- Haskell-Cafe - Haskell.org

Function to find a substring
by R J 08 Jun '10

08 Jun '10

What's an elegant definition of a Haskell function that takes two strings and returns "Nothing" in case the first string isn't a substring of the first, or "Just i", where i is the index number of the position within the first string where the second string begins? _________________________________________________________________ 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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is there a way to prove the equivalence of these two implementations of (Prelude) break function?
by Thomas Hartman 08 Jun '10

08 Jun '10

Here's two implementations of break, a snappy one from the prelude, and a slow stupid stateful one. They are quickchecked to be identical. Is there a way to prove they are identical mathematically? What are the techniques involved? Or to transform one to the other? import Control.Monad.State import Test.QuickCheck tThis = take 5 . show . mybreak (>4000000) $ [1..10^7] tPrel = take 5 . show . prelbreak (>4000000) $ [1..10^7] prelbreak p xs = (takeWhile (not . p) xs,dropWhile (not . p) xs) -- fast, more or less as implemented in prelude iiuc mybreak p xs = evalState (brk p) ([],xs) -- stateful, slow brk p = do (notsat,remaining) <- get case remaining of [] -> return (notsat,remaining) (r:rs) -> if p r then return (notsat,remaining) else do put (notsat++[r],rs) -- ++ is probaby a major reason brk p

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Fwd: [Gtk2hs-users] Installation problem with cairo.
by Magicloud Magiclouds 08 Jun '10

08 Jun '10

Hi, I cannot subscribe to libraries maillist. So I forward this here. I cannot install gtk2hs-cairo by cabal using cairo 1.9.6 compiled myself. Cabal always said that the cairo library/devel files were missing. ---------- Forwarded message ---------- From: Axel Simon <Axel.Simon(a)in.tum.de> Date: Tue, Jun 8, 2010 at 7:15 PM Subject: Re: [Gtk2hs-users] Installation problem with cairo. To: Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> Cc: gtk2hs-users <gtk2hs-users(a)lists.sourceforge.net> Hi again, it's very difficult for me to say what is happening. Could you ask on the cabal mailing list to see if anybody there has an idea. Cheers, Axel On 08.06.2010, at 11:01, Magicloud Magiclouds wrote: > I've tried a little, the error was not thrown out in gtk2hssetup.hs. > And even I faked a version (1.8.10), it did not work. > > On Tue, Jun 8, 2010 at 3:47 PM, Axel Simon <Axel.Simon(a)in.tum.de> wrote: >> >> Hi, >> >> On Jun 8, 2010, at 9:21, Magicloud Magiclouds wrote: >> >>> Yes, seems like so. >>> Here is the output: >>> >>> # /usr/bin/pkg-config --modversion cairo-pdf >>> 1.9.6 >>> # /usr/bin/pkg-config --modversion cairo-ps >>> 1.9.6 >>> # /usr/bin/pkg-config --modversion cairo-svg >>> 1.9.6 >>> # /usr/bin/pkg-config --modversion cairo >>> 1.9.6 >>> >> >> Oh dear. >> >> Since cabal manages once to get the correct version and fails later on, I >> assume it's the magic Duncan added to out Gtk2HsSetup.hs file that requests >> the package version again. If you look for >> >> --FIXME: Cabal should tell us the selected pkg-config package versions in >> the >> -- LocalBuildInfo or equivalent. >> -- In the mean time, ask pkg-config again. >> >> in the cabal file and try to add some debugging output then maybe you can >> find out more. >> >> I suspect that either the LocalBuildInfo or the PackageDescription value is >> not the most up-to-date one but some default value that lack some path or >> some such. >> >> cheers, >> Axel >> >>> On Tue, Jun 8, 2010 at 2:56 PM, Axel Simon <Axel.Simon(a)in.tum.de> wrote: >>>> >>>> Hi, >>>> >>>> On Jun 8, 2010, at 8:54, Magicloud Magiclouds wrote: >>>> >>>>> /usr/bin/pkg-config --modversion cairo >>>>> setup: Missing dependency on a foreign library: >>>>> * Missing C library: cairo >>>>> This problem can usually be solved by installing the system package that >>>>> provides this library (you may need the "-dev" version). If the library >>>>> is >>>>> already installed but in a non-standard location then you can use the >>>>> flags >>>>> --extra-include-dirs= and --extra-lib-dirs= to specify where it is. >>>> >>>> well, what's the output of >>>> >>>>> /usr/bin/pkg-config --modversion cairo >>>> >>>> ? Maybe cabal fails to parse the version for some reason. >>>> >>>> Axel >>>> >>>> >>> >>> >>> >>> -- >>> 竹密岂妨流水过 >>> 山高哪阻野云飞 >> >> > > > > -- > 竹密岂妨流水过 > 山高哪阻野云飞 -- 竹密岂妨流水过山高哪阻野云飞

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Updates in the regions library
by Bas van Dijk 08 Jun '10

