August 2006 - Haskell-Cafe

Filtering a big list into the IO monad
by Gabriel Sztorc 03 Aug '06

03 Aug '06

Hello, I want to filter a list with a predicate that returns a IO value, something that filterM is supposed to do. The problem is, filterM overflows the stack for really big lists and I couldn't come up with a simple replacement for filterM that would work for lists of any size (the truth is, I can't come up with anything at all :). The question is: how to do it? Any help is appreciated.

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Translating C to English Using Haskell
by Shannon -jj Behrens 03 Aug '06

03 Aug '06

I've written a two-part series for "Linux Journal" called "Translating C to English Using Haskell". Here are links to the two parts: http://www.linuxjournal.com/article/9096 http://www.linuxjournal.com/article/9242 The catchline is: Write a program in Haskell that translates C type declarations into English. Manually translate the Haskell into English. I've tried very hard to make it an entertaining article for Haskell newbies, so if your head exploded when you tried to read "A Gentle Introduction to Haskell", give my article a shot! A special thank you goes to all the members of this who helped me when I was writing the article!

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Bits and pieces needed to build a web server
by Johan Tibell 03 Aug '06

03 Aug '06

I've got an idea for a web server model I would like to try and since I'm quite new to Haskell it would be great if someone could enumerate a couple of useful libraries to me. Here's what I need (I've put what I've found so far inside parenthesis): * Parsing of incoming HTTP request and creation of replies (http://www.haskell.org/http/) * XML parsing and generating (http://www.cs.york.ac.uk/fp/HaXml/) * Database support (mainly SQLite but perhaps also MySQL/PostgreSQL) (There seems to be several options for this one. I don't know the names of the table nor the columns at compile time so I need something that doesn't require that.) A big plus if the library is still being developed/maintained. Thanks in advance. Johan

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Edison StandardSet has inefficient function implementation
by Ahn, Ki Yung 03 Aug '06

03 Aug '06

Edision does not yet have all the asymtotic description of its functions. I got the Edision 1.2 source and looked into the code whether the container implementations meet the expected asymtotic bounds. In the module Data.Coll.StandardSet which packages Data.Set, some functions which can be O(log n) is implemented as O(n). Data.Set has a split and splitMember running in O(log n). With those functions we can implement OrdCollX operations like filterLT, filterLE, filterGT, filterGE, partitionLT_GE, partitionLE_GT, partitionLT_GT all in O(log n). However, only partitionLT_GT was O(log n) implemended using split. All other function implmentation just used its axiomaic description using CollX operations like filter and partition, which is O(n). It needs to be fixed. P.S. I haven't checked the darcs version yet. -- Ahn, Ki Yung

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[Fwd: Re: [Haskell-cafe] Variants of a recursive data structure]
by Conor McBride 03 Aug '06

03 Aug '06

Hi folks Sorry, meant to send this around, not just to Klaus Conor

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FW: Reviving wxHaskell (was: Re: [wxhaskell-users] [Haskell-cafe] Troublecompiling wxhaskell)
by Daan Leijen 03 Aug '06

