August 2011 - Haskell-Cafe

Parsing workflow
by Nils Schweinsberg 21 Jan '12

21 Jan '12

Hi! I'm having a really hard time to write a correct parser for a small language I've developed. I have been trying to write a parser using parsec, but always get a lot of error messages like "unexpected "\n", expected ..., new-line or..." when trying to run the parser. Then I read about the happy parser and really liked the separation of lexing the text into tokens and parsing the actual logic behind those tokens. Since I couldn't get familiar with the lexer "alex" I gave up on the alex-happy-approach again and went back to parsec. But with that lexer->parser idea on my mind, my parser currently looks a lot like a lexer. So I came up with the idea of using a combination of parsec and happy, where I generate a list of tokens for my text via parsec and analyse it with happy. My questions would be: - Is this a valid approach? - What is your workflow on parsing complex data structures? - What about performance? Since my project is going to be an interpreted language parsing performance might be interesting aswell. I've read that happy is in general faster than parsec, but what if I combine both of them as I said above? I guess that parsing a simple list of tokens without any nested parser structures would be pretty fast? - Do you have any other ideas on how to improve my parser? - What are your general thoughts on happy vs. parsec? Thanks for any replies, Nils

10 17

Proposal to incorporate Haskell.org
by Don Stewart 16 Dec '11

16 Dec '11

Hello everyone. The haskell.org committee[1], in the interest of the long-term stability of the open source Haskell community infrastructure, has decided to incorporate haskell.org as a legal entity. This email outlines our recommendation, and seeks input from the community on this decision. The committee's proposal is that haskell.org incorporate as an entity under the Software Freedom Conservancy umbrella organization (the same group that Darcs joined recently): http://sfconservancy.org/ If we proceed with this move, haskell.org will be a legal entity, and registered as a non-profit, allowing us to more directly accept (US tax-deductible) donations, and to invest in assets that benefit the Haskell open source community. We welcome your feedback on the proposal attached below. -- Don Stewart (on behalf of the Haskell.org committee) ------------------------------------------------------------------------ = A proposal for the incorporation of Haskell.org = In recent years, haskell.org has started to receive assets, e.g. money from Google Summer Of Code, donations for Hackathons, and a Sparc machine for use in GHC development. We have also started spending this money: in particular, on hosting haskell.org itself. There is also interest in running fundraising drives for specific things such as Hackathon sponsorship and hosting fees. However, haskell.org doesn't currently exist as a legal entity, meaning that these assets have had to be held on our behalf by other entities, such as Galois and various universities. This leads to tricky situations, with no-one being sure who should decide how the haskell.org assets can be used. To solve these problems, we propose that haskell.org applies to become a member project of the Software Freedom Conservancy (SFC) <http://conservancy.softwarefreedom.org/>. The SFC is a non-profit organization that provides free financial and administrative services to open source projects. Additionally, it has 501(c)(3) status, meaning donations from the US are tax-deductible. The SFC would hold haskell.org's money and other assets, and would be able to accept donations on behalf of haskell.org. The haskell.org committee, as described here [2], will make decisions on spending assets and other decisions related to governing the non-profit. Before proceeding, we are inviting input from the community in the form of specific objections or queries regarding the plan. We've tried to answer some of the most likely questions: Q: Does this mean that my Haskell project must now be covered by a copyleft licence such as GPL? A: No, but Haskell projects using haskell.org resource should use an Open Source licence <http://www.opensource.org/licenses/alphabetical>. Q: Will it still be possible to use community.h.o to host non-open-source material, such as academic papers? A: An overall minority of such content, as is the current situation, is not a problem. Q: Will it still be possible to have job ads on the haskell.org mailing lists and website? A: Yes. Q: Will this affect our ability to host the Haskell Symposium <http://www.haskell.org/haskell-symposium/> and Industrial Haskell Group<http://industry.haskell.org/> webpages within haskell.org? A: No. Q: What will be the relationship between haskell.org and other organizations such as the Haskell Symposium and Industrial Haskell Group? A: Those organisations will continue to exist as separate entities. Q: If an umbrella non-profit organisation "The Haskell Foundation" was created, would haskell.org be able to join it? A: Yes. It's likely that in such a scenario, the Haskell Foundation would become the owner of the haskell.org domain name, with the cost divided between the members. The entity that is part of the SFC would be renamed "community.haskell.org" in order to avoid confusion. [1]: http://www.haskell.org/haskellwiki/Haskell.org_committee [2]: http://www.haskell.org/haskellwiki/Haskell.org_committee#Operation