08 Jun '10

Hi Arie (and others who are interested in the regions library), I would like to let you know that I'm working on a new version of my regions package and its reverse dependencies: regions-monadsfd, regions-monadstf, safer-file-handles, regional-pointers and usb-safe. The major change is that I removed the Resource class: class Resource resource where data Handle resource ∷ * open ∷ resource → IO (Handle resource) close ∷ Handle resource → IO () in favor of a much more simple interface, namely: -- | An 'IO' computation that closes or finalizes a resource. For example -- 'hClose' or 'free'. type CloseAction = IO () -- | A handle to a 'CloseAction' that allows you to duplicate the action to a -- parent region using 'dup'. newtype CloseHandle (r ∷ * → *) -- | Register the 'CloseAction' in the region. When the region terminates all -- registered close actions will be perfomed if they're not duplicated to a -- parent region. register ∷ MonadIO pr ⇒ CloseAction → RegionT s pr (CloseHandle (RegionT s pr)) Here's an example how to use this new interface from my updated regional-pointers package: import qualified Foreign.Marshal.Alloc as FMA -- | A regional handle to memory. This should provide a safer replacement for -- Foreign.Ptr.Ptr data RegionalPtr α (r ∷ * → *) = RegionalPtr (Ptr α) (CloseHandle r) {-| Allocates the given number of bytes and returns a regional pointer to them. This should provide a safer replacement for: Foreign.Marshal.Alloc.mallocBytes. -} mallocBytes ∷ MonadCatchIO pr ⇒ Int → RegionT s pr (RegionalPtr α (RegionT s pr)) mallocBytes size = block $ do ptr ← liftIO $ FMA.mallocBytes size let closeAction = free ptr ch ← register closeAction return $ RegionalPtr ptr ch And here's an example from safer-file-handles: -- | A regional handle to an opened file parameterized by the 'IOMode' in which -- you opened the file and the region in which it was created. data RegionalFileHandle ioMode (r ∷ * → *) = RegionalFileHandle (Handle ioMode) (CloseHandle r) openFile ∷ MonadCatchIO pr ⇒ FilePath → IOMode ioMode → RegionT s pr (RegionalFileHandle ioMode (RegionT s pr)) openFile = openNormal E.openFile -- | Opens a file in binary mode then yields a regional handle to it. This -- provides a safer replacement for System.IO.openBinaryFile. openBinaryFile ∷ MonadCatchIO pr ⇒ FilePath → IOMode ioMode → RegionT s pr (RegionalFileHandle ioMode (RegionT s pr)) openBinaryFile = openNormal E.openBinaryFile openNormal ∷ MonadCatchIO pr ⇒ (FilePath → IOMode ioMode → IO (E.Handle ioMode)) → FilePath → IOMode ioMode → RegionT s pr (RegionalFileHandle ioMode (RegionT s pr)) openNormal open filePath ioMode = block $ do h ← liftIO $ open filePath ioMode let closeAction = sanitizeIOError $ hClose h ch ← register closeAction return $ RegionalFileHandle h ch I haven't released it yet because I need to write some more documentation. However all the repositories contain the new code: darcs get http://code.haskell.org/~basvandijk/code/regions darcs get http://code.haskell.org/~basvandijk/code/regions-monadsfd darcs get http://code.haskell.org/~basvandijk/code/regions-monadstf darcs get http://code.haskell.org/~basvandijk/code/regional-pointers darcs get http://code.haskell.org/~basvandijk/code/safer-file-handles darcs get http://code.haskell.org/~basvandijk/code/usb-safe and some examples: darcs get http://code.haskell.org/~basvandijk/code/usb-safe-examples darcs get http://code.haskell.org/~basvandijk/code/safer-file-handles-examples darcs get http://code.haskell.org/~basvandijk/code/usb-safe-and-safer-file-handles-ex… Regards, Bas On Wed, Jun 2, 2010 at 3:13 PM, Bas van Dijk <v.dijk.bas(a)gmail.com> wrote: > On Wed, Jun 2, 2010 at 2:57 PM, Arie Peterson <ariep(a)xs4all.nl> wrote: >>> On Wed, Jun 2, 2010 at 2:28 PM, Bas van Dijk <v.dijk.bas(a)gmail.com> >> wrote: >>>> Before answering your questions I would like to make sure I understand >>>> your Resource type. When I want to create a memory Resource for >>>> example is the following what you have in mind? >>>> >>>> {-# LANGUAGE Rank2Types #-} >>>> >>>> -- from base: >>>> import Foreign.Ptr ( Ptr ) >>>> import Foreign.Marshal.Alloc ( mallocBytes, free ) >>>> >>>> -- from transformers: >>>> import Control.Monad.IO.Class ( liftIO ) >>>> >>>> -- from MonadCatchIO-transformers: >>>> import Control.Monad.CatchIO ( MonadCatchIO, bracket ) >>>> >>>> newtype Resource cap m = Resource { with :: forall a. (cap -> m a) -> m >>>> a } >>>> >>>> type Memory m a = Resource (Ptr a) m >>>> >>>> memory :: MonadCatchIO m => Int -> Memory m a >>>> memory size = Resource $ bracket (liftIO $ mallocBytes size) (liftIO . >>>> free) >> >> Yes, exactly. I also create type aliases for resources providing a >> specific capability. >> >> On Wed, 2 Jun 2010 14:45:08 +0200, Bas van Dijk <v.dijk.bas(a)gmail.com> >> wrote: >>> The previous can also be generalized using my Resource class: >>> >>> -- from regions: >>> import qualified Control.Resource as C ( Resource(..) ) >>> >>> resource :: (MonadCatchIO m, C.Resource resource) >>> => resource -> Resource (C.Handle resource) m >>> resource r = Resource $ bracket (liftIO $ C.open r) (liftIO . C.close) >> >> Yes, definitely. >> >> (This is not a literal generalisation of the 'memory' function, unless you >> make 'Int' an instance of 'C.Resource'; one would probably create a special >> type 'data Memory = Memory Int' instead. This is the difference I alluded >> to in my earlier email.) > > Indeed. > > My regional-pointers package provides this Memory type and an instance > for Resource: > http://hackage.haskell.org/packages/archive/regional-pointers/0.1.0.2/doc/h… > > Now I'm beginning to understand the problem with your 'fallback' > function. If I specialize the capability to Handles to Memory as in: > > -- from regional-pointers: > import Foreign.Ptr.Region ( Memory ) > > fallback :: Resource (C.Handle (Memory a)) IO > -> Resource (C.Handle (Memory a)) IO > -> Resource (C.Handle (Memory a)) IO > fallback (Resource primary) (Resource backup) = Resource l > where > l c = primary c `catch` (\(SomeException _) -> backup c) > > then the only way to create a value of type 'Resource (C.Handle > (Memory a))' is to apply my previously posted 'resource' function to > 'Memory nrOfBytes'. There's no other way to create Handles to Memory. > Is that your problem? > > The problem is that Handle is an associated _data_ type not an > associated _type synonym_: > > class Resource resource where > data Handle resource ∷ * > open ∷ resource → IO (Handle resource) > close ∷ Handle resource → IO () > > So 'Handle' is an injective type function which means that every > 'Handle resource' has only one 'resource'. > > I could try turning 'Handle' into an associated type synonym. > > Regards, > > Bas >

