August 2007 - Haskell-Cafe

Re: [Haskell-cafe] Re: Parsec is being weird at me
by Andrew Coppin 26 Aug '07

26 Aug '07

Chris Casinghino wrote: > On 8/25/07, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: > >> >> I thought the whole *purpose* of the endBy combinator was to keep >> applying one parser until the other one succeeds? >> > > > I don't think this is a lookahead problem, but rather just a > misreading of the spec of endBy. Here it is: > > (endBy p sep) parses zero or more occurrences of p, seperated and > ended by sep. Returns a list of values returned by p. > > The key text is "seperated and ended by sep". Not just ended by. > ...so then what does sepEndBy do?

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nested datatypes
by Thomas Girod 26 Aug '07

26 Aug '07

Hi there. recently I was trying to represent complex data by defining several datatypes and nesting them, such as data Foo = Foo { foo :: Bar } deriving (Eq,Show) data Bar = Bar { bar :: Int } deriving (Eq,Show) To change only a part of the data, syntactic sugar is quite convenient. But it seems to be quite painful with nested datatypes. b = Bar 10 f = Foo b foobar :: Int -> Foo -> Foo foobar i f = let nb = (foo f){bar = i} in f{foo = nb} So, my question is : is there a nifty way to modify data within a nested datatype, similar to the f{foo = bar} style ? If not, anyone is using some kind of workaround for this ?

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Hugs on Zaurus
by Ulrich Vollert 26 Aug '07

26 Aug '07

> Haskell could run on such a platform". Ofcourse this is completly > other way around in terms of power and memory. :) > > But anyway, did anyone do it? I compiled Hugs for my Sharp Zaurus SL-C3200 (http://www.trisoft.de) which is a PDA with an ARM processor. (System: Lineo uLinux , CPU: Intel® XScale™ (PXA270, 416 MHz)) Hugs was compiled by gcc 2.95.3 and can be used in the console, additionally under a special X/Qt-Environment called pdaXqtrom (http://www.users.on.net/~hluc/myZaurus/) So, it is possible to use graphics on the Zaurus, too. I didnt dare to port ghc or Yhc - which would be great to use on the Zaurus, too. Regards, Ulrich

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renderString problems
by Dave Tapley 26 Aug '07

26 Aug '07

Hi all, I'm having a lot of trouble using renderString from Graphics.UI.GLUT.Fonts. All my attempts to render a StrokeFont have so far failed. Using a BitmapFont I can get strings to appear but they demonstrate the odd behaviour of translating themselves a distance equal to their length every time my displayCallback function is evaluated. My requests are: * Does anyone know how to keep the position fixed? * Are there any good examples of (working) GLUT code available on the web, I'm finding it very hard to make any progress at the moment. Certainly not at Haskell speed :( I am using the following code: > import Graphics.UI.GLUT > main = do > getArgsAndInitialize > createWindow "" > displayCallback $= update > actionOnWindowClose $= ContinueExectuion > mainLoop > > update = do > clear [ColorBuffer] > renderString Fixed8By13 $ "Test string" > flush Cheers, Dave

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is there a way to run tests at compile time using TH
by Alex Jacobson 26 Aug '07

26 Aug '07

I'd like to have code not compile if it doesn't pass the tests. Is there a way to use TH to generate compiler errors if the tests don't pass? -Alex-

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:, infix operator, infix constructor, et cetera
by dbast0s＠yahoo.com.br 26 Aug '07

26 Aug '07

There is something called infix constructors and something else called infix operators. I'm guessing that an infix operator is really a function, and an infix constructor I don't know what it is. How would you guys describe them? (*) More questions. I learned how to define (++), and then I wanted to see how (:) would be defined. The Haskell 98 Report mentions that -- The (:) operator is built-in syntax, and cannot legally be given -- a fixity declaration; but its fixity is given by: -- infixr 5 : What does ``built-in syntax'' mean? Paul Hudak, in ``The Haskell School of Expression'' mentions that he defines (:) legally, in Appendix A. After writing data [a] = [] | a : [a] -- more pseudo-code infixr 5 : and making a couple of observations about it, he writes: ``The way (:) is defined here is actually legal syntax. Infix constructors are permitted in *data* declarations, and are distinguished from infix operators (for pattern-matching purposes) by the fact that they must begin with a colon (a property trivially satisfied by ":").'' So I'm not sure what ``legal syntax'' and ``pseudo-code'' exactly mean. The program > module Main where > data [a] = [] | a : [a] > infixr 5 : > main = putStrLn "hello world" gives %runhugs.exe Colon.lhs runhugs: Error occurred ERROR "Colon.lhs":3 - Syntax error in data declaration (unexpected `[') (*) The 5. What does that 5 do in ``infixr 5 :''?

