September 2010 - Glasgow-haskell-users

GHCi: scripting and multiline input
by Vivian McPhail 14 Sep '10

14 Sep '10

Hi, I am attempting to mimic matlab/octave using ghci, including a facility for updateable graphs of data. 1) It would be nice to be able to load a script of commands from the prompt \begin{file} script Sine where import "hmatrix" Numeric.Vector import "plot" Graphics.Rendering.Plot let ts = linspace 1000 (0,1 :: Double) let ss = sin (15*2*pi*ts) -- 15 Hz sine wave figure1 <- display $ plot (ts,[line ss blue]) \end{file} The idea is (apart from the import commands) is each line is equivalent to an input line in ghci. Note that the last line binds a variable from an action in the top level IO monad, something not possible in a module. 2) It would be nice to have multiline inputs if the parser can tell that the input is incomplete rather than malformed. One case is an empty 'do' construct: \begin{interaction} Prelude> :load Sine Sine> :show bindings ts :: Data.Vector.Storable Double ss :: Data.Vector.Storable Double figure :: Graphics.Rendering.Plot.Figure.Figure () Sine> withFigureMVar figure1 $ do Figure a > withTitle $ do Text a > setText "15 Hz sine wave" Text a > setFontSize 16 Text a > Figure a > Sine> \end{interaction} (an empty line ends the 'do' construct and returns to next higher level). Is this a feasible addition to GHCi? This way the full power of GHCi is accessible to an interactive mathematics session rather than developing a custom interface on top of the GHC API. Vivian

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unicode characters in operator name
by Greg 11 Sep '10

11 Sep '10

Hi-- I'm creating a family of methods for a class, and to prevent name clashes I've annotated the function names with º. The files are saved UTF-8, so the character is accepted. Method names of the form fº are handled fine. GHC consumes them quite happily. Where I'm having trouble is in operator names. I'd like to create the operator (*º) but I get a type error: Invalid type signature for this line: (*º) :: (Real b, Floating b) => b -> a -> a I've tried other (ascii) symbol combinations and they're accepted, so I think the general form of my type declaration is fine. If I read the Haskell Report correctly, operators are named by (symbol {symbol | : }), where symbol is either an ascii symbol (including *) or a unicode symbol (defined as any Unicode symbol or punctuation). I'm pretty sure º is a unicode symbol or punctuation. I know I could get around this by using a different name, but this naming convention is more intuitive for what I'm trying to do. Am I misinterpreting the report, or am I doing something else wrong? Thanks-- Greg

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HEAD: Deterioration in ByteString I/O
by Daniel Fischer 09 Sep '10

09 Sep '10

Trying out HEAD (specifically, ghc-6.13.20100831-src.tar.bz2 built with 6.12.3) investigating an issue with the text package, I found that I/O of ByteStrings has become significantly slower (on my machine at least: $ uname -a Linux linux-mkk1 2.6.27.48-0.2-pae #1 SMP 2010-07-29 20:06:52 +0200 i686 i686 i386 GNU/Linux Pentium 4, 3.06GHz). Timings for reading and outputting a 74.3MB file: cat: $ time cat bigfile > /dev/null 0.00user 0.04system 0:00.06elapsed 83%CPU ghc-6.12.3: $ time ./nbench lazyBSNull bigfile a b > /dev/null 0.01user 0.09system 0:00.10elapsed 100%CPU ghc-6.13.20100831: $ time ./hdbench lazyBSNull bigfile a b > /dev/null 0.07user 0.10system 0:00.18elapsed 96%CPU In addition to the slowdown, the allocation behaviour has become quite bad: ghc-6.12.3: 89,330,672 bytes allocated in the heap 15,092 bytes copied during GC 35,980 bytes maximum residency (1 sample(s)) 29,556 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) ghc-6.13.20100831: 475,305,720 bytes allocated in the heap 89,272 bytes copied during GC 68,860 bytes maximum residency (1 sample(s)) 29,444 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation)

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alignment and the evil gc assertion failure
by Evan Laforge 09 Sep '10

09 Sep '10

So a long time ago (I think when 6.10 first came out, the problem didn't happen with the previous version, and I think 6.10 changed how the FFI used alignment) I filed a ghc ticket about a gc assertion failure. Unfortunately it was so hard to reproduce and reduce to a manageable example that I wound up thinking I had fixed it and closing it as user error. However, I'm pretty sure I tracked down what the problem was since I changed something and I haven't had that crash since. To recap, the crash is an assertion failure in the gc, I've seen two positions: seq: internal error: ASSERTION FAILED: file rts/dist/build/sm/Evac_thr.c, line 298 seq: internal error: ASSERTION FAILED: file rts/dist/build/sm/Evac_thr.c, line 369 The problem was in the marshalling of a certain struct. The struct has 3 Color fields and two char fields. The Colors are simply triples of chars. I'd been using an alignment macro I've seen around: #let alignment t = "%lu", (unsigned long)offsetof(struct {char x__; t (y__); },y__) Since my struct is made out of chars, this macro returns an alignment of 1. However, an alignment of 1 seems to be what leads to the GC crash. After setting the alignment to 4 I've never had the crash again. So... what's going on here? Is the macro wrong? Alignment 1 seems correct for something built of chars... or does a 'struct { char, char, char }' embedded in another struct turn it into alignment 4 somehow? Alignment seems like a particularly problematic part of the FFI, which is in all other respects very easy to use. It's low level, poorly understood (well, by me at least), not super well documented, and one little mistake can either be harmless, or lead to a *very* hard to track down bug. If the alignment macro is correct, can it be built into hsc2hs? Copy and pasting some magic I saw on the net into every hsc file doesn't give me a good feeling. If it's incorrect, what would a correct one be? One odd thing about that macro is that if you mis-spell a field name, gcc gets a bus error (OS X 10.5.8, gcc 4.0.1, does anyone else see this?) Or is the alignment correct and it really is a ghc bug? If so, how can I help track it down? The best I can think of is to make a small program with that same data structure, pass it around some, and then generate and collect a lot of garbage, but this bug has been really hard to pin down in the past, change one little thing and it disappears only to pop up again in 6 months.