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PDF generation?
by Jim Tittsler 08 Jun '10

08 Jun '10

What is the easiest way to create PDF files from Haskell? Is gtk2hs's PDF output the preferred way?

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Re: [Haskell-cafe] is there a way to prove the equivalence of these two implementations of (Prelude) break function?
by Alberto G. Corona 08 Jun '10

08 Jun '10

> This isn`t a manifestation of the Curry-Howard isomorphism? > > 2010/6/8 Alexander Solla <ajs(a)2piix.com> > > >> On Jun 7, 2010, at 4:10 PM, Alexander Solla wrote: >> >> >>> You might note how much like evaluating the function generating the >>> analysis is. >>> >> >>

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haskell.org down?
by Ozgur Akgun 08 Jun '10

08 Jun '10

http://downforeveryoneorjustme.com/www.haskell.org -- Ozgur Akgun

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Haddock on HP failing
by Chris Dornan 08 Jun '10

08 Jun '10

Hi, I am trying to use Haddock on the Haskell Platform 2010.1.0.0 and getting haddock.exe: can't find a package database at E:\ghc\ghc-6.12.1\lib\package.conf.d I have tried uninstalling and re-installing HP (just in case) to no avail. Has anybody else seen this? Chris

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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.

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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.

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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

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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

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Re: [Haskell-cafe] Issue with "connectTo" function in "network" package
by aditya siram 08 Jun '10

08 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 >

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Reminder: Deadline to submit talks on your commercial use of Haskell to CUFP in 7 days
by Don Stewart 08 Jun '10

08 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 08 Jun '10

08 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 08 Jun '10

08 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

2 3

Is www.haskell.org down?
by DekuDekuplex＠Yahoo.com 07 Jun '10

07 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^

1 0

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

3 5

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).

10 23

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

1 0

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.

9 19

Proposal: Sum type branches as extended types (as Type!Constructor)
by Gabriel Riba 05 Jun '10

05 Jun '10

Extending sum types with data constructors would spare runtime errors or exception control, when applying functions to inappropriate branches, as in the example ... data List a = Nil | Cons a (List a) -- List!Nil and List!Cons -- as extended types * Actual system, with runtime errors (as in GHC Data.List head) or exception throwing hd :: List a -> a hd (Cons x _) -> x hd Nil -> error "error: hd: empty list" -- error or exception throwing * Proposed system extending types with constructors as Type!Constructor: User must do pattern matching before applying the constructor-specific type function. In ''var @ (Constructor _ _)'' the compiler should append the constructor to the type as a pair (Type, Constructor) as an extended type for ''var'' No need for runtime errors or exception control hd :: List!Cons a -> a hd (Cons x _) = x using it: headOf :: List a -> Maybe a headOf list = case list of li @ (Cons _ _) -> Just hd li -- extTypeOf li == ( 'List', 'Cons') -- should pass typechecker for List!Cons li @ Nil -> Just hd li -- compiler error !! -- extTypeOf ('List','Nil') don't match _ -> Just hd list -- compiler error !! -- extTypeOf ('List',Nothing) don't match Maybe we could take out importance on the number of _ wildcards (constructor arity) with a syntax like. li @ (Cons ...) li @ (Nil ...) Cheers! Gabriel Riba Faura.

7 9

Cabal -j
by Louis Wasserman 05 Jun '10

05 Jun '10

What, if anything, stands in the way of parallelizing Cabal installs, make -j style? Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

5 7

Re: [Haskell-cafe] Issue with "connectTo" function in "network" package
by aditya siram 05 Jun '10

05 Jun '10

Great! If there is a wiki or FAQ page for networking with Haskell it might be worth updating. -deech On 6/4/10, Roman Cheplyaka <roma(a)ro-che.info> wrote: > /etc/hosts is the file responsible for resolving 'localhost' hostname. > As you can see from it, there are two entries for localhost: one > contains IPv4 address (127.0.0.1) and another contains IPv6 address > (::1). > > The behaviour you observe is kinda bug in connectTo function. Instead of > trying all of the addresses, this function just tries the first one. > Until it is fixed (I'm gonna write and send a patch soon), can you live > with this temporary hack? If for some reason the answer is "no", you > have to use low-level functions from Network.Socket module for now. > > > * aditya siram <aditya.siram(a)gmail.com> [2010-06-04 14:15:47-0500] >> 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 >> > > > -- > Roman I. Cheplyaka :: http://ro-che.info/ > "Don't let school get in the way of your education." - Mark Twain >

