- Haskell-Cafe - Haskell.org

Interpreting fields in a buffer
by Gregory Wright 26 Jan '04

26 Jan '04

Hi, I have a question related to a program I'm writing. I have to handle IP packets, which will be read into a buffer. What is the best haskell idiom for getting access to the fields in the buffer? The IP header has a number of fixed format fields. In C, I would define a struct, then cast the pointer to the beginning of the buffer to a pointer to the struct. I would then be able to access each field in the header as <packet_struct_ptr> -> <field>. Is there a way in haskell to declare the format of the header, then access the components of the buffer as elements of those types? I'm only going to do read access on the buffer (an unboxed array). Most fields won't be examined but I can't tell in advance which ones will have to be looked at. I've not seen an example of this kind and was wondering if this was especially awkward. Thanks. Best Wishes, Greg Gregory Wright Antiope Associates LLC 18 Clay Street Fair Haven, New Jersey 07704 gwright(a)antiope.com

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Re: [Haskell] Re: Use of tab characters in indentation-sensitive code
by Graham Klyne 26 Jan '04

26 Jan '04

[Switching to Haskell-cafe] At 11:38 26/01/04 +0100, Andreas Rossberg wrote: >The most flexible but safe solution is to simply define the indentation as >the sequence of indentation characters used. Two consecutive lines are >indented consistently whenever one indentation is a prefix of the other. >Hence you may freely mix different indentation characters, but you must be >consistent across lines. Any decent editor should be able to ensure that. > >With this solution, tab width is irrelevant and indentation may include >whatever Unicode has. Neat. But unfortunately I don't think this, or any of the alternatives suggested, actually addresses the problem I have encountered, namely when an editing environment set up to handle local editing preferences is presented with code imported from another environment using different conventions. Conversion of all tabs to spaces (which I do to avoid this kind of problem when exporting my code to other environments) can cause correct code to become incorrect. There is, of course, a reasonable argument that my editing environment is at fault. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Converting String
by Alex Gontcharov 26 Jan '04

26 Jan '04

Hi, I'm fairly new to Haskell, I'd like to know how to convert a list of Strings of type IO [String] to Int. I used map read p, where p is a list of IO Strings ["1", "2"] Thanks

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RE: Library Infrastructure code & help wanted
by Simon Marlow 22 Jan '04

22 Jan '04

> > Having a single package database for all Haskell > implementations seems a > > hard thing to implement: when GHC is installed, it will > have to check > > whether the database and haskell-config already exist, and > similarly for > > the other Haskell implementations. When a Haskell system is > > deinstalled, it has to decide whether to de-isntall > haskell-config and > > the package database. > > I don't think that's so bad. Figuring out if a package database > already exists should be a matter of having an agreed upon > System.Directory.systemConfigDir. Then maybe the package database > itself can tell you where to find haskell-config. What about multiple versions of GHC installed on a system, each with different sets of packages installed? Can I still install GHC in my own home directory, without needing to modify files in /etc? > I think that the compilers / interpreters installed on the system will > also be stored in the "package config" database. If you're > deinstalling yourself, the package database can tell you whether > you're the only compiler left and thus whether to get rid of > haskell-config. > > That said, I think there are still issues with having a single package > database, for instance, what if a package configuration for one > Haskell Implementation has a field that conflicts with the package > configuration for another implementation. Yuck! But maybe that won't > come up. For instance, right now there are separate flags for ghc, > nhc, and hugs opts, and probably there should be flags for > shared opts. Also, there doesn't seem to be provision in the package configuration for naming libraries specific to a particular implementation (eg. to use this package on GHC you must link in libHSwibble.a from /foo/bar, but on nhc98 you link in libHSnhc_wibble.a from ...). It seems to me that every field needs to be duplicated for each compiler installation; either that or you could put the compiler name in the package identifier, so you'd have packages like "ghc-6.2-base-1.0", "nhc98-1.3-base-1.0". Incedentally, GHC never uses the extra_ghc_opts in the package configuration. In fact, I don't think it should be there: it doesn't make sense in conjunction with auto packages (if I don't know whether I'm using a package or not, I don't know whether to add the flags). The flags fields should be disabled if auto is True. > > I'd rather just admit that the Haskell systems are > separate, and each > > provides its own implementation of Distribution.Package. Yes, this > > means that if you run Setup.lhs with Hugs it is different > than running > > it with GHCi: if I want to install a package for GHC I > have to run the > > Setup.lhs script using GHC. > > I think this would dash my hopes of being able to install a package > with a single command. The idea so far has been to have some kind of > haskell-interactive / runhugs-type executable /usr/bin/haskell which > will execute the setup script to install and configure the package > based on the already-installed compilers. It doesn't prevent you doing this. Here's how it might work: - Each compiler has their own package database, and their own implementation of Distribution.Package with an implementation-defined location for the package database(s). - There is a suite of tools, including haskell-config and /usr/bin/haskell, which are installed only once. These tools know about all the installed compilers on the system. We could install these tools with a compiler, or separately; installs/deinstalls of compilers need to update some state so that the tools are aware. - /usr/bin/haskell could run the script it is given using each compiler on the system, if that's what you want. eg. $ /usr/bin/haskell script.hs -- runs script.hs using the default compiler $ /usr/bin/haskell --all script.hs -- runs the script using each compiler on the system so to do an installation for all compilers, you say './Setup.lhs --all --install', assuming that Setup.lhs begins with '!#/usr/bin/haskell'. > Likewise, when you install a new compiler, it knows about what > packages are there already for the other ones and might take a crack > at building them for itself. > > This isn't so far fetched: elisp for emacs and xemacs already do > something similar on Debian at least. We can aim for this in the long term if you like - I still believe this is something that the package management system should do for us, though. The end result is the same, and it's a lot less work. Cheers, Simon

