May 2011 - Haskell-Cafe

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

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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

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AVL Tree in a pattern matching way
by larry.liuxinyu 16 Aug '11

16 Aug '11

Hi, I browsed the current AVL tree implementation in Hackage http://hackage.haskell.org/packages/archive/AvlTree/4.2/doc/html/src/Data-T… AVL tree denote the different of height from right sub-tree to left sub-tree as delta, to keep the balance, abs(delta)<=1 is kept as invariant. So the typical implementation define N (Negative), P (Positive), and Z (zero) as the tree valid nodes (and the Empty as the trivial case). When a new element is inserted, the program typically first check if the result will break the balance, and process rotation to keep the balance of the tree. Some other pure functional implementation takes the same approach, for example: Guy Cousineau and Michel Mauny. ``The Functional Approach to Programming''. pp 173 ~ 186 Consider the elegant implementation of Red-black tree in pattern matching way by Chris Okasaki, I tried to use the same method in AVL tree, and here is the result. module AVLTree where -- for easy verification, I used Quick Check package. import Test.QuickCheck import qualified Data.List as L -- for verification purpose only -- Definition of AVL tree, it is almost as same as BST, besides a new field to store delta. data AVLTree a = Empty | Br (AVLTree a) a (AVLTree a) Int insert::(Ord a)=>AVLTree a -> a -> AVLTree a insert t x = fst $ ins t where -- result of ins is a pair (t, d), t: tree, d: increment of height ins Empty = (Br Empty x Empty 0, 1) ins (Br l k r d) | x < k = node (ins l) k (r, 0) d | x == k = (Br l k r d, 0) -- For duplicate element, we just ignore it. | otherwise = node (l, 0) k (ins r) d -- params: (left, increment on left) key (right, increment on right) node::(AVLTree a, Int) -> a -> (AVLTree a, Int) -> Int -> (AVLTree a, Int) node (l, dl) k (r, dr) d = balance (Br l k r d', delta) where d' = d + dr - dl delta = deltaH d d' dl dr -- delta(Height) = max(|R'|, |L'|) - max (|R|, |L|) -- where we denote height(R) as |R| deltaH :: Int -> Int -> Int -> Int -> Int deltaH d d' dl dr | d >=0 && d' >=0 = dr | d <=0 && d' >=0 = d+dr | d >=0 && d' <=0 = dl - d | otherwise = dl -- Here is the core pattern matching part, there are 4 cases need rebalance balance :: (AVLTree a, Int) -> (AVLTree a, Int) balance (Br (Br (Br a x b dx) y c (-1)) z d (-2), _) = (Br (Br a x b dx) y (Br c z d 0) 0, 0) balance (Br a x (Br b y (Br c z d dz) 1) 2, _) = (Br (Br a x b 0) y (Br c z d dz) 0, 0) balance (Br (Br a x (Br b y c dy) 1) z d (-2), _) = (Br (Br a x b dx') y (Br c z d dz') 0, 0) where dx' = if dy == 1 then -1 else 0 dz' = if dy == -1 then 1 else 0 balance (Br a x (Br (Br b y c dy) z d (-1)) 2, _) = (Br (Br a x b dx') y (Br c z d dz') 0, 0) where dx' = if dy == 1 then -1 else 0 dz' = if dy == -1 then 1 else 0 balance (t, d) = (t, d) -- Here are some auxiliary functions for verification -- check if a AVLTree is valid isAVL :: (AVLTree a) -> Bool isAVL Empty = True isAVL (Br l _ r d) = and [isAVL l, isAVL r, d == (height r - height l), abs d <= 1] height :: (AVLTree a) -> Int height Empty = 0 height (Br l _ r _) = 1 + max (height l) (height r) checkDelta :: (AVLTree a) -> Bool checkDelta Empty = True checkDelta (Br l _ r d) = and [checkDelta l, checkDelta r, d == (height r - height l)] -- Auxiliary functions to build tree from a list, as same as BST fromList::(Ord a)=>[a] -> AVLTree a fromList = foldl insert Empty toList :: (AVLTree a) -> [a] toList Empty = [] toList (Br l k r _) = toList l ++ [k] ++ toList r -- test prop_bst :: (Ord a, Num a) => [a] -> Bool prop_bst xs = (L.sort $ L.nub xs) == (toList $ fromList xs) prop_avl :: (Ord a, Num a) => [a] -> Bool prop_avl = isAVL . fromList . L.nub And here are my result in ghci: *AVLTree> test prop_avl OK, passed 100 tests. The program is available in github: http://www.google.com/url?sa=D&q=https://github.com/liuxinyu95/AlgoXY/blob/… I haven't provided delete function yet. Cheers. -- Larry, LIU https://github.com/liuxinyu95/AlgoXY

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Re: [Haskell-cafe] FFI for a beginner
by Jason Dagit 14 Jul '11

