- Haskell-Cafe - Haskell.org

The applicative instances for Either?
by Ketil Malde 19 Sep '11

19 Sep '11

Hi, I have a program that makes use of the applicative instance for Either String. I used to define these instances locally, but at some point, they became part of Control.Applicative. I have limited the dependencies to 'base >= 4', but apparently, some version 4s of base include this instance, some do not, and it causes problems for people trying to compile the program. Is there any information, or otherwise accessible source specifying exactly when this was changed, so that I can have more precise dependencies? And is there a simple way to handle this conditionally, either within cabal, or using CPP? -k -- If I haven't seen further, it is by standing in the footprints of giants

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Enumeratees are killing me
by tsuraan 19 Sep '11

19 Sep '11

I'm trying to write what I think should be a simple Enumeratee, but I'm just not having any luck getting it to typecheck, and I'm coming to the realization that I still have no idea what I'm doing. The essential thing I want to do is adapt Data.Enumerator.List.concatMapAccum so that the accumulator function has the type (s -> ao -> Either (ao, [ai]) (s, [ai]) ) . The accumulator returning Right (s, [ai]) behaves as normal, while returning Left (ao, [ai]) will cause the Enumeratee to call the upstream Iterator one last time with (Chunks ai) and then yield with the (ao : xs). I just can't make it work though. Here's the guts of my attempt: step _ k EOF = yield (Continue k) EOF step s k (Chunks xs) = loop s k xs loop s k [] = continue (step s k) loop s k (x:xs) = case decoder s x of Left (extra, ai) -> k (Chunks ai) >>== checkDone (\k' -> yield () $ Chunks (extra:xs)) Right (s', ai) -> k (Chunks ai) >>== checkDoneEx (Chunks xs) (\k' -> loop s' k' xs) At this point I've tried putting a bunch of random stuff into the part to the right of (>>==) in the Left case of loop, but without any luck. Any help would be much appreciated. It seems really weird to me that none of the built-in functions for Enumeratees let you stop before the Enumerator is done. That seems like a really common thing that a person would want; in my case, I'm parsing messages out of a byte stream, so I want to stop each Enumeratee as it finishes its message, but that doesn't seem possible without dropping down to this level.

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Function code generation (Template Haskell).
by Serguey Zefirov 19 Sep '11

19 Sep '11

The task is that I have some function and I need to create another function alongside with it. The second function is based on first one. As a matter of fact, I already did this with Template Haskell. TH is quite good at that task, because I can load my module in ghci and have both functions available for experiments. I just wondering, is it possible to use newly introduced ghc plugins to do something like that? Could I create new functions while inspecting defined ones? Will they be visible in ghci?

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ANN: usb-1.0, bindings-libusb-1.4.4 and usb-iteratee-0.4
by Bas van Dijk 19 Sep '11

19 Sep '11

Fellow Haskellers, I would like to announce the 1.0 release of the usb library! This library lets you to communicate with USB devices from userspace. It is implemented as a high-level wrapper around bindings-libusb[1] which is a low-level binding to the portable C library: libusb-1.0 ( http://libusb.org ). I think a 1.0 release is warranted since I have been successfully using the library in an industrial application at Sensor Sense for some time now. This release also adds the last piece of functionality that was missing from previous releases: isochronous transfers. These let you communicate with USB video or audio devices for example. A second major change is that all I/O functions, while still having a synchronous interface (they block), are now implemented asynchronously. This makes them more efficient (no busy-loops) and interruptible (just throw an asynchronous exception to the thread executing a transfer and the transfer is automatically canceled). The implementation uses the GHC event manager for event handling. If the event manager is not available, because you're either: not using GHC, building your program without -threaded or you're on Windows the implementation degrades gracefully to the synchronous implementation. Other changes: * Added writeControlExact :: DeviceHandle -> ControlAction WriteExactAction. * All I/O functions which previously returned the boolean TimedOut now return a: data Status = Completed | TimedOut. * Added the timeout constant: noTimeout :: Timeout. * Added function: maxIsoPacketSize :: EndpointDesc -> Size Which calculates the maximum packet size which a specific endpoint is capable of sending or receiving in the duration of 1 microframe. This function is mainly useful for setting up isochronous transfers. * Added some specific IOExceptions: ioException :: USBException incompleteReadException :: USBException incompleteWriteException :: USBException * Renamed System.USB.IO.Synchronous to just System.USB.IO. * Renamed System.USB.Unsafe to System.USB.Internal and exported more functions from it which are primarily needed in the usb-iteratee package. * Fixed some bugs. * Switched from darcs to git and hosted the project on github: https://github.com/basvandijk/usb/ API Docs: http://hackage.haskell.org/package/usb-1.0 Installing: $ cabal update $ cabal install usb I would like to thank John Obbele for pushing me to write the asynchronous implementation and for testing the library. Also thanks to Joris Putcuyps for testing the library on Windows. Unfortunately the library doesn't yet work on that platform due to a weird segmentation fault: http://hackage.haskell.org/trac/ghc/ticket/5254. Hopefully we will solve it in the near future. bindings-libusb =============== The usb library is based on bindings-libusb which is a low-level binding to the libusb C library. I recently took over maintenance of this package from Maurício Antunes. Maurício, thanks for creating and maintaining this package! It greatly simplified the task of writing the high-level wrapper. API Docs: http://hackage.haskell.org/package/bindings-libusb-1.4.4 usb-iteratee ============ I also released a new library: usb-iteratee which provides iteratee enumerators for the usb package. Note that this package was previously called usb-enumerator. However I'm planning to rewrite the latter to offer enumerators for the enumerator package instead. API Docs: http://hackage.haskell.org/package/usb-iteratee-0.4 Regards, Bas

