July 2007 - Haskell-Cafe

"How to Help Mailing Lists Help Readers"
by Justin Bailey 18 Jul '07

18 Jul '07

Andy Oram over at O'Reilly just posted an article analyzing how mailing list readers are helped or hindered in the Perl and Ruby on Rails communities. His most interesting conclusion is that many posters come without the background needed to understand answers. That is certainly been the case on the cafe in many instances. I believe if he had analyzed the Haskell-Cafe list he would have found a much higher percentage of user questions are answered, and sometimes in incredible depth. His conclusion seems to be that, for newbies at least, books and articles that lead the reader gently into the new domain is the answer. As I've learned Haskell over the last year I've found that to definitely be the case. The article is at http://praxagora.com/andyo/professional/mailing_list_follow_up He'll also be speaking at OSCON next week and I look forward to hearing what he has to say. Justin

1 0

RE: haskell for web
by Bayley, Alistair 18 Jul '07

18 Jul '07

> From: glasgow-haskell-users-bounces(a)haskell.org > [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of ??? > > help > haskell for web code http://www.haskell.org/haskellwiki/Applications_and_libraries/Web_programmi… Try a few of these out (whatever meets your needs). For web apps WASH and HAppS seem popular. Feel free to ask the haskell-café list for further help. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

8 10

Re: [Haskell-cafe] Time consumption nub
by Arie Groeneveld 18 Jul '07

18 Jul '07

Miguel Mitrofanov wrote: > AG> Wondering about time space consuming: 'nub' vs 'map > AG> head.group.sort' > > Prelude> :t Data.List.nub > Data.List.nub :: (Eq a) => [a] -> [a] > Prelude> :t Data.List.sort > Data.List.sort :: (Ord a) => [a] -> [a] > > nub uses less information than sort, so it MUST be slower. Ok, so when do I use nub instead of 'map head.group.sort' ? Using nub gave me a lot of trouble in terms of time consumption while handling long lists. @@i=arie

4 3

GHC 6.6.1: Where is Graphics.SOE ?
by Dmitri O.Kondratiev 18 Jul '07

18 Jul '07

*Andrea Rossato* wrote: <haskell-cafe%40haskell.org?Subject=%5BHaskell-cafe%5D%20GHC%206.6.1%3A%20Wh…> >Hi! > >as far as I know what you are looking for (Graphics.SOE) is part of >HGL. Have a look here: >http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HGL-3.1 >Hope I got it right and that this helps. > >All the best, >Andrea Andrea thanks! I tried to install HGL on Win32 and got this unresolved dependency: HGL-3.1>runghc Setup.hs configure Configuring HGL-3.1... configure: Dependency base-any: using base-2.1.1 Setup.hs: cannot satisfy dependency X11-any Any ideas? Thanks, Dmitri -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

3 2

"no-coding" functional data structures via lazyness
by Dave Bayer 18 Jul '07

18 Jul '07

Learning Haskell, the Prelude.ShowS type stood out as odd, exploiting the implementation of lazy evaluation to avoid explicitly writing an efficient concatenable list data structure. This felt like cheating, or at least like using a screwdriver as a crowbar, to be less judgmental. Recently I was playing with prime sieves and various heap data structures, while rereading Chris Okasaki's "Purely Functional Data Structures", and it dawned on me: > merge xs@(x:xt) ys@(y:yt) = case compare x y of > LT -> x : (merge xt ys) > EQ -> x : (merge xt yt) > GT -> y : (merge xs yt) > > diff xs@(x:xt) ys@(y:yt) = case compare x y of > LT -> x : (diff xt ys) > EQ -> diff xt yt > GT -> diff xs yt > > merge' (x:xt) ys = x : (merge xt ys) > > primes = ps ++ (diff ns $ foldr1 merge' $ map f $ tail primes) > where ps = [2,3,5] > ns = [7,9..] > f p = [ m*p | m <- [p,p+2..]] The code is very fast for its size; I haven't seen Haskell code posted on the web that comes close, and it is faster than any of my other tries (I posted this code to http://www.haskell.org/haskellwiki/ Prime_numbers). Effectively, it steals a heap data structure out of thin air by exploiting the implementation of lazy evaluation. It would seem that GHC's core data structures are coded closer to the machine that anything I can write _in_ Haskell. So much for studying how to explicitly write a good heap! So is there a name for this idiom, "no-coding" a classic data structure through lazy evaluation? Are there other good examples?