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RE: Library Infrastructure code & help wanted
by Simon Marlow 22 Jan '04

22 Jan '04

> 2) I also would like someone to help out with the package > configuration file IO. Right now, it uses HaXmL, with no generated > code. Someone who has seen HaXmL before could give me advice about > how my code could be better, submit patches, or rewrite it. As > discussed on libraries(a)haskell.org, I'm open to not using XML if > someone wants to redo what I've got there (not a huge task). This > parsing code is in Distribution.Package. I do want something more > flexible than straight-up read/show. I rather think that this should be considered a "sample implementation" of the Distribution.Package API. A consumer of the API should neither know nor care about the on-disk representation of the package database. I think we should specify the form of the library meta-data, at least for the default build system (by meta-data I mean the form of toolInfo in the example in sec. 3.4 of the doc). We can take the approach that we specify a very simple form, which works for most straightforward Haskell-only libraries, or we could define a rich form which will provide enough for all but the most complex library packages. I think we should start with the simple form - complex libraries will probably continue to use their own build systems for a while yet. The idea, as I understand it, is that we're going to provide two main APIs: (b) The high-level API (Distribution.Core in the example). Used by simple Setup.lhs scripts to provide generic build, install, and package registration services. Consumes Distribution.Package internally. (a) The low-level API (Distribution.Package). This is for custom build-systems which provide their own build & install, so that they can register a package. Also used by the command-line tool (haskell-config?). There may also be a mid-level API (Distribution.Build, Distribution.Install), but I'm not sure I can see a clear use for that at the moment. If not, then we don't need to specify these APIs, and they can be internal to the default build system. Also, I really think we should clarify the situation as regards multiple Haskell implementations, which seems to be glossed over at the moment. Having a single package database for all Haskell implementations seems a hard thing to implement: when GHC is installed, it will have to check whether the database and haskell-config already exist, and similarly for the other Haskell implementations. When a Haskell system is deinstalled, it has to decide whether to de-isntall haskell-config and the package database. I'd rather just admit that the Haskell systems are separate, and each provides its own implementation of Distribution.Package. Yes, this means that if you run Setup.lhs with Hugs it is different than running it with GHCi: if I want to install a package for GHC I have to run the Setup.lhs script using GHC. I believe that's the right way to go, and it's much easier from our point of view: all we have to do is agree on the Distribution.* APIs, we don't have to have any clever install/deinstall magic to deal with shared components. Cheers, Simon

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RE: MVar swap
by Simon Marlow 20 Jan '04

20 Jan '04

> I took a look at the implementation of the MVar function swap > and I am > just woundering where the atomicy comes from. > > -- |Swap the contents of an 'MVar' for a new value. > swapMVar :: MVar a -> a -> IO a > swapMVar mvar new = > block $ do > old <- takeMVar mvar > putMVar mvar new > return old > > Is it really just the Exception block, that makes it not > interruptable? swapMVar is only atomic with respect to other well-behaved threads. By well-behaved I mean that they only do takeMVar followed by putMVar on the MVar, never a putMVar on its own. Does that answer your question? Cheers, Simon