1 0

Issue with "connectTo" function in "network" package
by aditya siram 04 Jun '10

04 Jun '10

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 >

1 0

Tricks with GMap -- question about conflicts w/ indexed type families
by Ryan Newton 04 Jun '10

04 Jun '10

GMaps -- families of map implementations indexed by the key type -- are an example on the wiki: http://www.haskell.org/haskellwiki/GHC/Type_families I've been using something like this myself. It sure would be nice to have a fully developed version on Hackage, and I may try to submit this myself if no one else has already done it. But I'm running into a problem in achieving a particular optimization that I want -- namely, small tuple keys packed into Ints. For example, with a key type (Int16,Int16) it's possible to pack the key into a plain Int and use Data.IntMap. In fact, in the rough/partial implementation here (http://people.csail.mit.edu/newton/gmap/GMap.hs) I have a class for things that fit in a word: class FitInWord v where toWord :: v -> Word fromWord :: Word -> v (Or fitInInt for that matter.) Which includes things like small tuples: instance FitInWord (Int16,Int16) where toWord (a,b) = shiftL (fromIntegral a) 16 + (fromIntegral b) fromWord n = (fromIntegral$ shiftR n 16, fromIntegral$ n .&. 0xFFFF) (If you know a better idea than using templates/program generation to generate every combination of small scalar tuples that fit in a word.... let me know.) What I would next *like* to do is something like the following: import qualified Data.IntMap as DI instance FitInWord t => GMapKey t where data GMap t v = GMapInt (DI.IntMap v) deriving Show empty = GMapInt DI.empty lookup k (GMapInt m) = DI.lookup (wordToInt$ toWord k) m insert k v (GMapInt m) = GMapInt (DI.insert (wordToInt$ toWord k) v m) alter fn k (GMapInt m) = GMapInt (DI.alter fn (wordToInt$ toWord k) m) toList (GMapInt m) = map (\ (i,v) -> (fromWord$ intToWord i, v)) $ DI.toList m The problem is that there's already a more general instance of GMapKey that handles pairs by representing them as nested GMaps: instance (GMapKey a, GMapKey b) => GMapKey (a, b) where data GMap (a, b) v = GMapPair (GMap a (GMap b v)) .... Ideally, I want both of these to coexist (and to prioritize the more specific one). With normal type classes, OverlappingInstances can handle this, but with type families I get an error like the following: Conflicting family instance declarations: data instance GMap t v -- Defined at Intel/GMap.hs:108:7-10 data instance GMap (a, b) v -- Defined at Intel/GMap.hs:225:7-10 Any fixes? Thanks, -Ryan

1 0

alphanum in infix constructors (and possibly functions as well) - why not?
by Ozgur Akgun 04 Jun '10

04 Jun '10

Hi all, The constructor names in Haskell need to obey one of the two following rules: - either starts with a capital letter and contains alphanumeric characters afterwards, (including _ I guess) - or starts with a colon (:) and only contains symbols afterwards The first one is used Prefix by default, and the second ose is used infix by default. What stops us from allowing alphanum characters appear in the Infix version (after the colon)? Can't it be relaxed to only start woth a colon? So I want to be able to say something like: data Expr = Expr :< Expr -- checks for LT betwen two Expr's | Expr :<2 Expr -- a different implementation of the same thing maybe | Expr :<veryfast Expr -- and the veryfast implementation of it (just a silly example I could think of now.) Best, Ozgur Akgun

3 4

Strange GHC messages?
by Liam O'Connor 04 Jun '10

04 Jun '10

Hi all, I've got some strange GHC output that doesn't seem to be affecting my program adversely but I thought I should post about it cos it's disquieting mkUsageInfo: internal name? r{tv aIR} mkUsageInfo: internal name? s{tv aIS} It seems to be complaining about this newtype declaration: > newtype ThreadStateM r s v = ThreadState { unbox :: StateT s (ReaderT r IO) v } > deriving (Monad, Functor, MonadIO, MonadState s, MonadReader r) (uses newtype deriving) I'm not sure what the problem here is? Cheers. ~Liam

1 0

Re: [Haskell-cafe] Weird socket problem on Mac OS X
by Antoine Latter 04 Jun '10

04 Jun '10

On Fri, Jun 4, 2010 at 12:11 AM, Arnaud Bailly <arnaud.oqube(a)gmail.com> wrote: > Yes, I have also seen the other post on the same topic, but it seemed > to refer to the PortNumber in Network.Socket with a PortNum > constructor I think, not the one in Network (but I am probably > misleading myself as one is a reexportation of the other...). > But then I got. > > Slim\SlimClientIO.hs:26:51: > No instance for (Num PortID) > arising from a use of `fromInteger' > at Slim\SlimClientIO.hs:26:51-61 > Possible fix: add an instance declaration for (Num PortID) > In the first argument of `($)', namely `fromInteger' > In the expression: fromInteger $ slimport $ slimConfig st > In the definition of `port': > port = fromInteger $ slimport $ slimConfig st > Oops! I didn't read carefully enough. That's what I get for trying to program without haddocks or GHCi in front of me. I am glad you have something that works. Antoine

