- Haskell-Cafe - Haskell.org

hoogle broken?
by Michael Vanier 07 Nov '07

07 Nov '07

It looks as if hoogle isn't working. I get 404s whenever I try to do any search on hoogle. Mike

4 4

Haskell home page not updated
by Paul Johnson 07 Nov '07

07 Nov '07

The Haskell home page hasn't been updated since 23rd September, even though a "Haskell Weekly News" came out on October 25th. Paul.

2 1

Problem linking with GHC 6.8.1
by david48 07 Nov '07

07 Nov '07

This is a program that works under ghci. GHC was installed using the tarball on hashell.org. When I try to compile the program, it can't link. It used to work with 6.6.1. ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.1 ghc --make -O2 -o edimail Main.hs [1 of 5] Compiling Decode ( Decode.hs, Decode.o ) [2 of 5] Compiling Net ( Net.hs, Net.o ) [3 of 5] Compiling Smtp ( Smtp.hs, Smtp.o ) [4 of 5] Compiling Pop3 ( Pop3.hs, Pop3.o ) [5 of 5] Compiling Main ( Main.hs, Main.o ) Linking edimail ... /usr/bin/ld: cannot find -lgmp collect2: ld a retourné 1 code d'état d'exécution

3 6

GHC 6.8 and Mac OS X Leopard
by Tim Clark 07 Nov '07

07 Nov '07

Has anyone got GHC 6.8.1 building on Leopard? Unfortunately macports isn't even letting me build GHC 6.6.1 since the bootstrap GHC isn't working. Does anyone have any ideas or am I just too impatient? Thanks, Tim Clark

2 2

Monte Carlo Pi calculation (newbie learnings)
by Alex Young 07 Nov '07

07 Nov '07

Hi all, I'm new to Haskell, and don't quite understand how IO and lazy evaluation work together yet. In order to improve my understanding, I thought I'd try to knock together a translation of a fairly simple problem I can code up in C, that of calculating pi by throwing random numbers about. The C code I started with is as follows: ////////////////////////////////////////////////// #include <stdio.h> #include <stdlib.h> #include <time.h> int main(int argc, char *argv[]) { if(argc != 2) { printf("usage: %s iteration_count", argv[0]); return 1; } int total; if(sscanf(argv[1], "%d", &total) != 1) return 1; int count = 0; int unit_radius = RAND_MAX * RAND_MAX; int i; srand(time(NULL)); for(i = 0; i < total; i++) { int x, y; x = rand(); y = rand(); count += x*x + y*y < unit_radius; } double pi = (count << 2) / (double)total; printf("Count: %d\nTotal: %d\n", count, total); printf("%f", pi); return 0; } /////////////////////////////////////////////////// All very simple - the key is that I'm counting the result of a conditional evaluated inside a loop. Ignore whether it's correct, or accurate, for now; it happily runs and gives an accurate-looking result when run with an argument of 10000000. My (thoroughly ugly, not currently working) Haskell version looks like this: {--------------------------------------------------} module Main where import Random import System.Environment import List import Monad randMax = 32767 unitRadius = randMax * randMax rand :: IO Int rand = getStdRandom (randomR (0, randMax)) randListTail accum 0 = accum randListTail accum n = randListTail (rand : accum) (n - 1) randList :: Int -> [IO Int] randList n = randListTail [] n randPairs :: Int -> [(IO Int, IO Int)] randPairs n = zip (randList n) (randList n) pairIsInside x y = if x*x + y*y < unitRadius then 1 else 0 doCountPair :: (IO Int, IO Int) -> IO Int doCountPair (a, b) = do x <- a y <- b return (pairIsInside x y) fSumListTail :: Int -> [(IO Int, IO Int)] -> IO Int fSumListTail total [] = do return total fSumListTail total (x:xs) = do y <- doCountPair x fSumListTail (total+y) xs fSumList :: [(IO Int, IO Int)] -> IO Int fSumList l = fSumListTail 0 l piAsDouble :: Int -> Int -> Double piAsDouble a b = (fromInteger (toInteger a)) / (fromInteger (toInteger b)) calculatePi total = do count <- fSumList (randPairs total) return (piAsDouble (4*count) total) main = do args <- getArgs (calculatePi (read (args !! 0))) >>= (putStr . show) {--------------------------------------------------} Now, I have two questions. The easy question is, how can I make this more idiomatic? I seem to be jumping through hoops rather a lot to achieve what should be rather simple operations. The piAsDouble and fSumListTail functions are perfect examples, but I'm not wildly keen on doCountPair either. The second question is performance-related. The Haskell code above overflows the stack when run with an argument of 1000000, so either somewhere a list I'm intending to be lazily evaluated and chucked away is being retained, or one of my recursive functions isn't tail-optimising. Is it obvious to a more trained eye where the problem is? If not, how should I start to diagnose this? I'm not really sure where I should look for more information. More information, if it's relevant: Haskell: Glasgow Haskell Compiler, Version 6.8.0.20070917, for Haskell 98, stage 3 booted by GHC version 6.6 OS: Windows Vista, 32bit Any guidance gratefully appreciated... Thanks, -- Alex

12 18

Re: [Haskell-cafe] Monte Carlo Pi calculation (newbie learnings)
by Yitzchak Gale 07 Nov '07

07 Nov '07

Hi Alex, > I think I would have avoided most of the problems > in the first place. I'm finding the documentation > to be rather undiscoverable at the moment, but I'm > sure it's just a question of learning my way around it. Make sure to keep http://haskell.org/ghc/docs/latest/html/libraries/ on your favorites list. >> Represent >> your big iteration as a list. Try to stick >> to fairly simple operations on the list. Then the >> compiler will figure out how to keep things >> lazy and also release memory as it goes along. > See, this is what I thought I was doing, but I > couldn't see where to simplify it further. Sorry, you're right, I was begging the question. Hmm. Well, for starters: o Try to keep your big list far away from the IO monad. o Whenever possible, use the following library functions rather than manual recursion for iterating over your big list: take, takeWhile, drop, dropWhile, map, concatMap, filter, zip, zipWith. Browse around the wiki in places like: http://haskell.org/haskellwiki/Stack_overflow http://haskell.org/haskellwiki/Performance/Laziness http://haskell.org/haskellwiki/Performance/Strictness http://haskell.org/haskellwiki/Category:Style http://haskell.org/haskellwiki/Category:Tutorials -Yitz