10 16

Hexpat: Lazy I/O problem with huge input files
by Aleksandar Dimitrov 10 Dec '11

10 Dec '11

Hello Haskell Cafe, I really hope this is the right list for this sort of question. I've bugged the folks in #haskell, they say go here, so I'm turning to you. I want to use Hexpat to read in some humongous XML files (linguistic corpora,) since it's the only Haskell XML library (I could find) that takes ByteStrings as input. I stumbled on a problem when using one of the examples from the docs of Text.XML.Expat.Tree. The "cookbook recipe" there suggests *first* processing the data, and only then looking into the parser error to see if there has been an error. I understand this should prevent the parse tree from being fully evaluated before use. Unfortunately, that is not what happens on my system (ghc 6.12.1, if that's of importance.) This is the code from the docs, that I modified to read files: > import Text.XML.Expat.Tree > import System.Environment (getArgs) > import Control.Monad (liftM) > import qualified Data.ByteString.Lazy as C >· > -- This is the recommended way to handle errors in lazy parses > main = do > f <- liftM head getArgs >>= C.readFile > let (tree, mError) = parse defaultParseOptions f > print (tree :: UNode String) >· > -- Note: We check the error _after_ we have finished our processing > -- on the tree. > case mError of > Just err -> putStrLn $ "It failed : "++show err > Nothing -> putStrLn "Success!" Given a 42M test file, an invocation like this: % ghc --make -O2 Hexpat.hs % ./Hexpat input.xml > dump.xml will gobble up some 2Gigs of RAM (at least. I usually kill it before it starts thrashing the swap space, since that almost crashes my entire machine.) If I remove the last 3 lines: > import Text.XML.Expat.Tree > import System.Environment (getArgs) > import Control.Monad (liftM) > import qualified Data.ByteString.Lazy as C > > main = do > f <- liftM head getArgs >>= C.readFile > let (tree, mError) = parse defaultParseOptions f > print (tree :: UNode String) the same invocation and input file barely uses a megabyte or two of RAM and finishes really quickly. Why is that? Is this a mistake in the Hexpat docs, or am I doing something wrong? Lazy IO has always been a little bit of a mystery to me, and just when I thought I had it... Thanks for any help on the matter! Aleks

3 3

extending and reusing cmdargs option specs ?
by Simon Michael 13 Sep '11

13 Sep '11

Hi Neil, I just spent a day converting hledger from getopt to cmdargs. cmdargs feels more high level and featureful and nicer. And yet... I haven't reduced the line count that much - nothing like your HLint 3:1 ratio. And, I may have made things worse for myself in the reuse/avoiding boilerplate department: I'm not sure how to reuse a cmdargs options data structure, extending it with a few more options. Using getopt I was able to do this without repeating myself (as long as I defined the full set of Opt constructors in one place.) I've looked at the more advanced cmdargs api, but don't see a way yet - would you have any ideas ? I want this because I have multiple executables (hledger, hledger-vty, hledger-web etc.) which should share most (but not all) options. Also, I'd like to move a generic subset of report options, without the ui- specific ones, into hledger-lib for use by all apps. Also, the hledger executable has multiple commands, so I'd like to define a mode for each, but not have to redeclare all the same options for each mode - I didn't see how to do that. As always, thanks a lot for cmdargs! -Simon

4 6

hledger subcommand detection, RFC
by Simon Michael 11 Sep '11

11 Sep '11

When I split up the hledger package, I always intended to make the hledger program act as a single front end for hledger-* executables. I finally got around to trying that, just pushed to darcs [1]. So hledger now searches your PATH at startup and offer any hledger-* executables as subcommands. To make this fully modular, so that options can be reused and third-party add-ons don't need to be baked in to hledger in any way, there has also been a rather extensive options overhaul, using cmdargs. Below is an example [2] of the new help output with all hledger-* packages plus a local hledger-report.hs script installed. I'm not sure if this is fully safe, cross-platform, robust, quick enough to be unnoticeable, etc. I have about 20 dirs and 4k files in my PATH. I haven't noticed a slowdown on macbook or vps, but have not measured. More testing of this on diverse platforms and machines, or code review, would be very welcome. -Simon [1] http://joyful.com/darcsden/simon/hledger/browse/hledger/Hledger/Cli/Options… [2] $ hledger hledger [COMMAND] ... [OPTIONS] run the specified hledger command. hledger COMMAND --help for more detail. In general, COMMAND should precede OPTIONS. Misc commands: add prompt for new transactions and append them to the journal convert show the specified CSV file as hledger journal entries test run self-tests, or just the ones matching REGEXPS Report commands: accounts (or balance) show matched accounts and their balances entries (or print) show matched journal entries postings (or register) show matched postings and running total activity show a barchart of transactions per interval stats show quick statistics for a journal (or part of it) Add-on commands found: chart [-- OPTIONS] run the hledger-chart program interest [-- OPTIONS] run the hledger-interest program report [-- OPTIONS] run the hledger-report program vty [-- OPTIONS] run the hledger-vty program web [-- OPTIONS] run the hledger-web program -? --help Display help message --debug Show extra debug output -V --version Print version information

