March 2007 - Haskell-Cafe

Pls help about stupid question!
by iliali16 06 Mar '07

06 Mar '07

Hi folks I have this funciton to implement : leftJustified Now this function shoud justify the text acording to the number of characters per line.So for example I u put the following leftJustified :: Integer -> String -> String leftJustified n xs | acctually my problem is that I have for example the following string Hi my name is X. And what I want is to leftJustified it by 3 characters and I have to do it whithout choping words and I have to do it on the next line and what I came up is very stupid becouse it doesn't work at all like what I need but atleast it shows the string eventhough it counts not the characters but the words. leftJustified :: Integer -> String -> String leftJustified 0 xs = [] leftJustified _ [] = [] leftJustified n xs |lList (words xs) <= n = xs |otherwise = x : \n (leftJustified xs) This doesnt work but if I delete the last line it owrks as shown above. Please help me to understand how this should be done. -- View this message in context: http://www.nabble.com/Pls-help-about-stupid-question%21-tf3357906.html#a933… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Recursion
by unknown＠example.com 06 Mar '07

06 Mar '07

Does the following code increase the level of recursion once for each input line or is the recursive construct converted to an iteration? Thanks, Dave Feustel main :: IO () main = do line <- getLine processIt line main processIt :: String -> IO () processIt s = do print (length s)

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StaticDTD - Type safe markup combinator libraries from DTDs
by mm 06 Mar '07

06 Mar '07

Hello, I would like to inform you that i am currently working on a tool that transforms a Document Type Definition to a library. The resulting library contains combinators that assure proper nesting of elements to some extend. I am planning to add more constraints, that will also take care of the order of occurence of children (content model). The parsing of the DTD is done with HaXml's DTD parser. The whole thing is very much inspired by HList, HaXml, WASH/HTML and HSXML. The darcs repo is at: http://m13s07.vlinux.de/darcs/StaticDTD/ Marcel Manthe -- No one wins It's a war of man.

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Pulling a monad out of a functor
by Bryan Burgers 06 Mar '07

06 Mar '07

Haskellers, Recently, I have come across a couple of situations in which I need to "pull a monad out of a functor" so to speak. The first time it was needed, I didn't think much of it, but the second time started me thinking whether there is already an existing construct for this. The type signatures I have in mind are: pull :: (Monad m, Functor f) => f (m a) -> m (f a) fmapM :: (Monad m, Functor f) => (a -> m b) -> f a -> m (f b) fmapM f = pull . fmap f You may notice that these are already defined for the List functor as 'sequence' and 'mapM'. It seems to me that if this were a class, the implementor would be defining a traversing order by defining pull; in the list case this traversing sequence is clearly defined as head to tail. I am wondering if there is already a construct for this that I am not aware of (a class, a way to defined it using monads and functors that I am not aware of, an idiom)? Is this something that frequently happens for others, or is it a sign a few of my designs have been flawed recently? Bryan

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haskell zlib on win32
by Tim Newsham 05 Mar '07

05 Mar '07

Building haskell's zlib library for win32 is not trivial since it relies on an external C library. Most of the required tools are shipped with the ghc binary distribution though. I finally bit the bullet and jumped through the build hoops to install it. My notes are attached below. The build could be nicer if it was more cabalized -- ie. have the cabal file directly reference the zlib-1.2.3 sources and headers.. Problems are possible reliance on particular versions, duplicating already installed code (although in this case its pretty small), and possibly figuring out how to perform the "configuration" process without a shell.... Any opinions on this approach? --- WIN32 BUILD INSTRUCTIONS ---- I built from cygwin. Probably a little easier if you have mingwin installed. I have ghc-6.6 installed in /c/ghc/ghc-6.6 (/c is a mount point for c:\ "mount c:\\ /c"). Fetch sources $ darcs get http://haskell.org/~duncan/zlib $ cd zlib $ wget http://www.zlib.net/zlib-1.2.3.tar.gz $ tar -xzf zlib-1.2.3.tar.gz Prefer ghc's gcc (mingwin) rather than cygwins. $ export PATH=/c/ghc/ghc-6.6:/c/ghc/ghc-6.6/bin:$PATH Build zlib C code $ ./configure --prefix=/c/ghc/ghc-6.6 --libdir=/c/ghc/ghc-6.6 $ make install build haskell zlib $ cd .. $ runhaskell Setup.py configure $ runhaskell Setup.hs build $ runhaskell Setup.hs install Tim Newsham http://www.thenewsh.com/~newsham/

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Lambdabot on Windows?
by Iain Alexander 05 Mar '07

05 Mar '07

Does anyone know of a standalone version of lambdabot which will run on Windows? (When I try to configure lambdabot-4.0, I get Setup.hs: cannot satisfy dependency unix-any.) -- Iain Alexander ia(a)stryx.demon.co.uk

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Haskell sequence hosting
by John Goerzen 05 Mar '07

05 Mar '07

Hi everyone, I've been hosting sequence.complete.org on my own server for awhile, but have found that I haven't had time to maintain it lately. I'm looking for someone else that might be willing/able to host and maintain it. It's using Drupal on MySQL, though a fairly old version of Drupal at the moment. -- John

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ANN: New release of regex packages
by Chris Kuklewicz 05 Mar '07

