packages that improve on this, please add them.) acl-dev attr-dev binutils-dev cfitsio-dev expect-dev freeglut3-dev libadns1-dev libalure-dev libasound-dev libaspell-dev libatlas-dev libaugeas-dev libavcodec-dev libavfilter-dev libavformat-dev libavutil-dev libbibutils-dev libbluetooth-dev libbz2-dev libcal3d12-dev libcmph-dev libcrack2-dev libcrypto++-dev libcsound64-dev libctemplate-dev libcurl-dev libcv-dev libcwiid-dev libdb-dev libdevil-dev libdpkg-dev libev-dev libevent-dev libexif-dev libfam-dev libfcgi-dev libfftw3-dev libfltk1.3-dev libfreenect-dev libftgl-dev libfuse-dev libgd2-xpm-dev libgeoip-dev libglfw-dev libglpk-dev libgmime-2.6-dev libgnome-keyring-dev libgnutls-dev libgnutls-dev libgpcl-dev libhighgui-dev libimlib2-dev libinsighttoolkit4-dev libjasper-dev libjudy-dev libkyotocabinet-dev liblapack-dev libldap2-dev libldap2-dev libleveldb-dev libmagic-dev libmarkdown2-dev libmecab-dev libmpfr-dev libmtp-dev libmx-dev libmysqlclient-dev libncurses-dev libncurses-dev libnotmuch-dev libobjc-4.7-dev libodbc1 libode-dev libogg-dev libois-dev libopenal-dev libpam0g-dev libpcap-dev libpoker-eval-dev libportaudio-dev libpulse-dev libqd-dev libqrencode-dev libraw1394-dev libreadline-dev libscsynth1 libselinux-dev libsnappy-dev libsndfile1-dev libsqlite3-dev libssl-dev libssl-dev libst-dev libsvm-dev libtalloc-dev libtbb-dev libtheora-dev libtiff-dev libtokyocabinet-dev libtokyotyrant-dev libtre-dev libuuid-dev libv4l-dev libvxl1-dev libxapian-dev libxen-dev libxerces-c-dev libxine-dev libxmmsclient-dev libxosd-dev libxqilla-dev libxss-dev libxtst-dev libyaml-dev libz-dev libzephyr-dev libzmq-dev linux-libc-dev tcl-dev wx2.8-headers xmms2-dev

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Haskell for Programmers Workshop: Denver, Colorado
by Peter Jones 14 Jun '13

14 Jun '13

Since this is shameless self-promotion I'll keep it short. I'm teaching a Haskell workshop for imperative programmers in Denver, Colorado, September 16-18. If you want more information please take a look at the workshop website: http://www.devalot.com/workshops/haskell/index.html -- Peter Jones --- Love to Develop Devalot: http://www.devalot.com

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Automating Hackage accounts
by Andrew Pennebaker 14 Jun '13

14 Jun '13

Could we add an HTML form for creating new Hackage accounts? Right now, our community is small enough that emailing ross(a)soi.city.ac.uk and waiting for a manual response isn't too bad of a problem, but as we grow, it would be nice for these sorts of things to be handled by a server, like with RubyGems and NPM. -- Cheers, Andrew Pennebaker www.yellosoft.us

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ANN: Angel 0.4.2
by Michael Xavier 13 Jun '13

13 Jun '13

I'm pleased to announce the release of Angel 0.4.2 and that I have officially taken over maintainership of this project. Thanks to Jamie Turner for starting such a great project and allowing me to take over this project. angel is a daemon that runs and monitors other processes. It is similar to djb's daemontools or the Ruby project god. It's goals are to keep a set of services running, and to facilitate the easy configuration and restart of those services. 0.4.1 added the "count" option to the config to control the number of instances of a particular process to start. 0.4.2 added the "pidfile" option to specify the path of a pidfile to generate when monitoring processes. -- Michael Xavier http://www.michaelxavier.net

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Examples of MVars usage
by Francisco M. Soares Nt. 13 Jun '13

13 Jun '13

Hello, everyone. I am looking for packages on hackage which use MVars extensively. Those which create plenty of MVars -- not just one or two for conditional synchronization or to keep track of a value throughout the program. My purpose is to analyze usage patterns of MVars. Does anybody have any suggestions? So far I have analyzed a few packages: * conjure (0.1) * distributed-process (0.4.2) * distributed-process-p2p (0.1.1.0) * leksah (0.12.1.3) * manatee-core (0.1.1) * urlcheck (0.1.1) Nonetheless, I feel like I still haven't covered a good enough range of usage, since some examples are small, and some use MVars just for a small number of cases, while the heavy lifting is done with STM. And surely because there are only 6 of them. Any suggestion will be very much appreciated. []'s -- Francisco Soares Nt.