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[Haskell] [ANNOUNCE] Haskdogs-0.1
by Sergey Mironov 17 Sep '11

17 Sep '11

Hi! I am pleased to announce haskdogs - project-level ctag file generator. haskdogs is a small shellscript-like tool which creates tag file for entire haskell project directory. It takes into account first-level dependencies by recursively scanning imports and adding matching packages to the final tag list. As a result, programmer can use his/her text editor supporting tags (vim, for example) to jump directly to definition of any standard or foreign function he/she uses. Note, that haskdogs calls some Unix shell commands like test or mkdir so this tool will likely fail to work on pure Windows platforms. To use it, do 0) cabal install hasktags haskdogs && mkdir -p ~/.cabal/var/haskdogs 1) cabal unpack TrickyProject-4.2 && cd TrickyProject-4.2 2) haskdogs 3) enjoy the tagfile with references to every function http://hackage.haskell.org/package/haskdogs-0.1 Sergey

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Error in Installing Data.deriveTH
by mukesh tiwari 17 Sep '11

17 Sep '11

Hello all I am trying to install Data.deriveTH . When i tried cabal install derive , i got this error. ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/assignment_haskell/work$ cabal install deriveResolving dependencies...<command line>: cannot satisfy -package Cabal-1.10.2.0: Cabal-1.10.2.0-072fce39d68f011c6a364057a3d15270 is unusable due to missing or recursive dependencies: containers-0.3.0.0-ee442470d8dcc9e45f31677c400c5379 (use -v for more information)cabal: Error: some packages failed to install:derive-2.5.4 depends on haskell-src-exts-1.11.1 which failed to install.haskell-src-exts-1.11.1 failed during the configure step. The exception was:ExitFailure 1 ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/assignment_haskell/work$ cabal install haskell-src-extsResolving dependencies...<command line>: cannot satisfy -package Cabal-1.10.2.0: Cabal-1.10.2.0-072fce39d68f011c6a364057a3d15270 is unusable due to missing or recursive dependencies: containers-0.3.0.0-ee442470d8dcc9e45f31677c400c5379 (use -v for more information)cabal: Error: some packages failed to install:haskell-src-exts-1.11.1 failed during the configure step. The exception was:ExitFailure 1 ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/assignment_haskell/work$ cabal install containersResolving dependencies...No packages to be installed. All the requested packages are already installed.If you want to reinstall anyway then use the --reinstall flag. I tried to resolve this issue on #haskell and i got suggestion that it was due to conflict in global and local namespace [ see here for more detail http://hpaste.org/51376 ]. One idea is i should wipe ~/.ghc and install all the libraries individually. Could some one please suggest if there is another idea to resolve this issue. Currently i have ghc-6.12.3 and installing ghc-7.0 will resolve the issue ? Thank you Mukesh Tiwari .

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Packaged my log visualization tools
by Eugene Kirpichov 17 Sep '11

17 Sep '11

Hello cafe, I've packaged my log visualization tools in binary form for several platforms. Perhaps those of you who have better things to do than struggle installing them from source will find this useful :) Here you go: http://jkff.info/presentations/two-visualization-tools.pdf Documentation http://jkff.info/software/timeplotters-0.1-1_win32.zip Windows (any architecture) http://jkff.info/software/timeplotters-0.1-1_i386.deb Debian (i386) http://jkff.info/software/timeplotters-0.1-1_amd64.deb Debian (amd64) http://jkff.info/software/timeplotters-0.1-1_i386.tar.gz *nix binaries (i386), require gtk2.0, gmp3, libc6. http://jkff.info/software/timeplotters-0.1-1_amd64.tar.gz *nix binaries (amd64), require gtk2.0, gmp3, libc6. -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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ANNOUNCE: qrcode: QR Code encoder in pure Haskell
by Chris Yuen 17 Sep '11

17 Sep '11

Hello cafe, My first Hackage upload is a pure Haskell QR Code library ( http://hackage.haskell.org/package/qrcode-0.1) This is intended to be a full featured QR Code library with encoding and decoding and other advanced features. Currently it can only do encoding but I figured it would be nice to get some early feedback so here we go :) Cheer, Chris

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pattern matching instead of prelude.head
by Michael Litchard 17 Sep '11

17 Sep '11

Someone commented on StackOverflow that pattern matching the first element of a list was preferable to head. This makes sense intuitively. Could someone articulate the reason why this is true?

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VirtuaHac - online Haskell hacathon
by Chris Smith 17 Sep '11

17 Sep '11

Overview ======== VirtuaHac is a Haskell hacathon being planned using Google+ and a Wiki. The rules will be that if you have a project you'd like to participate in, then: 1. Create a darcs or git repository. 2. Post a Hangout in Google+. 3. Add a name, brief description, and links to the preceding two items to the Wiki. This way, we hope to recreate the idea of a Hacathon, but without the travel expenses or inconvenience. What Next? ========== I'm taking a poll of availability, at https://docs.google.com/spreadsheet/viewform?formkey=dDZ3Tjh1RzVDeHdRYWhNTk… The poll will remain open for the next week, after which I'll choose a date so we can all start planning. -- Chris Smith

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