April 2010 - Haskell-Cafe

instance reification in TH
by Dmitry Olshansky 23 Apr '10

23 Apr '10

Hello cafe, Could you say can I determine in Template Haskell that datatype X has instance of class Y? Regards, Dmitry

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ANNOUNCE: c2hs 0.16.2
by Duncan Coutts 23 Apr '10

23 Apr '10

All, c2hs version 0.16.2 is out and is available from hackage http://hackage.haskell.org/package/c2hs The only significant change in this release is that it works with ghc-6.12. Note that it is not currently possible to bind to C functions that use "long double" as the corresponding Haskell type (CLDouble) is not provided by the version of the base library that comes with ghc-6.12. See ghc ticket http://hackage.haskell.org/trac/ghc/ticket/3353 What is c2hs ============ The c2hs tool assists in the development of Haskell bindings to C libraries. It extracts interface information from C header files and generates Haskell code with foreign imports and marshaling. The major advantages of using c2hs compared to writing foreign imports by hand (or using hsc2hs) are: * Cross-language type safety: C functions are imported with the correct Haskell types. * Saves time: boilerplate marshaling code is generated. Links ===== Home page: http://www.cse.unsw.edu.au/~chak/haskell/c2hs/ Bug tracker: http://hackage.haskell.org/trac/c2hs/ Duncan

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ANN: forkable-monad 0.1
by David Anderson 23 Apr '10

23 Apr '10

Dear Haskellers, I'm happy, and only slightly intimidated, to announce the initial release of forkable-monad. The short version is that forkable-monad exports a replacement forkIO that lets you do this: type MyMonad = ReaderT Config (StateT Ctx IO) startThread :: MyMonad ThreadId startThread = forkIO threadMain threadMain :: MyMonad () threadMain = forever $ liftIO $ putStrLn "Painless monad stack forking!" Note the lack of monad stack deconstruction and reconstruction to transport it over to the new thread. You'll find the details in the Haddock documentation for the module. forkable-monad is available: * On hackage: http://hackage.haskell.org/package/forkable-monad * Via cabal: cabal install forkable-monad * Source and issue tracker: http://code.google.com/p/forkable-monad/ Feedback is of course welcome. As this is my first published Haskell code and Hackage upload, I expect there will be quite a bit! - Dave

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Haskell (wiki) Logo license
by Jens Petersen 23 Apr '10

23 Apr '10

Hi, We use the Haskell Logo in Fedora OS installer for the Haskell packages group: http://git.fedorahosted.org/git/?p=comps-extras.git;a=tree I trying to get the logo updated to the nice "new" logo designed by Jeff Wheeler: http://haskell.org/sitewiki/images/a/a8/Haskell-logo-60.png To do that we need to update the license in http://git.fedorahosted.org/git/?p=comps-extras.git;a=blob;f=COPYING Can someone tell me what the correct license for the above image file is? Is it the wiki license or something else? Thank you, Jens -- on behalf of Fedora Haskell SIG https://fedoraproject.org/wiki/Haskell_SIG

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Does anyone know the reason for differeng measurements in profiling under -threaded?
by Jesper Louis Andersen 22 Apr '10

