April 2008 - Haskell-Cafe

deriving
by PR Stanley 09 Apr '08

09 Apr '08

Hi data Bool = False | True deriving (Eq, Or, Show, Read) Bool is an instance of Eq, Ord, Show and Read. It is derived form these classes. If that is the meaning of the keyword deriving then wouldn't a different keyword such as from or derivative or even derivation be closer to the semantics of the concept? Discuss :-) Cheers, Paul

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ANN: graphics-drawingcombinators 0.1
by Luke Palmer 09 Apr '08

09 Apr '08

graphics-drawingcombinators 0.1 has just been uploaded to hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphics-drawing… Have you ever tried to write a simple 2D interface or game but found: * wxHaskell and gtk2hs are hard to install and learn * SDL does not provide enough by itself * OpenGL is gross and imperative? Then you're in luck, with the release of this very immature new module, Graphics.DrawingCombinators! This module is a wrapper around OpenGL with a functional interface, handling a small subset but still common set of 2D OpenGL functions. It currently handles some simple geometric primitives and transformations, loading images from files and SDL.Surfaces, and drawing text using SDL.TTF. I plan to add more support for lots of stuff in the future, the next priorities being: * A clean "picking" interface. * Ability to draw non-regular polygons (yeah, that's kinda important isn't it...) If there are any features which you would find useful (as long as it's in the 2D realm, I'm still contemplating how to do 3D stuff), I'd be glad to hear it and implement it. Look at example.hs in the distro for an example of how to use this with SDL. Enjoy! Luke

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ANN: Hayoo! beta 0.1
by Timo B. Hübel 09 Apr '08

09 Apr '08

Hello, we are pleased to announce the first beta release of Hayoo!, a Haskell API search engine providing advanced features like suggestions, find-as-you-type, fuzzy queries and much more. Visit Hayoo! here: http://holumbus.fh-wedel.de/hayoo Please bear in mind that this is still a beta release and we are continuously working on further improvements. Our plans for the future include: - Covering all documentation available at Hackage. - Compatibility with non-JavaScript enabled browsers. - Providing a web interface where people can point Hayoo! to an URI linking to Haddock documentation which will be automatically included in Hayoo!. Hayoo! was developed as a use-case for the Holumbus framework, which aims to help at the creation of very flexible and highly specialized search engines. Although Holumbus is still under heavy development and we have no official release yet, some informations and a Darcs repository are available at the Holumbus homepage: http://holumbus.fh-wedel.de Any suggestions and feedback is highly welcomed. Cheers, Timo B. Hübel & Sebastian M. Schlatt

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Re: [Haskell-cafe] problems building hpodder
by gwern0＠gmail.com 09 Apr '08

09 Apr '08

On 2008.04.08 15:19:12 -0500, John Goerzen <jgoerzen(a)complete.org> scribbled 1.1K characters: > On Mon April 7 2008 9:31:04 pm gwern0(a)gmail.com wrote: > > > > Well, changing the deps at least would be a good idea. > > Right. I've uploaded a new version of hpodder to hackage that will require > the correct HaXml versions. Great. > > And actually, there is a method - Cabal supports the 'stability:' field, > > but I don't believe it's enforced by cabal-install or anything. > > I think part of the problem is visibility in Hackage. Unless I'm wrong, > Hackage always takes you to the release with the highest version number. > There is no "latest stable version" list and "latest development version" > list, which would be helpful. Something akin to, say, > http://packages.debian.org/hpodder Yes, that's true. There is no good way of doing in Hackage that doesn't involve self-discipline or messing with release schedules. I do have a thought about the 'stability:' field, though. Feel free to weigh in at <http://hackage.haskell.org/trac/hackage/ticket/265>. > > (Personally, I'd update it, not just because of issues like this, but > > because there looked to be a number of changes necessary when I commented > > all the type sigs, and the longer you wait... &etc.) > > The thing is -- I don't want a stable hpodder to jump to a development > version of HaXml. The released version is working just fine, and when there > is a new stable release, I'll update then. XML parsing is quite central to > hpodder. > > -- John Sure, but there are costs to not transitioning just as there are costs to so doing. I was just saying which cost I found preferable. -- gwern JANET Salsa GSA TRANSEC LASINT A/B DJC Al BIOLWPN Xandros

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testing for same characters in lists of strings
by Jackm139 09 Apr '08