3 4

Regular Expression Parsing via derivatives
by Alex Clemmer 02 Sep '11

02 Sep '11

Hi Haskell people, I've been snooping through various mailing lists and the current Haskell implementation of regular expressions and I was wondering if there has been a discussion about implementing regex parsing with derivatives. If so, I haven't seen it. If not, I'd like to have a discussion about it -- if for no other reason than to decide whether I should implement it as a library, or (to attempt to implement it) as a core feature. For those of you who don't know, recent work by Might and Darais<http://arxiv.org/abs/1010.5023>indicates that parsing CFGs can be done better ( *i.e.*, significantly faster) than more "traditional" approaches<http://swtch.com/%7Ersc/regexp/regexp1.html>. Might's presenting at ICFP later in September about it. I guess the first thing I should ask is, which mailing list is actually the right place to field this inquiry. I considered dropping it in the main haskell list, but wasn't sure how people would respond. -- Alex

5 5

Pointed, but not Applicative
by Sönke Hahn 01 Sep '11

01 Sep '11

Hi! I was reading through the Typeclassopedia ([1]) and I was wondering which type could be an instance of Pointed, but not of Applicative. But I can't think of one. Any ideas? Sönke [1] http://www.haskell.org/haskellwiki/Typeclassopedia

8 16

Data structure
by yrazes 01 Sep '11

01 Sep '11

Hi guys, I want to compare data structure between Haskell, Java, Lisp and C. I am wondering if I could compare list comprehention in haskell with the vector class in Java, macros in common lisp and dynamic arrays in C. Thanks, Julita

