September 2013 - Haskell-Cafe

Product Profunctor and Contravariant
by Tom Ellis 20 Nov '13

20 Nov '13

Does anyone recognise these typeclasses: import Data.Profunctor (Profunctor) import Data.Functor.Contravariant (Contravariant) class Profunctor p => ProductProfunctor p where empty :: p () () (***!) :: p a b -> p a' b' -> p (a, a') (b, b') class Contravariant f => ProductContravariant f where point :: f () (***<) :: f a -> f b -> f (a, b) They are both a bit like Applicative, and ProductProfunctor is basically Arrow without the Category part. I'm finding ProductProfunctor useful for marshalling data from a database into Haskell, and both of them come up a lot inside my database library. Has anyone ever seen these before? Has Edward Kmett written a library for these already? Thanks, Tom

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Best practices for Arrows?
by Tom Ellis 13 Nov '13

13 Nov '13

I feel I may be doing a lot of programming with Arrows in the near future. Currently I'm delighted that Arrow notation[1] exists. It makes using Arrows much less painful. Are there any best-practices I should be aware of with Arrows? Or is it just a case of getting on with it? Tom 1. http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html

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Trouble installing haskeline: ExitFailure 139
by David Banas 26 Oct '13

26 Oct '13

Has anyone else hit an unexplained *ExitFailure 139* when trying to install the *haskeline* package? Thanks, -db dbanas@dbanas-lap:~/prj$ cabal install -v haskeline Reading available packages... Choosing modular solver. Resolving dependencies... Extracting /home/dbanas/.cabal/packages/ hackage.haskell.org/haskeline/0.7.0.3/haskeline-0.7.0.3.tar.gz to /tmp/haskeline-0.7.0.3-16574... creating /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup creating /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist creating /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup /usr/local/bin/ghc --make /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/Setup.hs -o /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup/setup -odir /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup -hidir /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup -i -i/tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3 -package Cabal-1.16.0 [1 of 1] Compiling Main ( /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/Setup.hs, /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup/Main.o ) Linking /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup/setup ... /tmp/haskeline-0.7.0.3-16574/haskeline-0.7.0.3/dist/setup/setup configure --verbose=2 --ghc --prefix=/home/dbanas/.cabal --user --flags=terminfo --flags=-libiconv --flags=-legacy-encoding --constraint=unix ==2.6.0.1 --constraint=transformers ==0.3.0.0 --constraint=terminfo ==0.3.2.5 --constraint=filepath ==1.3.0.1 --constraint=directory ==1.2.0.1 --constraint=containers ==0.5.0.0 --constraint=bytestring ==0.10.0.2 --constraint=base ==4.6.0.1 --disable-tests --disable-benchmarks World file is already up to date. cabal: Error: some packages failed to install: haskeline-0.7.0.3 failed during the configure step. The exception was: ExitFailure 139 dbanas@dbanas-lap:~/prj$

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hdbc-odbc not getting any data using mssql stored procedure
by grant 10 Oct '13

