December 2006 - Haskell-Cafe

Halp- haskell logic prover
by Dan Mead 06 Dec '06

06 Dec '06

Hey all, I've just finished my logic prover. I've been working on this as an undergraduate project with my advisor. Right now it only supports simple first order logic but in the spring I'll be adding predicates. Theres also a simple gui written in java 5 which is in the darcs repository. Comments are welcome! -Dan http://haskell.org/haskellwiki/Libraries_and_tools/Theorem_provers http://taz.cs.wcupa.edu/~dmead/code/halp/

1 0

HOgg 0.2.0 Released
by Conrad Parker 06 Dec '06

06 Dec '06

HOgg 0.2.0 Released ------------------- The HOgg package provides a commandline tool for manipulating Ogg files, and a corresponding Haskell library. http://snapper.kfish.org/~conrad/software/hogg/ This is the initial public release. The focus is on correctness of Ogg parsing and production. The capabilities of the hogg commandline tool are roughly on par with those of the oggz* tools[0], although hogg does not yet provide an equivalent to oggz-validate. HOgg supports chained and multiplexed Ogg bitstreams conformant with RFC3533[1]. HOgg can parse headers for CMML, FLAC, OggPCM, Speex, Theora and Vorbis media codecs, and can read and write Ogg Skeleton bitstreams. [0] Oggz: http://www.annodex.net/software/liboggz/index.html [1] RFC3533: http://www.ietf.org/rfc/rfc3533.txt Installation ------------ I am very interested in hearing about problems with building or installing the package, particularly from people who are not yet familiar with building from Haskell source. You need ghc instead of gcc; it compiles to a binary: $ ./Setup.hs configure $ ./Setup.hs build $ sudo ./Setup.hs install Building of this release has been tested with: * GHC versions 6.4 and 6.6 [2] * The Haskell build system "Cabal" versions 1.1.3, 1.1.4, 1.1.6, and the current development trunk. [3] * fps (Data.ByteString.Lazy) version 0.7, and development trunk. [4] Note that if you are using the recently-released GHC 6.6 then you will not need separate installs of Cabal or fps. You will however need to remove the word "fps" from the hogg.cabal file; see the README for details. The GHC and Cabal versions listed above span the packages available in most current distributions. I've tested on Debian unstable, Ubuntu Dapper and Ubuntu Edgy. I'm particularly interested to hear reports of build success or failure on other distributions or operating systems. [2] GHC: http://www.haskell.org/ghc/ [3] Cabal: http://www.haskell.org/cabal/ [4] fps: http://www.cse.unsw.edu.au/~dons/fps.html Usage ----- $ hogg help Usage: hogg <subcommand> [options] filename ... Commands: help Display help for a specific subcommand Reporting: info Display information about the file and its bitstreams dump Hexdump packets of an Ogg file pagedump Display page structure of an Ogg file dumpraw Dump raw (unparsed) page data Extraction: rip Rip selected logical bistreams from an Ogg file (default: all) reconstruct Reconstruct an Ogg file by doing a full packet demux Editing: merge Merge, interleaving pages in order of presentation time addskel Write a Skeleton logical bitstream Source ------ Source code is available from the darcs repository at: darcs get http://snapper.kfish.org/~conrad/software/hogg/ cheers, Conrad.

1 0

Download locations for former-base libraries?
by John Goerzen 05 Dec '06

05 Dec '06

Hi, I am working on updating and splitting things out of MissingH. MissingH is going to depend on regex-compat, HUnit, QuickCheck, and FilePath, among others. For each such library, I need to be able to tell users where they can go to download the dependency. But I'm having a lot of trouble finding canonical download locations. For instance, the homepage of regex-compat, according to http://darcs.haskell.org/packages/regex-compat/regex-compat.cabal, is http://sourceforge.net/projects/lazy-regex. But the software available for download there doesn't even provide Text.Regex. Of course, just saying "download the latest version with darcs" isn't useful, either. People that want to use my package shouldn't have to have darcs, and they also shouldn't have to fight with broken development software. Or should I be telling people to download the GHC 6.6 extralibs source? That doesn't seem quite right either; it's packaged as a tar.bz2, but what would a Windows user with HUGS do? That person probably couldn't even unpack it. Thanks, -- John

