July 2007 - Libraries

Overflow in Ratio
by David Benbennick 22 Jul '07

22 Jul '07

The Ratio data type suffers from numeric overflow. For example, consider the Ord declaration in the Haskell 98 Report (I'm looking at page 153, section 12.1 of http://haskell.org/definition/haskell98-report.pdf) (x:%y) <= (x':%y') = x * y' <= x' * y It is easy to see that Ratio Int is not well-ordered under this definition, because of overflow. This is a self-contradiction in the Report, since page 84 (section 6.3.2) says that "The Ord class is used for totally ordered datatypes". I think the best way to resolve this contradiction is to avoid overflow. For example, the following definition works: (x:%y) <= (x':%y') = (toInteger x) * (toInteger y') <= (toInteger x') * (toInteger y) I submitted a complete patch at http://hackage.haskell.org/trac/ghc/ticket/1517, but that was apparently the wrong place, since the bug was closed as wontfix. Is there a chance of getting this contradiction resolved? I'll be happy to provide unit tests for the change if that will help get it applied.

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Typeable<n> classes
by Matthew Pocock 21 Jul '07

21 Jul '07

Hi, I've been trying to use `deriving (Typeable, Data)' on some datatypes with arity 8 and 15, but GHC has been complaining. There aren't any Typeable<n> classes defined in Data.Typeable that go beyond 7. Is there a reason for this particular limit? If not, could it be extended further e.g. to 15? Is this likely to let GHC derive Typeable instances for my data types? Thanks, Matthew

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Re: Data.Map.updateAt problem
by Andriy Palamarchuk 21 Jul '07

21 Jul '07

Bertram, Christian, thank you for help. Can somebody apply the patch posted by Bertram in the previous message? Please let me know if it is better to submit a ticket for it. Here are the relevant changes: [fix Data.Map.updateAt Bertram Felgenhauer <int-e(a)gmx.de>**20070718150340 See http://haskell.org/pipermail/libraries/2007-July/007785.html for a piece of code triggering the bug. updateAt threw away parts of the tree making up the map. ] { hunk ./Data/Map.hs 526 - LT -> updateAt f i l - GT -> updateAt f (i-sizeL-1) r + LT -> balance kx x (updateAt f i l) r + GT -> balance kx x l (updateAt f (i-sizeL-1) r) } Andriy ____________________________________________________________________________________ Park yourself in front of a world of choices in alternative vehicles. Visit the Yahoo! Auto Green Center. http://autos.yahoo.com/green_center/

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Proposal: expose the drive functions in the filepath package
by Ian Lynagh 19 Jul '07

19 Jul '07

Hi all, This is proposal 1535: http://hackage.haskell.org/trac/ghc/ticket/1535 This comes out of http://hackage.haskell.org/trac/ghc/ticket/1490 where, in createDirectoryIfMissing, we use stat to determine if a directory exists. (I don't think catching an exception is a possibility in this case, as mkdir returns EEXIST if "pathname already exists (not necessarily as a directory)", so we can't distinguish between the directory already existing and a file existing with the name that we want. Short of doing something ugly like trying to make (path ++ "/.") and seeing if we get ENOTDIR, anyway). The problem is that mingw's stat succeeds for C:\ and C:\foo but fails for C: and C:\foo\. The current code strips a trailing \ before calling stat, and thus fails if asked to create C:\ (which we always do if "make parents" is true, and of course this path could be given explicitly by a user). What we want is something like if isDrive x then addTrailingPathSeparator x else dropTrailingPathSeparator x except filepath currently doesn't export isDrive, for reasons I'm not entirely clear on. Thus I propose exposing the *Drive functions from the filepath package. The patch, attached to the bug, is rather trivial as the code is already written, tested etc, just not exposed. Let's try until 21 July for a discussion period. Please try to accompany any objections with a (clean) alternative solution to the original problem. Thanks Ian

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Proposal: add exeExtension to System.Info
by Dimitry Golubovsky 19 Jul '07

19 Jul '07

Hi, While adopting my older program to the newer (GHC 6.6) infrastructure, I came across the issue: OS-dependent functions like exeExtension are missing in the newer version of the FilePath package. I checked with Neil who currently maintains FilePath, and he agreed with me on this idea. I am proposing to add the following to the System.Info module (which seems to be the best place to collect OS-specific things) - the code was found in Distribution.Compat.FilePath and slightly modified (#if directive) in the same way as the current System.FilePath distinguishes between Posix and Windows interfaces: ------------------------------- begin -------------------------------------- -- | Extension for executable files -- (typically @\"\"@ on Unix and @\"exe\"@ on Windows or OS\/2) exeExtension :: String #if defined(mingw32_HOST_OS) || defined(__MINGW32__) exeExtension = "exe" #else exeExtension = "" #endif -- | Extension for object files. For GHC and NHC the extension is @\"o\"@. -- Hugs uses either @\"o\"@ or @\"obj\"@ depending on the used C compiler. objExtension :: String objExtension = "o" -- | Extension for dynamically linked (or shared) libraries -- (typically @\"so\"@ on Unix and @\"dll\"@ on Windows) dllExtension :: String #if defined(mingw32_HOST_OS) || defined(__MINGW32__) dllExtension = "dll" #else dllExtension = "so" #endif ------------------------------- end -------------------------------------- Thanks -- Dimitry Golubovsky Anywhere on the Web

