- Beginners - Haskell.org

Compiling shared (dll) library
by Alexander.Vladislav.Popov 05 Dec '11

05 Dec '11

Hi, Haskellers. I'm trying to compile following program (where Regex.Genex is a package what I need to produce all possible expresions by the given pattern and `adder' is just FFI sample): -- genexlib.hs {-# LANGUAGE BangPatterns, ForeignFunctionInterface #-} module GenexLib where import Regex.Genex import System.IO import System.Environment adder :: Int -> Int -> IO Int -- gratuitous use of IO adder x y = return (x+y) foreign export stdcall adder :: Int -> Int -> IO Int -- genexlib.hs end // start.c #include <Rts.h> void HsStart() { int argc = 1; char* argv[] = {"ghcDll", NULL}; // argv must end with NULL // Initialize Haskell runtime char** args = argv; hs_init(&argc, &args); } void HsEnd() { hs_exit(); } // start.c end I'm using ghc >ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 compiling: >ghc -c genexlib.hs >ghc -c start.c >ghc -shared -o genexlib.dll genexlib.o genexlib_stub.o start.o genexlib.o:fake:(.text+0xd1): undefined reference to `__stginit_regexzmgenexzm0zi3zi2_RegexziGenex_' Creating library file: genexlib.dll.a collect2: ld returned 1 exit status and get undefined reference. But If I try to compile the executable from similar code: -- genexlib.hs {-# LANGUAGE BangPatterns, ForeignFunctionInterface #-} -- module GenexLib where import Regex.Genex import System.IO import System.Environment defaultRegex :: String defaultRegex = "a(b|c)d{2,3}e*" main :: IO () main = do hSetBuffering stdout NoBuffering args <- getArgs case args of [] -> do prog <- getProgName if prog == "<interactive>" then run [defaultRegex] else do fail $ "Usage: " ++ prog ++ " regex [regex...]" rx -> run rx run :: [String] -> IO () run regex = do let s = genexPure regex mapM_ print s -- genexlib.hs end >ghc --make genexlib.hs -O2 it's ok, no errors, and you can see in GHCi: *Main> :main "abdd" "acdd" "abddd" "acddd" "abddeee" "acddeee" "abdddeee" "acdddeee" "abddee" "acddee" "abdddee" "acdddee" "abdde" "acdde" "abddde" "acddde" Where is my mistake? What am I doing wrong? In first case, when compiling shared dll, I tried to link libraries what I've found in `cabal' directory (like `libHSregex-genex-0.3.2.a') to work around errors but all in vain. -- Alexander Popov

2 3

Diagnosing : Large memory usage + low CPU
by Hugo Ferreira 05 Dec '11

05 Dec '11

Hello, I am testing a simple algorithm and find that during part of the execution this is fast and uses acceptable memory. However, when it gets to a certain point the memory climbs to 75-80 % of the OS's memory and CPU plummets to a mere 5% at most. In the part of the application that executes ok I have calls as follows: let r9' = evalTagger'' (tagFun suffixCapsFreq) $ test let a9' = tagginAccuracy r9' putStrLn ("Suffix(3) + Caps + Freq tag result done = " ++ (show a9')) The slow one has: let r12' = evalTagger'' ruleSuffixCapsFreq $ test let a12' = tagginAccuracy r12' putStrLn ("Rules + Suffix(3) + Caps + Freq tag result done = " ++ (show a12')) The difference lies in the following functions: ruleApplication :: TransformationRule -> POSTags -> Maybe Tag ruleApplication (NextTagRule (Replacement old new) next) z = do (_, _, proposed) <- Z.safeCursor z (_, _, nextProposed) <- rightCursor z if proposed == old && nextProposed == next then Just new else Nothing .... updateState :: (TransformationRule,Int) -> POSTags -> POSTags updateState r = Z.fromList . reverse . Z.foldlz' (update r) [] where update (r,_) !xs z = case ruleApplication r z of Just tag -> (token, correct, tag):xs Nothing -> e:xs where e@(token, correct, _proposed) = Z.cursor z rulesT :: [(TransformationRule, Int)] -> POSTags -> POSTags rulesT rs state = L.foldl' tag state rs where tag !s rule = updateState rule s (.>) :: (POSTags -> Tag) -> (POSTags -> Tag) -> POSTags -> Tag f .> g = \ x -> case (g x) of "" -> f x t -> t (|>) :: (POSTags -> POSTags) -> (POSTags -> Tag) -> POSTags -> POSTags f |> g = \ x -> f $ tagFun g x let ruleT = rulesT top10Rules let suffixCapsFreq = suffixT .> capitalizeT .> freqT let ruleSuffixCapsFreq = ruleT |> suffixCapsFreq where top10Rules is a list. Note that suffixCapsFreq and tagFun are common to both calls. I have placed bangs in many places and used eager evaluation in the folds. However nothing seems to help. Can anyone tell me how I should diagnose this problem. Any suggestions are welcome. TIA, Hugo F.

