- Glasgow-haskell-users

GHCi can't see 'atexit', can't load module
by Maurício CA 13 Sep '09

13 Sep '09

(I left this in cafe, but I think it is actually ghc related.) I just checked it seems not to be possible to load any module in GHCi that uses FFI to wrap the standard C function 'atexit'. When trying that, we get a message saying the symbol 'atexit' can't be found. (This is not a problem when building an executable with ghc, though.) It's easy to reproduce the problem. Just try something like this: ----- module XXX where import Foreign import Foreign.C foreign import ccall atexit :: FunPtr (IO ()) -> IO CInt ----- Load it in ghci (after ':set -XForeignFunctionInterface' to get FFI) and you will get the error. If we replace the line below for the above we don't get any error message: foreign import ccall exit :: CInt -> IO () Here, we just replaced 'atexit' by 'exit', which, according to 'nm' tool, is at the same 'libc' library that 'atexit' belongs to. Thanks, Maurício

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Build error in users guide in HEAD of today
by "Philip K.F. Hölzenspies" 11 Sep '09

11 Sep '09

Dear GHC-ers, I'm trying to put together a virtual machine with a default development for GHC hacking. To test it, I was running "sh validate" in the build tree. There were build errors in the doc/users_guide, however. Google gave me an hpaste page with the error on it, but I couldn't find the referring message. Does this ring a bell for anyone: latex failed users_guide_tmp.tex:1380: Undefined control sequence \Documents. users_guide_tmp.tex:1380: leading text: } users_guide_tmp.tex:1380: Undefined control sequence \user. users_guide_tmp.tex:1380: leading text: } users_guide_tmp.tex:5782: Undefined control sequence \Documents. users_guide_tmp.tex:5782: leading text: ...kurl{C: \Documents~And~Settings\user\ghc} users_guide_tmp.tex:5782: Undefined control sequence \user. users_guide_tmp.tex:5782: leading text: ...kurl{C: \Documents~And~Settings\user\ghc} users_guide_tmp.tex:5782: Undefined control sequence \ghc. users_guide_tmp.tex:5782: leading text: ...kurl{C: \Documents~And~Settings\user\ghc} users_guide_tmp.tex: File ended while scanning use of \hyper@n@rmalise. users_guide_tmp.tex: Emergency stop. Error: latex compilation failed make[2]: *** [docs/users_guide/users_guide.ps] Error 1 make[1]: *** [all_docs/users_guide] Error 2 I understand the LaTeX errors, but I'm not so well versed in docbook. Should the path in the ghci.xml and packages.xml be forward slashed, instead of backslashed or does this indicate there's something wrong with my docbook/latex setup? Even when I change the backslashes to forward slashes, the last error (scanning use of \hyper@n@rmalise) remains. Any ideas? Regards, Philip

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Libraries in the repo
by Simon Marlow 11 Sep '09

11 Sep '09

Simon and I have been chatting about how we accommodate libraries in the GHC repository. After previous discussion on this list, GHC has been gradually migrating towards having snapshots of libraries kept as tarballs in the repo (currently only "time" falls into this category), but I don't think we really evaluated the alternatives properly. Here's an attempt to do that, and to my mind the outcome is different: we really want to stick to having all libraries as separate repositories. Background: * Scope: libraries that are needed to build GHC itself (aka "boot libraries") * Boot libraries are of several kinds: - INDEPENDENT: Independently maintained (e.g. time, haskeline) - COUPLED: Tightly coupled to GHC, but used by others (base) - SPECIFIC: Totally specific to GHC (e.g. template-haskell, DPH) * Most boot libraries are INDEPENDENT. INDEPENDENT libraries have a master repository somewhere separate from the GHC repositories. * We need a branch of INDEPENDENT libraries, so that GHC builds don't break when the upstream package is modified. * Sometimes we want to make local modifications to INDEPENDENT libraries: - when GHC adds a new warning, we need to fix instances of the warning in the library to keep the GHC build warning-free. - to check that the changes work, before pushing upstream Choices for how we deal with libraries in the GHC repository: (+) is a pro, (-) is a con. (1) Check out the library from a separate repo, using the darcs-all script. The repo may either be a GHC-specific branch [INDEPENDENT], or the master copy of the package [SPECIFIC/COUPLED]. (+) we can treat every library this way, which gives a consistent story. Consistency is good for developers. (+) [INDEPENDENT] makes it easy to push changes upstream and sync with the upstream repo (unless upstream is using a different VCS). (-) [INDEPENDENT] we have to be careful not to let our branches get too far out of sync with upstream, and we must sync before releasing GHC. (2) Put a snapshot tarball of the library in libraries/tarballs, but allow you to checkout the darcs repo instead. (-) [SPECIFIC/COUPLED] this approach doesn't really make sense, because we expect to be modifying the library often. (-) updating the snapshot is awkward (-) workflow for making a change to the library is awkward: - checkout the darcs repo - make the change, validate it - push the change upstream (bump version?) - make a new snapshot tarball - commit the new snapshot to the GHC repo. (-) having tarballs in the repository is ugly (-) we have no revision history of the library (3) The GHC repo *itself* contains every library unpacked in the tree. You are allowed to check out the darcs repo instead. (+) atomic commits to both the library and GHC. (+) doing this consistently would allow us to remove darcs-all, giving a nice easy development workflow (-) [INDEPENDENT/COUPLED] still need a separate darcs repo. (-) [INDEPENDENT/COUPLED] pushing changes upstream is hard (-) [INDEPENDENT/COUPLED] manual syncing with upstream (-) [COUPLED] (particularly base) syncing with upstream would be painful. (3) works best for SPECIFIC libraries, whereas (1) works best for INDEPENDENT/COUPLED libraries. If we want to treat all libraries the same, then the only real option is (1). Experience with Cabal and bytestring has shown that (1) can work for INDPENDENT libraries, but only if we're careful not to get too out-of-sync (as we did with bytestring). In the case of Cabal, we never have local changes in our branch that aren't in Cabal HEAD, and that works well. Comments/thoughts? Cheers, Simon

