September 2011 - Haskell-Cafe

Strange No instance error with cabal install
by Rune Harder Bak 20 Sep '11

20 Sep '11

Sometimes when one of our developers (using Arch-linux) tries to cabal install packages he gets a "no instance for" error. Code that compiles fine on my computer, and other computers I try it on. Even some packages on hackage (trying to install aeson-native for instance gives Data/Aeson/Types.hs:196:22: No instance for (NFData Object) arising from a use of `rnf' Possible fix: add an instance declaration for (NFData Object) In the expression: rnf o In an equation for `rnf': rnf (Object o) = rnf o In the instance declaration for `NFData Value') I tried removing the .ghc and .cabal folder, even reinstalled ghc-7.04 and cabal-install (most recent), but the problem persisted (might not have been a clean install though, I'm no Arch expert) I know this is kind of vauge, and not in she shape of filling a bug report, heck I can't even reproduce the error on my computer. But perhaps somebody else has run in to this strange behavior before? Best, Rune

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ANNOUNCE: lushtags, haskell ctags for Vim Tagbar plugin
by Bit Connor 20 Sep '11

20 Sep '11

I would like to announce the first release of my program lushtags. Available on hackage: cabal install lushtags At github: https://github.com/bitc/lushtags Screenshot: https://github.com/bitc/lushtags/raw/master/doc/screenshot-tagbar-2011-09-1… What is lushtags? From the README: lushtags ======== Create ctags compatible tags files for Haskell programs Similar programs are [hasktags][1], [gasbag][2], [hothasktags][3], and GHC's builtin ctags generation. lushtags differs from these by being designed to have smooth integration with the [Vim Tagbar][4] plugin. Tagbar is nice because it deals with creating tags automatically. There is no need to manually run commands or keep track of tag files. Just open any Haskell file in Vim and the Tagbar window will instantly show an interactive browsable list of all the functions and declarations in the file. This window also updates automatically as you edit the file. The tags created by lushtags are marked with several extensions, so that when used with Tagbar you get these features: - Type signatures are displayed for functions. - Tags are properly scoped so that, for example, data declarations appear as a tree with their constructors scoped as children. - Definitions that are exported from the module are marked as "public" and appear emphasized in Tagbar. - Tag locations are internally stored as patterns, not line numbers, so that you can correctly jump to tags even if they have moved in the source code during editing. ![screenshot](https://github.com/bitc/lushtags/raw/master/doc/screenshot-tagbar-2011-09-19.png) [1]: http://hackage.haskell.org/package/hasktags [2]: http://kingfisher.nfshost.com/sw/gasbag/ [3]: http://hackage.haskell.org/package/hothasktags [4]: http://majutsushi.github.com/tagbar/

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Data.Text.IO.hGetContents problem on windows
by David Virebayre 20 Sep '11

20 Sep '11

Bonjour Café, bonjour Bryan I have a program that works fine on linux, but doesn't on windows. On windows XP with the latest Haskell platform, I get: <socket: 1860>: hFileSize: invalid argument (Bad file descriptor) I think the problem is with hGetContents from Data.Text.IO, but my google-fu failed to help me find any information. Is there something I'm doing wrong ? David. The source code of the part that fails is as follows: -- Execute a command on a distant server using the rexec protocol. rexec :: HostName -- server to connect to -> Text -- user -> Text -- password -> TextEncoding -- server's text encoding -> Text -- command to execute -> IO (Maybe Text) rexec !ip !ru !rp !enc cmd = handle rexec_error $ do hdl <- connectTo ip (PortNumber 512) let end_param = T.singleton (chr 0) ctrl_string = T.concat [ "0", end_param, ru, end_param, rp, end_param, cmd, end_param ] hSetEncoding hdl enc TIO.hPutStr hdl ctrl_string -- make sure the control string is sent. hFlush hdl -- 1st char read is actually a error code which we ignore for now hGetChar hdl !res <- TIO.hGetContents hdl hClose hdl return (Just res) rexec_error :: SomeException -> IO (Maybe Text) rexec_error err = do putStrLn $ show err return Nothing

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Library support for sorting with Text.Data.ICU.Collate.MCollator?
by Richard Cobbe 19 Sep '11

19 Sep '11

I'm trying to sort a list of Text values using a collator obtained from the Text.Data.ICU.Collate module in the text-icu package on Hackage. Unfortunately, I can't use the normal Data.List.sortBy function with one of these collators, because the collators return (IO Ordering), not Ordering. It's easy to write sortWithCollator :: [Text] -> IO [Text], but I'd rather not have to. I haven't been able to find library support for this anywhere -- am I overlooking it, or do I need to write the sort function myself? Thanks, Richard

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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 18 Sep '11

18 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 18 Sep '11

18 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 18 Sep '11

18 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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