January 2009 - Haskell-Cafe

how out of memory is handled in Haskell
by Manlio Perillo 06 Jan '09

06 Jan '09

Hi. Here: http://damienkatz.net/2008/03/what_sucks_abou.html I found how Erlang (or at least old versions of Erlang) handles out of memory failure: it just calls exit(1). How is this handled in GHC? - exit(1)? - abort()? - IO exception? Thanks Manlio Perillo

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Lack of inlining -> slow parsing with Data.Binary
by Eugene Kirpichov 06 Jan '09

06 Jan '09

Hi, I'm parsing Java classfiles with Data.Binary, the code is here: http://paste.org/index.php?id=4625 The problem is that the resulting code parses rt.jar from JDK6 (about 15K classes, 47Mb zipped) in 15 seconds (run the program with main -mclose rt.jar, for instance), which is 10 times slower than my Java version of the same code. I compile the program with -O2 ; I tried -ddump-inlinings and it turns out that my readByte/readWord16/readWord32 functions don't get inlined, despite being simply aliases for 'get::Get WordXX'; so, in places where my Java version does a pointer access (after being JIT-compiled), the Haskell version does two function calls. What can be the reason of this lack of inlining? Or how do I understand the output of -ddump-inlinings? -- Eugene Kirpichov

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Pattern combinators
by Andrew Wagner 06 Jan '09

06 Jan '09

All, Wadler posted a blog entry the other day about a paper on pattern-matching in Haskell (http://wadler.blogspot.com/) I've taken a first stab at turning it into actual code for hackage (http://hpaste.org/13215) There are two commented-out definitions that don't type-check, though, and the types are too wild for me to grok. Anybody have any suggestions for 1.) How to fix it and/or 2.) How to use data/type/newtype to simplify the types and make it more manageable? Thanks!

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Re: bug in HPDF?
by Roman Cheplyaka 06 Jan '09

06 Jan '09

Thanks for helping! * alpheccar <misc(a)alpheccar.org> [2009-01-06 19:47:21+0100] > Roman, > > Here is the source code to do what you want: > > import Graphics.PDF > import Complex > > main = runPdf "bug.pdf" standardDocInfo (PDFRect 0 0 100 100) pdf > where > pdf = do > p <- addPage Nothing > drawWithPage p $ do > moveto $ 10 :+ 10 > sequence $ replicate 10 $ drawText $ text (PDFFont Helvetica > 10) 0 0 (toPDFString "ABC") > > > It is the Text monad that needs to be replicated. Each copy will start > drawing the text lines at (10,10) as specified by the moveto. > But, inside the text monad, each "text" will create a new line. > > Thanks, > Christophe. > > >> Here is a program which illustrates an unexpected behaviour: >> >> import Graphics.PDF >> >> main = runPdf "bug.pdf" standardDocInfo (PDFRect 0 0 100 100) pdf >> where >> pdf = do >> p <- addPage Nothing >> drawWithPage p $ drawText $ >> sequence $ replicate 10 $ >> text (PDFFont Helvetica 10) 10 10 (toPDFString "ABC") >> >> What I expect here is "ABC" printed 10 times on the same place >> (starting >> at (10,10)). What I see (in Okular) is ABC printed each time in the new >> place like this: >> >> ABC >> ABC >> ABC >> ABC >> >> What's happening here? I'm using HPDF-1.4.1 from Hackage. >> >> -- >> Roman I. Cheplyaka :: http://ro-che.info/ >> "Don't let school get in the way of your education." - Mark Twain > -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

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bug in HPDF?
by Roman Cheplyaka 06 Jan '09

06 Jan '09

Here is a program which illustrates an unexpected behaviour: import Graphics.PDF main = runPdf "bug.pdf" standardDocInfo (PDFRect 0 0 100 100) pdf where pdf = do p <- addPage Nothing drawWithPage p $ drawText $ sequence $ replicate 10 $ text (PDFFont Helvetica 10) 10 10 (toPDFString "ABC") What I expect here is "ABC" printed 10 times on the same place (starting at (10,10)). What I see (in Okular) is ABC printed each time in the new place like this: ABC ABC ABC ABC What's happening here? I'm using HPDF-1.4.1 from Hackage. -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

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HDBC-Sqlite3 "attaching databases"
by Günther Schmidt 06 Jan '09

06 Jan '09

Hi, has anybody here successfully tried to "attach" another database to an Sqlite database with HDBC-Sqlite3? I keep failing, so I'd be grateful for a hint how to do it. Günther

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Threads with high CPU usage
by Günther Schmidt 06 Jan '09

