June 2010 - Haskell-Cafe

Reminder: Deadline to submit talks on your commercial use of Haskell to CUFP in 7 days
by Don Stewart 07 Jun '10

07 Jun '10

This is a final call for speakers at CUFP 2010 -- the Commercial Users of FP Workshop, in Baltimore. CUFP is a chance for you to talk about your use of functional programming in a commercial setting -- and we'd love to see some Haskell speakers, given the recent spike in recruiting :-) You can see the fine videos from last year's CUFP on the all new CUFP site: http://cufp.org Time to stand up and be counted! -- Don ------------------------------------------------------------------------ 2010 Call for Presentations http://cufp.org Sponsored by SIGPLAN Co-located with ICFP 2010 Baltimore, MD USA Oct 1-2 Submission Deadline 15 June 2010 Functional programming languages have been a hot topic of academic research for over 35 years, and have seen an ever larger practical impact in settings ranging from tech startups to financial firms to biomedical research labs. At the same time, a vigorous community of practically-minding functional programmers has come into existence. CUFP is designed to serve this community. The annual CUFP workshop is a place where people can see how others are using functional programming to solve real world problems; where practitioners meet and collaborate; where language designers and users can share ideas about the future of their favorite language; and where one can learn practical techniques and approaches for putting functional programming to work. Giving a CUFP Talk If you have experience using functional languages in a practical setting, we invite you to submit a proposal to give a talk at the workshop. We're looking for two kinds of talks: Experience reports are typically 25 minutes long, and aim to inform participants about how functional programming plays out in real-world applications, focusing especially on lessons learned and insights gained. Experience reports don't need to be highly technical; reflections on the commercial, management, or software engineering aspects are, if anything, more important. You do not need to submit a paper! Technical talks are expected to be 30-45 minutes long, and should focus on teaching the audience something about a technical technique or methodology, from the point of view of someone who has seen it play out in practice. These talks could cover anything from techniques for building functional concurrent applications, to managing dynamic reconfigurations, to design recipes for using types effectively in large-scale applications. While these talks will often be based on a particular language, they should be accessible to a broad range of functional programmers. If you are interested in offering a talk, or nominating someone to do so, send an e-mail to francesco(at)erlang-consulting(dot)com or yminsky(at)janestreet(dot)com by 15 June 2010 with a short description of what you'd like to talk about or what you think your nominee should give a talk about. Such descriptions should be about one page long. There will be no published proceedings, as the meeting is intended to be more a discussion forum than a technical interchange. Program Committee Francesco Cesarini, Erlang Training and Consulting (Co-Chair) Tim Dysinger, Sonian Networks Alain Frisch, LexiFi Nick Gerakines, Chegg Adam Granicz, IntelliFactory Amanda Laucher Romain Lenglet, Google Japan Yaron Misky, Jane Street (Co-Chair) Mary Sheeran, Chalmers Don Stewart, Galois Dean Wampler, DRW Trading More information For more information on CUFP, including videos of presentations from previous years, take a look at the CUFP website at http://cufp.org. Note that presenters, like other attendees, will need to register for the event. Presentations will be video taped and presenters will be expected to sign an ACM copyright release form. Acceptance and rejection letters will be sent out by July 15th. Note that ICFP proper is on Sep. 27-29th.1G

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Span function
by R J 07 Jun '10

07 Jun '10

Can someone provide a hand calculation of: span (< 0) [-1, -2, -3, 0, 1, 2, -3, -4, -5]? I know the result is ([-1, -2, -3], [0, 1, 2, -3, -4, -5]), but the recursion flummoxes me. Here's the Prelude definition: mySpan :: (a -> Bool) -> [a] -> ([a], [a]) mySpan _ [] = ([], []) mySpan p xs@(x:xs') | p x = (x:ys, zs) | otherwise = ([], xs) where (ys, zs) = mySpan p xs' Thanks. _________________________________________________________________ Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

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How to name a mapped function?
by Martin Drautzburg 07 Jun '10

07 Jun '10

Hello all, I like some of the naming conventions in haskell quite a lot, like calling a list of something "xs", or function which takes a function as a parameter "..By" as in sortBy or groupBy. If I have a function, say "compute" whose last parameter is some value ... and I create another function, which applies "compute" to a list of values, how would I call this function? I was tempted to use "all", but my original function already returns a list, so this would be misleading. Also note that the mapped version, does some additional things (remove duplicates), otherwise a new function would hardly be justified. -- Martin

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Issue with "connectTo" function in "network" package
by aditya siram 07 Jun '10

07 Jun '10

Hi all, I had an issue where using the connectTo [1] function would fail to connect to "localhost" if my wireless card was turned off. The moment I turned on my wireless connection it worked. But if I use connectTo with "127.0.0.1" [2] with my network off it works. I confirmed that "localhost' resolves to "127.0.0.1" by pinging "localhost" from command line with the Internet off. Is this a bug in "connectTo"? -deech [1] connectTo "localhost" (PortNumber some_number) [2] connectTo "127.0.0.1" (PortNumber some_number)

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RE: [Haskell-cafe] GHC Api typechecking
by Phyx 07 Jun '10

