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ANN: cabal-dev 0.7.3.1 -- now with ghci!
by Rogan Creswick 23 Dec '10

23 Dec '10

Cabal-dev is now capable of launching ghci with the project's package database and local modules (if the package under development exposes a library). For example: # First, invoke cabal-dev install the package to populate the # package database: $ cabal-dev install .... <snip> .... $ cabal-dev ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> The ghci shell should have access to all the libraries your application/library is using, as well as any modules that your library exposes. Note that this is not quite as natural as your traditional ghci shell, namely: Source modifications are not visible without exiting, re-issuing `cabal-dev install` *and* `cabal-dev ghci`. This will eventually get better, but that's where things are right now. The reason for this is that `cabal-dev ghci` just issues ghci with the cabal-dev package database (and excluding the user package db, to best reflect what cabal-dev does when it causes compilation). Cabal-dev is available on hackage and github: - http://hackage.haskell.org/package/cabal-dev - https://github.com/creswick/cabal-dev There is a short write-up with a (slightly) longer example here: - http://blog.ciscavate.org/2010/12/cabal-dev-sandboxing-your-haskell-develop… --Rogan

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Behaviour of System.Directory.getModificationTime
by Arnaud Bailly 23 Dec '10

23 Dec '10

Hello, I am writing a program which scans a bunch of directories to detect changes (added, modified, deleted files). To detect modification, I would like to use getModification, doing something like the following: -- |Stores last modified timestamp of the file -- Yes, I come from Java... type HashMap = M.Map FilePath ClockTime data (Show a, Eq a) => Edit a = Mod a | Add a | Del a deriving (Eq,Show) checkChanges :: [String] -> HashMap -> IO ([Edit FilePath], HashMap) checkChanges [fs] m = do addedFilesList <- lsRecursive fs timestamps <- mapM getModificationTime addedFilesList let allts = zip addedFilesList timestamps let ret = (findDeletedFiles allts.updateScannedFiles allts) ([],m) return ret -- | Returns an updated map and a list of modified/added/deleted files updateScannedFiles :: [(FilePath,ClockTime)] -> ([Edit FilePath], HashMap) -> ([Edit FilePath], HashMap) updateScannedFiles [] r = r updateScannedFiles ((path,ts):files) (updates,m) = case M.lookup path m of Nothing -> updateScannedFiles files ((Add path:updates), M.insert path ts m) Just ts' -> if ts' < ts then updateScannedFiles files ((Mod path:updates), M.adjust (const ts) path m) else updateScannedFiles files (updates, m) -- omitting findDeletedFiles which is obvious and works I got the following test case that fails: "recursively marks changed files as modified" `for` do complexFileSetup -- create a small FS tree with 3 files root <- tempDir state <- checkChanges [root] M.empty writeFile (root </> "subdir" </> "cFile.txt") "this is another toast" checkChanges [root] (snd state) >>= (\r -> assertEqual "there should be 1 changed file in 3 files" 1 ((length.modified.fst) r)), In words: the getModificationTime of the overwritten file appears identical before and after the write. What am I doing wrong ? BTW, I checked the content of the file which has been correctly updated. Thanks for your help Arnaud

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$ do?
by Jonathan Geddes 23 Dec '10

23 Dec '10

Quick question: Why do I need the $ in the following bits of code? > main = withSocketsDo $ do > --do something with sockets > foo = fromMaybe 0 $ do > --do something in the maybe monad I don't see (after admittedly only a minute or so thinking about it) where any grammar ambiguities would be if 'do' had an implicit $ in front of it: > foo = fromMaybe 0 do > --do something in maybe Though now that I've written it down, that is hard for me to visually parse at a glance. I'm still curious though. --Jonathan

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Re: [Haskell-cafe] Proof in Haskell
by Patrick Browne 22 Dec '10

22 Dec '10

On 22/12/2010 14:48, Artyom Shalkhakov wrote: > ..Do you want to prove a property of > a function formally, using some kind of formal logic? I am aware that functional languages do not do proofs at term level, but the motivation for my question is to get a precise reason why this is so. The replies from the café have clearly articulated the reasons. Thanks to all, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Darcs failure
by Andrew Coppin 22 Dec '10

22 Dec '10

I'm running a VM with Ubuntu 10.10 (Maverick Meerkat). I installed Darcs 2.4.4 using apt-get, but it keeps CONSTANTLY failing with the error message darcs: bug at src/URL.hs:246 compiled Sep 12 2010 20:24:56 Another possible bug in URL.waitNextUrl: curl_multi_perform() - no running handles See http://wiki.darcs.net/BugTracker/Reporting for help on bug reporting. Does anybody have any clue how I can get the hell rid of this stupid message and actually get some stuff done? (At this point, Darcs has hung, and I have to use the kill command with just the right signal number to make Darcs exit. Ctrl+C just tells me "Couldn't handle interrupt since darcs was in a sensitive job." So every single time this error happens, I have to get a process list, find Darcs, issue the kill command, and then reissue the original Darcs command and hope it actually completes this time...)

