- Haskell-Cafe - Haskell.org

"data" and classes question
by Chaim Friedman 29 Jan '08

29 Jan '08

Hello, I had a question about type classes and data types. I want to create one class that has a function which depends upon a parameter from another class. I would assume that this can be done, but I can't seem to write code that does it. Can anybody tell me what is wrong? Here is an example of what I want to work: module Main where class Change c class State st where transitions :: (Change c) => st -> [(c, st)] data BinState = On | Off data BinChange = OnToOff | OffToOn -- get a list of (transition, newState) pairs: --binTransitions :: BinState -> [(BinChange, BinState)] -- doesn't work --binTransitions :: (State state, Change change) => state -> -- [(change, state)] -- doesn't work --binTransitions :: (State state1, State state2, Change change) => -- state1 -> [(change, state2)] -- doesn't work binTransitions On = [(OnToOff, Off)] binTransitions Off = [(OffToOn, On)] instance Change BinChange instance State BinState where transitions = binTransitions main = putStrLn "success." -- Thanks, -- -Chaim

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Performance Issues
by Adrian Neumann 29 Jan '08

29 Jan '08

Hello Haskell-Cafe! I've read about Control.Parallel and wanted to give it a try. Here's what I wrote: ----------- import Control.Parallel import Data.List splitList :: [Integer] -> [[Integer]] splitList = unfoldr f where f [] = Nothing f ~x = Just (splitAt 30000 x) map' :: (a->b) -> [a] -> [b] map' f (x:xs) =fx `par` (seq mfxs (fx:mfxs)) where fx = f x mfxs = map' f xs map' f [] = [] product' :: [Integer] -> Integer product' = foldr1 (\x y -> seq x (x*y)) fak :: Integer -> Integer fak n = product' $ map' product' (splitList [1..n]) main = putStrLn $ seq (fak 1000000) "done" ----------- This computes 1000000!. This version takes 8m29.189s to execute. Replace foldr1 with foldr and that goes down to 7m4.315s. Replace product' with the Prelude product and it takes only 6m17.685s. Why is that so? I'm using ghc 6.8.1 on Mac OS X. Regards, Adrian

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Enterprise Haskell AMQP library initial start and would like to pass it off to someone.
by Berlin Brown 29 Jan '08

29 Jan '08

I started a AMQP library; there really isn't a lot there but at least I was able to connect to the server. Here is the code and hopefully someone else can continue with the project. The AMQP protocol is moderately complex. HTTP is simple and stuff like RMI, JMS, Database Protocols are really complicated. AMQP seems to be in the middle. There is a lot of documentation and at least two good implementations. The included java implementation and I used the python implementation as a guide. http://www.iona.com/opensource/amqp/ http://barryp.org/software/py-amqplib/ Here is my source (in subversion): http://openbotlist.googlecode.com/svn/trunk/botlistprojects/botspider/spide… If were to make a suggestion; Barry's python code is really easy to follow. I would suggest using that as a guide. and if you are brave, you can really analyze the protocol and just go off the docs. Either way. My code got as far as connecting to the server, sending the protocol out and getting back an initial response. I didnt build a queue message (frame) or much else. -- Berlin Brown http://botspiritcompany.com/botlist/spring/help/about.html

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How to make GHC 6.6 and 6.8 co-exist -- was: First go at reactive programming
by Steve Lihn 29 Jan '08

29 Jan '08

I want to raise this issue again since I disagreed (see quoted text below) that cabal-install can solve the problem of moving from one version to another completely. For most /pure/ packages, it is probably true. But for those packages that have external depedencies, this is hardly so. For example, HDBC-OBDC package depends on where the odbc driver is installled. You actually have to modify the <package.cabal> file. The other package came to mind is the pcre-regex package depending on pcre lib. On my linux server, it is not obvious pcre-lib can be found in any standard location specified in <package.cabal> file. I believe there are and will be more packages like these. How can cabal-install install this kind of packages without knowing these external dependencies? They can be a path (prefix kind of things) or an environment variable, or even an option to GHC compiler or linker (I am not so sure about this one). In my brief experience with Ocaml's GODI, GODI has a way to specify them in a so-called config package. The install package then reads what it needs from the config package. In perl's CPAN shell, you can specify them in the cpan config file (to some extent). I suggest cabal-install looks into this so that moving from one version to another is indeed painless. 6.8.(1|2) is never the last Haskell version. The fact that Cabal records the 3rd digit version in the path indicates a diligent user has to re-install all the packages on every new 3rd-digit dot release. That is a lot of work! Regards, Steve On Jan 19, 2008 12:38 PM, <gwern0(a)gmail.com> wrote: > ... > > I am asking this question in another thread. The problem is -- I've > > got many modules compiled under 6.6, some with much agony. If I switch > > to 6.8, I have to recompile them again. Two issues I image: > > > > (1) It may take lots of effort to recompile all the modules. I have > > forgetten how I got around some of the modules! Too bad... Got to take > > notes next time... > > These days, every package you'd want to install (with the exception of GHC, Darcs, and the large graphics toolkits) should be available on Hackage or at least in Cabalized form. > > If they aren't, then that's a bug or at least missing feature. The whole point of Cabal was so you don't have to take notes! >