07 Jun '10

Hi, I checked out how Hint is doing it, but unfortunately they're calling a function in the GHC api's interactive part to typecheck a single statement, much like :t in ghci, So I can't use it to typecheck whole modules. I've tried working around not being able to construct a TargetId but ran into another wall. I can't find anyway to do dependency analysis on the in-memory target, so the dependency graph would be empty which is ofcourse a big problem. Does anyone know if Leksah uses the GHC api for typechecking? And if it only gives type errors after you save a file? The code I've been trying is typeCheckStringOnly :: String -> IO (ApiResults Bool) typeCheckStringOnly contents = handleSourceError processErrors $ runGhc (Just libdir) $ do buffer <- liftIO $ stringToStringBuffer contents clock <- liftIO getClockTime dflags <- getSessionDynFlags setSessionDynFlags dflags let srcLoc = mkSrcLoc (mkFastString "internal:string") 1 1 dynFlag = defaultDynFlags state = mkPState buffer srcLoc dynFlag parsed = unP Parser.parseModule state pkgId = stringToPackageId "internal" name = mkModuleName "Unknown" mod' = mkModule pkgId name location = ModLocation Nothing "" "" summary = ModSummary mod' HsSrcFile location clock Nothing [] [] "" dynFlag Nothing (\a->setSession $ a { hsc_mod_graph = [summary] }) =<< getSession case parsed of PFailed _ _ -> return $ ApiOk False POk newstate mdata -> do let module' = ParsedModule summary mdata check <- typecheckModule module' return $ ApiOk True this fails with a ghc panic : panic! (the 'impossible' happened) (GHC version 6.12.1 for i386-unknown-mingw32): no package state yet: call GHC.setSessionDynFlags Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug :( Cheers, Phyx -----Original Message----- From: Gwern Branwen [mailto:gwern0@gmail.com] Sent: Saturday, April 17, 2010 20:59 To: Phyx Subject: Re: [Haskell-cafe] GHC Api typechecking On Sat, Apr 17, 2010 at 1:49 PM, Phyx <lonetiger(a)gmail.com> wrote: > Hi all, I was wondering if someone knows how to do the following: > > > > I’m looking to typecheck a string using the GHC Api, where I run into > problems is that I need to construct a Target, but the TargetId only seem to > reference physical files. > > > > Ofcourse I can write the string to a file and typecheck that file, but I > would like to do it all in memory and avoid IO if possible. > > > > Does anyone know if this is possible? > > > > For the record I’m trying to create the target as follows > > > > createTarget :: String -> IO Target > > createTarget content = > > do clock <- getClockTime > > buffer <- stringToStringBuffer content > > return $ Target { targetId = TargetModule (mkModuleName > "string:internal") ß problem > > , targetAllowObjCode = True > > , targetContents = Just (buffer,clock) > > } > > > > typeCheckStringOnly :: String -> IO (ApiResults Bool) > > typeCheckStringOnly contents = handleSourceError processErrors $ > > runGhc (Just libdir) $ do > > dflags <- getSessionDynFlags > > setSessionDynFlags dflags > > target <- liftIO $ createTarget contents > > addTarget target > > load LoadAllTargets > > let modName = mkModuleName "string:internal" ß problem again, don’t know > how to create the dependency graph then. > > graph <- depanal [modName] True > > (\a->setSession $ a { hsc_mod_graph = graph }) =<< getSession > > value <- fmap typecheckedSource (typeCheck modName) > > return $ ApiOk True > > > > Cheers, > > Phyx Have you looked at how the Hint package does things? -- gwern

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Is www.haskell.org down?
by DekuDekuplex＠Yahoo.com 06 Jun '10

06 Jun '10

Since yesterday, I have been unable to connect to www.haskell.org. Is the site down? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Reporting a problem with binary-0.5
by Pete Chown 05 Jun '10

05 Jun '10

I've been trying to get in touch with the maintainers of the Binary package, to report an issue. When I emailed the addresses given on Hackage, I got an automated response saying I had used an address that was no longer current. I don't want to put pressure on anyone to fix my bug -- I didn't pay anything for Binary, so it wouldn't be fair for me to have that kind of expectation. At the same time, I don't really want my bug report to go missing just because someone's email address has changed. Does anyone know who I should be talking to? Or is there a bug tracker for the Hackage packages somewhere? I noticed this problem when I ran into some trouble with the network-dns package. It would hang up as soon as I tried to send a query. Eventually I traced the problem to the binary module, and reduced it to this short test case: module Main where import qualified Data.Binary.Get as G import qualified Data.ByteString.Lazy as B main = do urandom <- B.readFile "/dev/urandom" let urandomParser :: G.Get [Int] urandomParser = do v <- G.getWord32be rest <- urandomParser return $ fromIntegral v : rest seeds = G.runGet urandomParser urandom print $ take 4 seeds This code attempts to create an infinite list of random numbers -- a technique also used by network-dns. It turns out that this code works with binary-0.4.4 but not with binary-0.5.0.2. Both were built with ghc-6.12.1 on Ubuntu. (I haven't tested with the intermediate versions of the binary module.) It seems that with binary-0.5.0.2 there is some unwanted strictness; something is evaluated for the whole list, even though it is only the first few elements that are required. Incidentally, if the test case is changed like this: --- get_monad.hs 2010-05-28 11:31:02.399202535 +0100 +++ get_monad2.hs 2010-05-28 13:44:25.515486013 +0100 @@ -1,10 +1,12 @@ module Main where +import Control.Monad + import qualified Data.Binary.Get as G import qualified Data.ByteString.Lazy as B main = do - urandom <- B.readFile "/dev/urandom" + urandom <- liftM (B.take 64) $ B.readFile "/dev/urandom" let urandomParser :: G.Get [Int] urandomParser = do v <- G.getWord32be the program exits with an error: get_monad2.hs: too few bytes. Failed reading at byte position 68 This seems to demonstrate that the program is reading more data than it needs to. Thanks, Pete