1 0

Re: Best Linux for Haskell?
by Ivan Lazar Miljenovic 07 Nov '07

07 Nov '07

Seth Gordon wrote: > Peter Verswyvelen wrote: > > Each year I give Linux a try. And usually I kick it off my harddrive > > after a month, and stick to Windows. However, it does get better each > > year, so > > > > > > > > So which kind Linux works best for running GHC (6.8.1) and related > > tools? (I want to give Yi a go, I cant get it to work on Windows). > > Debian? Fedora? Ubuntu? > > I don't know which Linux distribution is best *for Haskell*, but in my > experience, Ubuntu has been the best distribution for getting up and > running on Linux without fuss. The latest version lets you boot into > Linux from a CD-ROM, so you can try it out and confirm that all your > hardware is compatible before you touch anything on your hard drive. > > I haven't tried 6.8.1 yet, but I have downloaded and used the 6.6.x > binaries on an Ubuntu system back when the repositories were still using > an earlier version. I strongly suspect that if you got hold of 6.8.1 > packages that were built for Debian, they would also install on Ubuntu, > but I haven't tried doing that yet. Well, we're testing out ghc-6.8.1 on the Gentoo-Haskell overlay right now... but it's probably not as good a distribution for a newbie :p -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com

1 0

ghc -e with standard input
by Maurício 07 Nov '07

07 Nov '07

Hi, Is there a way to run 'ghc -e' taking input from standard input? I would like to use it in a pipe. Thanks, Maurício

6 10

Go parallel
by Andrew Coppin 07 Nov '07

07 Nov '07

Hi folks. Take a look at this: render :: IOArray Point Colour -> (Point -> Colour) -> IO () render framebuffer fn = mapM_ (\p -> writeArray framebuffer p (fn p)) all_points How do I alter this to compute fn in multiple threads in parallel? (As you can see from the type signature, the calculation is independent for each pixel.) Thanks.

7 13

Is there a module for multivariate linear regression?
by Lihn, Steve 06 Nov '07

06 Nov '07

I am looking for a Haskell module that will do multivariate linear regression. Does someone know which module will do it? That is, the equivalent of Perl's Statistics::Regression.pm. http://search.cpan.org/~itub/PerlMol-0.35_00.ppm/lib/Statistics/Regressi on.pm Thanks, Steve ------------------------------------------------------------------------------ Notice: This e-mail message, together with any attachments, contains information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, New Jersey, USA 08889), and/or its affiliates (which may be known outside the United States as Merck Frosst, Merck Sharp & Dohme or MSD and in Japan, as Banyu - direct contact information for affiliates is available at http://www.merck.com/contact/contacts.html) that may be confidential, proprietary copyrighted and/or legally privileged. It is intended solely for the use of the individual or entity named on this message. If you are not the intended recipient, and have received this message in error, please notify us immediately by reply e-mail and then delete it from your system. ------------------------------------------------------------------------------

3 5

speeding up fibonacci with memoizing
by Thomas Hartman 06 Nov '07

06 Nov '07

I just thought this was interesting, so I would share it. Thanks to whoever it was on #haskell who helped me, sorry I can't remember who. -- horribly slow. try slow_fibs 30, not too much higher than that and it hangs slow_fibs = map slow_fib [1..] slow_fib 1 = 1 slow_fib 2 = 1 slow_fib n = ( slow_fib (n-2) ) + (slow_fib(n-1)) -- versus, try memoized_fibs !! 10000 memoized_fibs = map memoized_fib [1..] memoized_fib = ((map fib' [0 ..]) !!) where fib' 0 = 0 fib' 1 = 1 fib' n = memoized_fib (n - 1) + memoized_fib (n - 2)

14 16

Strange Type Inference
by C.M.Brown 06 Nov '07

06 Nov '07

Hi, I was given a quandary this evening, suppose I have the following code: module Test1 where import qualified Data.Map as Map testFunction :: Ord a => Map.Map a b -> Map.Map a b -> a -> (Maybe b, Maybe b) testFunction m0 m1 k = (lookup0 k, lookup1 k) where lookup0 x = Map.lookup x m0 lookup1 x = Map.lookup x m1 This compiles and type checks fine. However, the only way I could add type signatures to lookup0 and lookup1 was to do something along the lines of this: testFunction :: Ord a => Map.Map a b -> Map.Map a b -> a -> (Maybe b, Maybe b) testFunction m0 m1 k = (lookup0 k m0, lookup1 k m1) where lookup0 :: (Monad m, Ord a) => a -> Map.Map a b -> m b lookup0 x m0 = Map.lookup x m0 lookup1 :: (Monad m, Ord a) => a -> Map.Map a b -> m b lookup1 x m1 = Map.lookup x m1 Is there a way to give lookup0 and lookup1 explicit type signatures without passing in m0 and m1 as parameters? (So their definitions are the same as in the first example) If ghc can infer the type, surely it must be possible? Regards, Chris.

