- Beginners - Haskell.org

Call for Contributions - Haskell Communities and Activities Report, November 2008 edition
by Janis Voigtlaender 10 Oct '08

10 Oct '08

Dear Haskellers, so much has happened in the Haskell world in the past months. Therefore, I would very much like to collect contributions for the 15th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 31 October 2008 (please send your contributions to hcar(a)haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in any project related to Haskell that has not previously been mentioned in the HC&A Report, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the May 2008 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The best format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2008/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2008/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should have received your old entry as a template already (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Do not worry about writing correct LaTeX, I will be able to handle your file. Please do not use HTML or even DOC. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or ``historic'' overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep entries up. There is no minimum length of an entry! The report aims at being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports over distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write ``wishlist'' items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

3 2

Cryptography
by Jeffrey Drake 08 Oct '08

08 Oct '08

I have an application where I would like to use AES to have a public/private key pair and digitally sign/verify documents. I have found documentation for a library that does the encryption: http://www.haskell.org/crypto/doc/html/Codec-Encryption-AES.html However, it seems to have only two methods: encrypt :: AESKey a => a -> Word128 -> Word128 decrypt :: AESKey a => a -> Word128 -> Word128 A problem for me, I don't know where AESKey is supposed to come from, or how to use this to sign things. My ignorance of this topic does not help. Would it be correct to say that signing a document is similar to an MD5 hash on a document? In addition, this project requires a matching python component (that works together with the haskell). I am in a similar situation, I have found this: http://bitconjurer.org/rijndael.py that provides a similar functionality. But it seems to provide insights (and thus portability to haskell) on how to generate the public/private keys by providing a single key and performing a lot of math on it to generate the two pairs. But it still does not help me sign anything. Any help whatsoever on this topic is welcome. With regards, Jeffrey.

3 2

Perfect numbers
by Matthew Williams 03 Oct '08

03 Oct '08

Hi Guys, I'm new to Haskell and I was wondering if you can help me: One of the first program's I tend to write when I'm looking at a new language is a program to generate a list of perfect numbers: --My First Perfect Number Generator factors :: Integer -> [Integer] factors x = [z | z <- [1..x-1], x `mod` z == 0] is_perfect :: Integer -> Bool is_perfect x = if sum(factors x) == x then True else False do_perfect :: [Integer] -> [Integer] do_perfect x = [z |z <- x, is_perfect z ] Then to run it: > do_perfect [1..9000] I'm using GHC to run it. My problem / question is this: It's running quite a lot slower than equivalent programs in erlang and python. I suspect it's down to the way I've written it. Any thoughts (or comments in general) Many thanks Matt ______________________________________________________________________ This email may contain privileged or confidential information, which should only be used for the purpose for which it was sent by Xyratex. No further rights or licenses are granted to use such information. If you are not the intended recipient of this message, please notify the sender by return and delete it. You may not use, copy, disclose or rely on the information contained in it. Internet email is susceptible to data corruption, interception and unauthorised amendment for which Xyratex does not accept liability. While we have taken reasonable precautions to ensure that this email is free of viruses, Xyratex does not accept liability for the presence of any computer viruses in this email, nor for any losses caused as a result of viruses. Xyratex Technology Limited (03134912), Registered in England & Wales, Registered Office, Langstone Road, Havant, Hampshire, PO9 1SA. The Xyratex group of companies also includes, Xyratex Ltd, registered in Bermuda, Xyratex International Inc, registered in California, Xyratex (Malaysia) Sdn Bhd registered in Malaysia, Xyratex Technology (Wuxi) Co Ltd registered in The People's Republic of China and Xyratex Japan Limited registered in Japan. ______________________________________________________________________