2 2

Haskell Weekly News: Issue 197
by Daniel Santa Cruz 31 Aug '11

31 Aug '11

Welcome to issue 197 the HWN, a newsletter covering developments in the Haskell community. This release covers the week of August 21 to 27, 2011. [1] http://goo.gl/8hDku Announcements Liyang HU announced HakkaTaikai, a Haskell hackathon in Tokyo, on Sunday September 25th, 2011. [2] http://goo.gl/tJ4F4 Qiuchi Jian wondered if anyone would be interested in joining him in translating the 2010 Haskell Report into Chinese. [3] http://goo.gl/QDdeF Johan Tibell announced the results of this year's "State of Haskell" survey. [4] http://goo.gl/26Evg New and Updated Projects * network-address (New - Sebastian Nowicki) Provides data structures for IP addresses as well as subnetworks. [5] http://goo.gl/LTuUK * shelltestrunner (Update - Simon Michael) Tests command-line programs or arbitrary shell commands. [6] http://goo.gl/uKHMo * netwire (Update - Ertugrul Soeylemez) An arrowized functional reactive programming library written with networking applications in mind. [7] http://goo.gl/yM0hy * instinct (New - Ertugrul Soeylemez) Library for neural networks. [8] http://goo.gl/yM0hy Quotes of the Week * elliott: i'm here to prove theorems and compile code and I'm all out of code * shachaf: To deepSeq is a fundamental right * djahandarie: I wonder if De Bruijn was any good at remembering names. * JaffaCake: In the beginning there were 8 registers, and That Was Enough For Everyone. * frezik: "The compiler accepted it, why can't you?" * Cale: the units of the monoids in the algebras for the monoid monad * _Ray_: fucking hell, math and cs need to stop having awesome interesting things. it's ruining my life. * accel: next time I'll read a few research papers before I start trolling hakell * ddarius: isJust :: Maybe a -> Bool; isJust = unsafeCoerce Top Reddit Stories * Interested in a 2nd edition of "Real World Haskell"? Domain: self.haskell, Score: 130, Comments: 33 On Reddit: [9] http://goo.gl/fNS6P Original: [10] http://goo.gl/fNS6P * GHC on ARM: Merged into GHC HEAD Domain: ghcarm.wordpress.com, Score: 78, Comments: 7 On Reddit: [11] http://goo.gl/BvFvX Original: [12] http://goo.gl/ZKDYx * Why Haskell is Kinda Cool Domain: amtal.github.com, Score: 56, Comments: 27 On Reddit: [13] http://goo.gl/9ealD Original: [14] http://goo.gl/0AJIn * Haskell for Kids: Week 2! Domain: cdsmith.wordpress.com, Score: 55, Comments: 20 On Reddit: [15] http://goo.gl/0Qm5z Original: [16] http://goo.gl/DzdUy * Results from the State of Haskell, 2011 Survey Domain: blog.johantibell.com, Score: 54, Comments: 18 On Reddit: [17] http://goo.gl/BAQye Original: [18] http://goo.gl/l6sc1 * PyPy: We need Software Transactional Memory Domain: morepypy.blogspot.com, Score: 49, Comments: 2 On Reddit: [19] http://goo.gl/6dPjT Original: [20] http://goo.gl/kNCma * mmmodularity! - A modular package language for Haskell Domain: skilpat.tumblr.com, Score: 48, Comments: 8 On Reddit: [21] http://goo.gl/6dfSf Original: [22] http://goo.gl/os9IJ * A Tutorial Introduction to the Lambda Calculus Domain: utdallas.edu, Score: 34, Comments: 9 On Reddit: [23] http://goo.gl/OtPrs Original: [24] http://goo.gl/zTIx4 * Functional representation of Rubik's cube Domain: pnyf.inf.elte.hu, Score: 30, Comments: 3 On Reddit: [25] http://goo.gl/TMOnc Original: [26] http://goo.gl/EoYbu * monad-control, for more flexible transformers Domain: yesodweb.com, Score: 27, Comments: 14 On Reddit: [27] http://goo.gl/XF6GZ Original: [28] http://goo.gl/ijd6V * A different haskell emacs mode (work in progress) Domain: chrisdone.com, Score: 26, Comments: 5 On Reddit: [29] http://goo.gl/lfmxF Original: [30] http://goo.gl/7qvho Top StackOverflow Questions * Haskell: How does non-strict and lazy differ? votes: 25, answers: 5 Read on SO: [31] http://goo.gl/2KJVY * forall in Scala votes: 18, answers: 5 Read on SO: [32] http://goo.gl/ejOwF * What is the name of this monad-like functional programming pattern? votes: 16, answers: 1 Read on SO: [33] http://goo.gl/Rv1BC * Is (reverse . f . reverse) efficient? votes: 11, answers: 4 Read on SO: [34] http://goo.gl/U7qLu * Haskell Space Overflow votes: 8, answers: 4 Read on SO: [35] http://goo.gl/5uBY2 * How to write recursive lambda expression in Haskell? votes: 8, answers: 2 Read on SO: [36] http://goo.gl/TokK2 * Is the random number generator in Haskell thread-safe? votes: 8, answers: 4 Read on SO: [37] http://goo.gl/ERVpl * How can I create a tuple where each of the members are compared by an expression? votes: 7, answers: 5 Read on SO: [38] http://goo.gl/BiUpC * Haskell cartesian product of infinite lists votes: 7, answers: 4 Read on SO: [39] http://goo.gl/DC9M5 * How to implement a generic hierarchy of structures with injected functionality votes: 7, answers: 6 Read on SO: [40] http://goo.gl/IoMT6 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18899 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91853 4. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18894 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18893 6. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18900 7. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91845 8. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91845 9. http://www.reddit.com/r/haskell/comments/jtdrz/interested_in_a_2nd_edition_… 10. http://www.reddit.com/r/haskell/comments/jtdrz/interested_in_a_2nd_edition_… 11. http://ghcarm.wordpress.com/2011/08/27/current-status-merged-into-ghc-head/ 12. http://www.reddit.com/r/haskell/comments/jwgxj/ghc_on_arm_merged_into_ghc_h… 13. http://amtal.github.com/2011/08/25/why-haskell-is-kinda-cool.html 14. http://www.reddit.com/r/haskell/comments/juemg/why_haskell_is_kinda_cool/ 15. http://cdsmith.wordpress.com/2011/08/23/haskell-for-kids-week-2/ 16. http://www.reddit.com/r/haskell/comments/js4kd/haskell_for_kids_week_2/ 17. http://blog.johantibell.com/2011/08/results-from-state-of-haskell-2011.html 18. http://www.reddit.com/r/haskell/comments/jqfey/results_from_the_state_of_ha… 19. http://morepypy.blogspot.com/2011/08/we-need-software-transactional-memory.… 20. http://www.reddit.com/r/haskell/comments/jrprb/pypy_we_need_software_transa… 21. http://skilpat.tumblr.com/post/9411500320/a-modular-package-language-for-ha… 22. http://www.reddit.com/r/haskell/comments/jvlrw/mmmodularity_a_modular_packa… 23. http://www.utdallas.edu/~gupta/courses/apl/lambda.pdf 24. http://www.reddit.com/r/haskell/comments/jt0m1/a_tutorial_introduction_to_t… 25. http://pnyf.inf.elte.hu:8000/Rubik_en.xml 26. http://www.reddit.com/r/haskell/comments/jwfz2/functional_representation_of… 27. http://www.yesodweb.com/blog/2011/08/monad-control 28. http://www.reddit.com/r/haskell/comments/jtuk9/monadcontrol_for_more_flexib… 29. http://chrisdone.com/posts/2011-08-21-haskell-emacs.html 30. http://www.reddit.com/r/haskell/comments/js3sm/a_different_haskell_emacs_mo… 31. http://stackoverflow.com/questions/7140978/haskell-how-does-non-strict-and-… 32. http://stackoverflow.com/questions/7213676/forall-in-scala 33. http://stackoverflow.com/questions/7135540/what-is-the-name-of-this-monad-l… 34. http://stackoverflow.com/questions/7152183/is-reverse-f-reverse-efficient 35. http://stackoverflow.com/questions/7140732/haskell-space-overflow 36. http://stackoverflow.com/questions/7141229/how-to-write-recursive-lambda-ex… 37. http://stackoverflow.com/questions/7153255/is-the-random-number-generator-i… 38. http://stackoverflow.com/questions/7136921/how-can-i-create-a-tuple-where-e… 39. http://stackoverflow.com/questions/7141287/haskell-cartesian-product-of-inf… 40. http://stackoverflow.com/questions/7157841/how-to-implement-a-generic-hiera…

