December 2003 - Glasgow-haskell-users

RE: "Fail: unknown exception"
by Simon Marlow 06 Jan '04

06 Jan '04

> This has to be one of the most irritating ways a program can > fall over. > Can't the Haskell RTS try just a /little/ harder to help the poor > programmer? For example by saying what sort of exception it is, and > (if it's a dynamic exception) what type it has? An "unknown exception" is a dynamic exception. GHC just uses the ordinary instance of Show on Exception to print out the value of an exception, and it so happens that all it prints out for a dynamic exception is "unknown exception". Arguably the show instance should print out the type too. It could; I just looked into it and unfortunately there's an annoying module loop in the libraries which means doing this would entail some restructuring. Why can't GHCi just print out the type itself? Because it has to be done in the context of the interpreted program, which means GHCi would have to compile a little snippet of code to show the exception. Annoying, but doable. Cheers, Simon

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-xc giving very little information
by Stefan Reich 05 Jan '04

05 Jan '04

A complex program of mine fails with this message: Fail: Maybe.fromJust: Nothing I tried to extract more information about the error by compiling with -prof -auto-all and running the program with +RTS -xc, as advised on http://www.haskell.org/hawiki/TipsAndTricks . This yielded exactly one additional line: <Data.Maybe.CAF>. Fail: Maybe.fromJust: Nothing (Same result for GHC 6.0.1 and 6.2.) Is there another option I have to use to get a full stack trace? Or is my longing for stack traces a side-effect of writing too much Java code that I have to overcome? :) -Stefan

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RE: classes and template haskell (bug?)
by Simon Peyton-Jones 02 Jan '04

02 Jan '04

In GHC 6.2, Template Haskell has various bugs. I think they are all fixed in the HEAD, so you can either build from source or grab a development snapshot from the GHC site. The HEAD version of TH has a slightly different programming interface too -- see http://research.microsoft.com/~simonpj/tmp/notes2.ps Simon | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell-users- | bounces(a)haskell.org] On Behalf Of Jeremy Shaw | Sent: 31 December 2003 02:19 | To: glasgow-haskell-users(a)haskell.org | Subject: classes and template haskell (bug?) | | Hello, | | I have loaded the following from a file into ghci 6.2: | | module Main where | | import Language.Haskell.THSyntax | | class Test a where | test :: a -> a | | instance Test (a,b,c) where | test x = x | | main = putStrLn "Hello, World!" | | This works for me: | | *Main> runQ [d| instance Test (Int,Int) |] >>= putStrLn . show | [InstanceD [] (AppT (ConT "Main:Test") (AppT (AppT (TupleT 2) (ConT "GHC.Base:Int")) (ConT | "GHC.Base:Int"))) []] | | But this doesn't: | | *Main> runQ [d| instance Test (a,b) |] >>= putStrLn . show | ghc-6.2: panic! (the `impossible' happened, GHC version 6.2): | Failed binder lookup: a {- tv a20x -} | | Please report it as a compiler bug to glasgow-haskell-bugs(a)haskell.org, | or http://sourceforge.net/projects/ghc/. | | | Am I doing something wrong, or is this a bug? | | Thanks! | Jeremy Shaw. | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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RE: Running a "final" finaliser
by Simon Marlow 01 Jan '04

01 Jan '04

> Assuming the weak pointers solution is the way to go, I've been > re-aquainting myself with System.Mem.Weak and now I'm now wondering > what is an appropriate key for each ForeignPtr. Before we go down that route, I want to be sure that it's actually necessary to use weak pointers. It sounds like your application has the following properties: - there is a library that can allocate some resources, where each resource is represented by a ForeignPtr - a resource needs to be released when it is no longer referenced - at some point, we would like to free *all* outstanding resources (either at the end of the program, or when the library is no longer required). If this is the case, I'd do it something like this: - keep a global list of the pointers still to be released, probably a doubly-linked list. Lock the whole thing with an MVar. Elements are Ptrs, not ForeignPtrs. - the finaliser on each ForeignPtr removes the corresponding Ptr from the list. - the final cleanup routine explicitly releases all the remaining Ptrs in the list, holding the MVar lock as it does so to avoid race conditions with finalisers. Weak pointers aren't required, AFAICT. Cheers, Simon