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Identity type
by Pierre-Etienne Meunier 22 Dec '10

22 Dec '10

Hi, Is there something like an identity type, transparent to the type-checker, in haskell ? For instance, I'm defining an interval arithmetic, with polynomials, matrices, and all that... defined with intervals. The types are : Polynomial Interval (instead of Polynomial Double for instance) Matrix Interval etc. Now I'd like to be able to define some typeclass "intervalize" that would convert a IEEE-754 version of my types into the interval ones : class Intervalize a where intervalize::a Double-> a Interval Wouldn't it be very natural wanting to use this class on Doubles directly : intervalize' :: Double->Interval intervalize'=intervalize Does this exist in haskell ? in GHC ? Cheers, Pierre

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Missing Parsec library in latest stable GHC
by Aaron Gray 22 Dec '10

22 Dec '10

Missing Parsec library :- scheme.o(.text+0x4fa):fake: undefined reference to `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziCombinator_skipMany1_closure' scheme.o(.text+0x501):fake: undefined reference to `parseczm2zi1zi0zi0_TextziPaserCombinatorsziParsecziChar_space_closure' scheme.o(.text+0x5c2):fake: undefined reference to `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziChar_oneOf_closure' scheme.o(.text+0x63a):fake: undefined reference to `mtlzm1zi1zi0zi0_ControlziMonadziTrans_zdf1_closure' GHC from :- http://www.haskell.org/ghc/dist/stable/dist/ ghc-7.0.1.20101221-i386-windows.exe<http://www.haskell.org/ghc/dist/stable/dist/ghc-7.0.1.20101221-i386-windows…> and earlier. Aaron

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Re: [Haskell-cafe] Missing Parsec library in latest stable GHC
by Antoine Latter 22 Dec '10

22 Dec '10

ghc doesn't, by default, go searching for packages to link in to the resulatant executable.. If you try 'ghc --make scheme.hs' your example will work better. You can also specify what to link manually, but --make works pretty well most of the time. In ghc version 7 '--make' is the default, but until that's more widely distributed you'll want to get used to using the switch. Take care, Antoine On Wed, Dec 22, 2010 at 10:36 AM, Aaron Gray <aaronngray.lists(a)gmail.com> wrote: > On 22 December 2010 16:27, Antoine Latter <aslatter(a)gmail.com> wrote: >> >> What commands did you enter to produce this error? >> > > ghc scheme.hs > Aaron > >> >> Ahanks, >> Antoine >> >> On Wed, Dec 22, 2010 at 9:54 AM, Aaron Gray <aaronngray.lists(a)gmail.com> >> wrote: >> > Missing Parsec library :- >> > scheme.o(.text+0x4fa):fake: undefined reference to >> > >> > `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziCombinator_skipMany1_closure' >> > scheme.o(.text+0x501):fake: undefined reference to >> > `parseczm2zi1zi0zi0_TextziPaserCombinatorsziParsecziChar_space_closure' >> > scheme.o(.text+0x5c2):fake: undefined reference to >> > `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziChar_oneOf_closure' >> > scheme.o(.text+0x63a):fake: undefined reference to >> > `mtlzm1zi1zi0zi0_ControlziMonadziTrans_zdf1_closure' >> > GHC from :- >> > http://www.haskell.org/ghc/dist/stable/dist/ >> > ghc-7.0.1.20101221-i386-windows.exe >> > and earlier. >> > Aaron >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> > > >

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Re: [Haskell-cafe] Functor => Applicative => Monad
by Isaac Dupree 22 Dec '10

22 Dec '10

On 12/14/10 03:13, John Smith wrote: > I would like to formally propose that Monad become a subclass of > Applicative, with a call for consensus by 1 February. The change is > described on the wiki at > http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal, That page isn't written as a proposal yet, it's written as a bunch of ideas. I would be happy to see something along the lines of Bas van Dijk's work http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/14740 . -Isaac

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Rendering of <hask> in new wiki (MSIE6)
by Dimitry Golubovsky 22 Dec '10

22 Dec '10

Hi, In MSIE6, <hask> tags are rendered like this (from the Monad_Transformers page): transformers: provides the classes MonadTrans and MonadIO , as well as concrete monad transformers such as StateT ... etc. The Wiki source: [http://hackage.haskell.org/package/transformers transformers]: provides the classes <hask>MonadTrans</hask> and <hask>MonadIO</hask>, as well as concrete monad transformers such as <hask>StateT</hask>. HTML (a small piece of it): provides the classes <div class="inline-code"><div dir="ltr" style="text-align: left;"><div class="source-haskell" style="font-family: monospace;">MonadTrans</div></div></div> Words MonadTrans, MonadIO, StateT etc are enclosed in <hask> tags. They show up in monospace, each starting a new line. Is this only MSIE6, or this is how it is supposed to render? Thanks. PS I am not attaching a screenshot to the mailing list; if anyone needs to see it I'll send via personal e-mail. -- Dimitry Golubovsky Anywhere on the Web

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