06 Jan '09

Hi, in an application of mine I start a long-running operation in a thread via forkIO so that the UI process doesn't get blocked. It just so happens, that the long-running process also takes the CPU to nearly 100% while it runs. During that time the run-time system does *not* switch back and forth between the UI-process and the long-running task, it seems that the UI process only gets woken up *after* the high CPU thread finishes completely. To the effect of course that it makes no difference at all to the UIs responsiveness whether I use forkIO or not. The long running process is pretty atomic, it's a single query to the database which takes up to a minute to complete so I don't see a chance to squeeze a "mainIteration" in there. What can I do? Günther

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building HSQL MySQL on windows (Was: FFI imported function names)
by Daniil Elovkov 06 Jan '09

06 Jan '09

Ok, enough talking to myself :) If anybody ever wants to build hsql-mysql on windows and has the same problems as I had, here's how it should be done. The problem I had seemed to be that libmysql.dll uses stdcall, but names its functions without @<n> decoration. Thus, when linking a Haskell program against it, names with @<n> cannot be resolved. The solution is pretty much a hack. Somebody from the Ruby community used to run into the same problem, from the same situation -- namely building ruby mysql bindings. Here is the procedure one person suggests http://rubydotnetproxy.rubyforge.org/mysql_win.html It worked perfectly for me. Of course, with some cabal specifics, which is obviously not described in the above page. The solution was to create a def file listing the names which could not be resolved and creating a .a file based on the dll and this def file. 2008/12/24 Daniil Elovkov <daniil.elovkov(a)googlemail.com>: > Mm, actually I didn't change the calling convention ffi imports when I > thought I did. I tried to do it through defines... > > Well, by explicitly saying ccall I get the names without @<n> > decoration. And it all links well. But I get segault when I run the > code, which should mean that calling conventions didn't match. > > Actually I found some ramblings on the internet that there's something > wrong with libmysql.dll and libmysql.lib in this respect, but they > were from 2004. > > Thus the question: how do I link against a lib file in ghc? When I say > -lmysql on command line it strictly searches for dll and complains if > it's not found. Adding lib exlpicitly on the cmd line doesn't seem to > have any effect. > > Thanks > > > 2008/12/24 Daniil Elovkov <daniil.elovkov(a)googlemail.com>: >> Hello >> >> How is decided whether the name of imported function gets the ending >> of the form @4 in ghc? >> >> I'm having this problem on Windows trying to use HSQL MySQL on windows. >> >> I compile HSQL Oracle backend and I get names without that. It's ok. >> With HSQL MySQL I get names with that stuff. It prevents me from >> further linking to dll. Supplying in the ghc command-line the "lib" >> file where names have those marks doesn't have any effect. >> >> The linker says that fname@NN cannot be resolved. >> >> Can I control that? >> >> The ffi calling convention doesn't (and shouldn't as I understand) >> affect this. The only difference is that in the case of mysql the >> header file itself where functions are described marks it STDCALL. Is >> that the reason? >> >> Also I changed ffi import line from "hsmysql.h func" to just "func", >> to no avail. >> >> ghc 6.8.2 >> >> I'm lost. Please help. >> >> -- >> Daniil Elovkov >> > > > > -- > Daniil Elovkov > -- Daniil Elovkov

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Can I destructive rebind a local variable in haskell?
by Wang, Chunye (NSN - CN/Beijing) 06 Jan '09

06 Jan '09

Dear haskeller, Can I destructive rebind a local variable like this import System.Directory test filename = do is_dir <- doesDirectoryExist filename let filename = if not is_dir then filename else filename putStrLn $ " filename " ++ filename main = test "." in GHCi 6.10.1 on WinXP, the ghci aborts silencely when I executes "main" after compile it into a executable file. c:\USERS\home\learning_haskell\>testDirectory testDirectory C stack overflow in generated code I have some code in scheme which works very well (define (func x) (let ((x x)) x)) By rebinding a formal variable, we can save naming some varaibles. I am not good at naming a variable. Thank you in advance Best Regards Chunye Wang <chunye.wang(a)nsn.com>

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ANN bytestring-trie 0.1.2 (bugfix)
by wren ng thornton 06 Jan '09

06 Jan '09

-------------------------------------------- -- bytestring-trie 0.1.2 (bugfix) -------------------------------------------- Another bugfix release for efficient finite maps from (byte)strings to values. Release early, release often. -------------------------------------------- -- Changes -------------------------------------------- * Fixed a bug in alterBy pointed out by Mark Wotton. Same bug as the previous one pointed out by Maxime Henrion (a missing case). * Added an Eq instance, in case anyone cares. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie Darcs: http://code.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://code.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring-trie/ -- Live well, ~wren

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