03 Aug '06

Dear wxHaskell users, First of all, I apologize for not being responsive on the wxHaskell users mailing list. I recently changed jobs and countries and didn't properly take care of older email aliases. Anyway, even though I am motivated to support wxHaskell, practice proves that the project is too large for me to do alone. It would be great if someone or a group of people feel motivated enough to take over the project and release new versions that are compatible with the latest Ghc versions. One potential challenge is to find a group of testers that are willing to help compiling wxHaskell on different target systems: Windows, MacOS X, and Unix/GTK variations. I am happy to give volunteers administrator privileges on the sourceforge site and help out with the initial transition and building the initial new release (which is generally a lot of careful work in compiling and packaging correctly). All the best, -- Daan Leijen. Ps. Include me on the reply list as I am not subscribed properly to either mailing list at the moment. From: Simon Peyton-Jones Sent: Tuesday, August 01, 2006 4:46 AM To: Daan Leijen Subject: FW: Reviving wxHaskell (was: Re: [wxhaskell-users] [Haskell-cafe] Troublecompiling wxhaskell) Daan Have you seen this thread? Would you care to respond to it? Simon From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Jeremy O'Donoghue Sent: 01 August 2006 09:37 Cc: haskell-cafe(a)haskell.org; wxhaskell-users(a)lists.sourceforge.net Subject: Reviving wxHaskell (was: Re: [wxhaskell-users] [Haskell-cafe] Troublecompiling wxhaskell) Hi list, Apologies for continuing with the cross-posting, but wxhaskell-users is not exactly active. It seems like there may be enough interest in wxHaskell to justify trying to revive the project. At present, from what I can tell, Daan Leijen, the principal developer of wxHaskell, no longer has much interest in the project - the CVS log shows little activity for about 15 months now - and using wxHaskell with newer wxWidgets versions or on Unicode systems requires the application of various patches (for which the interested potential user must search diligently). Ideally, it would be best Daan for to enable new CVS committers to the project (or even to hand over project administration), but if this is not an option, is there a possibility of hosting a fork on hackage.haskell.org (I think there's a benefit in hosting Haskell-specific projects in a Haskell-friendly location). I don't have the time to nurture huge ambitions for wxHaskell, but I think it is realistic to aim for the following in the short term: * Patches to ensure that wxHaskell compiles against latest wxWidgets versions on Mac, Linux and Windows (exists today) * Add Eric Kow's Unicode patches (exist today) * Produce suitable binary packages for whatever targets we can get maintainers for, compatible with up-to-date versions of both GHC and wxWidgets. * Improve samples and documentation. In the longer term, we could look at: * Fixing operation with sash windows (a personal gripe ;-) - easy * Wrapping some of the more complex widgets - easy. * Subclassing of widgets in Haskell ( i.e. allow C++ virtual methods to be redefined in Haskell) - moderately hard * Move from the Eiffel wxc bindings to wxC project to give better maintained interface to wxWidgets - time consuming, but easy * Porting to other Haskell implementations (Hugs, YHC etc) Taking on a project like this is a very significant commitment, and only makes sense if there are a number of willing contributors. I'm happy to kick this off and try to cover some of the boring 'administrative' bits, as well as contributing on documentation and smaller bug fixes, but I don't have time to save wxHaskell from permanent bit-rot. If there's enough interest, I'll push ahead with trying to organize this. Regards Jeremy On 01/08/06, shelarcy <shelarcy(a)gmail.com> wrote: > On 7/31/06, David F. Place <d(a)vidplace.com> wrote: >> Hi: >> >> I'm having trouble compiling wxhaskell 0.9.4 under both ghc 6.4.2 and >> ghc 6.5. Does anyone know where I should direct my queries? If you use ghc 6,4,2 under Windows platform, you can use my patched version of wxHaskell. And you can get my patch from Kamiariduki's source. http://www.haskell.org/pipermail/haskell/2006-June/018043.html https://sourceforge.net/project/showfiles.php?group_id=168626 I also know Eric Y. Kow's unicode patch doesn't include db problems fix. These problems has not only wxWidgets 2.6.x change, but also unicode change. I try to fix latter case, use attached after unicode patch, this fixes compile problem ... but it has another problem that can't use non-ascii characters correctly. So, if want to accesss DB by wxHaskell's ODBC, data is garbled and cause problems. I don't know how to fix that. Attached is already sent wxhaskell-user's list, but you can't get this file. So I send again. http://sourceforge.net/mailarchive/message.php?msg_id=16222530 My patch where I put Kamiariduki's project page doesn't include unicode support that I noticed above Mailng-List's log. Don't worry about this. You can use both my patch and unicode patch, if you want. These cause conflict message but this is not problem. On Tue, 01 Aug 2006 08:18:27 +0900, Jason Dagit <dagit(a)eecs.oregonstate.edu> wrote: > I've done this recently. I put my notes on the wiki: > http://www.haskell.org/haskellwiki/WxHaskell/Install#Windows I don't know this problem when I made patch. So, my patch doesn't fix this problem. -- shelarcy <shelarcy capella.freemail.ne.jp> http://page.freett.com/shelarcy/ ------------------------------------------------------------------------ - Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys -- and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDE V _______________________________________________ wxhaskell-users mailing list wxhaskell-users(a)lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wxhaskell-users _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Can Your Programming Language Do This?
by Stephane Bortzmeyer 03 Aug '06

03 Aug '06

>From the excellent programming blog "Joel on software", a very good text if you need to convince Java or C programmers that functional programming is a A Good Thing. Probably all the readers of this list will find it brings nothing new (that's perfectly right) but it is oriented towards ordinary programmers :-) http://www.joelonsoftware.com/items/2006/08/01.html

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ANN: TextRegexLazy-0.56, (=~) and (=~~) are here
by Chris Kuklewicz 02 Aug '06