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Data.Map.updateAt problem
by Andriy Palamarchuk 18 Jul '07

18 Jul '07

I'm currently improving Data.Map documentation. Behavior of updateAt in this module differs from what I would expect. The documentation says: updateAt :: (k -> a -> Maybe a) -> Int -> Map k a -> Map k a O(log n). Update the element at index. Calls error when an invalid index is used. So I expected this code: let m = fromList [(3,"b"),(5,"a")] let f k a = Just "x" updateAt f 1 m to return fromList [(3,"b"),(5,"x")] but it returns fromList [(5,"x")] deleting the key 3. I tested this using ghci 6.6. Is this a bug or I missed something here? Thanks, Andriy ____________________________________________________________________________________ Fussy? Opinionated? Impossible to please? Perfect. Join Yahoo!'s user panel and lay it on us. http://surveylink.yahoo.com/gmrs/yahoo_panel_invite.asp?a=7

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Control.Concurrent.Chan: how do i close a channel?
by Claus Reinke 18 Jul '07

18 Jul '07

in principle, Chan looks like a nice way to do pipes within haskell code, or to avoid unsafeInterleaveIO, in combination with getChanContents (which explicitly mentions hGetContents in its API doc). however, i seem to be missing a way to close a channel? am i misinterpreting this library? claus

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ANNOUNCE: Haskell Hackathon 07 II: Freiburg: Oct 5-7
by dons＠cse.unsw.edu.au 17 Jul '07

17 Jul '07

------------------------------------------------------------------------ Hac 2007 II Haskell Hackathon 2007 II October 5-7, 2007 Freiburg, Germany http://haskell.org/haskellwiki/Hac_2007_II ------------------------------------------------------------------------ We are pleased to announce the 2nd Haskell Hackathon for 2007! The event will be held over 3 days, October 5-7 2007, at the University of Freiburg, in Germany, after ICFP. The plan is to hack on serious Haskell infrastructure, tools, libraries and compilers. To attend please register, and get ready to hack those lambdas! Code to hack on: * Hackage * Cabal * Porting foreign libraries * Bug squashing * You decide! At the last Hackathon, in January at Oxford, resulted in: * Cabal, Hackage, Haddock improvements * Data.Binary and DeferedBinary libraries * GHCi debugger improvements * ghc-api interface from Emacs * Crypto lirbary improvments * A native Haskell 'tar' implementation * And more And we hope to be similarly productive this time around. Before you attend, do start thinking and familiarising yourself with 1 or 2 projects you wish to work on, to ensure no wasted effort during the Hackathon. A list of possible projects is available on the website == Registration: We ask that you register you interest. Follow the instructions on the registration page: http://haskell.org/haskellwiki/Hac_2007_II/Register Once you've registered, do add your info to the attendees self-organising page, http://haskell.org/haskellwiki/Hac_2007_II/Attendees if you are looking to share costs, or meet up prior to the hackathon, with other attendees. N.B. if you already expressed interest via the wiki, do confirm by registering `officially' anyway. == Important dates: Hackathon: October 5-7, 2007 == Organisers: Duncan Coutts Ian Lynagh Don Stewart With local arrangements courtesy: Stefan Wehr Phillip Heidegger

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System.Posix.User.getAllUserEntries can be called only once
by Sascha Böhme 16 Jul '07

16 Jul '07

Hello, are there special constraints why the function System.Posix.User.getAllUserEntries of the unix package can be called only once? Beginning with the second call, it only returns the empty list. Thus, this function cannot be used for a long running process which needs to query the user database from time to time, again and again. Thanks, Sascha

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Functor (Either a)
by Conor McBride 16 Jul '07

16 Jul '07

Hi all I read TFM http://haskell.org/ghc/docs/latest/html/libraries/base/ Prelude.html#t%3AFunctor and it does rather suggest that there's an instance of Functor for (Either a). Yet, ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Prelude> fmap not (Right True) <interactive>:1:0: No instance for (Functor (Either a)) arising from use of `fmap' at <interactive>:1:0-20 Am I being dim? Conor

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