- Haskell-Cafe - Haskell.org

RE: [Haskell-cafe] elemt from list
by Bayley, Alistair 27 Jan '04

27 Jan '04

There is a Prelude function "!!" which I think would do what you want: <http://haskell.org/ghc/docs/latest/html/libraries/base/Data.List.html#v%3A! !> You may find other useful list functions in this library... > -----Original Message----- > From: Pajo Patak [mailto:brutalbanal@hotmail.com] > Sent: 27 January 2004 09:32 > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] elemt from list > > > I want to return a list, from and list of lists (all > integers), where is > the erroe in the code? > > nthList :: [[a]] -> Int -> [a] > nthList ([x]:xs) 1 = [x] > nthList ([x]:xs) (n+1) = nthListh xs n ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. ***************************************************************** ________________________________________________________________________ This email has been scanned for all viruses by the MessageLabs Email Security System. For more information on a proactive email security service working around the clock, around the globe, visit http://www.messagelabs.com ________________________________________________________________________

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Interpreting fields in a buffer
by Dominic Steinitz 27 Jan '04

27 Jan '04

Gregory, I don't know if this helps but I ended creating functions like the ones below (I didn't need to use the C structures because the IP definition is language independent). I'm sure there are better ways and I didn't test the throughput but I was able to develop a Haskell version of ping and traceroute (both multi-threaded). It would be nice if there were a library so that we didn't end up re-inventing the wheel. Dominic. ipHeaderLength :: String -> Int ipHeaderLength s = (fromIntegral (ord (s !! 0)) .&. 0x0f) * 4 ipTTL :: String -> Int ipTTL s = fromIntegral (ord (s !! 8)) ipProtocol :: String -> IPProto ipProtocol s = toEnum (ord (s !! 9)) ipDestAddr :: String -> HostAddress ipDestAddr s = fromIntegral (ord (s !! 16)) `shiftL` 24 + fromIntegral (ord (s !! 17)) `shiftL` 16 + fromIntegral (ord (s !! 18)) `shiftL` 8 + fromIntegral (ord (s !! 19)) ----- Original Message ----- > > Date: Sun, 25 Jan 2004 22:44:58 -0500 > From: Gregory Wright <gwright(a)antiope.com> > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] Interpreting fields in a buffer > Message-ID: <0356DB09-4FB2-11D8-A3D2-00039398F084(a)antiope.com> > Content-Type: text/plain; charset=US-ASCII; format=flowed > MIME-Version: 1.0 (Apple Message framework v609) > Content-Transfer-Encoding: 7bit > Precedence: list > Message: 1 > > > 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: Simple Fudgets-question
by Thomas Hallgren 27 Jan '04

27 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 27 Jan '04

27 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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