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Type Families and defaults
by Mitar 09 Sep '10

09 Sep '10

Hi! I see here: http://hackage.haskell.org/trac/ghc/wiki/TypeFunctionsStatus That defaults in type families are not yet implemented. But how far is this? And how far is overall implementation of this? There seems to be a lot of open bugs around that? Are type families meant to be used or is that still an experimental feature? For example, I have problems type checking the attached code (Test.hs). Is there a way to define a default function as I envisioned it? Or is this just broken code? ;-) It seems it is the later as also version with dependencies does not work (Test2.hs). Best regards Mitar

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SIGALRM, SIGVTALRM, and third party libraries
by Bryan O'Sullivan 09 Sep '10

09 Sep '10

I spent some time this morning trying to use HDBC-mysql to talk to a database. It uses the C mysql bindings, which talks over a blocking socket to the database server. Not surprisingly, it fails reliably when the thread it's running in is hit by an RTS-initiated alarm signal. I've managed to make the code *appear* to work successfully with some hacking: - Run in a bound thread - Use pthread_sigmask to temporarily block SIGALRM and SIGVTALRM to *only* this thread while issuing the sensitive native calls - Profit? What I am wondering is if there's a practical downside to doing this. Am I going to accidentally kill something? This is a very important gap in the usability of GHC with native libraries, and if this approach actually turns out to be safe in practice, that would be wonderful.

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Compiling base with custom compilation script
by Victor Nazarov 06 Sep '10

06 Sep '10

I have some custom compilation script that uses GHC API The aim is to extract some info from every module in dependency graph and to write this information to the file lying near module-file. Script goes like this: main :: IO () main = do args <- getArgs defaultErrorHandler defaultDynFlags $ runGhc (Just libdir) $ do sdflags <- getSessionDynFlags (dflags, fileargs', _) <- parseDynamicFlags sdflags (map noLoc args) when (null fileargs') $ ghcError (UsageError "No input files.") _ <- setSessionDynFlags dflags let fileargs = map unLoc fileargs' targets <- mapM (\x -> guessTarget x Nothing) fileargs setTargets targets mgraph <- depanal [] False let files = filter (not . isSuffixOf "boot") . map (extractPath . ms_location) $ mgraph extractPath l = fromMaybe (ml_hi_file l) (ml_hs_file l) setTargets [] flip mapM_ files $ \file -> do core <- compileToCoreSimplified file HscTypes.liftIO $ let info = show (generateInfo core) fp = replaceExtension file ".info" putStrLn $ "Writing " ++ fp writeFile fp program The problem is processing base-4 package. I'd like to run something like: $ cd base-4.2.0.1 $ compiler -fglasgow-exts -cpp -package-name base -I./include Prelude.hs and to receive Prelude.info and .info files for every other modules that prelude depends on. At first I've got errors like missing .h file. I've downloaded GHC source distribution and get missing headers from GHC. But now I get errors like "trying to load Prelude module which is not loadable". I don't remember the exact text and I have no access to my developing-machine. I think it's caused by circular dependencies between modules. And I think my compilation script is not quite correct for this case. What do you think? -- Victor Nazarov

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Confusing warnings from GHC HEAD
by Bryan O'Sullivan 05 Sep '10

05 Sep '10

I get a few dozen of these when building the text package, and they weren't present in 6.12 or earlier: SpecConstr Function `$wa1{v X5A2} [lid]' has three call patterns, but the limit is 2 Use -fspec-constr-count=n to set the bound Use -dppr-debug to see specialisations There are a few aspects to this that I find surprising. - I can't do anything with the Core name. - My cabal file contains -fspec-constr-count=5 just to be safe, but it doesn't help. I can't tell whether this is the compiler going a little off into the weeds, or whether I'm doing something wrong, but I appear to be unable to make the message go away. Thanks, Bryan.

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Difference between optimized and non-optimized version
by Mitar 02 Sep '10

02 Sep '10

Hi! I am attaching a program which locks (throws an exception <<loop>>) when compiled like: ghc --make -threaded -main-is Test.main Test.hs -o test and doesn't lock when compiled with ghc --make -threaded -main-is -O2 Test.main Test.hs -o test This should probably not happen. I am using 6.12.3 on Linux x86_64. Best regards Mitar

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FFI, signals and exceptions
by Edward Z. Yang 02 Sep '10

02 Sep '10

Hello all, I am currently investigating techniques to nicely handle SIGINTs when FFI code is running. Yes, I know it sounds kind of crazy. Ignoring the problems of cleaning up the unceremoniously terminated C computation, I'm having difficulty getting the FFI to /stop/ running when I get the signal. I currently have some code like this: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28422#a28422 which doesn't work, and which you guys can probably tell me doesn't work. :-) Throwing exceptions from the signal handler also doesn't work. Any suggestions for GHC source code to look at / techniques to try? Cheers, Edward

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