- Haskell-Cafe - Haskell.org

After `readFile`, why cannot i `writeFile` ?
by zaxis 08 Jun '10

08 Jun '10

hitSSQ :: String -> [Int] -> IO () hitSSQ no hitNum = do let newNum = unwords $ [no] ++ map (\n -> show n) hitNum hitNums <- fmap lines $ readFile "test.txt" writeFile "test.txt" $ unlines $ hitNums ++ [newNum] *** Exception: test.txt: openFile: resource busy (file is locked) Sincerely! ----- e^(π⋅i) + 1 = 0 -- View this message in context: http://old.nabble.com/After-%60readFile%60%2C-why-cannot-i-%60writeFile%60-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 4

How to set a GHC option for a single module and a specific version of GHC?
by Bernie Pope 08 Jun '10

08 Jun '10

Hi, I'm looking for a way to set a GHC compile option on a specific module (not every module in the program) but only for a specific version of GHC. Ideally within the confines of cabal, and in a portable way. GHC provides the OPTIONS_GHC pragma, but it does not appear to provide a way for the pragma to fire for specific versions of GHC. Also, I can't use an #ifdef trick because "File-header pragmas are read once only, before pre-processing the file (e.g. with cpp)." Cabal provides conditional statements and a way to check the version of GHC, but I can't see a way to get it to only use certain compiler options for a particular module. In summary, what I really want is a modified version of the OPTIONS_GHC pragma: {-# OPTIONS_GHC ==6.12.1 ... #-} or a way to tell cabal to use certain compiler options for a particular module under certain conditions. Cheers, Bernie.

3 2

A question on existential types and Church encoding
by Cory Knapp 08 Jun '10

08 Jun '10

Hello, A professor of mine was recently playing around during a lecture with Church booleans (I.e., true = \x y -> x; false = \x y -> y) in Scala and OCaml. I missed what he did, so I reworked it in Haskell and got this: >type CB a = a -> a -> a >ct :: CB aC >ct x y = x >cf :: CB a >cf x y = y >cand :: CB (CB a) -> CB a -> CB a >cand p q = p q cf >cor :: CB (CB a) -> CB a -> CB a >cor p q = p ct q I found the lack of type symmetry (the fact that the predicate arguments don't have the same time) somewhat disturbing, so I tried to find a way to fix it. I remembered reading about existential types being used for similar type-hackery, so I added quantification to the CB type and got >type CB a = forall a . a -> a -> a >ctrue :: CB a >ctrue x y = x >cfalse :: CB a >cfalse x y = y >cand :: CB a -> CB a -> CB a >cand p q = p q cfalse >cor :: CB a -> CB a -> CB a >cor p q = p ctrue q which works. But I haven't the faintest idea why that "forall" in the type makes things work... I just don't fully understand existential type quantification. Could anyone explain to me what's going on that makes the second code work? Thanks, Cory

7 10

ANN: random-fu 0.1.0.0
by mokus＠deepbondi.net 08 Jun '10

08 Jun '10

Announcing the 0.1.0.0 release of the "random-fu" library for random number generation[1]. This release hopefully stabilizes the core interfaces (those exported from the base module "Data.Random"). Warning to anyone upgrading from earlier releases: 'Discrete' has been renamed 'Categorical', the entropy source classes have been redesigned, and many things are no longer exported from the root module "Data.Random" (In particular, DevRandom - this is not available on windows, so it will likely move to its own package eventually so that client code dependencies on it will be made explicit). Unfortunately, Hackage appears to have choked on some of the package's dependencies (specifically, 2 dependencies also depend on time, and were built using different versions) so its documentation (which I put quite a bit of work into) is not displayed on the Hackage site. In the past I have dealt with that by uploading new versions with hacks to make sure the thing builds, but I really would rather not continue to do so. Is there any procedure by which I can request a manual rebuild of the package so that its documentation will be generated and displayed? Incidentally, this is a recurring problem I have run into several times for several packages. Can we *please* come up with a way for sdist or similar to just include pre-built documentation? Or if I were to spend some time working on such a thing, would it be accepted (assuming it was done up to all applicable standards of quality)? For now, I have added some pre-built haddock docs to the repository so that they may be browsed online[2] (if code.haskell.org ever starts responding to my HTTP requests. It's just not my day today, I guess). [1] http://hackage.haskell.org/package/random-fu-0.1.0.0 [2] http://code.haskell.org/~mokus/random-fu/doc/haddock/index.html

5 12

Re: [Haskell-cafe] Issue with "connectTo" function in "network" package
by aditya siram 07 Jun '10

07 Jun '10

Doing (1) solves my problem. What did I do and why did it work? Thanks a lot for your help! -deech On 6/4/10, Roman Cheplyaka <roma(a)ro-che.info> wrote: > Confirm if any of the following solves the problem: > > 1. commenting out (by prepending putting hash sign '#' in the beginning > of the line) the following line in /etc/hosts: > > ::1 localhost ip6-localhost ip6-loopback > > 2. executing as root the following command: > > rmmod ipv6 > > * aditya siram <aditya.siram(a)gmail.com> [2010-06-04 13:51:29-0500] >> Here's my GHCI session (with my network card off): >> Prelude Network Network.Socket> connectTo "localhost" (PortNumber 3000) >> *** Exception: connect: does not exist (Connection refused) >> Prelude Network Network.Socket> connectTo "127.0.0.1" (PortNumber 3000) >> {handle: <socket: 7>} >> >> The OS is Ubuntu (Lucid) Linux. The contents of "/etc/hosts" and >> "/etc/nsswitch.conf" are: >> >> > cat /etc/nsswitch.conf >> # /etc/nsswitch.conf >> # >> # Example configuration of GNU Name Service Switch functionality. >> # If you have the `glibc-doc-reference' and `info' packages installed, >> try: >> # `info libc "Name Service Switch"' for information about this file. >> >> passwd: compat >> group: compat >> shadow: compat >> >> hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4 >> networks: files >> >> protocols: db files >> services: db files >> ethers: db files >> rpc: db files >> >> netgroup: nis >> >> > cat /etc/hosts >> 127.0.0.1 localhost >> 127.0.1.1 deech-mac-ubuntu >> >> # The following lines are desirable for IPv6 capable hosts >> ::1 localhost ip6-localhost ip6-loopback >> fe00::0 ip6-localnet >> ff00::0 ip6-mcastprefix >> ff02::1 ip6-allnodes >> ff02::2 ip6-allrouters >> ff02::3 ip6-allhosts >> >> >> -deech >> >> On 6/4/10, Roman Cheplyaka <roma(a)ro-che.info> wrote: >> > * aditya siram <aditya.siram(a)gmail.com> [2010-06-03 22:30:33-0500] >> >> 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) >> > >> > How exactly did it fail? Did it throw an exception? If so, what was the >> > exception? >> > >> > Which operating system did you observe that under? >> > >> > If that was Linux, can you show the contents of the following files on >> > that system: >> > >> > /etc/nsswitch.conf >> > /etc/hosts >> > >> > -- >> > Roman I. Cheplyaka :: http://ro-che.info/ >> > "Don't let school get in the way of your education." - Mark Twain >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > -- > Roman I. Cheplyaka :: http://ro-che.info/ > "Don't let school get in the way of your education." - Mark Twain >

2 1

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

1 0

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:…

2 1

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

5 4

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)

3 4

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

2 3