January 2009 - Glasgow-haskell-users

unique identifiers as a separate library
by Sebastian Fischer 06 Jan '09

06 Jan '09

Hello, for a project I am using the libraries Unique, UniqSupply, and UniqFM from the package ghc. It seems odd to have a dependency on a whole compiler only in order to use its (highly efficient and, hence, preferred) implementation of unique identifiers. Would it be possible to put everything concerned with unique identifiers in GHC into a separate package on Hackage? This may also allow me to get a fix for <http://hackage.haskell.org/trac/ghc/ticket/2880 > without reinstalling GHC. Cheers, Sebastian

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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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black hole detection and concurrency
by Conal Elliott 06 Jan '09

06 Jan '09

I'm looking for information about black hole detection with ghc. I'm getting "<<loop>>" where I don't think there is an actual black hole. I get this message sometimes with the unamb package, which is implemented with unsafePerformIO, concurrency, and killThread, as described in http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice/… http://conal.net/blog/posts/smarter-termination-for-thread-racing/ . Suppose I have a definition 'v = unsafePerformIO ...', and v is used more than once. Evaluation (to whnf) of v is begun and the evaluation thread gets killed before evaluation is complete. Then the second use begins. Will the second evaluation be (incorrectly) flagged as a black hole? I haven't found a simple, reproducible example of incorrect black-hole reporting. My current examples are tied up with the Reactive library. I do have another strange symptom, which is "thread killed" message. I wonder if it's related to the <<loop>> message. Code below. Thanks, - Conal import Prelude hiding (catch) import System.IO.Unsafe import Control.Concurrent import Control.Exception -- *** Exception: thread killed main :: IO () main = print $ f (f True) where f v = (v `unamb` True) `seq` v -- | Unambiguous choice operator. Equivalent to the ambiguous choice -- operator, but with arguments restricted to be equal where not bottom, -- so that the choice doesn't matter. See also 'amb'. unamb :: a -> a -> a unamb a b = unsafePerformIO (evaluate a `race` evaluate b) -- | Race two actions against each other in separate threads, and pick -- whichever finishes first. See also 'amb'. race :: IO a -> IO a -> IO a race a b = do v <- newEmptyMVar let t x = x >>= putMVar v withThread (t a) $ withThread (t b) $ takeMVar v where withThread u v = bracket (forkIO u) killThread (const v)

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pthread mutex error building 6.10 on NetBSD
by Donn Cave 04 Jan '09

04 Jan '09

I tried to build GHC 6.10.1 on NetBSD 4.0 this afternoon, and ran into an error, where cabal-bin calls the stage2 ghc in utils/installPackage. pthread_mutex_unlock() is called, evidently from fileLock(), with an invalid mutex. NetBSD's pthread implementation differs from Linux in its intolerance of such things. I have found errors where someone forgets that a mutex was never used and tries to unlock it. I guess you could do that all day long on Linux without noticing anything, but NetBSD aborts the program. In the present case, I don't see anything like that wrong in rts/FileLock.c, though. I'm stumped. So I'm checking, has anyone else built 6.10 on NetBSD? Donn

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Preferred gcc for use with ghc
by Gregory Wright 03 Jan '09

03 Jan '09

Hi, What is the preferred gcc to use with ghc 6.10.1? I'm starting the long and doubtless slow process of putting together a more user friendly (and binary distributable) ghc distribution to be built using MacPorts. The guidance about gcc on the wiki is vague. Is there a best choice? Or at least some versions to be avoided? Best, Greg

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Re: [Haskell] Haskell profiler
by Tim Chevalier 01 Jan '09

01 Jan '09

On 12/29/08, Srinivas Nedunuri <nedunuri(a)cs.utexas.edu> wrote: > Could someone tell me what the expected accuracy of the Haskell profiler is? > I get quite widely varying results for the amount of time spent on a given > function across different runs (eg one random example, from 1.5% to 10%). I > even upped the size of the dataset in an attempt to increase the accuracy. [redirecting to ghc-users] I assume you mean the GHC profiler. It's normal when profiling any programs, Haskell or not, for timing numbers to vary. Memory allocation counts, however, should be stable across different runs. > Also a related q: why doesnt't the time spent in putStr show up? > You would have to rebuild your GHC libraries with the -auto-all flag in order to get cost centres attached to library functions (but this may defeat some optimizations). Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt "It's okay to be mean to the evil people who make life unbelievable, but try to be nice to me." -- Nerissa Nields

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