22 Apr '10

Hi, I am asking if anyone has seen the following behaviour from GHC under -threaded and heavy use of File I/O, network I/O and STM use. After having run Combinatorrent for a while and then terminating it, we get the following output from the RTS in GC statistics: INIT time 0.00s ( 0.00s elapsed) MUT time 0.22s ( 60.06s elapsed) GC time 0.05s ( 0.07s elapsed) RP time 0.00s ( 0.00s elapsed) PROF time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.26s ( 60.12s elapsed) %GC time 18.2% (0.1% elapsed) Alloc rate 431,635,651 bytes per MUT second Productivity 80.3% of total user, 0.4% of total elapsed Note that I am running it for exactly 60 seconds of wall-clock time. Yet, the profile information reports: total time = 11.20 secs (560 ticks @ 20 ms) total alloc = 78,696,568 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc ticks bytes MAIN MAIN 80.4 0.2 450 20033 PeerControl Process.Peer 11.2 1.0 63 95272 Sender Process.Peer.Sender 6.2 0.0 35 387 sha1_digest Digest 1.8 1.4 10 136377 checkPiece FS 0.2 92.3 1 9083658 TorrentManager Process.TorrentManager 0.2 1.8 1 179587 which is clearly wrong. Total time is nowhere near 11.20 secs with 560 ticks here. I am almost certain that the 450 ticks attributed to MAIN is wrong as well. I have SCCs at each possible forkIO-point in the program. -prof -auto-all -caf-all doesn't make any difference either, I've tried. Has anyone seen this kind of problem and in that case, what was the fix? Barring nobody knows, the only thing I have left to try is to dive into the GHC RTS and figure out what is going on in there. As an aside: Before writing combinatorrent, I wrote etorrent, A BitTorrent client in Erlang. The two clients follow more or less the same basic structure and largely does the same ting. Combinatorrent has better memory usage right now, mostly due to representation choices: Haskell can do much better data representation than erlang. On the other hand, etorrent still beats out combinatorrent in the CPU department, using less CPU. And that somewhat perplexes me. This is why I am looking at performance profiles and trying to find the culprit. But with the above problem, I can't find it, since I don't trust the profiler anymore to give me the right results. All logic suggest that Haskell is ripping apart Erlang in all parts that has no communication, mostly due to being compiled and having a static type system (The erlang code is interpreted, even though they have a HiPE JIT). In practice however, my guess is that the binary handling in Erlang is quite effective, as is the communication between processes, so this might turn the tides in the favor of the erlang-RTS. As a fun thing, I tried downloading the same file in the test environment and running strace -cf on both programs. Here are the top winners. I don't think you can trust all the columns: Erlang: % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 95.57 285.220815 2241 127300 10518 futex 2.16 6.453726 271 23828 9838 read 2.15 6.420512 279153 23 wait4 0.07 0.198851 3 64073 1 epoll_wait 0.02 0.048004 4000 12 poll 0.01 0.039131 2 16297 write 0.01 0.026917 0 88057 37842 recvfrom 0.01 0.018174 0 75803 36 epoll_ctl 0.00 0.008001 667 12 1 select 0.00 0.007133 1 5138 3 lseek 0.00 0.001931 64 30 6 execve 0.00 0.001912 0 5535 writev 0.00 0.001334 191 7 vfork 0.00 0.001209 0 2684 1995 open 0.00 0.001128 0 3094 2257 stat 0.00 0.000942 6 153 munmap Haskell/GHC: % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 32.20 15834.075616 176163 89883 10065 futex 21.72 10682.355809 176009 60692 26672 recvfrom 12.22 6009.064699 176023 34138 18 select 10.25 5038.622913 176009 28627 1 write 8.45 4153.652812 176010 23599 1 read 4.84 2380.702238 176009 13526 1 writev 3.59 1765.107536 175965 10031 rt_sigprocmask 1.95 961.185526 176009 5461 fstat 1.94 956.257032 176009 5433 lseek 1.80 883.829682 175922 5024 getrusage 0.85 419.781465 176009 2385 107 rt_sigreturn 0.04 19.713008 176009 112 fcntl 0.03 12.320630 176009 70 mmap 0.02 9.680495 176009 55 socket 0.02 9.504619 176011 54 53 connect 0.02 8.976459 176009 51 getsockopt 0.01 5.456279 176009 31 close Apart from Erlang using epoll, there is not too much difference from the perspective of the kernel, which I think is rather good :) -- J.

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wxHaskell not able to link to libstdc++.so
by Ahn, Ki Yung 22 Apr '10

22 Apr '10

Dear Haskellers, I heard from a guy who was having problems with wxHaskell using GHC 6.10.x. I tried it myself and had the exact same problem occurring using GHC 6.12.x as well. I am using the debian unstable version of ghc6 package. kyagrd@kyagrd:~/tmp$ head bb.hs import Graphics.UI.WX import Graphics.UI.WXCore (bitmapGetSize) main = start viewer viewer :: IO () viewer = do bm <- bitmapCreateFromFile "~/sample.jpg" size <- bitmapGetSize bm f <- frame [text := "viewer", on paint := onPaint bm, clientSize :=size] kyagrd@kyagrd:~/tmp$ runghc bb.hs bb.hs: <command line>: can't load .so/.DLL for: stdc++ (libstdc++.so: cannot open shared object file: No such file or directory) kyagrd@kyagrd:~/tmp$ ghc bb.hs bb.o: In function `r43M_info': (.text+0x48): undefined reference to `wxzm0zi12zi1zi4_GraphicsziUIziWXziWindow_zdfDimensionsObject_closure' bb.o: In function `r43O_info': (.text+0xa8): undefined reference to `wxzm0zi12zi1zi4_GraphicsziUIziWXziWindow_zdfPaintObject_closure' ... The problem is there is no libstdc++.so in my Debian system as well as the guy who uses Ubunto. What exists is libstdc++.so.6. kyagrd@kyagrd:~/tmp$ ls /usr/lib/libstdc++.* /usr/lib/libstdc++.so.6@ /usr/lib/libstdc++.so.6.0.13 I wonder how wxHaskell was even able to successfully configure and compile at all. I didn't have wx package installed before, and I installed it just today and it successfully configured and compiled but unable to dynamically link to libstdc++.so. More serious problem is that wxHaskell doesn't work even making a symbolic link of libstdc++.so from libstdc++.so.6. I've also tried running ldconfig -v and ldconfig -p as a root, but still get the same problem. Is this a commom problem with the libraries linking to libstdc++.so or is this just the wxHaskell problem? Or, is this a ghc problem? -- Ahn, Ki Yung