6 7

Exposing Ratio data constructor
by Casey Rodarmor 29 Sep '08

29 Sep '08

Hej hej! I was writing some code using Ratio, and even though I know it's tucked behind an abstraction barrier, I really wanted access to the Ratio data constructor ':%'. I wrote invertRatio like such: invertRatio r = denominator r % numerator But I really wanted to write it like this: invertRatio (n :% d) = d % n I understand that exposing ':%' causes problems, since it allows us not only to pick apart ratios, but to construct bad ones that would normally be caught when constructed with '%'. (Such as '1:%0'.) Is there any way to avoid this, while still letting the user benefit from the nice pattern matching syntax that exposing the data constructor allows? Kram, Casey Rodarmor

2 1

Exposing Ratio data constructor in prelude
by Casey Rodarmor 29 Sep '08

29 Sep '08

Hej hej! I was writing some code using Ratio, and even though I know it's tucked behind an abstraction barrier, I really wanted access to the Ratio data constructor ':%'. I wrote invertRatio like such: invertRatio r = denominator r % numerator But I really wanted to write it like this: invertRatio (n :% d) = d % n I understand that exposing ':%' causes problems, since it allows us not only to pick apart ratios, but to construct bad ones that would normally be caught when constructed with '%'. (Such as '1:%0'.) Is there any way to avoid this, while still letting the user benefit from the nice pattern matching syntax that exposing the data constructor allows? Kram, Casey

2 1

Re: [Haskell-beginners] Module import problem
by Casey Rodarmor 29 Sep '08

29 Sep '08

On Sun, Sep 28, 2008 at 9:09 PM, Chaddaï Fouché <chaddai.fouche(a)gmail.com> wrote: > 2008/9/28 Casey Rodarmor <caseyrodarmor(a)gmail.com>: >> The comparison I was thinking of was with python, where a module can >> be placed in an arbitrary directory, and then accessed using a >> relative path. If I have 'stuff.py' that contains class 'Foo', I can >> move it to 'hello/module/stuff.py', and then import it is >> hello.module.stuff. > > And if you have something in hello/ it must refer to the same module > by module.stuff instead. I'm not sure it's clearer... Yeah, I can see where you're coming from. > Anyway the hierarchical modules in Haskell means that every code must > import the same module with the same name which I find saner (though I > could wish the import/export syntax would be more advanced to allow > for shorter imports when I have a lot of imports in the same hierarchy > part). > >> A python module doesn't need to know where it >> lives. > > Well, it doesn't need to, but everyone else needs to, in Haskell, the > module need to know where it lives, but everyone else don't care. > >> >> I often find myself working on computers where I don't have >> administrative privileges, which means I might not be able to install >> libraries in the 'right' places. This approach also makes it simple to >> create self-contained projects that can easily be moved from machine >> to machine, where only the local directory structure is important. >> Would there be any downside to this in haskell? > > Note that if the modules are in the same project, they're all in the > same hierarchy (at least that's how I see a project) and so their > names are coherent with each other and with the relative directory > hierarchy, there is no problem there. > Now, I think your preoccupation has more to do with packaging several > libraries you're not sure will be available on the final machine with > your project, and reasonably, you don't want to put them all in the > same base directory (yeah, it would be a mess). I would argue that > Haskell being a compiled language, its distribution problematics > aren't exactly the same as Python... If you're just distributing an > application you presumably would just compile it on your development > setup and then distribute the executable. Very true, but I still really like source distributions, even for compiled languages. Even though it can mean headaches, a source distribution is sometimes more portable than a binary distribution. (Usually because a clever end user can fiddle a little bit and get it working, whereas an incompatible binary is a useless blob.) > But if you really want to distribute the code source in a self > sufficient package (assuming GHC on the final machine I guess ?), you > should use Cabal : it automates dependency checking, compilation and > installation with several source directory and configurable target > installation directory. It's pretty easy to use too (mainly I suggest > you do copy/paste of a good project file on Hackage). I'll definitely check out Cabal. I'm a student, and so I'm always developing on personal and school computers, as well coding with random project partners (who always seem to be running equally random OSs). And then, to top it off, I've got to turn in assignments without being exactly sure where the grader is going to run them. I'm *always* wanting to use some library in a project, and (apt-get|port|emerge) my_sweet_library is never an option :-( Thus my concern with portable source packages. :-) These days I usually write crazy makefiles that try to do their best to compile and link in all the libraries needed, wherever they might find themselves. > > -- > Jedaï >