14 Jul '11

I don't know how to make ghc load them without using either hsc2hs or c2hs. I've had better experiences with hsc, but your mileage may vary. On Apr 8, 2011 3:34 AM, "Andrew Pennebaker" <andrew.pennebaker(a)gmail.com> wrote: ncurses is proving too difficult to setup, so I'm working on a new library called charm. The C code works by itself, but I can't compile a Haskell wrapper for it. While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples>are helpful, they're outdated. Argh! The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…>say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error. GitHub: charm <https://github.com/mcandre/charm> and hscharm<https://github.com/mcandre/hscharm> <https://github.com/mcandre/charm>$ make cp /usr/include/charm.c . ghc --make -fforce-recomp -o hellocharm hellocharm.hs charm.hs charm.c -I/usr/include -dylib-install-name /usr/lib/libcharm.dynlib [1 of 2] Compiling Charm ( charm.hs, charm.o ) [2 of 2] Compiling Main ( hellocharm.hs, hellocharm.o ) Linking hellocharm ... ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o collect2: ld returned 1 exit status make: *** [hellocharm] Error 1 Cheers, Andrew Pennebaker www.yellosoft.us _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Weird multi-threading runtime behaviour of single-threaded program with GHC-7.0.3
by Herbert Valerio Riedel 24 Jun '11

24 Jun '11

Hello, I'm experiencing an unexpected behaviour with GHC-7.0.3/x86_64's multithreading runtime when running some more demanding single-thread computations. I've isolated the part of the larger application which exposes the problem down to the parsing of a ~8MiB JSON file which causes quite a bit of memory allocations (source at end of email). The program is compiled with ghc --make -O2 -threaded -eventlog -rtsopts parse_aeson.hs and I ran it with and w/o the -N12 option on a 12-core machine (the problem is visible on a 4-core machine as well): $ time ./parse_aeson test.json 0.000392s after readFile 0.001016s parse was a success... 6.050836s NFed parse-tree... 6.050968s NFed parse-tree... no exception real 0m6.100s user 0m5.680s sys 0m0.419s $ time ./parse_aeson test.json +RTS -N12 0.001249s after readFile 0.001777s parse was a success... 10.048607s NFed parse-tree... 10.048738s NFed parse-tree... no exception real 0m10.125s user 0m57.496s sys 0m39.927s When observing top(1)'s output, I saw that all HECs consumed a substantial amount of cpu cycle, so I loaded up threadscope to see what the remaining 11 HECs were doing all the time. I've put the resulting eventlog files and screenshots of threadscope's visualization online at http://www.narf.at/~hvr/Haskell/parse_aeson.N0.eventlog.png http://www.narf.at/~hvr/Haskell/parse_aeson.N0.eventlog.xz http://www.narf.at/~hvr/Haskell/parse_aeson.N12.eventlog.png http://www.narf.at/~hvr/Haskell/parse_aeson.N12.eventlog.xz What's happening there? The actual processing work seems to be done in a single HEC... but what are the remaining 11 HECs doing exactly? Am I doing something wrong? What does it mean when the GC-bars in threadscope becomes green as in the middle of the screenshot below? http://www.narf.at/~hvr/Haskell/parse_aeson.N4.eventlog.pdf ------------------------------------------------------------------------- -- also available at http://www.narf.at/~hvr/Haskell/parse_aeson.hs module Main where import Control.DeepSeq import Control.Exception import Data.Aeson import Data.Time import System.Environment import qualified Data.Attoparsec.Lazy as APL import qualified Data.ByteString.Lazy.Char8 as LB parseIt :: FilePath -> IO () parseIt fn = do t0 <- getCurrentTime let putStrLn' !s = do t0' <- getCurrentTime putStrLn $ show (t0' `diffUTCTime` t0) ++ " " ++ s json_text <- LB.readFile fn putStrLn' "after readFile" let Success p = decode json_text :: Result Value putStrLn' "parse was a success..." putStrLn' $ p `deepseq` "NFed parse-tree..." putStrLn' $ p `deepseq` "NFed parse-tree..." decode :: FromJSON a => LB.ByteString -> Result a decode = either Error fromJSON . APL.eitherResult . APL.parse json main :: IO () main = do fns <- getArgs case fns of [fn] -> do res <- try $ parseIt fn case res of Left e -> putStrLn $ "caught: " ++ show (e :: SomeException) Right () -> putStrLn "no exception" _ -> putStrLn "usage: parse_aeson <json-file>"

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Using cmake with haskell
by Robert Clausecker 22 Jun '11

22 Jun '11

Hello readers of haskell-cafe! I asked this question on Stackoverflow: http://stackoverflow.com/q/5969616/417501 Because I want to get a good answer, I cross post it here. It may be interesting: Is it possible to use cmake for Haskell projects? I am planning a project written in Haskell, maybe there are some parts in C as well. For the buildsystem I decided against the common choice for Haskell programs cabal, mainly because I want to learn how building programs in other languages works. I heard about CMake and I think it is a pretty cool product. Although I have no knowledge in how to use it, I want use CMake for that project, just to find out how it works. Googleing did not reveal any facts about how to use cmake with haskell, and all tutorials I read were rather confusing. Is it possible, and if yes, how is it possible, to compile a projectwritten in Haskell using CMake? Robert Clausecker