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classes and template haskell (bug?)
by Jeremy Shaw 30 Dec '03

30 Dec '03

Hello, I have loaded the following from a file into ghci 6.2: module Main where import Language.Haskell.THSyntax class Test a where test :: a -> a instance Test (a,b,c) where test x = x main = putStrLn "Hello, World!" This works for me: *Main> runQ [d| instance Test (Int,Int) |] >>= putStrLn . show [InstanceD [] (AppT (ConT "Main:Test") (AppT (AppT (TupleT 2) (ConT "GHC.Base:Int")) (ConT "GHC.Base:Int"))) []] But this doesn't: *Main> runQ [d| instance Test (a,b) |] >>= putStrLn . show ghc-6.2: panic! (the `impossible' happened, GHC version 6.2): Failed binder lookup: a {- tv a20x -} Please report it as a compiler bug to glasgow-haskell-bugs(a)haskell.org, or http://sourceforge.net/projects/ghc/. Am I doing something wrong, or is this a bug? Thanks! Jeremy Shaw.

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-keep-hc-files with --make?
by Antti-Juhani Kaijanaho 30 Dec '03

30 Dec '03

Hi, It seems that with GHC 6.2, -keep-hc-files and --make conflict: GHC deletes the HC files it makes. I found an old bug report about this, with a note from one of the developers claiming to have fixed this, but at least here the bug is reproduceable (a regression, perhaps?). This is on up-to-date Debian unstable. -- Antti-Juhani Kaijanaho, FM (MSc), http://www.mit.jyu.fi/antkaij/ ohjelmistotekniikan assistentti * assistant in software engineering Jyväskylän yliopisto * University of Jyväskylä Tietotekniikan laitos * Dept. of Mathematical Inf. Tech.

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RE: GHC Warning Request
by Simon Peyton-Jones 29 Dec '03

29 Dec '03

Good idea. Already done in the HEAD (which will become 6.4). I doubt we'll retrofit this to 6.2 though. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell-users- | bounces(a)haskell.org] On Behalf Of Ashley Yakeley | Sent: 28 December 2003 12:10 | To: glasgow-haskell-users(a)haskell.org | Subject: GHC Warning Request | | When -fwarn-unused-imports is switched on, "import M()" should not issue | a warning. In 6.2, it does. | | The idea is that I only wish to import instance declarations here, and | that's the obvious way of making that explicit. I'm using -Werror (new | in 6.2, thanks), and most of the time I'm interested in knowing about | superfluous imports. | | -- | Ashley Yakeley, Seattle WA | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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RE: Running a "final" finaliser
by Simon Marlow 28 Dec '03