7 9

Re: Happy Error
by Hugo Pacheco 17 Jul '07

17 Jul '07

I'm sorry, it turned out to be pretty simples. The error appears when there are references to undefined terminal or non-terminal productions. That's it. Sorry for the messages, at least it might be of some help to someone else. hugo On 7/18/07, Hugo Pacheco <hpacheco(a)gmail.com> wrote: > > Hi Simon, > > I'm having what I supose ti be a oarsec internal error when trying to > compile some grammar. > > > I get: > $ happy -g -a -c Parser.y > happy: parE > > > Is this a known bug? I could find no reference to it. > > > I'm using happy version 1.16 and I can't reduce the problem to a single > production. > The grammar file is attached. > > > Thanks in advance, > hugo > > > > > >

1 0

Fwd: Happy Error
by Hugo Pacheco 17 Jul '07

17 Jul '07

I'm forwarding this mail, in case anyone might know about the bug. ---------- Forwarded message ---------- From: Hugo Pacheco <hpacheco(a)gmail.com> Date: Jul 18, 2007 3:45 AM Subject: Happy Error To: simonmar(a)microsoft.com Hi Simon, I'm having what I supose ti be a oarsec internal error when trying to compile some grammar. I get: $ happy -g -a -c Parser.y happy: parE Is this a known bug? I could find no reference to it. I'm using happy version 1.16 and I can't reduce the problem to a single production. The grammar file is attached. Thanks in advance, hugo

1 0

Getting lambdabot to work with 6.6.1
by Vincenz Syntactically 17 Jul '07

17 Jul '07

Dear, After a suggestion from quicksilver, I decided to write this message. To get lambdabot working on 6.6.1 you need to: 1) ensure you have the regexp-base, regexp-compat and regexp-posix from hackage installed 2) If you install them from user, make sure to add --user in the build-script of lambdabot 3) Get hs-plugins from darcs http://www.cse.unsw.edu.au/~dons/code/hs-plugins/, not from hackage (Lemmih informed it's an issue with the .hi parser) 4) Fix Lib/Parser.hs and replace "> 606" with ">= 606" in the #if GLASSGOW_HASKELL around the definition of "pling_name" and co 5) Remove the Maybe-definition of arbitrary in scripts/ShowQ.hs 6) Make sure to uncomment the runplugs section in lambdabot.cabal 7) ... 8) Profit Cheers

3 3

F#
by Edward Ing 17 Jul '07

17 Jul '07

Has anyone tried out F#? Is this a taboo subject here?

4 3

Haskell shootout game
by Hugh Perkins 17 Jul '07

17 Jul '07

Had an idea: a real shootout game for Haskell. The way it would work is: - you email a haskell program to a specific address - it shows up on a web-page The webpage shows the last submitted solution for each person - anyone can select two solutions and click "Fight" -> the scripts "fight" in an arena for a second or so, and the results are published to the website The arena itself comprises: - a 2d grid, of a certain size (or maybe variable size) - each grid cell can be a wall, or one of the opponents - the boundaries of the grid are walls - random blocks of wall are placed around the grid The opponents only perceive what is in a section of space to their front, in a 45 degree arc from either side of the direction they are facing - each player can face along one of the four grid axes Each player takes it in turns to move - at each move the player can: - move one square - turn 90 degrees, in either direction - fire Firing will score one point if the opponent is in the line of fire at that time, and there are no intervening walls. Opponents can see the direction the other opponent is facing, as long as the other opponent is in their view arc, and there are no intervening walls. Each turn is represented by a function something like: doturn :: String -> [[GridValue]] -> (Action,String) -- [[GridValue]] is a map of what Me sees this turn, or has seen previously -- the Strings are a way for the function to pass state to itself between moves data GridValue = Opponent | Me | Wall | Empty data Action = Fire | MoveNorth | MoveSouth |MoveEast | MoveWest | TurnLeft | TurnRight | Wait -- (players can move backwards and sideways) The turn would be run as a separate thread, which either terminates successfully, or is aborted after a fixed time x milliseconds (maybe 10 milliseconds?) The String that doturn produces at the end of a turn is passed back in at the beginning of the next turn (so one could use gread/gshow to serialize/deserialize arbitrary data types, and there is no limitation on what data can be stored in the state). After say 1000 turns, the results are the points of each script. (or we could give each script a number of "lives" and if its loses them all the other script wins outright) This can run on a hosted webserver probably, because each match is part of a webpage request, and lasts a maximum of about a second, so shouldnt be terminated prematurely by cpu-monitoring scripts.

10 10