09 Apr '08

I'm new to Haskell, and I'm finding it is very different from any other language I have worked with. I have an assignment to make a program to test whether two lists use the same characters for each string. e.g. sameCharacter ["rock", "cab"] ["cork", "abc"] True My plan to tackle this was to use: nub to eliminate duplications, sort to put the characters in order, and map to put characters together.. and then somehow check to see if these characters are the same. My problem right now is just figuring out how to make a function that uses these three functions to give me a list of tuples. I know this is an awfully newbish question, and I'm not trying to get an answer to my homework problem. I just need some sort of working example syntax to off of. Any help would be appreciated! Thanks. -- View this message in context: http://www.nabble.com/testing-for-same-characters-in-lists-of-strings-tp165… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Function Precedence
by PR Stanley 09 Apr '08

09 Apr '08

Hi If f x = x and g y = y then f g x returns an error because f takes only one argument. Why can't we have function application implemented outwardly (inside-out). So f g x would be applied with gx first followed by its return value passed to f instead of putting g x in brackets. Cheers, Paul

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announcing the darcs 2.0.0 release
by David Roundy 09 Apr '08

09 Apr '08

Hello darcs users, I am pleased to announce the release of darcs 2.0.0! It's been a long time coming, and hopefully you will be pleased with the result. Notable new features include (in no particular order): * New ssh-connnection mode that dramatically improves connection times when darcs 2.0.0 is present on the server as well as the client. * New "hashed" format that improves darcs' robustness with regard to network or filesystem errors, and also allows for efficient (and safe) caching of patch data, greatly improving access times to a repository that you have had previous contact with--which is the common case of pulling, pushing or sending. This format allows patches to move back and forth with darcs1-format repositories, and is the recommended format (with an old-format mirror) if you do not wish to require darcs 2.0.0, but want to benefit from most of its improvements. * New "darcs-2" format, which features improved conflict handling, and also has all the benefits of the hashed format. Because this format features new and different semantics, it is not possible to interact with darcs 1.x, and care must be taken in switching to this format. It is the recommended format for new projects (although we haven't made it the default). * The --partial get is now replaced with a --lazy get when using hashed formats, and in fact if you forget to provide one of these flags on the command line, you can acheive a lazy get simply by hitting control-C after darcs prints a message suggesting you do this. Lazy repositories are identical to full repositories so long as you have a working network connection and the original repository is accessible, with patches downloaded as they are needed. When the original repository is no longer accessible, a lazy repository will behave like the old --partial repositories. * We have added support for pipelined http downloading via either libwww or a very recent version of libcurl. Neither of these is yet the default, but it is recommended that you try one of them if possible, as they greatly improve download times. * There is now a configurable global cache, which can help reduce network traffic if you have multiple local copies of the same--or similar--remote repositories. * I'm sure there are other new features, but this is all that comes to mind at the moment. BUGS There are numerous issues with the --darcs-2 repository semantics (see a discussion at http://wiki.darcs.net/DarcsWiki/DarcsTwo) In general, I believe the status is that anything you could do with darcs 1.x you can do with the darcs-2 semantics, but the high-level code hasn't been updated to take into account the new possibilities opened up by the new semantics. In particular, patch dependencies are much more complex. Alas, there has not been enough time (or sufficient contributors) to make the code take these complexities into account, and if you're clever you can reveal bugs. You have been warned. Darcs 2.0.0 contains some performance regressions when running with large repositories. It is possible that these regressions will be sufficient to prevent its effective use in some projects. If this describes your project--and the only way to know is to try it--then I recommend considering either switching to a different revision control system, or helping us to improve darcs. The former is certainly the easier path. If I knew how to easily make darcs dramatically faster, I would have done so. FUTURE DIRECTIONS FOR DARCS The existing model for darcs maintenance has been the use of two branches, a darcs-stable branch for well-tested changes, and a darcs-unstable branch for more experimental work. Each branch has a separate maintainer, so most patches will be reviewed by two maintainers before making it into a darcs release. This is quite a nice system. Unfortunately, it is also a rather labor-intensive process, and due to a lack of contributors, we've moving to a more streamlined process. Starting with the darcs 2.0.0 release, there will be just one central branch of darcs and only one maintainer: for now this is me, David Roundy. Moreover, I will be attempting to act as a much lower-profile maintainer than we've had previously. I will not be reading bug reports, reading the darcs-users email list or even the darcs-devel mailing list. I will only be reviewing patches that are contributed. I will not write up long user-friendly release announcements like this for future darcs releases. I will only be reviewing and applying contributed patches. What does this mean? It means that if darcs is to continue to improve, we need new contributors. If bug reports are to be read, responded to and bugs are to be fixed, we need new contributors. If darcs is to be made more efficient, we need new contributors. The new URL for the darcs repository is simply http://darcs.net. (Yes, this is a lame ending to the email, but I'm sending it anyhow, because I'm tired, and I've got to get this done.)

