December 2010 - Haskell-Cafe

dot-ghci files
by Tony Morris 12 Dec '10

12 Dec '10

I teach haskell quite a lot. I recommend using .ghci files in projects. Today I received complaints about the fact that ghci will reject .ghci if it is group-writeable. I didn't offer an opinion on the matter. I am wondering if these complaints have legitimate grounds i.e. maybe you want to have group write on that file for some reason. I'd appreciate some comments on this issue. Thanks. -- Tony Morris http://tmorris.net/

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haskell-2010 binary IO
by Permjacov Evgeniy 12 Dec '10

12 Dec '10

Does haskell 2010 include binary IO? If no, what was the reason?

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Issues Installing Vacuum-Cairo 0.4.1, gtk-0.12.0, GHC 6.12.3, Cairo 0.12.0
by aditya siram 12 Dec '10

12 Dec '10

Hi all, When I try to cabal install vacuum-cairo, I get the following error message : Resolving dependencies... Configuring vacuum-cairo-0.4.1... Preprocessing library vacuum-cairo-0.4.1... Building vacuum-cairo-0.4.1... [1 of 1] Compiling System.Vacuum.Cairo ( System/Vacuum/Cairo.hs, dist/build/System/Vacuum/Cairo.o ) System/Vacuum/Cairo.hs:239:17: Ambiguous occurrence `G.eventKeyName' It could refer to either `G.eventKeyName', imported from Graphics.UI.Gtk at System/Vacuum/Cairo.hs:21:0-36 or `G.eventKeyName', imported from Graphics.UI.Gtk.Gdk.Events at System/Vacuum/Cairo.hs:22:0-47 System/Vacuum/Cairo.hs:261:16: Ambiguous occurrence `G.eventKeyName' It could refer to either `G.eventKeyName', imported from Graphics.UI.Gtk at System/Vacuum/Cairo.hs:21:0-36 or `G.eventKeyName', imported from Graphics.UI.Gtk.Gdk.Events at System/Vacuum/Cairo.hs:22:0-47 cabal: Error: some packages failed to install: vacuum-cairo-0.4.1 failed during the building phase. The exception was: exit: ExitFailure 1 Can anyone shed some light? -deech

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: genprog-0.1
by Alberto G. Corona 12 Dec '10

12 Dec '10

DNK? I think you mean DNA. the genotype program that develips the fenotype is much more smooth and granular than a computer program. A chante un a gen does not make you to have a extra bone. It can make you to have your hand slighltly longer. or shorter. In fact there are metalevels of selection that discard abrupt changes. For example, when females ovulate there are a strong selection where thounsands of candidate cell ovules are tested and discarded. This is one of the reasons why anomalous mutations are scarce. > > 2010/12/8 Mitar <mmitar(a)gmail.com> > > Hi! >> >> On Wed, Dec 8, 2010 at 12:33 PM, Alberto G. Corona <agocorona(a)gmail.com> >> wrote: >> > But programs are non lineal. >> >> And DNK is? I doubt. ;-) >> >> I think the approach is valid, it simulates what is happening in >> nature (random insertions, deletions, changes, translations, copies, >> etc, without any "higher" meaning and guiding). Only problem is that >> people often do not want to wait millions of years for evolution of >> programs to achieve their goal. >> >> Of course mutations are only part of the story. Also combining >> different parents' genes in a way to get functional offsprings is also >> a question how to do with programs. >> >> >> Mitar >> > >

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Recent Package Updates on Haskell.org
by Gabríel A. Pétursson 12 Dec '10

12 Dec '10

Is it just me or has the list of recent package updates, as seen on the front page of haskell.org, not changed since the migration?

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Help with lhs2TeX
by Dominic Steintiz 12 Dec '10

12 Dec '10

Hi, I wonder if someone could point out what I am doing wrong. My understanding was that I should be able to create a .lhs file and run it e.g. with ghci and then use lhs2TeX to create a nice .pdf file, all from the same source. I can produce nice slides but unfortunately the .lhs does compile because of the special symbols I wish to use. So presumably I need to pre-process my file before submitting to ghci. However, I was unable to find any information about this in the excellent guide. For example: > \documentclass{beamer} > > %include polycode.fmt > %format m_ = "\mu" > %format ^ = " " > %format inv(a) = a^"^\circ" > %format in_ = "in" > > \begin{document} > > \title{Some Notes on Category Theory with Some Applications to Computer > Science} > > \author{Dominic Steinitz} > > \begin{frame} > \frametitle{Haskell Example: Total} > Initial algebras can be defined as follows: > \begin{code} > newtype m_^f = In {inv(in_) :: f (m_^f )} > \end{code} > \end{frame} > > > \end{document} produces a nice slide but does not compile. If I change the offending line to > newtype Mu f = In {in_ :: f (Mu f)} then all is well but the slide does not look as nice

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Re: [Haskell-cafe] [Haskell] haskell.org migration complete
by Henning Thielemann 11 Dec '10

11 Dec '10

It seems that the new Wiki interprets a line-feed as beginning of a new paragraph. Before only two LFs where interpreted as paragraph separators. I liked the former interpretation because it allowed to split a paragraph into lines, where each sentence or each part of a sentence is on an individual line. This is better for all the line-oriented Diff-tools. Also code like <haskell> class C a where method :: a instance C Int where method = bla </haskell> is formatted with strange indentation as class C a where method :: a instance C Int where method = bla

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Monads in the 'hood
by Greg Meredith 11 Dec '10

11 Dec '10

Dear Haskellians, Keepin' it light. For your amusement this weekend: monads in the hood<http://www.youtube.com/watch?v=rYANU61J5eY> . Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: [Haskell-cafe] A home-brew iteration-alike library: some extension quiestions
by Antoine Latter 11 Dec '10