1 0

Module import problem
by Casey Rodarmor 28 Sep '08

28 Sep '08

Hi there! I have a problem with importing a module I'd like to use. My working directory, ~/proj, contains: ./Haskore -- a folder containing a version of haskore, this music thingy ./test.hs -- random stuff using haskore The main file in ~/proj/Haskore is ~/proj/Haskore/Haskore.hs, which contains the following module declaration: > module Haskore(module HaskoreLoader) where > import HaskoreLoader I've tried to put all the following in ~/proj/test.hs, with no luck: > import Haskore -- Could not find module `Haskore': > import Haskore.Haskore -- file name does not match module name `Haskore' Am I doing something wrong? Is there a way to place a module in an arbitrary directory, without having to modify it? Thanks so much for your help! Best, Casey Rodarmor

5 7

Haskell Preprocessor?
by Mike Sullivan 25 Sep '08

25 Sep '08

Hi All, What is the easiest and/or best way to define global "constants" in Haskell (avoiding any side effects, of course). The functionality I'm thinking of is similar to the use of global "const" variables or #defines in C code. Judging from this article ( http://www.lochan.org/keith/publications/hspp-hw99.ps.gz) and the existence of the GHC -cpp command-line option, it seems that it is possible to use the C preprocessor on Haskell code. However, the article is fairly old ('99)... Is this still (or was it ever) the best way? The point of the article is that it's not... were any of the "Haskell preprocessor" suggestions in the paper (dubbed "HsPP") adopted by GHC, and can we use them? There also seems to be a (almost fully) native implementation of a Haskell preprocessor in Hackage ( http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cpphs) Does anybody use it? What would you use if you wanted such functionality? Thanks! Mike

4 4

Default values for Data Types?
by Mike Sullivan 25 Sep '08

25 Sep '08

Hi all, I was wondering if there is syntactic sugar in Haskell for defining a default value for fields in a data type. For instance, say I have a type that is defined in record syntax: type CustomerID = Int type Address = Maybe String data Customer = Customer { customerID :: CustomerID , customerName :: String , customerAddress :: Address } deriving (Show) Is there any way to define default values for some (or all) fields such that they may be omitted from a declaration, and still have it generate a valid object? e.g.) a = Customer{customerID = 12, customerName="Bill"} -- I would like a{customerAddress} to default to Nothing (for instance). It seems to me that this would be a nice feature to have, if it does not exist. Am I missing something? Mike

4 6

Why is there no notion of a one-tuple (e.g., a '([])' as opposed to a '[]') in Haskell?
by Benjamin L.Russell 24 Sep '08

24 Sep '08

I'm having difficulty in understanding the following behavior: In GHCi: Prelude> :type [] [] :: [a] but: Prelude> :type ([]) ([]) :: [a] I.e., the types of both the empty-list '[]' and the one-tuple containing the empty-list '[]' are '[a]' (a list of a generic type variable). According to "Chapter 2. Types and Functions" (see http://book.realworldhaskell.org/read/types-and-functions.html) of Real World Haskell (beta) (see http://book.realworldhaskell.org/beta/) >Haskell doesn't have a notion of a one-element tuple. Why not? It seems that a tuple is similar to a list, except that the elements need not be all of the same type, and that a tuple, unlike a list, cannot be extended. Yet: Prelude> :type [] [] :: [a] and Prelude> :type [[]] [[]] :: [[a]] so the types of the empty-list '[]' and the one-element list containing the empty-list '[[]]' are different. Forgive me if I am missing something, but something about this asymmetry bothers me.... -- Benjamin L. Russell

8 9