4 5

Web Application Sessions
by Lars Oppermann 06 Nov '07

06 Nov '07

Hello, I'm currently looking into web application programming with Haskell. Though I am aware of HAppS, a lighttpd/fastcgi based approach is what I'm currently considering. It seems rather straightforward to implement a session manager, that would maintian some server side state which is passed to the function handling the request along with the request information and that gets updated as a result of the request processing. Just like in Java servlets (maybe call it funclets ;). In fact, this seems so straight forward to me, that I was convinced that somebody would already have implemented a library for that, that takes care of associating requests with session-ID cookies - however, I haven't found anything. Does anyone know whether something like this has been done? Maybe I can't find anything because my state-infested notion of session-data is totally inappropriate in a functional programming environment. In that case I would like to know about that (and how to better myself)... Cheers, Lars

1 0

package maintainers: updating your packages to work with GHC 6.8.1
by Duncan Coutts 06 Nov '07

06 Nov '07

If you maintain a Haskell package this is for you. So now that GHC 6.8.1 is out you'll want to test your package with it. We'd especially like maintainers of packages that are distributed on hackage.haskell.org to test their packages and update them as necessary. However we would appreciate it if your packages will still work with GHC 6.6.x, so read below for details on how to do that. You'll most likely find that your package fails to build with GHC 6.8.1 with an error message like this: Could not find module `Data.Map': it is a member of package containers-0.1.0.0, which is hidden. This is the first symptom of the change in the base library in GHC 6.8.1. Several parts of the old base library got split out into separate packages. For example Data.Map is now in the containers package. Unfortunately this means most packages need updating. Of course you'd also like to make your packages continue to work with GHC 6.6.x (and perhaps older). If you go and just add containers to your package's build-depends then it will no longer work with GHC 6.6.x or older. So the solution at the moment is to use Cabal configurations like so: flag splitBase description: Choose the new smaller, split-up base package. library if flag(splitBase) build-depends: base >= 3, containers else build-depends: base < 3 Since this uses a new Cabal feature you'll have to make your packages require Cabal-1.2 or later: cabal-version: >=1.2 This is ok since Cabal-1.2.x comes with ghc-6.8 and can also be installed from hackage for older ghc versions. For a bigger example see Cabal's own .cabal file: http://hackage.haskell.org/packages/archive/Cabal/1.2.2.0/Cabal.cabal The new Cabal syntax is explained in the Cabal user guide: http://haskell.org/cabal/release/rc/doc/users-guide/x30.html#configurations This is all explained in more detail on the wiki: http://haskell.org/haskellwiki/Upgrading_packages And the details of other api changes in the libraries are in the GHC 6.8.1 release notes: http://haskell.org/ghc/docs/6.8.1/html/users_guide/release-6-8-1.html A list of packages on hackage that currently fail to build with GHC 6.8.1 is here: http://www.haskell.org/pipermail/libraries/2007-November/008529.html We realise that this change is rather disruptive, requiring changes in each package. We've started a wiki page with ideas on how to avoid this in future when the base package is shrunk again. http://hackage.haskell.org/trac/ghc/wiki/PackageCompatibility Duncan (Cabal release manager)

6 10

Compiling GHC on Scientific Linux V5 [http://ftp.scientificlinux.org/linux/scientific/50/iso/i386/SL-5.0-050407-i386-DVD.iso, Build on RHEL5]
by Animesh Sharma 06 Nov '07

06 Nov '07

For a long time I wanted to go 'strictly functional' and what better way then to start with the beast, ghc. So about a couple of months back, I downloaded version 6.6.1 [ http://www.haskell.org/ghc/dist/6.6.1/rpm/rhel5/i386/ghc661-6.6.1-2.el5.i38…] and started reading YAHT [ http://darcs.haskell.org/yaht/yaht.pdf ]. Then the 'laziness' took over and my 'idea of being strictly functional' took a back-seat. The news of release of ghc Version 6.8.1 (3 November 2007) reignited the desire. This time I thought of compiling the ghc from source [ Main - http://www.haskell.org/ghc/dist/6.8.1/ghc-6.8.1-src.tar.bz2 , Extra Libs - http://www.haskell.org/ghc/dist/6.8.1/ghc-6.8.1-src-extralibs.tar.bz2 ]. Make sure you have 4-5 hours and 1-1.5 GB of disc space! Become 'root' and extract the above two files: tar xvjf ghc-6.8.1-src.tar.bz2 tar xvjf ghc-6.8.1-src-extralibs.tar.bz2 Both of them will extract in the directory ./ghc-6.8.1. For compiling there are some dependencies to be taken care of: 1. ghc [I already had v 6.6.1, http://www.haskell.org/ghc/dist/6.6.1/rpm/rhel5/i386/ghc661-6.6.1-2.el5.i38…] , it is interesting as the compilers compiles its next version [ http://en.wikipedia.org/wiki/Bootstrapping_(compilers) ]: rpm -ivhf ghc661-6.6.1-2.el5.i386.rpm 2. cabal, Common Architecture for Building Applications and Libraries, which can be downloaded from http://www.haskell.org/cabal/release/rc/cabal-1.2.2.0.tar.gz and installation instructions need to be followed [ http://haskell.org/haskellwiki/Cabal/How_to_install_a_Cabal_package ]: tar xzf cabal-1.2.2.0.tar.gz cd cabal-1.2.2 runhaskell Setup.hs configure runhaskell Setup.hs build runhaskell Setup.hs install 3. readline-devel, readline library which in turn has terminfo as dependence, so better to use yum for installation: yum install readline-devel 4. alex, A lexical analyser generator for Haskell, whose rpm can be downloaded from http://www.haskell.org/alex/dist/2.1.0/RH9/alex-2.1.0-1.i386.rpm and installed easily: rpm -ivhf alex-2.1.0-1.i386.rpm 5. happy, the yacc of haskell [I read on http://tritium.livejournal.com/ to compile happy you need old version of happy]. But for me simply getting the source http://www.haskell.org/happy/dist/1.17/happy-1.17.tar.gz and installation worked fine: tar xvzf happy-1.17-html.tar.gz cd happy-1.17 runhaskell Setup.hs configure runhaskell Setup.hs build runhaskell Setup.hs install We are all set for the ghc compilation: cd ghc-6.8.1 ./configure make make install Clean up ~300MB from the older ghc (v 6.6.1) installation, I went the brute force way: find /usr -name ghc-6.6.1 > <list> rm -rf <list> Now I have to dig deeper into the release note (http://www.haskell.org/ghc/docs/6.8.1/html/users_guide/release-6-8-1.html ), one thing which has really got me interested is the introduction of debugger to GHCi. Any tutorials on how to go about it? Regards, Ani ---------The Answer lies in the Genome------------ fuzzylife.org

2 1

The question of ByteString
by Andrew Coppin 06 Nov '07

06 Nov '07

Somewhat related to the discussions about Haskell's performance... String. ByteString. Do we really need both? Can one replace the other? Why is one faster? Can't we make *all* lists this fast? [insert further variations here] Thoughts?