1 0

Fwd: [Haskell-cafe] Weird socket problem on Mac OS X
by Arnaud Bailly 04 Jun '10

04 Jun '10

---------- Forwarded message ---------- From: Arnaud Bailly <arnaud.oqube(a)gmail.com> Date: Fri, Jun 4, 2010 at 11:59 AM Subject: Re: [Haskell-cafe] Weird socket problem on Mac OS X To: Antoine Latter <aslatter(a)gmail.com> Hello, I managed to solve the problem using Network.Socket instead of Network. Is this a bug or a feature ? Arnaud On Fri, Jun 4, 2010 at 7:11 AM, Arnaud Bailly <arnaud.oqube(a)gmail.com> wrote: > Yes, I have also seen the other post on the same topic, but it seemed > to refer to the PortNumber in Network.Socket with a PortNum > constructor I think, not the one in Network (but I am probably > misleading myself as one is a reexportation of the other...). > But then I got. > > Slim\SlimClientIO.hs:26:51: > No instance for (Num PortID) > arising from a use of `fromInteger' > at Slim\SlimClientIO.hs:26:51-61 > Possible fix: add an instance declaration for (Num PortID) > In the first argument of `($)', namely `fromInteger' > In the expression: fromInteger $ slimport $ slimConfig st > In the definition of `port': > port = fromInteger $ slimport $ slimConfig st > > :-( > Thanks > > Arnaud > > On Fri, Jun 4, 2010 at 1:46 AM, Antoine Latter <aslatter(a)gmail.com> wrote: >> Hi Arnaud, >> >> One thing you might want to try is to stop using the PortNumber data >> constructor, and instead rely on 'fromInteger' to do the right thing. >> >> The data constructor assumes that it's argument is in network byte order, >> which won't always be the case. >> >> It's not obvious that the constructor should be avoided - this is the second >> time today I've seen this! >> >> Take care, >> Antoine >> >> On Jun 3, 2010 5:13 PM, "Arnaud Bailly" <arnaud.oqube(a)gmail.com> wrote: >> >> Hello, >> I have the following code which works ok on Linux and Windows XP, but >> fails on Mac OS X with error message: >> >> Connect: does not exist (connection refused) >> >> The server: >> >> doStartstate = do pr <- liftIO $ runProcess ... >> liftIO $ threadDelay 500000 >> st <- get >> put st { slimHandle = Just pr } >> >> >> The client: >> >> fetchAnswers st msgs = liftIO $! do let port = PortNumber $ >> fromInteger $ port $ config st >> cnx <- connectTo "localhost" port >> >> Help most welcome, thanks >> >> Arnaud >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

1 0

Haskell opportunity at investment bank
by Pearse, Chris 04 Jun '10

04 Jun '10

Hello, I'm looking for a developer with expertise in Haskell for an investment bank in London. The role will involve working on a multi-asset, multi-region, pricing, risk and trade lifecycle solution for the wholesale banking division. Solutions are developed using Java, C#/WPF, C++ and make heavy use of Haskell. The requirement is for an experienced developer with in-depth knowledge of functional programming in Haskell - ideally in a commercial environment. If interested, please feel free to send your CV through or call me to discuss. Thanks, Chris Pearse 0207 469 5050 http://www.linkedin.com/pub/3/9a1/142 ------------------------------------------------------------------------------- Huxley Associates - Professional Recruitment Solutions Established in 1995, Huxley Associates is an international recruitment consultancy. We provide bespoke recruitment services to the Accountancy & Finance, Banking Technology, Energy, Engineering, Global Markets, Human Resources, IT, Oil & Gas, Sales & Marketing, and Supply Chain sectors. With over 400 specialist recruitment professionals, we are consistently able to meet the high recruitment needs of a diverse range of companies across the UK, Europe, Asia and the United States. This email message may contain confidential and/or privileged information, and is intended solely for the addressee(s). If you have received this email in error, please notify Huxley Associates immediately. Unauthorised copying, disclosure or distribution of the material in this email is forbidden. If you do not want to receive emails from us about our services in future, or if you want to query or qualify how your personal data is held by us (for details on this see our privacy statement http://www.huxley.co.uk/privacy_policy.html ) please e-mail audit.data(a)huxley.com.

1 0

Efficient string construction
by Kevin Jardine 04 Jun '10

04 Jun '10

(I've done a basic Google search on this with no results. Apologies if this has been asked before.) I am coding a web application in which the content is a Unicode string built up over multiple functions and maintained in a State structure. I gather that the String module is inefficient and that Data.Text would be a better choice. Is it more efficient to build up a list of Text objects over time and combine them together with a single Data.Text.concat for the final output or to run Data.Text.append for each new string so that I am maintaining a single Text object rather than a list? As Data.Text.append requires copying both strings each time, my gut feeling is that concat would be much more efficient, but Haskell has surprised me before, so I wanted to check. Kevin

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ICFP Programming Contest
by Wouter Swierstra 04 Jun '10

04 Jun '10

This year's ICFP Programming Contest will begin on June 18th (12:00 Noon GMT) and will run till June 21st (12:00 Noon GMT). As in the previous editions, this is your chance to show that your favorite programming language is better than all others! The problem statement and further information will become available at: http://icfpcontest.org/2010/ Feel free to contact ifcpcont at imn dot htwk-leipzig dot de for further questions. Good luck! Wouter

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Simplest reference for how to build GHC head + get it running w/ cabal/haskell-platform?
by Ryan Newton 04 Jun '10