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PhD student / postdoc positions in PLT+AD
by Barak A. Pearlmutter 22 Apr '10

22 Apr '10

Seeking PhD students and postdocs interested in an elegant combination of functional programming and big-iron style numeric computing. Functional Programming and Automatic Differentiation PhD Studentships Postdoctoral Positions We are adding exact first-class derivative calculation operators (Automatic Differentiation or AD) to the lambda calculus, and embodying the combination into a production-quality optimising compiler. Our research prototype compiler generates object code competitive with the fastest current systems, which are based on FORTRAN. We are seeking PhD students and postdocs with interest and experience in relevant areas: programming language theory, numeric computing/numeric linear algebra, or differential geometry; and a burning drive to help lift big iron numeric computing out of the 1960s and into a newer higher order. Specific sub-projects include: compiler and numeric programming environment construction; writing, simplifying, and generalising numeric algorithms through the use of AD operators; and associated type/lambda calculus/PLT/real computation issues. The project headquarters will be in the Hamilton Institute, NUI Maynooth, Ireland, http://www.hamilton.ie/. Applications to: "Barak A. Pearlmutter" <barak+ad-fp-job(a)cs.nuim.ie> -- Prof Barak A. Pearlmutter Hamilton Institute & Dept Comp Sci, NUI Maynooth, Co. Kildare, Ireland http://www.bcl.hamilton.ie/~barak/

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GHC lexer questions
by Phyx 22 Apr '10

22 Apr '10

Hi all, I have a few questions about the GHC lexer (currently using GHC Glasgow Haskell Compiler, Version 6.13.20100320, for Haskell 98, stage 2 booted by GHC version 6.12.1) And I notice that when invoking the lexer I get a few unexpected results that I was hoping someone could clarify. The first part has to do with braces. Opening braces are lexed fine, but the closing ones throw an error. I assume the lexer is keeping some kind of state and trying to do brace matching? (but isn't this supposed to be done by the parser?) ?parseLine("{")->tag cStatelessParseResultSOk ?parseLine("}")->tag cStatelessParseResultSFailed and ?parseLine("{-")->tag cStatelessParseResultSOk ?parseLine("-}")->tag cStatelessParseResultSFailed and in a statement "let foo = .." "foo" isn't lexed as an identifier, "f" is lexed as a ITvocurly and the "oo" as an identifier. I find this rather odd, and maybe it's me doing something wrong but doubt it. The original Haskell code I call from FFI is lexSourceString :: String -> IO (StatelessParseResult [Located Token]) lexSourceString source = do buffer <- stringToStringBuffer source let srcLoc = mkSrcLoc (mkFastString "internal:string") 1 1 let dynFlag = defaultDynFlags let result = lexTokenStream buffer srcLoc dynFlag return $ convert result -- | convert the build in ParseResult to out custom StateLessParseResult -- mainly because the State is difficult to marshal and we don't really seem to need it convert :: ParseResult a -> StatelessParseResult a convert (POk _ ty) = SOk ty convert (PFailed src msg) = SFailed src (showSDocDump msg)

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Hughes' parallel annotations for fixing a space leak
by Heinrich Apfelmus 22 Apr '10

22 Apr '10

Dear haskell-cafe, I've been thinking about space leaks lately. In particular, I've been studying the tricky example with pairs break [] = ([],[]) break (x:xs) = if x == '\n' then ([],xs) else (x:ys,zs) where (ys,zs) = break xs which was discussed in the papers Jan Sparud. Fixing some space leaks without a garbage collector. http://bit.ly/cOxcVJ Philip Wadler. Fixing some space leaks with a garbage collector. http://homepages.inf.ed.ac.uk/wadler/topics/garbage-collection.html As I understand it, GHC implements the technique from Sparud's paper, so this is a solved problem. (It will only kick in when compiled with optimization, though, so -O0 will make the above leak in GHC 6.10.4 if I checked that correctly.) Now, I'm wondering about alternative solutions. Wadler mentions some sort of parallel combinators break xs = (PAR before '\n' ys, PAR after '\n' zs) where (ys,zs) = SYNCLIST xs which were introduced by John Hughes in his Phd thesis from 1983. They are intriguing! Unfortunately, I haven't been able to procure a copy of Hughes' thesis, either electronic or in paper. :( Can anyone help? Are there any other resources about this parallel approach? Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

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Adopting MissingPy?
by John Goerzen 21 Apr '10

21 Apr '10

Hi folks, MissingPy is a library I wrote a little while back that allows you to call Python code from Haskell. It's on Hackage and, as far as I know, still works. Trouble is, the need I used to have for it is gone. So I no longer use it myself for anything, and thus it is starting to bitrot. Would anyone like to take over maintenance of this project? Thanks, -- John

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