05 Mar '07

I would like to announce new versions of the regex-* packages. This announcement covers: Version Package Description 0.83 regex-base -- Type Classes and generic instances 0.90 regex-compat -- Uses regex-posix to provide old API 0.91 regex-dfa -- backend, pure haskell, no submatch capture 0.90 regex-parsec -- backend, pure haskell 0.91 regex-pcre -- backend, links against libpcre 0.91 regex-posix -- backend, links against standard c library 0.92 regex-tdfa -- backend, pure haskell (Posix semantics) 0.91 regex-tre -- backend, links against libtre (currently buggy) These all compile, install, and run a few test correctly. Most notably, I consider regex-tdfa to be of useful quality now. Summary of changes and recommendations: * all packages: ** import Text.Regex.XXX exposes (getVersion_Text_Regex_XXX :: Data.Version) which allows programs to access the current version number of the package ** LICENSE file provided (all are 3 clause BSD except regex-dfa is LGPL) * regex-base: ** BUGFIX: one of the RegexContext instances used tail unsafely ** RegexMaker now has makeRegexM and makeRegexOptsM for better error handling ** Extract has new instances for (Seq Char) and (ByteString.Lazy), as well as the previous [Char] and ByteString instances * all backends: ** Now support [Char], (Seq Char), ByteString, and ByteString.Lazy ** CHANGE: The (=~~) monadic match operators now use makeRegexM and will call 'fail' when a regular expression cannot be parsed. ** CHANGE: (import Text.Regex.BACKEND) now re-exports (module Text.Regex.Base) * regex-dfa: ** BUGFIX: No longer hangs on repeated nullable subpatterns * regex-tdfa: ** New backend in pure haskell that provides true Posix semantics ** Runs with excellent memory usage ** I recommend this backend for Posix extended regular expressions (leftmost longest). * regex-compat: No other changes, still uses regex-posix underneath, not recommended * regex-parsec: No other changes, I recommend regex-tdfa or regex-pcre instead * regex-pcre: No other changes, best provider of Perl's left-biased regular expressions * regex-posix: No other changes, very slow (on OS X the underlying C library is buggy) * regex-tre: No other changes, underlying libtre version 0.7.5 is still buggy Dependencies: All of the above packages have been updated to depend on regex-base>=0.80. I have only tested with GHC 6.6 on Mac OS X 10.4.8 (PPC, 32bit). Porting the backends to other Haskell compilers should be possible, though they may not support the polymorphic type class API that regex-base provides. Porting to GHC 6.4 should work once the support for (Seq Char) and ByteString[.Lazy] has been edited or externally obtained. I think only regex-tdfa actually uses bang patterns at the moment, and those could also be removed when porting. Where to get more information and the packages themselves: There is a slowly developing wiki page at http://haskell.org/haskellwiki/Regular_expressions for holding more documentation relating to these packages. I have uploaded tar.gz sources for each of the packages to hackage: http://hackage.haskell.org/packages/hackage.html They are listed under the "Text" Category: http://hackage.haskell.org/packages/archive/pkg-list.html#cat:Text Development and bug fixes continue in the darcs repositories under http://darcs.haskell.org/packages/regex-unstable/ To checkout one of the above versions with darcs you can use commands like darcs get --partial --tag=0.83 regex-base where the --tag=0.83 may be omitted to get the latest unstable version To install the packages once you have the source: For regex-pcre and regex-tre (and perhaps regex-posix) you might need to edit the end of cabal file to provide Include and Lib directories to the corresponding C library. # Compile Setup.hs for better startup speed ghc --make Setup.hs -o setup # I use my own path and "--user" .I recommend doing this to avoid overwriting # the global regex-* from GHC 6.6 ./setup configure --enable-library-profiling --prefix=YOUR_PATH --user ./setup build ./setup install Producing haddock documentation may not work and may not be up to date, with the important exception of regex-base. Future Plans: * regex-base: add support for generalized indices instead of the current Int * regex-tdfa: Improve DFA algorithm and further limit memory allocation. Try to improve performance of ByteString.Lazy matching. Cheers, Chris Kuklewicz

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Tricks for making low level haskell hacking easier
by dons＠cse.unsw.edu.au 05 Mar '07

05 Mar '07

For those of you who like your Haskell code fast, then, if you're like me, reading Core output all day can give you headaches. Here's a couple of tricks I use to make optimising low level stuff easier. 1) Use Hscolour to pretty-ifiy the Core so its more parsable: ghc -O Foo.hs -ddump-simpl | HsColour -tty | less -r will colourise the Core, and pipe it into less (which will display the metachars). A screenshot: http://www.cse.unsw.edu.au/~dons/tmp/screen-core.png 2) Use vimdiff on asm output, checking the results for different ghc / gcc flags. Yes, sometimes you get down to the asm level, and it can make all the difference. It is useful when writing (*cough*) entries for the great language shootout, to compare the asm output. Currently I do this by compiling a program with different flags, or slightly different code, with -keep-tmp-files -no-recomp set, and then vimdiff the resulting asm files: http://www.cse.unsw.edu.au/~dons/tmp/asm-diff.png (before on the left, after generating pentium4 instructions on the right). Do people have any other tricks for making the tweaking of low level ghc output easier (that we should incorporate into vim/emacs support)? -- Don

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Haskell 6.6 problem
by Dave Feustel 05 Mar '07

05 Mar '07

make and make install of ghc 6.6 completed successfully, but exe generated by ghc fails to load. ===test.hs=== main :: IO() main = putStr "This is a test\n" ============= ghc test.hs compilation IS NOT required /usr/bin/ld: cannot find -lgmp collect2: ld returned 1 exit status Is there a simple way to fix this? Thanks.

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