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[ANN] mysql-simple - your go-to package for talking to MySQL
by Bryan O'Sullivan 21 Jun '11

21 Jun '11

Hi, folks - Over the past few days, I've released two MySQL-related packages on Hackage that I think should be pretty useful. The first is mysql-simple: http://hackage.haskell.org/package/mysql-simple This is a mid-level binding to the MySQL client API. I aimed it squarely at being both fast and easy to use, and I'm very pleased with the results so far. - Performance: compared to HDBC-mysql, mysql-simple yields a 60% performance improvement on my real-world application. - Ease of use: compared again to HDBC, my real-world application's DB-specific code shrunk by 50% while achieving the above performance increase. - Type safety: it's intentionally hard to construct SQL queries by string concatenation, but of course I provide some nice safe APIs for formatting queries and converting results. That safety does not come at the expense of performance or expressive bloat, as the above results indicate. Because I know that some people favour interacting with their databases via a model such as iteratees, the mysql-simple library is built on top of a very lightweight library. That lower-level library is named mysql: http://hackage.haskell.org/package/mysql This is a low-level binding to the MySQL client API. It is aimed at high performance and simplicity, but more specifically for consumption by authors of higher-level database libraries. It is bare enough of features that it doesn't even perform conversion between Haskell and SQL types, but at the same time it uses bytestrings sensibly, cheaply avoids some nasty signal interruption problems with the MySQL client library, and abstracts some of the tiresome details of memory management. It gives a higher-level library complete control over result conversion, how to fetch results, and all that, so you can focus purely on building iteratees and not the lower-level gunk. Enjoy! B.

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Parsing Haskell in Parsec
by Alex Rozenshteyn 16 Jun '11

16 Jun '11

I'm trying to write a parser for a small functional language in Parsec, as part of a larger project of porting this<http://www.cs.jhu.edu/%7Escott/pl/book/dist/>from ocaml to Haskell. I was wondering if there was a parser for Haskell written in Parsec that I could use as a reference. -- Alex R

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Building Haskell Platform natively for 64bit Windows
by John Sneer 10 Jun '11

10 Jun '11

Hello all, I know it is not probably good question to this list, but anyway, could anyone point me to some more detailed "how to" where is described building of Haskell Platform natively to 64bit Windows? I have no problem using 32bit version until I need more than 2GB of RAM. I went through some older threads, but description like: 1) install gcc 2) install ghc 3) build platform is something that didn't work well. Is there some more detailed description or recommended versions etc? If there is none then there is no need to respond. Thanks, regards J. -- John Sneer johnsneer(a)operamail.com -- http://www.fastmail.fm - Does exactly what it says on the tin

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Fwd: [GeekUp] Functional Programming Night at GeekUp Liverpool May
by Hakim Cassimally 05 Jun '11

05 Jun '11

If anyone's in Northwest UK on Tuesday 31st May, why not come to Geekup's first ever "Functional Programming Night"? http://lanyrd.com/2011/geekup-liverpool-may/ It'll mostly consist of 3 short talks on Haskell, Lisp, and Clojure: * Haskell in the Real World (Hakim @osfameron) * Implenting a Lisp interpreter in .Net (Simon Johnson) * Clojure: why you should be interested in a 50 year old language (Tom Mortimer-Jones @morty_uk) There will also be beer! The talks are likely to be more "general interest" than assuming deep knowledge of FP, but it would be great to meet anyone around UK NW to gauge interest in more in-depth sessions sometime in the future! Hakim ---------- Forwarded message ---------- From: John McKerrell <mcknut(a)gmail.com> Date: 6 May 2011 15:10 Subject: [GeekUp] Functional Programming Night at GeekUp Liverpool May To: geekup(a)googlegroups.com Just wanted to announce May's GeekUp properly. We'll be having a Functional Programming Extravaganza with three (count em', 3!) talks from Hakim Cassimally, Simon Johnson and Tom Mortimer-Jones. Meeting in 3345 on Parr Street as ever, talks starting at 7pm but turn up from 6:30 and we'll be chatting over beer in the bar. Turn-out has been great for the recent events but we're always happy to welcome more people, there's more information on lanyrd: http://lanyrd.com/2011/geekup-liverpool-may/ Let us know you're coming by signing up on that link but feel free to turn up anyway, attendance is free and open to all comers of course. John -- http://geekup.org/ | http://geekup.org/wiki/ | http://jobboard.geekup.org/ To post e-mail: geekup(a)googlegroups.com Or go online: http://groups.google.com/group/geekup/ To unsubscribe e-mail: geekup+unsubscribe(a)googlegroups.com

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