7 11

Efficient and type safe flag sets
by Henning Thielemann 05 Nov '07

05 Nov '07

We have http://www.haskell.org/haskellwiki/EnumSet_EnumMap Is there also an efficient implementation for bit sets that fit into a machine word? This would be useful for foreign function interfaces. E.g. where C defines #define SND_SEQ_PORT_CAP_READ (1<<0) /**< readable from this port */ #define SND_SEQ_PORT_CAP_WRITE (1<<1) /**< writable to this port */ #define SND_SEQ_PORT_CAP_SYNC_READ (1<<2) /**< allow read subscriptions */ #define SND_SEQ_PORT_CAP_SYNC_WRITE (1<<3) /**< allow write subscriptions */ In Haskell we could define module SndSeq data PortCap = Read | Write | SyncRead | SyncWrite bit32 :: Enum a => a -> FlagSet32 a flags32 :: Enum a => [a] -> FlagSet32 a In C we have to write foo (SND_SEQ_PORT_CAP_SYNC_READ | SND_SEQ_PORT_CAP_SYNC_READ) which is efficient but unsafe, because flags of the wrong sort or plain numbers could be passed accidentally. In Haskell we would write foo (flags32 [Read, SyncRead]) which is both efficient and safe, because foo :: FlagSet32 PortCap -> IO () (Given that constant expression evaluation works ...)

2 1

Adding GLUT package to WinHugs
by Peter Verswyvelen 05 Nov '07

05 Nov '07

The precompiled WinHugs Sep 2006 does not seem to come with the GLUT package. Although I'm able to add packages to GHC (using Cabal), I failed doing the same for WinHugs. How can I add the GLUT package to WinHugs? I tried starting MSYS, typed configure, which worked fine, but then "make" fails. Also typing runhugs Setup.hs configure fails with runhugs: Error occurred ERROR "c:\program files\winhugs\packages\base\Text\ParserCombinators\ReadP.hs":156 - Syntax error in type expression (unexpected `.') Thanks for any help, Peter Verswyvelen

2 4

space leak?
by Massimiliano Gubinelli 04 Nov '07

04 Nov '07

> ( these two lines are just to fool the gmane post algorithm which > complains for top-posting....) Hi, i'm learning Haskell and trying to use the HPDF 1.2 library I've come across some large memory consumption for which I do not understand the origin. I've tried heap profiling but without much success. This is my code > module Main where > import Control.Monad.State > import Graphics.PDF > data Opcodes = Rect | Ship deriving (Show) > doPage (Rect:ops) = do > stroke $! Rectangle 10.0 10.0 10.0 10.0 > doPage ops > doPage l = return l > doOps [] = return () > doOps (Ship:ops) = {-# SCC "OPSHIP" #-} do > p <- addPage Nothing > ops' <- drawWithPage p $! do > strokeColor red > applyMatrix $ (translate 72.0 72.0) > doPage ops > doOps ops' > doOps (op:_) = error ("unexpected " ++ show op) > testpdf = do > let ops = concat $ replicate 100 (Ship : (replicate 1000 Rect )) > pageRect = PDFRect 0 0 > (floor $ 21.0/2.54*72.0) (floor $ 29.7/2.54*72.0) > runPdf "test1.pdf" (standardDocInfo { author=toPDFString "mgubi", > compressed = False}) > pageRect $ doOps ops > testpdf' = do > let pageRect = PDFRect 0 0 > (floor $ 21.0/2.54*72.0) (floor $ 29.7/2.54*72.0) > runPdf "full.pdf" (standardDocInfo { author=toPDFString "mgubi", > compressed = False}) > pageRect $ sequence_ $ foldM f [] $ replicate 100 $ > (\p -> sequence_ $ replicate 1000 $ > drawWithPage p $ stroke $ > Rectangle 0.0 0.0 10.0 10.0) > where f ps acts = do > p <- addPage Nothing > acts p > return $ p:ps > main = testpdf now, if I run testpdf' then memory profile is very low and everything is as expected while if I run testpdf then the profile grows up to 80MB and more. This is the stripped down version of the original program (which is a DVI interpreter) so there I will have also some StateT and more complicated opcodes. I would like to know what is wrong with the above code. Could someone help me? thanks, Massimiliano Gubinelli

2 2

torrent for 6.8.1?
by brad clawsie 04 Nov '07

04 Nov '07

do torrents exist for 6.8.1? my experience is that people will use torrents if they are offered and they really do lift the pressure from the origin domain (haskell.org)

7 7

Help please
by Ryan Bloor 04 Nov '07

04 Nov '07

hello, I am struggling with rose trees in Haskell. I need to construct an algebraic data type definition for "family trees" and a representation of the tree below. Also I need to construct a function that returns a persons children when given both a family tree and a name. The same is needed but for a getParents function. 1 2 3 4 5 6 7 8 9 10 11 That is the rose tree that I seek. Where each number above is equivalent to (String, String) Data Tree a = Empty | Leaf a | Node a [(Tree a)] example :: Tree (String, String) example = Node ("a","b") -- root node [ ...............define tree.........................] -- end of tree _________________________________________________________________ 100’s of Music vouchers to be won with MSN Music https://www.musicmashup.co.uk