28 Dec '03

> Thanks for your reply. I'm afraid it's left me even > more confused about which way to go with this :-( > > If it's possible that future Haskell FFI's don't guarantee > that all finalisers are run then this more or less rules > out the use of the reference counting solution (which > wasn't particularly attractive anyway because it needs to > be done in C AFAICS :-). If users who want this behaviour > are required to code it themselves, it seems to require that > they maintain a global list of all allocated ForeignPtrs. > But doing that naively will stop them being garbage collected > at all, unless it's possible to do something clever using weak > pointers. Perhaps it is possible (or maybe some tricks at the > C level could be used) but I think it's a significant extra > burden for FFI users. Yes, it would have to be a global list of weak pointers to ForeignPtrs. This topic has come up before, though not on this list. See this message, and the rest of the thread: http://www.haskell.org/pipermail/cvs-ghc/2003-January/016651.html the thread also moved on to ffi(a)haskell.org: http://www.haskell.org/pipermail/ffi/2003-January/001041.html and be sure to check out the paper by Hans Boehm referenced in that message, it's a good summary of the issues involved. > Also, while we're talking about this, maybe the semantics > of performGC should be clarified. Does it block until > all GC (and finalisation of garbage ForeignPtrs) is complete? > I would guess this was the original intention, but this > doesn't seem to be consistent with non-stop Haskell. If > it does block, are all Haskell threads blocked, or just > the calling thread? performGC doesn't do anything that you can rely on :-) In practice, it probably starts all the finalizers that are ready to run, but it certainly doesn't wait for their termination. > Also, I could you explain what you mean by a suitable > exception handler? I don't really understand this at all. > I'd expected I may well end up using bracket or similar, > but I'm not sure how exception handling is relevant to > this problem. Start your program something like this: import Control.Exception (finally) main = my_main `finally` clean_up my_main = ... put your program here ... clean_up = ... all the cleanup code goes here ... You can additionally use finalizers to perform incremental cleanup during program execution, but the right way to clean up at the end is to use an exception handler as above. Cheers, Simon

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ghc-6-2-1.msi broken?
by Gour 28 Dec '03

28 Dec '03

After having problem in building ghc-6.2 from source tarball, I installed ghc-6-2-1.msi build and then tried to compile darcs with it, but, strangely enough, configure script fails when trying to compile "Hello world!" program. I tried to compile: bash-2.05b# cat main.hs module Main(main) where main = putStrLn "Hello world!\n" but ghc only produces main.hi file - no output file "main". The same result within MSYS & MSDOS prompt. However, the same 'program' compiles and produces executable on my Gentoo box. It looks like Win32 build of ghc-6.2 is broken or am I missing something (being still a ghc newbie in the begining stages of learning Haskell :-( Sincerely, Gour -- Gour gour(a)mail.inet.hr Registered Linux User #278493

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ghc 6.2 build problem (MinGW/MSYS)
by Gour 28 Dec '03

28 Dec '03

I'm pretty new with GHC and I have prblem building ghc 6.2 with MinGW compiler in MSYS environment. After running "configure --prefix=/mingw ; make" make fails with: [snip] copying ./mpn/generic/gmp-mparam.h to gmp-mparam.h gcc -E -mno-cygwin -undef -traditional -I../includes -x c -DHAVE_LIBMINGWEX package.conf.in \ | sed 's/^#.*$//g' >package.conf.inplace gcc -E -mno-cygwin -undef -traditional -I../includes -DINSTALLING -x c -DHAVE_LIBMINGWEX package.conf.in \ | sed 's/^#.*$//g' >package.conf.installed ../utils/ghc-pkg/ghc-pkg-inplace --update-package <package.conf.inplace ../utils/ghc-pkg/ghc-pkg-inplace: /ghc/utils/ghc-pkg/ghc-pkg.exe: No such file or directory ../utils/ghc-pkg/ghc-pkg-inplace: exec: /ghc/utils/ghc-pkg/ghc-pkg.exe: cannot execute: No such file or directory make[2]: *** [../driver/stamp-pkg-conf-rts] Error 126 make[1]: *** [boot] Error 1 make[1]: Leaving directory `/home/gour/ghc-6.2/ghc' make: *** [build] Error 1 I have tried to modify the script by changing paths, but then I broke another thing. The same source tarball builds normally under Gentoo Linux with appropriate ebuild. Question: does anyone built ghc 6.2 with MinGW & MSYS (via configure)? (Since on Win98 I compile everything with MinGW/MSYS and install all the packages under /mingw tree,I'd like to do the same with ghc, and not to use binary installer since it comes with its own Perl ...) I'm using binary ghc-6.0.1 in building process. Sincerely, Gour -- Gour gour(a)mail.inet.hr Registered Linux User #278493

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