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Haskell Weekly News: Issue 270
by Daniel Santa Cruz 13 Jun '13

13 Jun '13

Welcome to issue 270 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 2 to 8, 2013. Quotes of the Week * shachaf: getLine :: IO String contains a String in the same way /bin/ls contains a list of files. Top Reddit Stories * Haskell's HLearn library cross-validates more than 400x faster than Java's Weka due to new monoid based algorithm Domain: izbicki.me, Score: 86, Comments: 13 On Reddit: [1] http://goo.gl/dAoe5 Original: [2] http://goo.gl/dLZJv * FP Complete Launches Haskell in Real World Competition with $1,000 Cash Prize Each Month Domain: fpcomplete.com, Score: 80, Comments: 10 On Reddit: [3] http://goo.gl/FthPX Original: [4] http://goo.gl/pzgQ2 * So I write compilers for a living now Domain: evincarofautumn.blogspot.it, Score: 76, Comments: 69 On Reddit: [5] http://goo.gl/hXp0G Original: [6] http://goo.gl/wMaab * Haskell for all: pipes-parse-1.0.0: Pushback, delimited parsers, resumable parsing, and lenses Domain: haskellforall.com, Score: 55, Comments: 77 On Reddit: [7] http://goo.gl/U3MzG Original: [8] http://goo.gl/3wEJ7 * "Thank you to the Haskell community for encouraging programmers to explore the relationship between algebra and programming." -- My accepted paper at the International Conference of Machine Learning (ICML13) Domain: jmlr.org, Score: 54, Comments: 3 On Reddit: [9] http://goo.gl/Ni0oF Original: [10] http://goo.gl/klOzM * GHCJS introduction – Concurrent Haskell in the browser, by Luite Stegeman Domain: weblog.luite.com, Score: 54, Comments: 27 On Reddit: [11] http://goo.gl/l9LDH Original: [12] http://goo.gl/avaez * Haskell from C: Where are the for Loops? Domain: fpcomplete.com, Score: 46, Comments: 28 On Reddit: [13] http://goo.gl/YqkSU Original: [14] http://goo.gl/pwYB3 * fficxx - FFI to C++ in Haskell Domain: ianwookim.org, Score: 45, Comments: 12 On Reddit: [15] http://goo.gl/y1awM Original: [16] http://goo.gl/wHF2U * Control.Category: now with kind polymorphism Domain: neocontra.blogspot.com, Score: 42, Comments: 7 On Reddit: [17] http://goo.gl/9gb6f Original: [18] http://goo.gl/ALvld * Pixel art editor in 600 lines of Haskell Domain: github.com, Score: 39, Comments: 5 On Reddit: [19] http://goo.gl/gmFOF Original: [20] http://goo.gl/w8yEg * Haskell for all: pipes-concurrency-1.2.0: Behaviors and broadcasts Domain: haskellforall.com, Score: 35, Comments: 35 On Reddit: [21] http://goo.gl/rojk1 Original: [22] http://goo.gl/aNK8l * Mastermind in Agda, Running in the Browser Domain: people.inf.elte.hu, Score: 28, Comments: 0 On Reddit: [23] http://goo.gl/N8yty Original: [24] http://goo.gl/7lhvT * Bittoll: A bitcoin transaction system written in Haskell. Domain: bittoll.com, Score: 25, Comments: 13 On Reddit: [25] http://goo.gl/9MFXB Original: [26] http://goo.gl/q39ps * standalone-haddock: host your Haskell documentation on your website Domain: feuerbach.github.io, Score: 22, Comments: 7 On Reddit: [27] http://goo.gl/dAsmP Original: [28] http://goo.gl/INyIe Top StackOverflow Questions * What is () in Haskell, exactly? votes: 18, answers: 4 Read on SO: [29] http://goo.gl/XkrXD * Haskell pre-monadic I/O votes: 15, answers: 1 Read on SO: [30] http://goo.gl/oryYA * Display function types in Haskell votes: 12, answers: 1 Read on SO: [31] http://goo.gl/qOtpV * Haskell Pattern Matching Fails on Negative Number votes: 12, answers: 2 Read on SO: [32] http://goo.gl/mw4WF * Debugging performance bottlenecks for longest common subsequence algorithm votes: 11, answers: 1 Read on SO: [33] http://goo.gl/mfa8S * What is a shrink, with regard to Haskell's QuickCheck? votes: 11, answers: 2 Read on SO: [34] http://goo.gl/eLlyO * Cross module optimizations in GHC votes: 10, answers: 1 Read on SO: [35] http://goo.gl/RyFQP * Functional composition with multi-valued functions in haskell? votes: 9, answers: 3 Read on SO: [36] http://goo.gl/10Xvd * A more succinct way to map functions onto fields of an algebraic datatype? votes: 9, answers: 1 Read on SO: [37] http://goo.gl/CfIOa * What's the history behind the Functor type class? votes: 8, answers: 1 Read on SO: [38] http://goo.gl/5BwZS Until next