04 Jun '10

What's the easiest reference for how to build GHC head and get it up and running with cabal/haskell-platform? I simply installed 6.12 + haskell-platform then built ghc-6.13.xx and rebuilt only the packages I needed with "cabal install --reinstall". Perhaps this is not the recommended way. Indeed, something must be wrong because I can't build certain packages. (I have problems with "base" dependencies.) For example: $ cabal install QuickCheck Resolving dependencies... cabal: cannot configure QuickCheck-1.2.0.0. It requires base ==3.* For the dependency on base ==3.* there are these packages: base-3.0.3.1 and base-3.0.3.2. However none of them are available. base-3.0.3.1 was excluded because of the top level dependency base -any base-3.0.3.2 was excluded because of the top level dependency base -any Thanks! -Ryan

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Re: [Haskell-cafe] Weird socket problem on Mac OS X
by Antoine Latter 04 Jun '10

04 Jun '10

Hi Arnaud, One thing you might want to try is to stop using the PortNumber data constructor, and instead rely on 'fromInteger' to do the right thing. The data constructor assumes that it's argument is in network byte order, which won't always be the case. It's not obvious that the constructor should be avoided - this is the second time today I've seen this! Take care, Antoine On Jun 3, 2010 5:13 PM, "Arnaud Bailly" <arnaud.oqube(a)gmail.com> wrote: Hello, I have the following code which works ok on Linux and Windows XP, but fails on Mac OS X with error message: Connect: does not exist (connection refused) The server: doStartstate = do pr <- liftIO $ runProcess ... liftIO $ threadDelay 500000 st <- get put st { slimHandle = Just pr } The client: fetchAnswers st msgs = liftIO $! do let port = PortNumber $ fromInteger $ port $ config st cnx <- connectTo "localhost" port Help most welcome, thanks Arnaud _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Language Shootout reverse-complement benchmark
by Louis Wasserman 04 Jun '10

04 Jun '10

Hey, I was looking at the reverse-complement benchmark on the Language Shootout, and among other things, I noticed that the Haskell implementation was using (filter (/= '\n')) on ByteStrings, and also using lists as queues. I had a few improvements which using -fasm seem to yield about a 19% improvement in speed, and a 35% reduction in allocation, on my computer. (If both programs are compiled with -fllvm -- I'm not sure whether or not that's fair game on the Shootout -- my implementation is 35% faster, and does 10% less allocation.) I've checked my code on the Shootout's test input, as well. Mostly, the improvement comes from a tightly specialized version of (filter (/= '\n')), although eliminating an intermediate list entirely (and one used in a queuelike fashion) didn't seem to hurt. I managed to cut the program to a point where the program size is about the same as before. The code is at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=25865; the previous implementation is at http://shootout.alioth.debian.org/u32/program.php?test=revcomp&lang=ghc&id=2 . Let the arguing begin? Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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Data.ByteString.Lazy.hPut and timeouts?
by Jeremy Shaw 04 Jun '10

04 Jun '10

Hello, I am using Data.ByteString.Lazy.hPut to write data to a network socket. Is there any portable way to have hPut timeout if no data has been sent after some time period (say 120s?). I know that some OSes provide a per socket timeout that I can set in my application code (instead of a system-wide timeout that can only be set by root). But that does not seem like a very portable solution. My only other thought so far is to modify hPut to take a timeout. The standard definition looks like this: hPut :: Handle -> ByteString -> IO () hPut h cs = foldrChunks (\c rest -> S.hPut h c >> rest) (return ()) cs Seems like it should be easy to modify it so that it forks another thread to act as a watch dog. After every S.hPut it can tickle the timer. Since chunks are 16k, I think this should be sufficiently ok. If it is taking longer than 120s to send 16k, then we might want to consider the connection dead, even if it is making painfully slow progress.. Once all the chunks have been sent, we can kill the watch dog thread.. But, I am wondering if I am missing a better solution ? (Or an existing implementation of this solution?) Thank you for your insights! - jeremy p.s. At this point, using lazy ByteStrings is a requirement. Though I believe I would still have a very similar question if I was using iteratees anyway.

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Weird socket problem on Mac OS X
by Arnaud Bailly 04 Jun '10

04 Jun '10

Hello, I have the following code which works ok on Linux and Windows XP, but fails on Mac OS X with error message: Connect: does not exist (connection refused) The server: doStartstate = do pr <- liftIO $ runProcess ... liftIO $ threadDelay 500000 st <- get put st { slimHandle = Just pr } The client: fetchAnswers st msgs = liftIO $! do let port = PortNumber $ fromInteger $ port $ config st cnx <- connectTo "localhost" port Help most welcome, thanks Arnaud

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Process priority
by Andrew Coppin 03 Jun '10

03 Jun '10

I've had a hunt around in System.Process, but I can't discover any way to adjust the priority of the process you just spawned. Am I missing something, or has this just not been implemented yet? (I must confess, I don't know whether Windows and Unix both have the same ideas about priority numbers...)