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Why isn't there a FAQ? (was: Re: [Haskell-cafe] anybody can tell me the pronuncation of "haskell"?)
by Tim Chevalier 29 Jan '08

29 Jan '08

I thought that the .wav file that Jeremy linked to should go in the Haskell FAQ, if there was one, but it doesn't seem to exist. There's a comp.lang.functional FAQ (that isn't maintained anymore) with a section on Haskell, and a GHC FAQ, but no general Haskell FAQ. A google search for "haskell faq" turns up this as one of the first ten hits: http://www.mail-archive.com/haskell@haskell.org/msg01153.html which is hardly current. So somebody should write one. Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt "What you call 'lying', other people would call 'abstraction'." -- Alex Aiken

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Crash in Data.ByteString.Lazy.hPut
by Jamie Love 29 Jan '08

29 Jan '08

Hi there, Not sure where to raise bugs in hackage libraries, so I'm posting here. If there is a better place, please let me know. The following code crashes with a divide by zero error when using the package 'binary-0.4.' module Main where import IO import Data.Binary import Data.Binary.Put import qualified Data.ByteString.Lazy as B simpleImage = take tot (map (\x -> x `mod` 256) [1..]) where tot = 640 * 480 main = do output <- openFile "test.tmp" WriteMode B.hPut output $ runPut $ mapM_ putWord8 simpleImage -- Jamie Love Senior Consultant Aviarc Australia Mobile: +61 400 548 048 ------------------------------------------------------------ This message has been scanned for viruses and dangerous content by MailScanner and is believed to be clean.

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anybody can tell me the pronuncation of "haskell"?
by Jeremy Apthorp 29 Jan '08

29 Jan '08

On 29/01/2008, Tim Chevalier <catamorphism(a)gmail.com> wrote: > On 1/28/08, Jeremy Apthorp <nornagon(a)gmail.com> wrote: > > On 29/01/2008, Tim Chevalier <catamorphism(a)gmail.com> wrote: > > > "Haskell", stress on the first syllable; the first syllable is like > > > the word "has" and the second syllable is pronounced with a schwa > > > where the "e" is written. > > > > > > Sometimes you will hear people stress the second syllable, but that is > > > not Preferred. > > > > > > > "Hass" (like in "hassle") "kell" (to rhyme with "fell") > > > > That is not correct. The second syllable does not rhyme with "fell". > In fact, the correct pronunciation sounds like "hassle" with a 'k' > inserted between the two syllables of that word. > > (And when I say it's not correct, I'm comparing to the speech of a few > People Who Should Know.) > The two are pretty similar, but I notice now that when I say it it's more like you describe than like I describe. I blame sleep deprivation :) Jeremy -- - Jem

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question about ghc run-time .a's
by Galchin Vasili 28 Jan '08

28 Jan '08

Hello, I am trying to build Haskell packages on RedHat RHEL 5. How do I tell ghc where the ".a" (Linux archives) are? Is there an enviroment variable? Regards, Vasili

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Haskell maximum stack depth
by istarex 28 Jan '08

28 Jan '08

Hi all, Does Haskell have a maximum stack depth restriction like Java & Python, or is it only limited by the maximum stack available on the system (i.e. what would be available to a C program)? Thanks, Istarex

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How to organize code
by L.Guo 28 Jan '08

28 Jan '08

Hi, How do you organize code ? Here is a sample. Acturally, I am thinking about using this plan. Any suggestions ? > -- BasicalType.hs > type Position = (Int,Int) > data Box = Box { pos :: Position } > data Chain = Chain { pos :: [Position] } > -- Object.hs > import BasicalType > class Object o where > pos :: o -> [Position] > -- Type.hs > import BasicalType > import Object > instance Object Box where > pos = return . BasicalType.pos > instance Object Chain where > pos = BasicalType.pos > -- Main.hs > import Type > ... Regards -------------- L.Guo 2008-01-28

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