April 2015 - Beginners

missing parallelism
by Maurizio Vitale 25 Apr '15

25 Apr '15

G'day, I have a test code that I compile with ghc -threaded -eventlog -rtsopts --make parallel.hs and run with ./parallel 2 +RTS -ls -N4 on a laptop with 4 physical cores. I would expect activities in two threads, but threadscope shows only one active thread. Can somebody explain me the program's behaviour? Thanks a lot Maurizio {-# LANGUAGE UnicodeSyntax #-} {-# LANGUAGE TupleSections #-} import Control.Applicative import Control.Concurrent.Async (async, waitAny, Async) import Data.List (delete, sortBy) import Data.Ord (comparing) import System.CPUTime import System.Environment import GHC.Conc (numCapabilities) concurrentlyLimited :: Int -> [IO a] -> IO [a] concurrentlyLimited n tasks = concurrentlyLimited' n (zip [0..] tasks) [] [] concurrentlyLimited' ∷ Int -- ^ number of concurrent evaluations → [(Int, IO b)] -- ^ still to run (ordered by first element) → [Async (Int,b)] -- ^ currently running → [(Int,b)] -- ^ partial results (ordered by first element) → IO [b] concurrentlyLimited' _ [] [] results = return . map snd $ sortBy (comparing fst) results concurrentlyLimited' 0 todo ongoing results = do (task, newResult) <- waitAny ongoing concurrentlyLimited' 1 todo (delete task ongoing) (newResult:results) concurrentlyLimited' _ [] ongoing results = concurrentlyLimited' 0 [] ongoing results concurrentlyLimited' n ((i, task):otherTasks) ongoing results = do t <- async $ (i,) <$> task concurrentlyLimited' (n-1) otherTasks (t:ongoing) results euler :: Int → Int euler n = length (filter (relprime n) [1..n-1]) where relprime x y = gcd x y == 1 sumEuler :: Int → Int sumEuler = sum . (map euler) . mkList where mkList n = [1..n-1] p ∷ IO Int p = return $ sumEuler 3000 numThreads ∷ [String] → IO Int numThreads [] = return numCapabilities numThreads [cores] = return $ read cores main ∷ IO() main = do threads ← getArgs >>= numThreads putStrLn $ "Running up to " ++ show threads ++ " threads in parallel (on " ++ show numCapabilities ++ " cores)" startTime ← getCPUTime f ← concurrentlyLimited threads $ replicate 10 p endTime ← getCPUTime putStrLn $ foldr ((++) . show ) "" f putStrLn $ "Evaluation took " ++ show (fromIntegral (endTime - startTime) / 1000000000000∷Double)

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Re: [Haskell-beginners] missing parallelism
by Grzegorz Milka 25 Apr '15

25 Apr '15

As far as I can tell there's nothing wrong with your code. My hypothesis is that Haskell optimizes call to sumEuler 5000 by calling it only once in one thread. Here's why I think so: The program I used for debugging this is: import Control.Concurrent.Async (async, wait) import System.IO.Unsafe (unsafePerformIO) sumEuler :: Int -> Int sumEuler = sum . map euler . mkList where mkList n = seq (unsafePerformIO (putStrLn "Calculating")) [1..n-1] euler n = length (filter (relprime n) [1..n-1]) where relprime x y = gcd x y == 1 p :: Int -> IO Int p b = return $! sumEuler b p5000 :: IO Int p5000 = return $ sumEuler 5000 main :: IO () main = do a <- async $ p5000 b <- async $ p5000 av <- wait a bv <- wait b print (av,bv) The two main modification are adding a debugging message "Calculating" when a list [1..(n-1)] is evaluated. The second one is making p into a function. Notice that it uses strict application ($! - check how it works with simple $). I will use that function in further examples. Running this program with "time ./Test 2 +RTS -ls -N2" gives me: Calculating (7598457,7598457) real 0m3.752s user 0m3.833s sys 0m0.211s Just to be sure I have almost the same time when doing only one computation with: main :: IO () main = do a <- async $ p5000 av <- wait a print av So it seems like the value returned by p5000 is computed only once. GHC might be noticing that p5000 will always return the same value and might try cache it or memoize it. If this hypothesis is right then calling sumEuler with two different values should run in two different threads. And indeed it is so: main :: IO () main = do a <- async $ p 5000 b <- async $ p 4999 av <- wait a bv <- wait b print (av,bv) Gives: Calculating Calculating (7598457,7593459) real 0m3.758s user 0m7.414s sys 0m0.064s So it runs in two threads just as expected. The strict application ($!) here is important. Otherwise it seems that the async thread returns a thunk and the evaluation happens in print (av, bv) which is evaluated in a single thread. Also the fact that p5000 is a top level binding is important. When I do: main :: IO () main = do a <- async $ p 5000 b <- async $ p 5000 av <- wait a bv <- wait b print (av,bv) I get no optimization (GHC 7.6.3). Best, Greg

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Strange "Not in scope" error message
by Matthew Moppett 24 Apr '15

24 Apr '15

I have a file with the following relevant lines: import qualified Control.Concurrent as C ... isCancelled = C.isEmptyMvar . stopConcurrentProcess Trying to compile this or run it in ghci yields a puzzling error message: Not in scope: `C.isEmptyMvar' Perhaps you meant one of these: `C.isEmptyMVar' (imported from Control.Concurrent), `C.isEmptyChan' (imported from Control.Concurrent) Any idea what could be going wrong?

