February 2005 - Haskell-Cafe

how to avoid overlapping instance error?
by S. Alexander Jacobson 25 Feb '05

25 Feb '05

Currently, the HAppS.DBMS lib requires the user to provide a Show instance for their table types. An example might be: instance (Show item) => Show (MyTable item) where showsPrec d table = showsPrec d $ Set.toList $ myTableSet table But the Table class itself defines a toSet function so I think I should be able to do this once for all Table instances: instance (Show item,Ord item, Table table item p) => Show (table item) where showsPrec d table = showsPrec d $ Set.toList $ toSet table But I get an error telling me I am overlapping with Show [a]. Since [] is not an instance of Table, I don't see why there should be an overlap. -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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RE: [Haskell-cafe] GHC compilation with postres libs
by Bayley, Alistair 25 Feb '05

25 Feb '05

> From: Edwin Eyan Moragas [mailto:haaktu@gmail.com] > > -compiled Main.hsc with: > $ hsc2hs -I/usr/local/pgsql/include Main.hsc > all went well with this > > - tried compiling this way: > $ ghc -o test -L/usr/local/pgsql/lib Main.hs Do you not also need the -l option? e.g. (wild guess, assuming library is libpg.so) > $ ghc -o test -L/usr/local/pgsql/lib -lpg Main.hs Alistair. ----------------------------------------- ***************************************************************** 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. *****************************************************************

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RE: [Haskell-cafe] GHC compilation with postres libs
by Bayley, Alistair 25 Feb '05

25 Feb '05

> From: Edwin Eyan Moragas [mailto:haaktu@gmail.com] > > eyan@tanis $ ghc -o test -L/usr/local/pgsql/lib/ -lpg Main.hs > compilation IS NOT required > /usr/lib/gcc-lib/i486-slackware-linux/3.3.4/../../../../i486-s > lackware-linux/bin/ld: > cannot find -lpg > collect2: ld returned 1 exit status > > still no go. :-) > thanks. Where is the library (the .so file) and what is it called? Can you show me the contents of /usr/local/pgsql/lib ? ----------------------------------------- ***************************************************************** 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. *****************************************************************

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YAWQ (Yet Another Wash Question)
by John Goerzen 24 Feb '05

24 Feb '05

Thanks for everyone that's helped me out with my Wash questions. I have one more. I have a textInputField that I only want to display on a form in certain cituations. I can't figure out how to make this work. For instance: ... do if cond then x <- textInputField ... else () ... submit ... Well, two problems there... first, the scope of the x doesn't reach to the submit. Second, there's a type issue. So I thought maybe I could figure out something like this: ... do x <- if cond then textInputField ... else () still stuck with the typing problem. So I thought maybe something like this... ... do let x = if cond then Just $ textInputField ... else Nothing Though I suspect this won't work either, since the textInputField won't actually be processed at the right place in the program. I then thought maybe I could kludge by this with a hidden field, but I can't find one of those, either. Ideas, anyone? Thanks, John

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new Haskell hacker seeking peer review
by Sean Perry 23 Feb '05

23 Feb '05

I am learning Haskell, so I decided to implement everyone's favorite, overused Unix command -- cat. Below is my simple implementation, comments about style, implementation, etc. are welcomed. In particular, is my untilEOF idiomatically ok? Is there a better way to accomplish this? Also, while talking about untilEOF, it is slightly annoying that hIsEOF returns IO Bool and that functions like 'not' only want Bool. Sure makes the logic tests feel like more work than they should be. cat.hs: module Main where import IO import System(getArgs) untilEOF :: Handle -> (Handle -> IO ()) -> IO () untilEOF hdl f = do eof <- hIsEOF hdl if eof then return () else do f hdl untilEOF hdl f cat :: Handle -> IO () cat hdl = do line <- hGetLine hdl putStrLn line catFile :: FilePath -> IO () catFile path = do hdl <- openFile path ReadMode untilEOF hdl cat main :: IO () main = do args <- getArgs if (length args) > 0 then mapM_ catFile args else untilEOF stdin cat

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module Crypt_Discordian - code critique requested
by Terrence Brannon 23 Feb '05