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Converting CTime -> Int
by Rob Hoelz 26 Aug '07

26 Aug '07

Hello everyone, I'm writing an interface to C code, and one of the functions I'm wrapping returns time_t. I see that this maps to CTime in Foreign.C.Types, but I can't figure out how to convert it to an Int (or any other useful Haskell type, for that matter) for the life of me. I've poured over the standard library docs, but to no avail. Could someone give me a hint? Thanks, Rob Hoelz

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Text.Xhtml.Strict
by Marco Túlio Gontijo e Silva 26 Aug '07

26 Aug '07

Hello there. I don't know if it's off topic, but I don't know where else to ask. I've been using Text.Xhtml.Strict, and I'm wondering why the functions are mostly Html -> Html and not HTML a => a -> Html, or something similar. If they were like this, << and toHtml would be not needed, what would make it simpler to call the functions with arguments that are not Html. The question is specific to this library, but I think it's a very general one: isn't it better to have more generic functions with type changing inside? It seems to me that it would make things better from the users point of view. What do you think? -- Marco Túlio Gontijo e Silva Blog: http://marcotmarcot.blogspot.com/ Página: http://marcotmarcot.googlepages.com/ Correio, Jabber, GTalk, MSN: marcotmarcot(a)gmail.com IRC: marcotmarcot(a)irc.freenode.net IRC: marcotmarcot(a)irc.indymedia.org Skype: marcotmarcot Telefone: 33346720 Celular: 98116720 Endereço: Rua Paula Cândido, 257/201 Gutierrez 30430-260 Belo Horizonte/MG Brasil

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Files for Hugs on Zaurus
by Ulrich Vollert 26 Aug '07

26 Aug '07

Hi Phillipa, Hugs is fast enough for me to work on the Zaurus. I packed together all the files which belong to Hugs on my Zaurus and I hope that I didnt miss something. Unpack them to /, the files will stay in usr/local/bin, usr/local/lib and usr/local/share. If you want to use Hugs not only under the console, but with pdaxqtrom (which includes a X-Server, a gcc-Compiler and tons of other stuff), go to http://www.users.on.net/~hluc/myZaurus/custom.html Please tell me, whether Hugs is working for you. Regards, Ulrich

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"GHC from source" makes a great hardware test
by Dave Bayer 26 Aug '07

26 Aug '07

I recently did the classic "push a shopping cart down the aisle at Fry's" to build a Core 2 Quad computer, with Linux swap and a soft raid array spread across three 750 GB sata hard disks. I had some potential "first build" issues, notably a mishap with the lawn of copper grass that passes for a 775 cpu socket, followed by an hour of brain-surgery with a magnifying glass and a tiny screwdriver. I was very curious to test the stability of this system when it booted up after all; the best hardware test I could think of was multiple processes building GHC from source, with each iteration using the previous GHC binary as the compiler for the new build. Four iterating GHC builds in parallel is enough to peg all four cores at 100% indefinitely, with considerable disk activity to the soft raid array. The most I had going at once was 30 GHC builds; the system remained responsive enough for me to gracefully change my mind in the morning. Building multiple copies of GHC generates a lot of heat; going full tilt, the computer was drawing 220 watts at the wall. I don't use air conditioning for my summer office, so I ended up taping a small bathroom exhaust fan and dimmer switch into the back of a cardboard box, to collect the hot air from the back of the computer and send it out the window through a dryer hose. This kept the cores at 40 C (the enclosure itself has all possible fans) and my office cooler. A previous passive dryer hose arrangement kept the computer at 50 C, which is cooler than my MacBook cpu at full tilt, but I like to build things. Cardboard is an awesome quick prototyping material. Someone else in the same boat might save some time by modifying my Bash script. I ran hundreds of GHC builds without a mishap, and concluded that my system is stable. > #!/bin/bash > > # ghc-test.sh > > # Bash script to iteratively build ghc from source > # http://www.haskell.org/ghc > > # usage: > # ghc-test iters [ghc] > > # Bash scripting reference: Advanced Bash-Scripting Guide > # http://tldp.org/LDP/abs/html/index.html > > # Customize these parameters to local installation: > > sourcedir="/home/me/ghc-6.6.1" > src1="${sourcedir}/ghc-6.6.1-src.tar.bz2" > src2="${sourcedir}/ghc-6.6.1-src-extralibs.tar.bz2" > > testdir="/media/raid/ghc-test" > log="${testdir}/log.txt" > ghcdir="ghc-6.6.1" > binarypath="driver/ghc/ghc" > > > # determine build directory > time=$(date +'%Y%m%d-%H%M%S') > builddir="${testdir}/${time}" > > > # determine number of iterations > if [[ -z "$1" ]] > then > iters=2 > else > iters=$1 > fi > > > # choose ghc binary to use > if [[ -n "$2" && -f $2 && -x $2 ]] > then > ghc=$2 > else > ghc=$(which ghc) > fi > > > # check ghc binary for pulse > fib=`${ghc} -e 'let x = 0 : 1 : zipWith (+) x (tail x) in x !! 99'` > if [[ ${fib} != "218922995834555169026" ]] > then > echo "** bad ** ${ghc} ${iters} ${time}" >> ${log} > ghc=$(which ghc) > else > echo "ok ${ghc} ${iters} ${time}" >> ${log} > fi > > > # do an iteration if $iters > 0 > let iters=iters-1 > if [[ ${iters} -gt 0 ]] > then > > # build new copy of ghc from source > mkdir -p ${builddir} > cd ${builddir} > tar -jxf ${src1} > tar -jxf ${src2} > cd ${ghcdir} > ./configure --with-ghc=${ghc} > make > > # delete previous build directory, now that we're done with $ghc > if [[ -n "$3" && -d "$3" ]] > then > rm -rf $3 > fi > > # iterate > newghc="${builddir}/${ghcdir}/${binarypath}" > ${sourcedir}/ghc-test.sh ${iters} ${newghc} ${builddir} > > else > > # delete previous build directory > if [[ -n "$3" && -d "$3" ]] > then > rm -rf $3 > fi > > fi

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