10 Oct '13

Hi, I am trying to call a stored procedure (exec gp_somestoredproc 123,22) using microsoft sql server 2008 R2 using hdbc-odbc. Here is the haskell code: import Database.HDBC import Database.HDBC.ODBC import Control.Monad import Data.Maybe import qualified Data.ByteString.Char8 as B test1 = do c <- connectODBC "Driver={SQL Server};Server=?;Database=?;uid=sa;pwd=?;" p <- prepare c "exec gp_somestoredproc 123,22" -- returns no data -- p <- prepare c "exec [sys].sp_datatype_info_100 0,@ODBCVer=4;exec gp_somestoredproc 123,22" -- all is good e <- execute p [] -- returns 6 (number of rows) putStrLn $ "execute " ++ show e r <- fetchAllRows' p putStrLn $ "fetchAllRows' " ++ show r The problem is that this code returns the number of rows correctly but doesn't return data nor are there any errors. However, I ran a perl program using perl dbi and got the data correctly. Here is the perl code: #!/usr/bin/perl use DBI; my $user = 'sa'; my $pw = '????'; my $dsn = '????'; my $dbh = DBI->connect("dbi:ODBC:$dsn", $user, $pw, {PrintError => 1, RaiseError => 1}); if (!$dbh) { print "error: connection: $DBI::err\n$DBI::errstr\n$DBI::state\n"; } my $type_info = $dbh->type_info(93); while(my($key, $value) = each(%$type_info)){ print "$key => $value\n"; }; my $sql = "exec gp_somestoredproc 123,22"; my $sth = $dbh->prepare($sql); my $r = $sth->execute; while (my ($db) = $sth->fetchrow_array) { print $db . "\n===\n"; } $dbh->disconnect if $dbh; I traced both versions and noticed that the perl dbi version first called exec [sys].sp_datatype_info_100 0,@ODBCVer=4 So I prefixed the stored proc call in haskell with "exec [sys].sp_datatype_info_100 0,@ODBCVer=4;" and it worked fine. In short: FAILS p <- prepare c "exec gp_somestoredproc 123,22" -- returns number of rows but no data WORKS p <- prepare c "exec [sys].sp_datatype_info_100 0,@ODBCVer=4;exec gp_somestoredproc 123,22" I have no idea why this works. sp_datatype_info_100 just dumps out the fields types ... More information: The stored procedure returns data with user defined field types. I have managed to do selects against tables with user defined field types without any problems using hdbc-odbc. I couldn't emulate this error on a local older version of mssql server (Microsoft SQL Server 2008 (SP1) - 10.0.2531.0 (X64) express) but the perl dbi prefixed the stored procedure call with "exec sp_datatype_info 0,@ODBCVer=3" I am running this against Microsoft SQL Server 2008 R2 (RTM) - 10.50.1797.0 (X64) I would appreciate any pointers you can give me. Thanks Grant

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Music update
by Mark Lentczner 07 Oct '13

07 Oct '13

Some might remember me asking about music packages a while back... An update: I ended up using Euterpea, which in turn uses both Codec.Midi and Sound.PortMidi. My working environment was to have my code loaded up in ghci, play MIDI into a software MIDI bus, and pipe that into MainStage 3 which ran the synths. The piece I was working on premiered last night at a concert in Wellington, NZ. The live recording will take a while, but you can hear a studio synth recording (from the above setup) here: https://soundcloud.com/mtnviewmark/plain-changes-2-all-synth-mix The code for the piece is all open source: https://github.com/mzero/PlainChanges2. In particular, there is a somewhat improved MIDI player<https://github.com/mzero/PlainChanges2/blob/master/src/Sound/MidiPlayer.hs> in there. Enjoy! - Mark P.S.: Yes, and now that that's done with, I can get on with the next Haskell Platform release!

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Poll & plea: State of GUI & graphics libraries in Haskell
by Conal Elliott 06 Oct '13

06 Oct '13

I'm polling to see whether there are will and expertise to reboot graphics and GUIs work in Haskell. I miss working on functional graphics and GUIs in Haskell, as I've been blocked for several years (eight?) due to the absence of low-level foundation libraries having the following properties: * cross-platform, * easily buildable, * GHCi-friendly, and * OpenGL-compatible. The last several times I tried Gtk2hs, I was unable to compile it on my Mac. Years ago when I was able to compile, the GUIs looked and interacted like a Linux app, which made them awkward and upleasant to use. wxHaskell (whose API and visual appearance I prefered) has for years been incompatible with GHCi, in that the second time I open a top-level window, the host process (GHCi) dies abruptly. Since my GUI & graphics programs are often one-liners, and I tend to experiment a lot, using a full compilation greatly thwarts my flow. For many years, I've thought that the situation would eventually improve, since I'm far from the only person who wants GUIs or graphics from Haskell. About three years ago, I built a modern replacement of my old Pan and Vertigo systems (optimized high-level functional graphics in 2D and 3D), generating screamingly fast GPU rendering code. I'd love to share it with the community, but I'm unable to use it even myself. Two questions: * Am I mistaken about the current status? I.e., is there a solution for Haskell GUI & graphics programming that satisfies the properties I'm looking for (cross-platform, easily buildable, GHCi-friendly, and OpenGL-compatible)? * Are there people willing and able to fix this situation? My own contributions would be to test and to share high-level composable and efficient GUI and graphics libraries on top of a working foundation. Looking forward to replies. Thanks, -- Conal