3 6

ANNOUNCE: Haskell Communities and Activities Report (21st ed., November 2011)
by Janis Voigtländer 04 Dec '11

04 Dec '11

On behalf of all the contributors, I am pleased to announce that the Haskell Communities and Activities Report (21st edition, November 2011) is now available in PDF and HTML formats: http://haskell.org/communities/11-2011/report.pdf http://haskell.org/communities/11-2011/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ========================================= End of April 2012: target deadline for contributions to the May 2012 edition of the HC&A Report ========================================= Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

1 0

Question about Interpreter
by Song Zhang 03 Dec '11

03 Dec '11

Hi I am a beginner of Haskell. I want to know why the interpreter was designed in a way that one function or a part fails the whole program fails. why we don't keep the parts that is correct functioning and also give error message for the wrong parts?

3 2

emacs setup for haskell
by Rustom Mody 02 Dec '11

02 Dec '11

I am using haskell inside emacs with the usual setup ie .hs file in one window, ghci in another. Now the most usual workflow one would do is to load the file into ghci (C-c C-l) and try out something in ghci. A small niggle is that after the C-c C-l Ive to manually switch to the other window (current window does not switch to ghci automatically) This seems to be a strange default More importantly the state of the workspace in ghci seems to get messed up unless I do: 0. Prepare some code in .hs 2. Switch to ghci 3. Clear the ghci workspace with :m <nothing> 4. switch back to .hs 5. C-c C-l 6. Switch to ghci to try out new code Is there something I am missing?

2 2

Parallelism?
by Michael Craig 02 Dec '11

02 Dec '11

I was writing some parallel code (asynchronous database writes for an event logger, but that's besides the point), and it seemed like the parallelized version (i.e. compiled with -threaded -with-rtsopts=-N2) wasn't running fast enough. I boiled it down to a dead-simple test: import Control.Concurrent import Data.Time.Clock.POSIX import System.Environment main :: IO () main = do n <- getArgs >>= return . read . head t1 <- getPOSIXTime work n t2 <- getPOSIXTime putStrLn $ show $ t2 - t1 putStrLn $ show $ (fromIntegral n :: Double) / (fromRational . toRational $ t2 - t1) work :: Integer -> IO () work n = do forkIO $ putStrLn $ seq (fact n) "Done" putStrLn $ seq (fact n) "Done" fact :: Integer -> Integer fact 1 = 1 fact n = n * fact (n - 1) (I know this is not the best way to time things but I think it suffices for this test.) Compiled with ghc --make -O3 test.hs, ./test 500000 runs for 74 seconds. Compiling with ghc --make -O3 -threaded -with-rtsopts=-N, ./test 500000 runs for 82 seconds (and seems to be using 2 cpu cores instead of just 1, on a 4-core machine). What gives? Mike S Craig

4 8

discover data constructor without pattern matching?
by Philippe Sismondi 02 Dec '11

02 Dec '11

Is it possible to discover the data constructor used to make a value without doing pattern matching? In my current small project I have a data type with a zillion constructors. These use record syntax so that I can access the payload without pattern matching. But I just discovered that it I want to print out the constructor name in my error handling function. Can I do this without, say, hacking around with a string from 'show'? - P -

3 3

Cabal on mac
by Tim Pizey 28 Nov '11

28 Nov '11

Hi, I have installed cabal and HaxMl cabal install HaXml but my .hs file cannot find import Text.XML.HaXml.Xml2Haskell (readXml) giving *** Chasing dependencies: Chasing modules from: *ch.hs ch.hs:3:8: Could not find module `Text.XML.HaXml.Xml2Haskell': locations searched: Text/XML/HaXml/Xml2Haskell.hs Text/XML/HaXml/Xml2Haskell.lhs It would seem only to be looking in the current directory. Hoping I have just missed something obvious. Tim -- Tim Pizey http://pizey.net/~timp

2 2

Multi-parameter type classes
by David Beacham 28 Nov '11

28 Nov '11

Hi all, I've been reading through Okasaki's purely functional data structures and noticed that the source he provides for the Set implementations uses multi-parameter type classes, whereas those for the Heap structures does not. class Set s a where empty :: s a insert :: a -> s a -> s a ... class Heap h where empty :: Ord a => h a insert :: Ord a => a -> h a -> h a ... The only difference I can see is that the instance declarations for Set contain the Ord constraint, whereas they are defined on the class functions for Heap. instance (Ord a) => Set RedBlackSet a where ... I realise that the type of `a` needs to be constrained to Ord instances at some point in the code, but is there any particular reason for doing it differently in each case? If not, should one method be preferred over the other? Thanks for the help, David.

2 1

help with a class definition
by Rustom Mody 28 Nov '11

28 Nov '11

The Data.Vector.Unboxed page on hackage has the following line class (Vector Vector a, MVector MVector a) => Unbox a Can someone explain whats the repeating Vector (and MVector) there?

2 1