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Difference between div and /
by Maciej Piechotka 05 Jun '10

05 Jun '10

I started to wonder what is the difference between div and / so they are 2 separate symbols. div: Take a Integral divide and round (down) (/): Take a Fractional divide and usually round In some applications I would like to use any of those but it is not possible. Is this unification taken into account while reworking numeric classes? I.e. why not: class Num a => Divisable a where (/) :: a -> a -> a class (Real a, Enum a, Divisable a) => Integral a where quot :: a -> a -> a rem :: a -> a -> a div = (/) mod :: a -> a -> a x `quotRem` y = (x `quot` y, x `rem y) x `divMod` y = (x `div` y, x `mod` y) toInteger :: a -> Integer class Divisable a => Fractional a where recip = (1/) :: a -> a fromRational :: Rational -> a (Example does not take into account other refactoring) Regards PS. Why is Fd/cPid etc. Integral or even Num? What does (stdin + stderr) `mod` stdout mean (result will be stdin).

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Proposal filed to GHC, expecting supporters vote: Type supplement for constructor specific uses of sum types
by Gabriel Riba 05 Jun '10

05 Jun '10

Hi! I have filed a new proposal to GHC bugs/whishes process based on the previous discussion here. The proposal address is http://hackage.haskell.org/trac/ghc/ticket/4116#comment:2 ( I had some formatting problems with the first posting, so it is better read at comment:2) Voters admitted: GHC prioritizes bugs/features based on the length of supporters email list at the CC field of the ticket as explained here: http://hackage.haskell.org/trac/ghc/wiki/ReportABug#Ifalreadyreportedvotefo… You may add also any change proposal. Cheers. Gabriel

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Problems with Haskell Platform
by Dominic Steintiz 05 Jun '10

05 Jun '10

I seem to be in some sort package dependency hell (which I thought the Haskell Platform did away with). I install ghc using my package manager (I'm on opensuse). > dom1@fourier:~> ghc --version > The Glorious Glasgow Haskell Compilation System, version 6.12.1 > dom1@fourier:~> cabal --version > cabal-install version 0.8.2 > using version 1.8.0.4 of the Cabal library > dom1@fourier:~> cabal update > Downloading the latest package list from hackage.haskell.org But > dom1@fourier:~> cabal upgrade cabal-install > Resolving dependencies... > Downloading HTTP-4000.0.9... > Configuring HTTP-4000.0.9... > Preprocessing library HTTP-4000.0.9... > Building HTTP-4000.0.9... > > Network/StreamSocket.hs:30:7: > Could not find module `Network.Socket': > There are files missing in the `network-2.2.1.7' package, > try running 'ghc-pkg check'. > Use -v to see a list of the files searched for. > cabal: Error: some packages failed to install: > HTTP-4000.0.9 failed during the building phase. The exception was: > ExitFailure 1 > cabal-install-0.8.2 depends on HTTP-4000.0.9 which failed to install. And trying to install the Haskell Platform via http://hackage.haskell.org/platform/2010.1.0.0/cabal/haskell-platform-2010.… > dom1@fourier:~> cabal install > Resolving dependencies... > cabal: dependencies conflict: ghc-6.12.1 requires directory ==1.0.1.1 > however > directory-1.0.1.1 was excluded because ghc-6.12.1 requires directory > ==1.0.1.0 Which is somewhat baffling. So I uninstalled ghc via the package manager and downloaded the linux binaries at http://haskell.org/ghc/dist/6.12.1/ghc-6.12.1-i386-unknown-linux-n.tar.bz2 But sadly > dom1@fourier:~/Downloads/ghc-6.12.1> ./configure > checking for path to top of build tree... ./configure: line 1687: > utils/ghc-pwd/ghc-pwd: cannot execute binary file > configure: error: cannot determine current directory And indeed it is true > dom1@fourier:~/Downloads/ghc-6.12.1> utils/ghc-pwd/ghc-pwd > bash: utils/ghc-pwd/ghc-pwd: cannot execute binary file I'm not sure where to go from here. I think my options are a) build ghc from source or b) find the source for ghc-pwd and compile it and then try configure again. Any help would be appreciated. Dominic.

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