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"plugins" fails on a simple example
by Petr Pudlák 03 Oct '13

03 Oct '13

Hi, I'm playing with "plugins", trying to evaluate a simple expression: |import Control.Monad import System.Eval.Haskell main =do let fExpr ="1 + 2 :: Int" r <- eval_ fExpr ["Prelude"] [] [] [] ::IO (Either [String] (Maybe Int)) case rof Right (Just f) ->do print $ f Left err -> putStrLn $"Error:" ++ unlines err _ -> putStrLn $"Unknown error."| However, it fails with |Error: on the commandline: Warning: -fglasgow-exts is deprecated: Use individual extensions instead| Am I doing something wrong? How can I turn off the flag? I'm using GHC 7.6.3. Thanks, Petr

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Roshask
by Tijn van der Zant 02 Oct '13

02 Oct '13

Hi, I want to use Haskell to program my robot using ros. I've found roshask https://github.com/acowley/roshask and I was wondering if someone on this list has experience with it and can tell me how good it is. Thanx! --Tijn

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HTML framework for web-ui
by Vlatko Basic 01 Oct '13

01 Oct '13

Hi, I'd like to start using web pages as the UI for apps. I found out for yesod, snapp and happstack as the candidates. Would you recommend any of them as better for app ui (not "classical" web pages)? Or maybe another one? thanks, vlatko

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The RTSOPTS "-qm" flag's impact on runtime
by Iustin Pop 30 Sep '13

30 Sep '13

Hi all, I found an interesting case where the rtsopts -qm flag makes a significant difference in runtime (~50x). This is using GHC 7.6.3, llvm 3.4, program compiled with "-threaded -O2 -fllvm" and a couple of language extension. Source is at http://benchmarksgame.alioth.debian.org/u64q/benchmark.php?test=chameneosre…, on the language shootout benchmarks. Running the code without -N results (on my computer) in around 4 seconds of runtime: $ time ./orig 6000000 … real 0m3.919s user 0m3.903s sys 0m0.010s This is reasonably consistent. Running -N4 (this is an 8-core machine) results in the surprising: $ time ./orig 6000000 +RTS -N4 … real 1m15.154s user 1m38.790s sys 2m7.947s The cores are all used very erratically (continuously changing 5%-20%-40%) and the overall cpu usage is ~27-28%. Note the surprising 2m7s of sys usage, which means the kernel is involved a lot… Note that removing the explicit forkOn and running with -N4 results in somewhat worse performance: real 2m6.548s user 2m13.470s sys 2m3.043s So in that sense the forkOn itself is not at fault. What I have found is that -qm is here a life saver: $ time ./orig 6000000 +RTS -N4 -qm real 0m2.773s user 0m5.610s sys 0m0.123s Adding -qa doesn't make a big difference. To summarise more runs (in terms of cpu used, user+sys): with forkOn: - -N4: 228s - -N4 -qa: 110s - -N4 -qm: 6s - -N4 -qm -qa: 6s without forkOn: - -N4: 253s - -N4 -qa: 252s - -N4 -qm: 5s - -N4 -qm -qa: 5s (Note that "without forkOn" is a bit slower in term of wall-clock, as the "with forkOn" version distributes the work a bit better, even if it uses overall a tiny bit more CPU.) So the question is, what does -qm actually do that it affects this benchmark so much (~50x)? (The docs are not very clear on it) And furthermore, could there be an heuristic inside the runtime such that automatic thread migration is suspended if threads are "over-migrated" (which is what I suppose happens here)? thanks for any explanations, iustin

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