1 0

Rose Tree
by Ryan Bloor 04 Nov '07

04 Nov '07

Hello, I need help... I am having trouble with rose trees. 1 2 3 4 5 6 7 8 9 10 11 That is the rose tree that I seek. Data Tree a = Empty | Leaf a | Node a [(Tree a)] example :: Tree (String, String) example = Node ("a","b") -- root node [ ........................................] -- end of tree What I want to do is create two functions that return either the children or parents of a given input, here a String. String -> Tree -> [String] I think _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

4 3

Could someone explain winhugs does not accept this argument?
by karle 04 Nov '07

04 Nov '07

type Address = Int data Port = C | D deriving(Eq,Show) data Payload = UP[Char] | RTDP(Address,Port) deriving(Eq,Show) data Pkgtype = RTD | U deriving(Eq,Show) type Pkg = (Pkgtype,Address,Payload) type Table = Signal (Address,Port) system inA inB = (outC,outD) where route = update_pr inA inB s1 = lookup_pr route inA s2 = lookup_pr route inB (s3,s4) = unzipSY s1 (s5,s6) = unzipSY s2 s7 = add_list_pr s3 s5 s8 = add_list_pr s4 s6 outC = fifoSY s7 outD = fifoSY s8 lookup_pr::Signal Table->AbstExt Pkg->AbstExt Pkg lookup_pr t (Prst(x,y,z)) |(delive_adr_A t == y && x == U) = (Prst(x,y,z)) |otherwise = look_if_RTD_pkG_A t (Prst(x,y,z)) the error message is Type error in application *** Expression : delive_adr_A t *** Term : t *** Type : Signal Table *** Does not match : Signal (Int,Port) -- View this message in context: http://www.nabble.com/Could-someone-explain-winhugs-does-not-accept-this-ar… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

Why does GHC limit stack size?
by Abhay Parvate 04 Nov '07

04 Nov '07

Hello all, Why is there a limitation on the stack size in GHC? Like heap where we can limit the size by -M RTS option but the default is unlimited, why not let the program use as big a stack as required? If not by default, then by a separate option? Some of the functions that we write in recursive fashion will usually cause a stack overflow, but will work fine if there is more stack (suppose we are not worried about efficiency). And these functions generally look nicer and compact than their tail recursive versions. Is this is a technical hurdle, or just a checkpoint for runaway programs? Can we have an RTS flag allowing unlimited stack size? Thanks, Abhay

4 4

Re: Why can't Haskell be faster?
by Isaac Gouy 04 Nov '07

04 Nov '07

Ketil Malde wrote: > [LOC vs gz as a program complexity metric] Do either of those make sense as a "program /complexity/ metric"? Seems to me that's reading a lot more into those measurements than we should. It's slightly interesting that, while we're happily opining about LOCs and gz, no one has even tried to show that switching from LOCs to gz made a big difference in those "program bulk" rankings, or even provided a specific example that they feel shows how gz is misrepresentative - all opinion, no data. (Incidentally LOC measures source code "shape" as much as anything else - programs in statement heavy languages tend to be longer and thinner, and expression heavy languages tend to be shorter and wider.) __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

10 14

Compile-time evaluation
by Nicholas Messenger 03 Nov '07

03 Nov '07

{-# OPTIONS_GHC -fglasgow-exts -fno-monomorphism-restriction #-} -- Many people ask if GHC will evaluate toplevel constants at compile -- time, you know, since Haskell is pure it'd be great if those -- computations could be done once and not use up cycles during -- runtime. Not an entirely bad idea, I think. -- -- So I set about allowing just that: for arbitrary expressions to be -- evaluated, and the expanded expression spliced into client code. -- -- If you had some data in a file just out of convenience, you could say: -- > yourData = $(compileTimeIO $ parseFile $ readFile "data.txt") -- -- Or if you had an expensive computation that you want done at compile: -- > result = $(compileTimeEval $ expensiveComputation) -- -- I would appreciate comments. I wrote this completely blind with just -- the TH and Generics haddocks, so if I'm doing something tremendously -- stupid that can be improved, let me know. :) Especially if you can -- think of a less awkward way to go from Generics' data to TH -- expressions than using 'showConstr'... -- -- I wrote this with 6.6.1, in case there's any incompatibilities. Copy/ -- paste this post into CompileTime.hs, load into ghci, :set -fth, and -- futz around with the splices. -- -- -- Nicholas Messenger (nmessenger at gmail.com, omnId on #haskell) module CompileTime(compileTimeEval, compileTimeIO) where import Data.Generics import Language.Haskell.TH import Control.Monad import Data.Tree import Data.Ratio -- Expands a datum into an expression tree to be spliced into -- client code. compileTimeEval :: Data a => a -> ExpQ compileTimeEval = return . toExp -- Runs the IO action and splices in the evaluated result datum. compileTimeIO :: Data a => IO a -> ExpQ compileTimeIO = liftM toExp . runIO -- Does the work. :) toTree gets us a tree of constructors, so -- we mostly just have to fold the tree with AppE, except for -- TH's bizarre TupE. toExp :: Data d => d -> Exp toExp = applyAll . toTree where applyAll (Node k args) | isTuple k = TupE (map applyAll args) | otherwise = foldl AppE k (map applyAll args) isTuple (ConE n) = all (==',') (nameBase n) isTuple _ = False -- Synonym to shorten the definition of exp below type Ex a = a -> Exp -- Turns some datum into a tree of TH expressions representing -- that datum. The Exp at each node represents the constructor, -- the subtrees are its arguments. toTree :: Data d => d -> Tree Exp toTree x = Node (exp x) (gmapQ toTree x) where -- The various ways to turn a (Data d => d) into an -- Exp representing its constructor. any = ConE . mkName . deparen . showConstr . toConstr char = LitE . CharL int = sigged $ LitE . IntegerL . toInteger rat = sigged $ LitE . RationalL . toRational sigged f x = SigE (f x) (ConT . mkName . show $ typeOf x) -- The above functions combined together for different types. -- This is what gives the constructor Exp at each Node. There -- are definitely more types to cover that 'any' gets wrong... exp = any `extQ` (int::Ex Int) `extQ` (int::Ex Integer) `extQ` char `extQ` (rat::Ex Float) `extQ` (rat::Ex Double) `extQ` (rat::Ex Rational) -- Generics' showConstr puts parens around infix -- constructors. TH's ConE doesn't like 'em. deparen s = (if last s == ')' then init else id) . (if head s == '(' then tail else id) $ s