23 Feb '05

Hi, I am getting into Haskell so I decided to convert a Perl module of mine: http://cpan.uwinnipeg.ca/htdocs/Crypt-Discordian/Crypt/Discordian.html into Haskell. I was pleased at the cleanliness and conciseness of the Haskell code. However, I am sure that it can be improved on and am soliciting any feedback you may have about this module. Thanks, metaperl on #haskell module Crypt_Discordian where import List vowel_list = "aeiouAEIOU" is_vowel c = c `elem` vowel_list move_vowels lis = move_vowels' lis [] [] move_vowels' [] c v = v ++ c move_vowels' (x:xs) c v | is_vowel x = move_vowels' xs c (x:v) | otherwise = move_vowels' xs (x:c) v remove_spaces str = filter (\x -> x /= ' ') str encrypt str = List.sort $ move_vowels $ remove_spaces str {- The algorithm for Discordian text encryption is given at: http://www.principiadiscordia.com/book/78.php After implementing this, I realized that all the early steps are a farce. But anyway, here is the algorithm in case you don't enjoy tilting your head to read a page: Step 1. Write out message (HAIL ERIS) and put all vowels at the end (HLRSAIEI) Step 2. Reverse order (IEIASRLH) Step 3. Convert to numbers (9-5-9-1-19-18-12-8) Step 4. Put into numerical order (1-5-8-9-9-12-18-19) Step 5. Convert back to letter (AEHIILRS) This cryptographic cypher code is GUARANTEED TO BE 100% UNBREAKABLE .. so says the Principia Discordia. But I think we can generate and test to break it. Many thanks to kosmikus and Pseudonym for their help in developing this module -} -- Carter's Compass: I know I'm on the right track when, by deleting something, I'm adding functionality.

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GUI components for displaying Html pages
by David Owen 23 Feb '05

23 Feb '05

Hi all, I'm looking for a Haskell GUI library which supports the display of rendered Html pages in the same way that web browsers do. I've been getting to grips recently with wxHaskell as it was recommended on haskell-cafe. I was hoping that a special pane component (or something similar) existed in wxHaskell for the purpose. I've looked through the API but can't seem to see anything which looks suitable. Does anybody know if wxHaskell can support this? If not is anybody aware of a Haskell GUI library which caters for this purpose. Also, I am hoping to make use of HaXmL to deal with the Html parsing or failing that attempt a very simple home-made parser. Should anybody out there know of a better way of attacking this aspect then that would be much appreciated too! (Apologies, this email might have been better suited to the libraries mailing list!) Cheers all! David (Swansea, UK)

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WASH defaults in inputFields
by John Goerzen 22 Feb '05

22 Feb '05

Hi, Hope this is not a stupid question, but I RTFM'd and couldn't find the answer. I am designing a form that will be used to edit some data that is in the database. I want users to pull up the form and have all the input fields pre-filled with the current state of the database (so they don't have to re-key all that), then the database gets updated with they hit submit. Simple to do in HTML, but I can't figure out how to do this with Wash. The inputFields don't seem to take a parameter giving a default, nor does there appear to be any way to set it later. Ideas? Thanks, John

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ANN: HAppS -- Haskell Application Server v 0.2
by S. Alexander Jacobson 22 Feb '05

22 Feb '05

HAppS is a Haskell library for building Internet applications, featuring: * HAppS.ACID: Guarantee application integrity in the face of unplanned outages using this module's integrated write-ahead logging and checkpointing framework. * HAppS.DBMS: Do relational operations in Haskell (rather than SQL) on Haskell sets (outside the IO Monad). Define custom indices for your Haskell datatypes (e.g. geographic/geometric types). Use in combination with ACID for a robust relational DBMS customized for your application. * HAppS.Protocols: Expose your application using as an HTTP server and/or by recieving and sending SMTP. - License HAppS is released by HAppS.org under the terms of the GNU General Public License Version 2, a copy of which is enclosed with this package. - How do I get HAppS? More information is available at http://HAppS.org A tarball of HAppS is available at http://HAppS.org/HAppS/happs.tar.gz. HAppS is also available via darcs: darcs get http://happs.org/HAppS ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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RE: [Haskell-cafe] Problems building GreenCard
by Simon Peyton-Jones 22 Feb '05

22 Feb '05

-fno-prune-tydecls is a dead flag. Just remove it. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Dmitri Pissarenko | Sent: 15 February 2005 21:04 | To: haskell-cafe(a)haskell.org | Cc: sof(a)galois.com; alastair(a)reid-consulting-uk.ltd.uk | Subject: Re: [Haskell-cafe] Problems building GreenCard | | Hello! | | I could fix the problem by replacing the line | | GHC_OPTS = -cpp -fglasgow-exts -fno-prune-tydecls | | in file lib/Makefile by this one | | GHC_OPTS = -cpp -fglasgow-exts | | However, I don't know what this -fno-prune-tydecls does and whether it is | important (for using Green Card to access Java). | | Best regards | | Dmitri Pissarenko | -- | Dmitri Pissarenko | Software Engineer | http://dapissarenko.com | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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