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Re: [Haskell-beginners] Error when running `cabal install cabal-install`
by Peng Yu 23 Apr '15

23 Apr '15

On Thu, Apr 23, 2015 at 7:39 PM, Ryan Schmidt <ryandesign(a)macports.org> wrote: > >> On Apr 23, 2015, at 7:23 PM, Peng Yu <pengyu.ut(a)gmail.com> wrote: >> >>> Long story short: Try >>> $> cabal install --extra-include-dirs=/opt/local/include --extra-lib-dirs=/opt/local/lib >> >> I tried it but it still shows the error messages. >> >> ~$ cabal install --extra-include-dirs=/opt/local/include >> --extra-lib-dirs=/opt/local/lib cabal-install >> >> Resolving dependencies... >> Configuring cabal-install-1.22.3.0... >> Failed to install cabal-install-1.22.3.0 >> Build log ( /Users/py/.cabal/logs/cabal-install-1.22.3.0.log ): >> cabal: Error: some packages failed to install: >> cabal-install-1.22.3.0 failed during the configure step. The exception was: >> user error ('/usr/bin/ghc' exited with an error: > > You've installed ghc into /usr/bin? That's probably not a good idea. Prefix /usr is only for the OS vendor to install software into. This is what installed from the haskell installer. I agree this might not be a good idea. But I have no control over it. -- Regards, Peng

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Re: [Haskell-beginners] Error when running `cabal install cabal-install`
by Peng Yu 23 Apr '15

23 Apr '15

> Long story short: Try > $> cabal install --extra-include-dirs=/opt/local/include --extra-lib-dirs=/opt/local/lib I tried it but it still shows the error messages. ~$ cabal install --extra-include-dirs=/opt/local/include --extra-lib-dirs=/opt/local/lib cabal-install Resolving dependencies... Configuring cabal-install-1.22.3.0... Failed to install cabal-install-1.22.3.0 Build log ( /Users/py/.cabal/logs/cabal-install-1.22.3.0.log ): cabal: Error: some packages failed to install: cabal-install-1.22.3.0 failed during the configure step. The exception was: user error ('/usr/bin/ghc' exited with an error: Undefined symbols for architecture x86_64: "_iconv", referenced from: _hs_iconv in libHSbase-4.7.0.1.a(iconv.o) (maybe you meant: _hs_iconv, _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc_info , _base_GHCziIOziEncodingziIconv_iconvEncoding8_info , _base_GHCziIOziEncodingziIconv_iconvEncoding9_closure , _hs_iconv_open , _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc1_closure , _hs_iconv_close , _base_GHCziIOziEncodingziIconv_iconvEncoding6_info , _base_GHCziIOziEncodingziIconv_iconvEncoding4_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding6_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding10_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding7_info , _base_GHCziIOziEncodingziIconv_iconvEncoding2_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding2_info , _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding3_info , _base_GHCziIOziEncodingziIconv_iconvEncoding10_info , _base_GHCziIOziEncodingziIconv_iconvEncoding9_info , _base_GHCziIOziEncodingziIconv_iconvEncoding8_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding7_closure , _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc1_info , _base_GHCziIOziEncodingziIconv_iconvEncoding3_closure ) "_iconv_close", referenced from: _hs_iconv_close in libHSbase-4.7.0.1.a(iconv.o) (maybe you meant: _hs_iconv_close) "_iconv_open", referenced from: _hs_iconv_open in libHSbase-4.7.0.1.a(iconv.o) (maybe you meant: _hs_iconv_open) "_locale_charset", referenced from: _localeEncoding in libHSbase-4.7.0.1.a(PrelIOUtils.o) ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) ) -- Regards, Peng

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Re: [Haskell-beginners] Error when running `cabal install cabal-install`
by Brandon Allbery 23 Apr '15

23 Apr '15

On Thu, Apr 23, 2015 at 6:00 PM, Clemens Lang <cal(a)macports.org> wrote: > From https://www.haskell.org/platform/ > > it sounds like the latest platform release > is 2014.2.0.0, and that's what's currently packaged in MacPorts. It took > me a > while to get it done, but it's there and all components of the platform > should be > included. I'd welcome a bug report if there's anything wrong with it. > Mrrr, I thought there was a 6.8.4-based release (with the bizarre new build system). We're packaging 6.8.3 still. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-beginners] Error when running `cabal install cabal-install`
by Brandon Allbery 23 Apr '15