11 Dec '10

Also, one thing that tripped me up is that your "Stream" type is fundamentally different from the "Stream" types in the iteratee/enumerator libraries - yours is more of a monadic list in the inner monad, with explicit errors. How does this change the operation of the Iterator type? I hope I am not pestering you too much :-) I think it is really fascinating how many different approaches people have to the left-fold-enumerator idea, and it is hard for me to grasp which differences are fundamental and what the differences mean. Also, in what way are the other libraries not Haskell-2010 compliant? I haven't experimented too much with this sort of thing, since Cabal defaults to the Haskell '98 language, and that's how I install most things. Thanks for your response, Antoine On Thu, Dec 9, 2010 at 2:07 PM, Permjacov Evgeniy <permeakra(a)gmail.com> wrote: > On 12/09/2010 10:54 PM, Antoine Latter wrote: >> I only have some surface level questions/comments - >> >> What existing packages is this similar to? How is it different from >> any previous work in the area? >> > Main idea was taken from Iteratees invented by Oleg Kiselev (there are > two packages on hackage implementing this ideas: data-iteraties and > enumerator packages) > The difference is, that I wished haskell-2010 compilant package for > left-foldable streams, including support for easy builing, transcoding, > merging and folding of streams relying on do-notation (see > Data.Iteration.Unicode.* for examples of transcoding streams: it is > quite clean and easily understandable) and ability to specify easily > monadic actions in stream processors. >> Also, likes looks like you don't need the 'Monad m' constraint on your >> various Monad and Functor instances in Data.Iteration.Types, which I >> think is one of the nicest properties of the continuation-based >> approach to something like this. > Errgh. That may be true, but I did not consider non-monadic context at > all, so I enforced this constrain mindlessly >> It's a mater of taste which way to go, but I prefer importing modules >> qualified rather than have type-suffixes on functions - so I would >> rather use 'I.next' and 'A.next' instead of 'nextI' and 'nextA'. But >> reasonable people can disagree on this. >> >> Take care, >> Antoine > Thanks! >> On Thu, Dec 9, 2010 at 1:42 PM, Permjacov Evgeniy <permeakra(a)gmail.com> wrote: >>> Hi. I Wrote a simple iteration library. It was not intensively tested, >>> so it MAY contatin bugs, but it is very unlikely. The library is >>> currently on github: https://github.com/permeakra/iteration >>> >>> I'm not ready to upload it to hackage, as some testing and extension is >>> really needed. However, I'd like to know about possible flaws. >>> >>> Current goal is addition of byte-stream (de)compression and IO functions >>> extenstion. After this package will be cabalized and uploaded to >>> hackage. So, while design is not frozen yet, I'm interested in criticism -)/ >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> > >

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ANN: archiver 0.1 and 0.2
by Gwern Branwen 11 Dec '10

11 Dec '10

I'd like to announce a small utility and library which builds on my WebArchive plugin for gitit: archiver http://hackage.haskell.org/package/archiver Source is available via `darcs get http://community.haskell.org/~gwern/archiver/`. The library half is a simple wrapper around the appropriate HTTP requests; the executable half reads a text file and loops as it (slowly) fires off requests and deletes the appropriate URL. That is, 'archiver' is a daemon which will process a specified text file, each line of which is a URL, and will one by one request that the URLs be archived or spidered by http://www.webcitation.org * and http://www.archive.org ** for future reference. That is, WebCite and the IA will store a copy of the HTML and hopefully all the non-dynamic resources the web pages need. (An example would be http://bits.blogs.nytimes.com/2010/12/07/palm-is-far-from-game-over-says-fo… and http://webcitation.org/5ur7ifr12) Usage of archiver might look like `while true; do archiver ~/.urls.txt gwern0(a)gmail.com; done`***. There are a number of ways to populate the source text file. For example, I have a script `firefox-urls` which is called in my crontab every hour, and which looks like this: #!/bin/sh set -e cp `find ~/.mozilla/ -name "places.sqlite"` ~/ sqlite3 places.sqlite "SELECT url FROM moz_places, moz_historyvisits \ WHERE moz_places.id = moz_historyvisits.place_id and visit_date > strftime('%s','now','-1 day')*1000000 ORDER by \ visit_date;" >> ~/.urls.txt rm ~/places.sqlite This gets all visited URLs in the last time period and prints them out to the file for archiver to process. Hence, everything I browse is backed-up. More useful perhaps is a script to extract external links from Markdown files and print them to stdout: import System.Environment (getArgs) import Text.Pandoc (defaultParserState, processWithM, readMarkdown, Inline(Link), Pandoc) main = getArgs >>= mapM readFile >>= mapM_ analyzePage analyzePage x = processWithM printLinks (readMarkdown defaultParserState x) printLinks (Link _ (x, _)) = putStrLn x >> return undefined printLinks x = return x So now I can take `find . -name "*.page"`, pass the 100 or so Markdown files in my wiki as arguments, and add the thousand or so external links to the archiver queue (eg. `find . -name "*.page" | xargs runhaskell link-extractor.hs >> ~/.urls.txt`); they will eventually be archived/backed up and when combined with a tool like link-checker**** means that there never need be any broken links since one can either find a live link or use the archived version. General comments: I've used archiver for a number of weeks now. It has never caught up with my Firefox-generated backlog since WebCite seems to have IP-based throttling so you can't request more often than once per 20 seconds, according to my experiments, so I removed the hinotify 'watch file' functionality. It may be I was too hasty in removing it. * http://en.wikipedia.org/wiki/WebCite ** http://en.wikipedia.org/wiki/Internet_Archive *** There are sporadic exceptions from somewhere in the network or HTTP libraries, I think **** http://linkchecker.sourceforge.net/ -- gwern http://www.gwern.net

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