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Sockets get bound to wrong port on Windows
by Matthias Reisner 03 Jun '10

03 Jun '10

Hi, there's something wrong with port numbers in the Network.Socket module of package network. Printing values gives: *Main> PortNum 8888 47138 *Main> PortNum 47138 8888 So I thought it's just an error in the show instance of PortNumber, which shows the bytes flipped. But if I use the following code snippet sock <- socket AF_INET Datagram 0 bindSocket sock $ SockAddrInet (PortNum 8888) iNADDR_ANY to bind a socket to port 8888, netstat and TCPView reveal that the socket is actually bound to the wrong port 47138. I'm using network-2.2.1.7 on Windows XP. Is that a bug or am I doing something wrong here? Regards, Matthias

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The state of Hackage: what are we doing about it?
by Don Stewart 03 Jun '10

03 Jun '10

I see fairly regular complaints about too many Haskell libraries, bewildering choice of difficult-to-determine quality. I've tried to summarize the state of Hackage, and what projects are active to make it easier to find high quality libraries: http://tinyurl.com/2cqw9sb Thoughts? -- Don

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design question/font metrics
by Gery Debongnie 03 Jun '10

03 Jun '10

Dear Haskell-Cafe list, Since I am learning Haskell, I decided to try to do a real program, to practice and give me some experience. I choose to do a basic typesetting program (like TeX/LaTeX). Now, obviously, such a program needs to manipulate font objects, and in particular, font metrics. Basically, I would like some function like this : stringWidth :: Font -> String -> Double charWidth :: Font -> Char -> Double which take some appropriate font type, string or char, and gives me a width (also height) in some given unit. I'd like it to take into account appropriate kerning. Question 1 : Now, I may be missing something, but a quick search on Google/Hackage didn't yield anything. Is there an existing package, or any nice and simple way to deal with that problem? Question 2 : In case the answer to the previous question is negative, I guess I will need to do it myself, the old-fashioned way. Now, this is where I have a design question. The thing is that font metrics information are encoded into some file (either .tfm or .pl files, for TeX fonts), so my function stringWidth will need to read a file, which is, gasp, an IO operation. Therefore, its type will be something like stringWidth :: Font -> String -> IO Double and it will "infect" pretty much everything else. It doesn't sound really appealing, so I'd like your opinions on this subject. To start the conversation, here are the solutions that I have in mind, so far : 1. well, do nothing, and let IO infect everything. 2. use unsafePerformIO to get rid of IO. (I am weary to do this, since I am a newbie, and I don't really want to start using that function everywhere... However, in this case, it looks kinda legitimate...) 3. Perform a reading of the font metrics file in the main program, put the results into some FontMetrics object, and give that to stringWidth :: FontMetrics -> Font -> String -> Double. Pros : allow me to avoid problems of solution 1 and 2, cons : it doesn't sound right for the main program to have to do this, it should be confined into a Font module. Any other ideas, suggestions? Gery D.

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Parselib sample
by C K Kashyap 03 Jun '10

03 Jun '10

Hi, Is there a not-so-trivial parser implementation with Parselib? Parser for a "C" like language would be good. I searched and found Haskell++ -> http://www.cs.chalmers.se/~rjmh/Software/h++.html However, I'd prefer to look at a parser for a "C" like language. -- Regards, Kashyap

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cabal update doesn't work on Windows XP
by John O'Donnell 03 Jun '10

03 Jun '10

Hi, Has anyone managed to get cabal to run on Windows? I'm running Windows XP. There are lots of other things, including old version of Haskell tools, installed, but it's too costly to try installing cabal on a fresh machine image. Whenever I do cabal update, the response is "Unsuccessful HTTP code 403". That looks like the hackage server isn't responding, but other software tools (browsers, etc) on the computer can download the hackage database; the only program that can't get it is cabal. This problem occurs with all cabal 1.6, 1.8, with downloaded executables, with versions I've compiled, with the version in the Haskell Platform. Needless to say, these problems don't occur on Linux. Best wishes, John

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Wire GUI
by Andrew Coppin 03 Jun '10

03 Jun '10

I'm looking at a project which involves a GUI where you can insert components and wire up connections between them. Obviously the details of what the components are and what code gets executed for them is domain-specific, however the general idea of graphically wiring things together is quite generic. Has anybody already put together a Haskell library for doing this kind of thing?

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How to build an "Indicator Type" for a type class?
by Steffen Schuldenzucker 03 Jun '10

03 Jun '10

Dear Cafe, let: > data True > data False > > class C a (arbitrary instances for C may follow) Now, how to obtain an "Indicator Type" for C, i.e. a type IndC that is defined via a type family / fundep / ... , so that IndC a = True forall a which are instances of C IndC a = False for all other a. I've collected some failed approaches here[1]. My key problem is that if I define (in the 3rd try): > instance (C a) => IndC3 a True , it does *not* mean "Define this instance for all a which are an instance of C", but "Define the instance IndC3 a True for all types a, but it's not gonna work if a is not an instance of C". Does anyone have another idea? Background: After having implemented type-level lists[2] and a quicksort on them[3], I'd like to have type-level sets. In their most simple implementation, sets are just (unsorted) lists like this: > data Nil > data Cons a b > class Elem x l (instances for Elem so that Elem x l iff x is an element of the list l) Now I want: > type family Insert x s :: * Insert x s = s forall (x, s) with (Elem x s) Insert x s = Cons x s for all other (x, s). Thanks a lot! Steffen [1] http://hpaste.org/fastcgi/hpaste.fcgi/view?id=25832#a25832 [2] Kiselyov, Peyton-Jones, Shan: "Fun with type functions" http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fu… [3] I rewrote this algorithm using type families instead of fundeps: http://www.haskell.org/haskellwiki/Type_arithmetic#An_Advanced_Example_:_Ty…

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Re: [web-devel] Re: [Haskell-cafe] Google Summer of Code: BlazeHTML RFC
by Jasper Van der Jeugt 03 Jun '10