3 4

Picking out elements of a heterogenous list
by Creighton Hogg 05 Dec '06

05 Dec '06

Hi Haskell-ers, So I think I understand the idea of creating a heterogenous list using typeclasses and existentials, but I don't see how to filter the list to retrieve elements of the list that are of only one type. More concretely, taking the example here<http://haskell.org/haskellwiki/Existential_type>how could we take a list of shapes [Shape] and pull out all objects that are Squares? I don't see an obvious way this makes sense. Is there a way of doing heterogenous lists that would make this possible?

4 3

Re: haddock not finding base lib docs -- $topdir ?
by Simon Marlow 05 Dec '06

05 Dec '06

[ moving to haskell-cafe(a)haskell.org ] Claus Reinke wrote: >> This is due to the way GHC is installed on Windows, the package >> database doesn't have hardcoded pathnames, the idea being that you can >> move your GHC anywhere in the filesystem and it will still work. > > this is an essential feature (for instance, running GHC from a USB or > network drive, or just unpacking snapshots without using installers), > please do not start splicing in absolute paths! Don't worry, we don't intend to do that. >> Unfortunately this means that Haddock can't find the documentation for >> the packages. > > this part I do not understand - if GHC and ghc-pkg can find the > packages, why can't Haddock? wouldn't it just be a case of making > $topdir be in a fixed relationship to the output of "ghc --print-libdir"? well yes, but Haddock doesn't invoke 'ghc --print-libdir'. I think it would be better for ghc-pkg to hide $topdir from everyone by replacing it with its value in any ghc-pkg output. After all, $topdir is just a hack to make the GHC tree location-independent on Windows, it's not a documented feature of the package system. > or should there be a way to query ghc-pkg for the list of package > location roots? as you say, the main docs will be in a known location > relative to GHC, but perhaps docs in general should be be in a known > location relative to their packages, which ghc-pkg (or other tools for > other implementations) should be able to locate? > >> One workaround is to specify the paths by hand, using Haddock's >> --read-interface flag. You're using Haddock via Cabal though, so that >> doesn't work too well. The other workaround is to find GHC's >> package.conf file and replace the string $topdir with the literal path >> ("c:/ghc/ghc-6.6" in your case - perhaps you have to append "/doc" for >> the haddock fields, though). > > will all docs be moved into "`ghc --print-libdir`\\doc"? No. Docs can be installed wherever you like. Cabal has a policy for installation locations on Windows, which you can override if you want. > what about > local/user package databases? Same here, you can install things wherever you like. In fact, installing things inside the GHC tree isn't recommended. Cheers, Simon

1 0

unexpected compiler behavior
by jepalomar23＠ono.com 05 Dec '06

05 Dec '06

Hi, I am newbie in Haskell and do not understand why the interpreted mode differs from the compiled program. I run this code main = do putStr "line1" x<-getLine putStr "line2" y<-getLine return (x++y) either under runhugs or runghc or in a console under ghci and works properly, that is, first displays "line1" and then prompts for the corresponding inputs through the keyboard, and so on. However, if I compile the code with the command line ghc --make Main.hs -o main and launch the compiled program main, then at first prompts for the input lines through the keyboard and then displays the strings "line1" "line2". I have read in the tutorials that the command "do" implies an ordered execution and this only occurs in interpreted mode. Using the monadic notation >> , >>= occurs the same unordered behavior. ¿How can I avoid the unordered execution of this code in a compiled program? Thanks in advance. jepalomar