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Resolving overloading loops for circular constraint graph
by Stefan Holdermans 10 Sep '09

10 Sep '09

Manuel, Simon, I've spotted a hopefully small but for us quite annoying bug in GHC's type checker: it loops when overloading resolving involves a circular constraint graph containing type-family applications. The following program (also attached) demonstrates the problem: {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} type family F a :: * type instance F Int = (Int, ()) class C a instance C () instance (C (F a), C b) => C (a, b) f :: C (F a) => a -> Int f _ = 2 main :: IO () main = print (f (3 :: Int)) My guess is that the loop is caused by the constraint C (F Int) that arises from the use of f in main: C (F Int) = C (Int, ()) <= C (F Int) Indeed, overloading can be resolved successfully by "black-holing" the initial constraint, but it seems like the type checker refuses to do so. Can you confirm my findings? I'm not sure whether this is a known defect. If it isn't, I'd be more than happy to issue a report. Since this problem arises in a piece of very mission-critical code, I would be pleased to learn about any workarounds. Thanks in advance, Stefan

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Functor <constant>
by Serge D. Mechveliani 09 Sep '09

09 Sep '09

People, I have data DBit = Bit0 | Bit1 deriving (Eq, Ord, Enum) data BNatural = BNat [DBit] deriving (Eq) and want to apply things like fmap reverse (bn :: BNatural). GHC reports an error on this usage of fmap. It also does not allow instance Functor BNatural where fmap f (BNat ds) = BNat $ f ds, -- it reports that a type constructor for Functor must have kind * -> *. Probaly, GHC agrees with Haskell at this point (?). Now, I deceive the compiler by declaring instead newtype BNatAux a = BNat a deriving (Eq) type BNatural = BNatAux [DBit] instance Functor BNatAux where fmap f (BNat ds) = BNat $ f ds So, a parasitic BNatAux is introduced. I wonder: is there a simpler way to have fmap for BNatural ? Thank you in advance for advice, ----------------- Serge Mechveliani mechvel(a)botik.ru

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fromEnum (c :: Char)
by Serge D. Mechveliani 09 Sep '09

09 Sep '09

People, I need to convert Char <-> Int in a possibly _standard_ way for Haskell -- and also in an efficient way. In particular, it must not spend 100 comparisons in a look through the listing of Char. I use ord :: Char -> Int and chr :: Int -> Char. Is this all right? Thank you in advance for advice, ----------------- Serge Mechveliani mechvel(a)botik.ru

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no backspace, delete or arrow keys in ghci
by Gabriel Schwanen 08 Sep '09

08 Sep '09

Hello to everybody, I'm currently working with Haskell via the ghc interpreter (version 6.10.4 on Ubuntu 9.10). The problem, if it is really a problem and not a normal comportement, happens when I try, as example, to enter some characters after having execute getLine. Actually, if I make a mistake when typing the string, I can't use neither backspace, delete or even arrow keys to correct it before hitting enter to finish input. In fact, as example, if I hit backspace, I get "^?" in place of deleting the previous character ... :^( As I wrote above, I don't know if it is a "bug" or not. Maybe it is normal. Can someone help me ? Thanks in advance, Gabriel

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ghc --help displays obsolete URL
by Ahn, Ki Yung 08 Sep '09

08 Sep '09

$ ghc --help ... The User's Guide has more information about GHC's *many* options. An online copy can be found here: http://www.haskell.org/ghc/documentation.html ------------------------------------------------------------------------ The above URL does not exist! Hope it gets fixed in 6.12.x

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few questions about GHC RTS
by Peter Hercek 07 Sep '09

07 Sep '09

Hi, I thought I may look at the :next command for GHCi debugger again. If I do, I will not make it before 6.12.1 is out. I have few questions about RTS in relation to :next implementation. If they is easy to answer I would appreciate it, if not, don't bother since I'm not sure I'll do it ... and If I decide to do it I may just figure it out myself or fail again :) Why is stg_nofoceIO_info added as last argument to IO actions in unregistered mode? Do I still need to pass it in even when (I think) my IO action does not need it? E.g. is it required for every IO action by some stack walking code or something? Are there any Cmm functions in RTS with signature (IO()) or signature (Int->IO()) which I could check out as examples to see how they are done? ... and ideally also how they are called from the stack. Are there any special things to do when adding a new field (e.g. simulated stack size) to StgTSO? Thanks, Peter.

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Plans for GHC 6.12.1: release candidate 14 September 2009
by Ian Lynagh 26 Aug '09

26 Aug '09

Hi all, This is a summary of our plans for GHC 6.12.1. We are aiming to have the first release candidate out on the 14th September 2009. Until then, we plan to focus on the bugs in the 6.12.1 milestone, marked high priority; they are listed here: http://hackage.haskell.org/trac/ghc/query?status=new&status=assigned&status… If there is a bug not in that list that is important to you, please let us know. Thanks Ian

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