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Problem building HXT
by ln 09 Apr '08

09 Apr '08

Hi, I would like to try HXT, but I can't manage to build it. I resolved all the dependencies, but I get the following error: > > ln@igemyvuci:~/Desktop/hxt$ make all > > make -C src all VERSION=7.5 > > make[1]: Entering directory `/home/ln/Desktop/hxt/src' > > make install_local_hxt > > make[2]: Entering directory `/home/ln/Desktop/hxt/src' > > ghc -Wall -O2 -fglasgow-exts -ignore-package hxt -package-name hxt-7.5 > > -package parsec -package HTTP -package tagsoup --make HXT.hs > > [ 1 of 143] Compiling Text.XML.HXT.Validator.RE ( > > Text/XML/HXT/Validator/RE.hs, Text/XML/HXT/Validator/RE.o ) > > (.....) > > [100 of 143] Compiling Text.XML.HXT.Parser.TagSoup ( > > Text/XML/HXT/Parser/TagSoup.hs, Text/XML/HXT/Parser/TagSoup.o ) > > Text/XML/HXT/Parser/TagSoup.hs:305:21: Not in scope: `options' > > make[2]: *** [libHShxt.a] Error 1 The alternative installation method with cabal leads to the same result. Using Ubuntu 8.04 Beta and GHC6.8. Maybe there are incompabilities with newer versions of the dependencies? Would appreciate any help. Regards Lukas

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Parsec expressions with alphaNum operators
by Paul Keir 08 Apr '08

08 Apr '08

Hi, I'm using buildExpressionParser, and I'd like to use alphanumeric operator characters. I get an (unexpected "a") error though. With a test string like "-a" if "a" is used in any of the "reservedOpNames". I'm aiming for the Fortran operators like ".and.". The listing below may be helpful. It's taken from the Haskell wiki's "Parsing expressions and statements" article (minus the statement part).I've added an ":a:" operator. The article uses "~" as a unary operator (I'm heading for +/-). It can be tested with: $ parseTest exprparser "~a" -- code begins module Main where import Control.Monad(liftM) import Text.ParserCombinators.Parsec import Text.ParserCombinators.Parsec.Expr import Text.ParserCombinators.Parsec.Token import Text.ParserCombinators.Parsec.Language data Expr = Var String | Con Bool | Uno Unop Expr | Duo Duop Expr Expr deriving Show data Unop = Not deriving Show data Duop = And | Iff deriving Show data Stmt = Nop | String := Expr | If Expr Stmt Stmt | While Expr Stmt | Seq [Stmt] deriving Show def = emptyDef{ commentStart = "{-" , commentEnd = "-}" , identStart = letter , identLetter = alphaNum , opStart = oneOf "~&=:" , opLetter = oneOf "~&=:a" , reservedOpNames = ["~", "&", "=", ":=", ":a:"] , reservedNames = ["true", "false", "nop", "if", "then", "else", "fi", "while", "do", "od"] } TokenParser{ parens = m_parens , identifier = m_identifier , reservedOp = m_reservedOp , reserved = m_reserved , semiSep1 = m_semiSep1 , whiteSpace = m_whiteSpace } = makeTokenParser def exprparser :: Parser Expr exprparser = buildExpressionParser table term <?> "expression" table = [ [Prefix (m_reservedOp "~" >> return (Uno Not))] , [Infix (m_reservedOp "&" >> return (Duo And)) AssocLeft] , [Infix (m_reservedOp "=" >> return (Duo Iff)) AssocLeft] , [Infix (m_reservedOp ":a:" >> return (Duo Iff)) AssocLeft] ] term = m_parens exprparser <|> liftM Var m_identifier <|> (m_reserved "true" >> return (Con True)) <|> (m_reserved "false" >> return (Con False)) play :: String -> IO () play inp = case parse exprparser "" inp of { Left err -> print err ; Right ans -> print ans } -- code ends Cheers, Paul Keir Research Student University of Glasgow Department of Computing Science pkeir(a)dcs.gla.ac.uk

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Type Families: infinite compile process?
by Hugo Pacheco 08 Apr '08

08 Apr '08

Hi guys, I have been experimenting some weird stuff (risky, yes I know) but the behaviour was certainly not the one I expected: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module Nest where data Nest a = Nil | Cons a (Nest (a,a)) type family F a x :: * type instance F (Nest a) x = Either () (a,F (Nest (a,a)) x) fnn :: F (Nest Int) (Nest Int) fnn = Left () The following module fails to compile (or better, compilation never ends). Maybe there is something very bad going on due to the undecidable-instances extension? Any clue? hugo

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