time, [39]+Daniel Santa Cruz References 1. http://izbicki.me/blog/hlearn-cross-validates-400x-faster-than-weka 2. http://www.reddit.com/r/haskell/comments/1fl33k/haskells_hlearn_library_cro… 3. http://www.fpcomplete.com/blog/2013/06/call-for-submissions 4. http://www.reddit.com/r/haskell/comments/1fvtbf/fp_complete_launches_haskel… 5. http://evincarofautumn.blogspot.it/2012/07/so-i-write-compilers-for-living-… 6. http://www.reddit.com/r/haskell/comments/1fsbs2/so_i_write_compilers_for_a_… 7. http://www.haskellforall.com/2013/06/pipes-parse-100-pushback-delimited.html 8. http://www.reddit.com/r/haskell/comments/1fjcpg/haskell_for_all_pipesparse1… 9. http://jmlr.org/proceedings/papers/v28/izbicki13.pdf 10. http://www.reddit.com/r/haskell/comments/1fk8fl/thank_you_to_the_haskell_co… 11. http://weblog.luite.com/wordpress/?p=14 12. http://www.reddit.com/r/haskell/comments/1fvv5q/ghcjs_introduction_concurre… 13. https://www.fpcomplete.com/blog/2013/06/haskell-from-c 14. http://www.reddit.com/r/haskell/comments/1fm55g/haskell_from_c_where_are_th… 15. http://ianwookim.org/fficxx/ 16. http://www.reddit.com/r/haskell/comments/1fy5gy/fficxx_ffi_to_c_in_haskell/ 17. http://neocontra.blogspot.com/2013/06/controlcategory-now-with-kind.html 18. http://www.reddit.com/r/haskell/comments/1fjjt0/controlcategory_now_with_ki… 19. https://github.com/mchakravarty/BigPixel 20. http://www.reddit.com/r/haskell/comments/1fqyrk/pixel_art_editor_in_600_lin… 21. http://www.haskellforall.com/2013/06/pipes-concurrency-120-behaviors-and.ht… 22. http://www.reddit.com/r/haskell/comments/1ft1sc/haskell_for_all_pipesconcur… 23. http://people.inf.elte.hu/divip/mastermind/index.html 24. http://www.reddit.com/r/haskell/comments/1fx4rb/mastermind_in_agda_running_… 25. https://www.bittoll.com/ 26. http://www.reddit.com/r/haskell/comments/1fqtmu/bittoll_a_bitcoin_transacti… 27. http://feuerbach.github.io/standalone-haddock/ 28. http://www.reddit.com/r/haskell/comments/1fuw5n/standalonehaddock_host_your… 29. http://stackoverflow.com/questions/16892570/what-is-in-haskell-exactly 30. http://stackoverflow.com/questions/17002119/haskell-pre-monadic-i-o 31. http://stackoverflow.com/questions/16971221/display-function-types-in-haske… 32. http://stackoverflow.com/questions/16976543/haskell-pattern-matching-fails-… 33. http://stackoverflow.com/questions/16952956/debugging-performance-bottlenec… 34. http://stackoverflow.com/questions/16968549/what-is-a-shrink-with-regard-to… 35. http://stackoverflow.com/questions/16908539/cross-module-optimizations-in-g… 36. http://stackoverflow.com/questions/16888222/functional-composition-with-mul… 37. http://stackoverflow.com/questions/16974868/a-more-succinct-way-to-map-func… 38. http://stackoverflow.com/questions/16880577/whats-the-history-behind-the-fu… 39. https://plus.google.com/105107667630152149014/about

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Re: [Haskell-cafe] Question about Newtype "op()" function arguments.
by David Banas 12 Jun '13

12 Jun '13

Ah, so is the idea, then, to use *op()* when `n` wasn't actually constructed formally, but rather "assembled" by the user, so as to match the type of the accessor function normally supplied as the argument to the constructor? On 6/7/2013 4:51 PM, Tom Ellis wrote: > On Fri, Jun 07, 2013 at 04:05:09PM -0400, Joe Q wrote: >> The phantom parameter solves the same problem as scoped type variables. >> Granted, if you find yourself in that kind of polymorphic soup you have >> deeper problems... > I don't understand this. Scoped type variables are used when you want to > use a type variable from the top level within the body of a function. If > you use "op" and specify a particular constructor then you don't have a > variable but a concrete instance of a type. But maybe I'm missing some more > powerful way this can be used ... > > Tom You can use scoped type variables to correct an ambiguous type error. You can think of op as a variation on asTypeOf, as documented here on http://www.haskell.org/haskellwiki/Scoped_type_variables#Avoiding_Scoped_Ty… . If I tried to come up with an example that's specific to op, it would only be horribly contrived.

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