3 3

Layout to non-layout code
by Maurício 03 Nov '07

03 Nov '07

Hi, I understand that many people like using layout in their code, and 99% of all Haskell examples use some kind of layout rule. However, sometimes, I would like not to use layout, so I can find errors easier (and maybe convert it to layout for presentation after all problems are solved). So, I wonder: would it be possible to implement a feature in, say, ghc, that would take code from input and output the same code with layout replaced by delimiting characters? For instance, it could take (untested code warning here): main = do s <- readLn putStrLn s and convert it to: main = do {s<-readLn; putStrLn s}; I imagine that would be easy, since the compiler has to do that as a first step anyway. If it's possible, how can I sugest that feature? Thanks, Maurício

2 2

Announce: xmonad 0.4.1
by Don Stewart 03 Nov '07

03 Nov '07

The xmonad strike force is pleased to announce the 0.4.1 release of xmonad! (a tiling window manager) This is a maintainence release with simpler build dependencies. The X11-extras library is no longer required, and instead xmonad uses the improved X11 binding: X11-1.3.0 In other respects this release is functionally identical to xmonad 0.4. As usual, you can find the latest and greatest xmonad codes on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11 Brought to you by the xmonad team: Spencer Janssen Don Stewart Jason Creighton Andrea Rossato David Roundy

1 0

Please help from a newby
by karle 03 Nov '07

03 Nov '07

The same declaration.- type Address = Int data Port = C | D deriving(Eq,Show) data Payload = UP[Char] | RTDP(Address,Port) deriving(Eq,Show) data Pkgtype = RTD | U deriving(Eq,Show) type Pkg = (Pkgtype,Address,Payload) type Table = [(Address,Port)] update_table1::Table -> Pkg -> Table update_table1 [] (t,d,y) = [(t,d,y)] Error is Type error in explicitly typed binding *** Term : update_table1 *** Type : Table -> Pkg -> [(Pkgtype,Int,Payload)] *** Does not match : Table -> Pkg -> Table Please kindly suggest, thanks in advance. -- View this message in context: http://www.nabble.com/Please-help-from-a-newby-tf4740192.html#a13555338 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 3

Re: Semantics of uniqueness types for IO
by Edsko de Vries 03 Nov '07

03 Nov '07

Hey, Just to continue the academic nitpicking.. :-) > Linear logic/typing does not quite capture uniqueness types since a term > with a unique type can always be copied to become non-unique, but a linear > type cannot become unrestricted. Actually, that isn't quite accurate. In linear logic, a term with a non-linear type can always be regarded as having a linear type, i.e. U -o !U is a theorem (my favourite reading of this theorem is "if you have an unlimited supply of bank notes, then you also have a single one"). The implication in the opposite direction is a falsity (from the fact that we have a single bank note, we cannot decude that we have an unlimited supply). By contrast, in uniqueness typing we certainly don't want to allow treat a term with a non-unique type as having a unique type, but we *may* want to allow to treat a term with a unique type as having a non-unique type (as you mention). However, you have to be very careful when you do that: if you allow to treat functions with a unique type as having a non-unique type, you loose all uniqueness guarantees unless you indicate in the type of the term whether or not the function has any unique elements in its closure. If you don't, you could for example construct a term that duplicates unique elements using dup x = (\f -> (f _|_, f _|_)) (\y -> x) (which would get type "forall a, unique a to pair of unique a's" if you allow to copy unique functions and then apply them). Edsko

2 4

Re: Why can't Haskell be faster?
by Jeff.Harper＠handheld.com 03 Nov '07

03 Nov '07

Peter Hercek wrote: > * it is easy to mark stuff strict (even in function signatures > etc), so it is possible to save on unnecessary CAF creations Also, the Clean compiler has a strictness analyzer. The compiler will analyze code and find many (but not all) cases where a function argument can be made strict without changing the behavior of the program.