3 2

using greencard to write COM in haskell
by Anatoly Yakovenko 05 Dec '06

05 Dec '06

One of the goals for greencard ( http://www.cs.york.ac.uk/fp/nhc13/greencard.html#SEC2 ) is: A convenient way to write COM software components in Haskell, and to call COM components from Haskell. Are there any examples of the first goal. I would like to be able to write COM components in Haskell, so far I found this example of calling haskell form C: http://www.cs.york.ac.uk/fp/nhc13/CcallingHaskell.html >From what i understand form that example is that you register a bunch of haskell functions with your C program. So is there a way to start haskell form C? Ideally i would like something thats as easy to use as a dll, that the end use wouldn't be aware that they are using a component written in haskell. Also, I would like to have my haskell programs use my implementation of realloc which takes a context, is there an easy way to change ghc to use it? I guess I should start looking at the ghc source. Thanks, Anatoly

3 3

SYB, ext1Q, and deriving custom Typeable and Data instances
by Vyacheslav Akhmechet 05 Dec '06

05 Dec '06

Hi, I'm trying to use SYB to implement some basic relational persistance. I define a PKey datatype to represent primary keys like this: newtype PKey a = PKey a deriving (Show, Typeable, Data) I then define a sample data type like this: data User = User { id :: PKey Int, firstName :: String, lastName :: String, } deriving (Show, Typeable, Data) Finally, I try to iterate over the structure like this: test = gmapQ (gshow `ext1Q` aux) where aux :: PKey a -> String aux _ = "test" The problem is that for id gmapQ calls gshow instead of my customized version (aux). I've seen a discussion on this on haskell cafe at the following URL: http://www.opensubscriber.com/message/haskell-cafe@haskell.org/2167871.html I couldn't understand the following details: 1. Do I need to derive both Typeable and Data instances? 2. Do I need to derive them for both PKey and User or just PKey? 3. Where can I find sample code to derive these instances? Documentation shows a sample of how GHC derives Data, is that a good template to use? Also, what's a good example to derive Typeable (if in fact it needs to be done manually)? Thanks, - Slava.

1 0

'+++' and 'interleave'
by Greg Fitzgerald 04 Dec '06

04 Dec '06

Could ReadP's '+++' (symmetric choice) operator be implemented for all Monads with the 'interleave' function mentioned in Backtracking, Interleaving, and Terminating Monad Transformers<http://portal.acm.org/ft_gateway.cfm?id=1086390&type=pdf&coll=GUIDE&dl=GUID…>? This paper was a tough read for me, but it looks like they solve the same problem, but one interleaves computations within a monad and the other does it from the outside. Or am I just confused? Thanks, Greg

1 0

Four Fours problem in Haskell
by Joachim Breitner 04 Dec '06

04 Dec '06

Hi, This might be interesting for some of you (I hope): http://www.joachim-breitner.de/blog/archives/210-FourFours-in-Haskell.html Preview: An interesting programming puzzle appeard in the Blogosphere: To describe every integer from 1 to 100 as a calculation involving only and exactly four fours, and standard operations. There are solutions in C# (code) and in python (code), so I tried to do mine in Haskell. This seems to be meant for Haskell. The Type system makes the program really great to read. Functional programming allows me to conveniently create all possible associations of four operands with three binary operations. (I found five. Why are the other two sources using four?) And, in this program particular, the use of the list monad to quickly generate a list of all possible combination is really a programming-time-saver. The program is not too slow either: $ time ./fourfour>/dev/null real 0m0.917s user 0m0.852s sys 0m0.040s And it could probably be sped up by using some kind of hashing or similar stuffs. Note that the program has only 73 mostly quite short lines, while the python code has 119 and the C# code has 138 of relatively long lines (comments and empty lines not counted). I think this is impressive (unless you have worked with haskell a bit, then you are used to these kind of results). Comments welcome! Thanks, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: joachimbreitner(a)amessage.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

1 0