02 Aug '06

Announcing: TextRegexLazy version 0.56 Where: Tarball from http://sourceforge.net/projects/lazy-regex darcs get --partial [--tag=0.56] http://evenmere.org/~chrisk/trl/stable/ License : BSD, except for DFAEngine.hs which is LGPL (derived from CTK light) Development/unstable version is at: darcs get [--partial] http://evenmere.org/~chrisk/trl/devel/ This is the version that has eaten John Meacham's JRegex library and survived to become strong. Thanks John! It now compiles against the posix regexp provided by the c library and the pcre library, in addition to the "full lazy" and the "DFA" backends. All 4 backends can accept regular expressions given as String and as ByteString. All 4 backends can run regular expressions against String and ByteString. In particular, the PosixRE and PCRE can run very efficiently against ByteString. (Though the input for the PosixRE needs to end in a \NUL character for efficiency). So there are 4*2*2 = 16 ways to use to provide input to this library. And the RegexContext class has at least 11 instances that both (=~) and (=~~) can target. So that is 4*2*2*11*2 = 352 things you can do with this library! Get your copy today! To run with cabal before 1.1.4 you will need to comment out the "Extra-Source-Files:" line in the TextRegexLazy.cabal file. The Example.hs file: > {-# OPTIONS_GHC -fglasgow-exts #-} > import Text.Regex.Lazy > import Text.Regex.Full((=~),(=~~)) -- or DFA or PCRE or PosixRE > > main = let b :: Bool > b = ("abaca" =~ "(.)a") > c :: [MatchArray] > c = ("abaca" =~ "(.)a") > d :: Maybe (String,String,String,[String]) > d = ("abaca" =~~ "(.)a") > in do print b > print c > print d This produces: > True > [array (0,1) [(0,(1,2)),(1,(1,1))],array (0,1) [(0,(3,2)),(1,(3,1))]] > Just ("a","ba","ca",["b"]) You can also use makeRegex and makeRegexOpts to compile and save a regular expression which will be used multiple times. Each of the 4 backends has a separate "Regex" data type with its own option types. For low level access, the WrapPCRE and WrapPosix modules expose a typesafe layer around the c libraries. You can query the "getVersion :: Maybe String" to see if the have been compiled into the library. It may be possible to use WrapPCRE and the UTF8 option flags to do unicode regex matching with PCRE. ( The Full and DFA backends use the Haskell unicode Char already ). Adding new types to String/ByteString is a matter of adding instances to the existing classes. Feedback and comments of any length is welcome. -- Chris Kuklewicz

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Re: [Haskell-cafe] ANN: TextRegexLazy-0.56, (=~) and (=~~) are here
by Chris Kuklewicz 02 Aug '06

02 Aug '06

Bulat Ziganshin wrote: > Hello Chris, > > Wednesday, August 2, 2006, 3:16:58 PM, you wrote: > >> Announcing: TextRegexLazy version 0.56 > > your feature list is really strong! it will be great now to make it > a part of GHC standard distribution > > afaiu, selection of regex engine implemented via import statements? > You can actually choose which backend (=~) uses at runtime: > {-# OPTIONS_GHC -fglasgow-exts #-} > import Text.Regex.Lazy > > import qualified Text.Regex.PCRE as R > import qualified Text.Regex.PosixRE as S > import qualified Text.Regex.Full as F > > -- Choose which library to use depending on presence of PCRE library > > (=~) :: (RegexMaker R.Regex R.CompOption R.ExecOption a,RegexContext R.Regex b t > ,RegexMaker F.Regex F.CompOption F.ExecOption a,RegexContext F.Regex b t > ,RegexMaker S.Regex S.CompOption S.ExecOption a,RegexContext S.Regex b t) > => b -> a -> t > (=~) = case R.getVersion of > Just _ -> (R.=~) > Nothing -> case S.getVersion of > Just _ -> (S.=~) > Nothing -> (F.=~) > > main = print (("ba" =~ "(.)a") :: (String,String,String,[String])) The R.getVersion and S.getVersion detect whether it was compiled against PCRE or PosixRE. The (=~) is then chosen at run time.

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[Parsec] Backtracking with try does not work for me?
by Stephane Bortzmeyer 02 Aug '06

02 Aug '06

I'm trying to write a Parsec parser for a language which authorizes (this is a simplified example) "a" or "a,b,c" or "a,c" or "a,b". (I can change the grammar but not the language.) The first attempt was: ***** CUT HERE **** import Text.ParserCombinators.Parsec import System (getArgs) comma = char ',' minilang = do char 'a' optional (do {comma ; char 'b'}) optional (do {comma ; char 'c'}) eof return "OK" run parser input = case (parse parser "" input) of Left err -> putStr ("parse error at " ++ (show err) ++ "\n") Right x -> putStr (x ++ "\n") main = do args <- getArgs run minilang (head args) ***** CUT HERE **** Of course, it fails for "a,c": parse error at (line 1, column 3): unexpected "c" expecting "b" for a reason explained in Parsec's documentation (the parser "optional (do {comma ; char 'b'})" already consumed the input, do note the column number). What puzzles me is that the solution suggested in Parsec's documentation does not work either: ********* CUT HERE ******************* minilang = do char 'a' try (optional (do {comma ; char 'b'})) optional (do {comma ; char 'c'}) eof return "OK" ********* CUT HERE ******************* parse error at (line 1, column 2): unexpected "c" expecting "b" Apparently, "try" was used (do note that the column number indicates that there was backtracking) but the parser still fails for "a,c". Why?

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