28 47

Hiding side effects in a data structure
by C Rodrigues 03 Nov '07

03 Nov '07

While thinking about how to generate unique integer IDs on demand without using a state variable, I came up with an interesting design pattern. It's a way of doing side-effecting computation outside IO. Referential transparency is preserved by making the side effects spatial rather than temporal: by hiding side effects behind lazy thunks in a data structure, they can be disguised as the output of a single, apparently nondeterministic IO function used to data structure. This lets pure code use nondeterministic computation without the monadic plumbing required to maintain state. The getContents function works this way, but I came up with a more interesting example. The code below is a source of unique integer IDs that is modeled after the RandomGen class. It uses unsafeInterleaveIO under the hood, preserving referential transparency but not determinism. It seems to work. However, I'm not entirely sure how safe my use of unsafeInterleaveIO is. In particular, could the two branches of the tree get CSE'd? I'm also curious what people think about the general design pattern. module Unique where import Control.Monad(liftM) import Data.IORef import System.IO.Unsafe -- The goal is to produce an infinite tree of integers where each node in the -- tree has a unique value. type Unique = Int data Supply = Supply Unique Supply Supply -- The tree can be used in a stateful manner as a source of unique integers. getUnique :: Supply -> (Unique, Supply) getUnique (Supply u s1 _) = (u, s1) -- The tree can also be split into independent sources of unique integers. split :: Supply -> (Supply, Supply) split (Supply _ s1 s2) = (s1, s2) -- The catch is, the tree will probably be visited very sparsely, with most of -- it being skipped. Assigning every node its own integer is very bad, because -- that will waste most of the 2^32 available integers very quickly. In fact, -- it can get used up in just 32 calls to getUnique. -- -- Instead, we'll create a tree where integers magically appear only in places -- where they are actually used. -- First, we need an IO-bound supply of integers. newtype IOSupply = IOSupply (IORef Unique) newIOSupply :: IO IOSupply newIOSupply = liftM IOSupply $ newIORef 0 getUniqueIO :: IOSupply -> IO Unique getUniqueIO (IOSupply s) = do u <- readIORef s writeIORef s $ u+1 return u -- Now we'll use the IO-bound supply to create a tree having the desired -- properties. {-# NOINLINE getPureSupply #-} getPureSupply :: IOSupply -> IO Supply getPureSupply s = do s1 <- unsafeInterleaveIO $ getPureSupply s s2 <- unsafeInterleaveIO $ getPureSupply s n <- unsafeInterleaveIO $ getUniqueIO s return $ Supply n s1 s2 _________________________________________________________________ Climb to the top of the charts! Play Star Shuffle: the word scramble challenge with star power. http://club.live.com/star_shuffle.aspx?icid=starshuffle_wlmailtextlink_oct

14 21

using an external application
by Petr Hoffmann 03 Nov '07

03 Nov '07

Hi, I'm solving the following problem - I need to use an external application - give it the input data and receive its output. However, when multiple calls are made, the results are not as expected. The simplified version of the problem is given below: import System.Cmd main = do System.Cmd.system "echo hello >output.txt" -- use the external application to create an output file o1 <- readFile "output.txt" System.Cmd.system "echo bye >output.txt" -- the second call to the external application o2 <- readFile "output.txt" putStr o1 -- "hello" expected, but "bye" printed return 0 Can you please give me some hint to solve this problem? I'm a beginning haskell developer and I'm still a bit confused by the IO monad. Thank you in advance. Petr

9 10

Re: [Haskell-cafe] using an external application
by Andrew Butterfield 02 Nov '07

02 Nov '07

> On 11/2/07, Petr Hoffmann <spy.hunter(a)email.cz> wrote: > >> import System.Cmd >> main = do >> System.Cmd.system "echo hello >output.txt" -- use the external >> application to create an output file >> o1 <- readFile "output.txt" >> System.Cmd.system "echo bye >output.txt" -- the second call to >> the external application >> o2 <- readFile "output.txt" >> putStr o1 -- "hello" expected, but "bye" printed >> return 0 >> >> Can you please give me some hint to solve this problem? >> I'm a beginning haskell developer and I'm still a bit confused >> by the IO monad. >> > > I'm puzzled - when I run this on GHCi (v6.4, Windows XP) I get the following outcome *Main>main The process cannot access the file because it is being used by another process. hello *Main> This is certainly the behaviour I would expect - seeing "bye" being printed seems to me to be an error and may even constitute a violation of referential transparency. What implementation of GHC are you using ? > This looks like yet another case of the lazy-I/O goblins. > Yes, but not quite what everyone has being addressing.... -- -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Foundations and Methods Research Group Director. Course Director, B.A. (Mod.) in CS and ICT degrees, Year 4. Department of Computer Science, Room F.13, O'Reilly Institute, Trinity College, University of Dublin, Ireland. http://www.cs.tcd.ie/Andrew.Butterfield/ --------------------------------------------------------------------

3 2

Slightly off-topic
by PR Stanley 02 Nov '07

02 Nov '07

Hi folks Apologies for the off-topic post. If anyone knows anything about the rules of proof by deduction and quantifiers I'd be grateful for some assistance. Much obliged, Paul

3 2

do/if/then/else confusion
by David Carter 02 Nov '07

02 Nov '07

Another newbie question, but I can't seem to find any answers on the web... Can someone tell me what's wrong with this? import qualified System.Posix.Directory as PD readdirAll :: PD.DirStream -> IO [String] readdirAll d = do dir <- PD.readDirStream d if dir == "" then return [] else rest <- readdirAll d return (dir:rest) Compiling with GHC 6.6.1 gives me the not-very-useful message "Parse error in pattern", pointing to the "i" of "if". I've tried all kinds of alternative indentations (including indenting the "else" more), bracketings etc, but nothing helps. Thanks David -- The Wellcome Trust Sanger Institute is operated by Genome Research Limited, a charity registered in England with number 1021457 and a company registered in England with number 2742969, whose registered office is 215 Euston Road, London, NW1 2BE.

5 5

Need help from a newby
by karle 02 Nov '07

02 Nov '07

My declaration is as followed:- type Address = Int data Port = C | D deriving(Eq,Show) data Payload = UP[Char] | RTDP(Address,Port) deriving(Eq,Show) data Pkgtype = RTD | U deriving(Eq,Show) type Pkg = (Pkgtype,Address,Payload) type Table = [(Address,Port)] findport::[Pkg]->[Table]->[([Pkg],[Pkg])] findport [(pt,ad,pa)] [(a,p)] | ( p == C) = ([pt,ad,a],[]) | otherwise = ([],[pt,ad,a]) Error received:- Type error in explicitly typed binding *** Term : [(a,p)] *** Type : [(a,b)] *** Does not match : [Table] Can anyone please help? -- View this message in context: http://www.nabble.com/Need-help-from-a-newby-tf4735009.html#a13540531 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 4

