January 2004 - Haskell-Cafe

Re: Simple Fudgets-question
by Thomas Hallgren 26 Jan '04

26 Jan '04

Henrik Berg wrote: >Hi! > >I can't find any function in the Fudgets-library to do this: > >(F a b) -> (F a (a, b)) > >... All I want to do is to resend the input out on the output. > > If that is all you want, this combinator is the right choice: throughF :: F a b -> F a (Either b a) -- Thomas H

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Matrix as list of list
by Pajo Patak 26 Jan '04

26 Jan '04

Hello i am new to Haskell and i could need some help :) I want to make a function getElement which returns me an Element from a martix, which ist represented as a list of lists, e.g. [[1,2,3] [4,5,6]] is a 2x3 Matrix. The Type of the function getElement is Int->Int->[[a]]->a Can someone help me please? _________________________________________________________________ 5 neue Buddies = 50 FreeSMS. http://messenger-mania.msn.de MSN Messenger empfehlen und kräftig abräumen!

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[jimlewis@miclog.com: Haskell web]
by John Peterson 26 Jan '04

26 Jan '04

Here are some comments regarding Haskell and the website. I think there are some things we can do to make it a little easier on people that enter haskell.org from outside the sheltered halls of academia. I think the comment about the organization of haskell.org is fairly accurate - we have been adding more and more content without a really systematic way of searching for things. I would really like to be able to sift through the information at haskell.org in a more useful way. Much of the best content on haskell.org is hidden in Mailman archives - I also have problems digging through old topics. I'm not sure we should change over to different discussion software but maybe we could do something better to access the archives. John ------- Start of forwarded message ------- Subject: Haskell web To: "peterson-john(a)cs.yale.edu" <peterson-john(a)cs.yale.edu> From: "Jim Lewis" <jimlewis(a)miclog.com> Hi John I would like to offer my input on how to make Haskell more successful. In running a software publishing company over the years I got a bit behind in language technology and only discovered FP and Haskell a few days ago. FP in general and Haskell specifically appear to be a great advance over OOPs (we use c++). So it's surprising that Haskell is not more mainstream. With years of experience in software publishing, I thought it might be helpful to offer some strategic feedback (aka constructive criticism), knowing well that I am probably not the first to make these points. Actually I am excited enough over the potential of Haskell that I would consider trying to help commercialize the language but unfortunately my plate is rather full at the moment. In summary, the problem is that the site and tools nearly stopped me from learning Haskell. I had a lot of trouble getting over the initial hump - not so much because of the concepts but because of the site, tools, and docs. (I still only know about 10% of the language but I can now get some trivial apps (a few pages of code) to run.) Here are some of the problems I encountered: 1) Site organization - I wasted hours finding SOEGraphics as I missed the link about older versions. It seems unlikely I am the first with this problem and must conclude that someone is not carefully listening to feedback. I almost gave up with Haskell due to this small glitch. 2) The documentation on the site and in two books I have starts out slow and then picks up speed too fast leaving the reader in the dust. 3) The archaic forum UI makes it impractical to find anything and the zip archive is a broken link. And the topics are mostly too advanced for a novice. For about $150 one can buy a modern forum management package. Downloading zip archives went out of style close to a decade ago. 4) The search options are usually unproductive. For example I could not find how to resolve how to handle the Maybe construct in my app. I only found advanced discussions that did not help. My use was very simple. 5) Someone is not listening to bugs. Hugs crashes on startup after changing any value in the options dialog. This also wasted hours. I can't be the first to experience this. I also almost gave up after this experience. 6) The language reference has numerous functions with no description or examples. I know you know this but find it hard to understand how it can remain so. The Prelude Tour on the other hand is a model of the proper approach - I found it very helpful in learning the language as each function showed the description, code, syntax, and examples. 7) There is so much complexity to the language that it should be made clear what minimal subset one needs to learn to start writing some code. It took a while to figure out what I needed. Once the user starts writing code successfully he will be much more hooked. I realize it must be difficult in a non-commercial environment to depend on volunteers to fix such problems but these types of glitches will stop all but the most dedicated newbies. I would like to see Haskell succeed and offer any advice on setting a better strategy for managing the spread of the language. No doubt you've heard most of the above many times before but perhaps I've added something new from the perspective of someone with almost two decades of experience in commercializing software. Regards, Jim Lewis President Micro Logic New Jersey 201-447-6991 x 113 ------- End of forwarded message -------

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RE: [Haskell] Gallopping Tab characters
by Simon Marlow 26 Jan '04

26 Jan '04

> Simon Marlow wrote: > > As for the width of the tab character: tab stops are every > 8 columns. > > Period. The Haskell report says so > > Yes, true. I think it was Leslie Lamport who wrote in TeXHaX > that anyone > defining an input format which includes tabs should be > sentenced to ten years > programming COBOL in Nijny-Novgorod. Off you go, Simon ... :) /me grabs COBOL book and bottle of Vodka sniff Simon

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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 21 Jan '04

21 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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