1 0

Re: [Haskell-cafe] Project Euler: request for comments
by KC 31 Aug '11

31 Aug '11

You also don't need mutual recursion for this explicit recursion since I imagine it would use up more stack space. -- Doing the one dimensional case. f011 :: [Int] -> Int f011 (t:u:v:xs) = f011helper t u v xs f011helper :: Int -> Int -> Int -> [Int] -> Int f011helper t u v (w:ws) | ws == [] = t*u*v*w | otherwise = max (t*u*v*w) (f011helper u v w ws) -- Note: f011helper does not call f011. On Mon, Aug 29, 2011 at 9:40 AM, Oscar Picasso <oscarpicasso(a)gmail.com> wrote: > Got it. > > f :: [Int] -> Int > f (t:u:v:xs) = helper t u v xs > > helper :: Int -> Int -> Int -> [Int] -> Int > helper t u v (w:ws) > | ws == [] = t*u*v*w > | otherwise = max (t*u*v*w) (f (u:v:w:ws)) > > I tend to overlook mutual recursion in my toolbox. Good going! :) > > Thanks for the enlightenment. > > On Sun, Aug 28, 2011 at 4:54 PM, KC <kc1956(a)gmail.com> wrote: >> Try something like the following: >> >> -- Project Euler 11 >> >> -- In the 20×20 grid below, four numbers along a diagonal line have >> been marked in red. >> >> -- <snip> >> >> -- The product of these numbers is 26 × 63 × 78 × 14 = 1788696. >> >> -- What is the greatest product of four adjacent numbers in any >> direction (up, down, left, right, or diagonally) in the 20×20 grid? >> >> >> import Data.List >> >> -- Doing the one dimensional case. >> f011 :: [Int] -> Int >> f011 (t:u:v:xs) = f011helper t u v xs >> >> f011helper :: Int -> Int -> Int -> [Int] -> Int >> f011helper t u v (w:ws) >> | ws == [] = t*u*v*w >> | otherwise = yada nada mada >> >> -- What are yada nada mada? >> >> -- The 20x20 grid case will become: >> f0112D :: [[Int]] -> Int >> -- where [[Int]] is a list of lists of rows, columns, major diagonals, >> & minor diagonals. >> -- -- Regards, KC

2 1