23 Apr '15

On Thu, Apr 23, 2015 at 5:56 PM, Clemens Lang <cal(a)macports.org> wrote: > So if you get these link errors, the problem almost always is that you > adjusted your library search path, but not the corresponding header > search path and you're actually compiling against Apple's iconv.h but > MacPorts libiconv.dylib (or vice-versa). > Those are coming from a library already built against Apple's libiconv, but being linked against MacPorts' libiconv. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-beginners] Error when running `cabal install cabal-install`
by Peng Yu 23 Apr '15

23 Apr '15

Indeed, I uninstalled libiconv by running the following command. sudo port uninstall --follow-dependents libiconv Then, `cabal install cabal-install` runs without a problem. Hi MacPorts Users, Is there better solution to this problem? Please see the original question regarding the conflict of MacPorts and Haskell. (I don't what to use haskell from MacPorts as I want to keep updated with the latest version of Haskell.) https://mail.haskell.org/pipermail/beginners/2015-April/015214.html On Thu, Apr 23, 2015 at 3:34 PM, Brandon Allbery <allbery.b(a)gmail.com> wrote: > On Thu, Apr 23, 2015 at 4:19 PM, Peng Yu <pengyu.ut(a)gmail.com> wrote: >> >> It is installed from <https://www.haskell.org/platform/ >> >> >. The OS is Mac OS X. > > > Yes, as I suspected. The problem is that Apple ships an old version of the > iconv library; MacPorts and Homebrew generally install a newer, incompatible > one with symbols renamed to trigger that link error (because otherwise you > find out about the mismatch when the first iconv call dumps core at > runtime). You will need to arrange for ghc not to see that other iconv, only > Apple's, since it won't work with any but Apple's. Or use a ghc from > whichever package manager you have installed, so that it works with that > package manager's iconv library. > > (This is more likely to happen with Homebrew, since most things don't look > /opt/local but just about everything looks under /usr/local.) > > -- > brandon s allbery kf8nh sine nomine associates > allbery.b(a)gmail.com ballbery(a)sinenomine.net > unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -- Regards, Peng

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Error when running `cabal install cabal-install`
by Peng Yu 23 Apr '15

23 Apr '15

~$ cabal --version cabal-install version 1.18.0.5 using version 1.18.1.4 of the Cabal library ~$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.8.3 I got the following error. Does anybody know how to resolve it? Thanks. ~$ cabal install cabal-install Resolving dependencies... Configuring Cabal-1.22.3.0... Failed to install Cabal-1.22.3.0 Last 10 lines of the build log ( /Users/py/.cabal/logs/Cabal-1.22.3.0.log ): cabal: Error: some packages failed to install: Cabal-1.22.3.0 failed during the configure step. The exception was: user error (Undefined symbols for architecture x86_64: "_iconv", referenced from: _hs_iconv in libHSbase-4.7.0.1.a(iconv.o) (maybe you meant: _hs_iconv, _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc1_info , _base_GHCziIOziEncodingziIconv_iconvEncoding9_info , _base_GHCziIOziEncodingziIconv_iconvEncoding9_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding8_info , _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding10_info , _base_GHCziIOziEncodingziIconv_iconvEncoding6_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding10_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding3_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding7_info , _hs_iconv_close , _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc1_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding2_info , _base_GHCziIOziEncodingziIconv_iconvEncoding8_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding3_info , _base_GHCziIOziEncodingziIconv_iconvEncoding2_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding7_closure , _base_GHCziIOziEncodingziIconv_iconvEncodingzuloc_info , _base_GHCziIOziEncodingziIconv_iconvEncoding4_closure , _base_GHCziIOziEncodingziIconv_iconvEncoding6_info , _hs_iconv_open ) "_iconv_close", referenced from: _hs_iconv_close in libHSbase-4.7.0.1.a(iconv.o) (maybe you meant: _hs_iconv_close) "_iconv_open", referenced from: _hs_iconv_open in libHSbase-4.7.0.1.a(iconv.o) (maybe you meant: _hs_iconv_open) "_locale_charset", referenced from: _localeEncoding in libHSbase-4.7.0.1.a(PrelIOUtils.o) ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) ) cabal-install-1.22.3.0 depends on Cabal-1.22.3.0 which failed to install. ~/ports$ gvim ~/.cabal/logs/ -- Regards, Peng

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Re: [Haskell-beginners] Value from Monad
by Shishir Srivastava 22 Apr '15

22 Apr '15

Thanks ! that does work now. My next question was how do i only get "34" or "3" i.e. the Maybe value without wrapped in to a 'Maybe' ? Shishir

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