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MVar swap
by Georg Martius 20 Jan '04

20 Jan '04

Hi, I took a look at the implementation of the MVar function swap and I am just woundering where the atomicy comes from. -- |Swap the contents of an 'MVar' for a new value. swapMVar :: MVar a -> a -> IO a swapMVar mvar new = block $ do old <- takeMVar mvar putMVar mvar new return old Is it really just the Exception block, that makes it not interruptable? Thanks! Georg

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GHC kit for Windows
by Graham Klyne 20 Jan '04

20 Jan '04

I note that the GHC kit for Windows includes some of the (I presume) MinGW binutil components, but does not include 'ar'. I find that 'ar' is needed for building HaXml. May I suggest that 'ar' also be included in this kit? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Library Infrastructure code & help wanted
by Isaac Jones 20 Jan '04

20 Jan '04

Greetings, I'm looking for some help on the library infrastructure project. The project and its goals are explained here: http://www.haskell.org/libraryInfrastructure/ And the code is here: http://www.haskell.org/libraryInfrastructure/code.html In brief, the Library Infrastructure Project is an effort to provide a framework for developers to more effectively contribute their software to the Haskell community. I'm hoping to not have to implement this entire proposal myself, and I'm very flexible about implementation details. I've made some concrete progress, and I just mirrored my code repository on my web site, so now is the time to get involved! Specifically, at the moment, I want a couple of things: 1) A version parser. I implemented a parser which parses versions that are made of dates, or simple numeric versions. I'd like it to do more kinds of versions, though. Distribution.Version is attached. What is this version stuff used for? Well for one thing, you can use it to say something like: ./haskell-config --user list "MyPackage>1.2" where "MyPackage" is the name of a package. This will list all of the user-installed packages whose versions are greater than 1.2. If you look at the HUnit tests at the bottom of the attached file, you'll get a good idea of what it does right now. It uses Parsec. BTW, I understand that the Debian project has some standard version number parser which might give you some ideas. 2) I also would like someone to help out with the package configuration file IO. Right now, it uses HaXmL, with no generated code. Someone who has seen HaXmL before could give me advice about how my code could be better, submit patches, or rewrite it. As discussed on libraries(a)haskell.org, I'm open to not using XML if someone wants to redo what I've got there (not a huge task). This parsing code is in Distribution.Package. I do want something more flexible than straight-up read/show. 3) Someone to make it work with in nhc & hugs. The HaXmL page says that it doesn't work w/ Hugs yet (can anyone tell me why?), so that's one step. Also, at least for the version of NHC I have on my system, Parsec is different. So maybe we have to move away from Parsec, or maybe a different version thereof? It is absolutely vital for this project to be completely cross-platform and cross-compiler, so if we have to avoid depending on such things, I'm very willing to change. 4) Someone to make it work with Windows. Avoid using a filename that's called "Aux" ;) ABOUT THE CODE: It only works with GHC at the moment (see above) :( Right now what I've got is a first stab at Distribution.Package and haskell-config as explained in the proposal [1], though some things may have changed. It's hosted with Arch[2], a newish version control system. You can branch my code on your own machine and submit patches. Arch has some tools to help with patches. I've also provided a tarball. Arch is cool, though. You should try it. The HUnit tests in some of the modules should be pretty illuminating. The interactive help for haskell-config is pretty OK. Get the code here: http://www.haskell.org/libraryInfrastructure/code.html peace, isaac [1] http://www.haskell.org/libraryInfrastructure/proposal/index.html [2] http://regexps.srparish.net/www/

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LIP and the AAP project
by Isaac Jones 20 Jan '04

20 Jan '04

Greetings, Now that I have at least gotten something started on the Library Infrastructure Project, I'm going to bend some of my energy toward the build/install issues for Haskell modules. The plan at this point, which is summarized on the project web site[1] is to attempt to leverage HMake's usefulness as a non-compiler-specific build system. Since I wrote the proposal, I've been made aware of the AAP project[2]. Does anyone have any experience with this? Does anyone have a sense for how it might fit in with LIP? I know that Peter Simons seems to think it might. I haven't read through all of the documentation, but I suspect it might help us out with the CPAN-style distribution stuff, and perhaps the building stuff. One big objection I have with using it would be that Haskell modules would depend on Python (!), but I thought I'd let everyone know about it anyway. Seems like an interesting project, and we could quite possibly learn something from it. peace, isaac [1] http://www.haskell.org/libraryInfrastructure/ [2] http://www.a-a-p.org/

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