03 Jun '10

On Thu, May 27, 2010 at 8:31 PM, Matt Parker <moonmaster9000(a)gmail.com> wrote: > will it be possible to easily interleave IO values into the HTML? like > instead of the [1,2,3] > > ul $ forM_ [1, 2, 3] (li . string . show) > > what if it was a function that returned IO [1,2,3] (maybe 1,2,3 came out of > a database). will the forM_ handle that OK? Depends on how you write the code. More precisely, it depends on in which monad the forM_ takes place. Also, I would not recommend interleaving IO values into the HTML, because the HTML is pure -- you usually don't want to mix that with IO. I'd rather write something like: numbers <- getFromDataBase return $ ul $ forM_ numbers (li . string . show) Or it could be possible to store your templates in a separate modules, and they would all have a pure type signature: templateX :: Arg1 -> Arg2 -> Html a I think this would be the preferred way, because it allows a separation between the pure View code and the IO-interleaved Controller code in a web application. Kind regards, Jasper Van der Jeugt On Thu, May 27, 2010 at 8:31 PM, Matt Parker <moonmaster9000(a)gmail.com> wrote: > will it be possible to easily interleave IO values into the HTML? like > instead of the [1,2,3] > > ul $ forM_ [1, 2, 3] (li . string . show) > > what if it was a function that returned IO [1,2,3] (maybe 1,2,3 came out of > a database). will the forM_ handle that OK? > > > On Thu, May 27, 2010 at 12:19 PM, Matt Parker <moonmaster9000(a)gmail.com> > wrote: >> >> Blaze HTML looks wonderful to me (especially with the do notation), and >> better then Hamlet for one reason: writing idiomatic haskell for stuff like >> this: >> >> ul $ forM_ [1, 2, 3] (li . string . show) >> >> instead of the Hamlet style: >> >> %ul >> $forall children.person child >> %li $child$ >> >> which, i don't know, for some reason, made me feel like i was back in PHP >> land. >> >> -matt >> >> >> On Thu, May 27, 2010 at 7:08 AM, Alberto G. Corona <agocorona(a)gmail.com> >> wrote: >>> >>> As a user, I have too many HTML generators, a few of them with Ajax and >>> none with server-side event handling (like ASPX or JSPX). Ajax is >>> complicated but server side event handling is what I really miss because it >>> is simple from the user point of view, my ervents could be handled in >>> haskell code rather than in javaScript and I implicitly could use the >>> advantages of dinamic HTML and Ajax without the need to know them at all. >>> Imagine a dynamic Web application with 100% haskell code made with >>> dynamic widgets created by third party developers. >>> So, anyone want to create a HTML templating system with server side event >>> handling? It is not terribly hard to do. (I refer to ASP.NET documentation >>> or the JavaServer Faces framework). >>> By the way, I vote for XML templating or else, combinator templating that >>> produce XHML templating because it can be handled by a future graphical IDE. >>> >>> 2010/5/27 Jasper Van der Jeugt <jaspervdj(a)gmail.com> >>>> >>>> Hey Bas, >>>> >>>> > How about also providing an enumerator back-end? >>>> > >>>> > http://hackage.haskell.org/packages/archive/iteratee/0.3.5/doc/html/Data-It… >>>> > >>>> > Then your library can integrate more easily with the snap framework: >>>> > http://snapframework.com >>>> >>>> Sure, I can do that. But I already tested integration with the snap >>>> framework, the best path here seems to call the `writeLBS` function >>>> from the snap framework on the `L.ByteString` that BlazeHtml produces >>>> (`writeLBS` internally uses an enumerator). >>>> >>>> Kind regards, >>>> Jasper Van der Jeugt >>>> >>>> On Thu, May 27, 2010 at 10:38 AM, Bas van Dijk <v.dijk.bas(a)gmail.com> >>>> wrote: >>>> > Q14: Do you see any problems with respect to integrating BlazeHtml in >>>> > your favourite web-framework/server? >>>> > >>>> > How about also providing an enumerator back-end? >>>> > >>>> > http://hackage.haskell.org/packages/archive/iteratee/0.3.5/doc/html/Data-It… >>>> > >>>> > Then your library can integrate more easily with the snap framework: >>>> > http://snapframework.com >>>> > >>>> > Regards, >>>> > >>>> > Bas >>>> > >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> >>> _______________________________________________ >>> web-devel mailing list >>> web-devel(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/web-devel >>> >> > >

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Happy GLR parser creation problem
by Paul Jensen 02 Jun '10

02 Jun '10

[Apologies if this is the wrong place to ask -- there appears to be no Happy-specific list.] I'm interested in experimenting with using a GLR parser to parse some data that could be interpreted in different ways. I'm trying to use Happy's GLR mode but I am encountering an error building one of the examples (specifically http://darcs.haskell.org/happy/examples/glr/expr-eval/Expr.y) [Mac OS X 10.6.3] rhoda:expr-eval psj$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 rhoda:expr-eval psj$ happy --version Happy Version 1.18.4 Copyright (c) 1993-1996 Andy Gill, Simon Marlow (c) 1997-2005 Simon Marlow rhoda:expr-eval psj$ happy --glr --ghc Expr.y shift/reduce conflicts: 9 happy: src/ProduceGLRCode.lhs:190:11-76: Irrefutable pattern failed for pattern (pre, drop_me : post) It's the same error someone else encountered last year, described at http://www.haskell.org/pipermail/haskell-cafe/2009-July/063770.html . Does anyone have any insights into this? If Happy's GLR support isn't working just now, are there any other parser generators that produce GLR parsers that anyone would recommend having a look at? Many thanks, psj

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