Disjunctive Normal Form
by Jim Burton 02 Nov '07

02 Nov '07

I am trying to rewrite sentences in a logical language into DNF, and wonder if someone would point out where I'm going wrong. My dim understanding of it is that I need to move And and Not inwards and Or out, but the function below fails, for example: > dnf (Or (And A B) (Or (And C D) E)) And (Or A (And (Or C E) (Or D E))) (Or B (And (Or C E) (Or D E))) data LS = Var | Not LS | And LS LS | Or LS LS --convert sentences to DNF dnf :: LS -> LS dnf (And (Or s1 s2) s3) = Or (And (dnf s1) (dnf s3)) (And (dnf s2) (dnf s3)) dnf (And s1 (Or s2 s3)) = Or (And (dnf s1) (dnf s2)) (And (dnf s1) (dnf s3)) dnf (And s1 s2) = And (dnf s1) (dnf s2) dnf (Or s1 s2) = Or (dnf s1) (dnf s2) dnf (Not (Not d)) = dnf d dnf (Not (And s1 s2)) = Or (Not (dnf s1)) (Not (dnf s2)) dnf (Not (Or s1 s2)) = And (Not (dnf s1)) (Not (dnf s2)) dnf s = s Thanks, Jim -- View this message in context: http://www.nabble.com/Disjunctive-Normal-Form-tf4733248.html#a13534567 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

5 6

Embedding the GHC API
by Joel Reymont 02 Nov '07

02 Nov '07

Has anyone tried to embed GHC as a library recently? What is the size of the resulting binary? I'm assuming a bare minimum of needed libraries. Thanks, Joel -- http://wagerlabs.com

4 3

Why can't Haskell be faster?
by Paulo J. Matos 02 Nov '07

02 Nov '07

Hello all, I, along with some friends, have been looking to Haskell lately. I'm very happy with Haskell as a language, however, a friend sent me the link: http://shootout.alioth.debian.org/gp4/ which enables you compare several language implementations. Haskell seems to lag behind of Clean. >From what I've seen of Clean it seems almost like Haskell. It even distributes a Haskell->Clean translator so the obvious question is, why is Haskell slower? Being similar languages and being GHC a very good compiler, can't it get at least as fast as Clean? What am I missing here? (I wrote this mail assuming the results from the URL are trustworthy). Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

10 13

virtual lambda
by Alberto Ruiz 01 Nov '07

01 Nov '07

Hi, I think that you may like the following demo of a simple computer vision application powered by Haskell: http://covector.blogspot.com/2007/10/functional-augmented-reality.html Alberto

1 0

Haskell libraries for computer vision
by Don Stewart 01 Nov '07

01 Nov '07

http://alberrto.googlepages.com/easyvision "An experimental Haskell system for fast prototyping of computer vision and image processing applications." Looks ridiculously cool. -- Don

4 3

Re: Why can't Haskell be faster?
by david48 01 Nov '07

01 Nov '07

I'd like to see Supero and Jhc - compiled examples in the language shootout.

1 0

Type inference problem with division (/)
by noa 01 Nov '07

01 Nov '07

Hi! I have the following function: theRemainder :: [String] -> [String] -> Double theRemainder xs xt = sum( map additional (unique xs) ) where additional x = poccur * (inf [ppos,pneg]) --inf takes [Double] where xsxt = zip xs xt pi = countPos xr -- countPos returns an Int ni = (length xr) - pi len = length xs len2 = length xr ppos = pi/len2 -- THESE ARE THE PROBLEM pneg = ni/len2 -- THESE ARE THE PROBLEM poccur = (pi+ni)/len xr = (filter ((\y -> (fst y)==x)) (xsxt)) And I am getting this error message with ghc: matrix.hs:54:31: Couldn't match expected type `Double' against inferred type `Int' In the expression: ppos In the first argument of `inf', namely `[ppos, pneg]' In the second argument of `(*)', namely `(inf [ppos, pneg])' How can I change the declaration of ppos nad pneg so they are treated as Double for the inf function? Cheers, Nick -- View this message in context: http://www.nabble.com/Type-inference-problem-with-division-%28-%29-tf472211… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

5 5

does the order of splice definitions matter in template haskell, or is this a bug?
by Thomas Hartman 01 Nov '07

01 Nov '07

I have a situation where ... stuff... $(expose ['setState, 'getState] f = SetState compiles but f = SetState $(expose ['setState, 'getState] doesn't compile, with error: Not in scope: data constructor 'SetState. Is this a bug? expose is defined in HAppS.State.EventTH t,. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

3 2

help needed packaging curl bindings
by brad clawsie 01 Nov '07

01 Nov '07

i have decided to take on the task of packaging-up (for hackage) and documenting the curl bindings as available here: http://code.haskell.org/curl/ if the originators of this code are reading this and do not wish me to proceed please say so, i won't be offended otherwise i was wondering if people had good examples to point me to for providing the cross-platform support needed for a FFI-based module such as this. i have made the necessary changes to compile the code on freebsd, but for other platforms i am not sure at all, particularly non-unix style platforms like windows. my guess is that providing cross-platform support requires autoconf etc prior to the hackage build process (?) any info/references appreciated thanks brad

2 3

Re: [Haskell-cafe] Why can't Haskell be faster?
by manu 31 Oct '07

31 Oct '07

>> From what I've seen of Clean it seems almost like Haskell. It even >> > distributes a Haskell->Clean translator so the obvious question is, > why is Haskell slower? > It's also something I've wondered about, and I'm curious about the answer... One of the differences between Haskell and Clean is how side-effects are allowed (Uniqueness Types for Clean, and Monadic I/O for Haskell) GHC also supports a lot of extensions beyong Haskell98. Does it explain the difference in performances ? I don't know... Experts please ! Manu

1 0

Re[2]: [Haskell] Image manipulation
by Bulat Ziganshin 31 Oct '07

31 Oct '07

Hello Bjorn, Wednesday, October 31, 2007, 11:54:33 AM, you wrote: > data fields. Data Field Haskell had a function to force the evaluation of > all elements in a data field at once. this looks like a parallel arrays? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

1 0

Re: [Haskell-cafe] Strictness leak
by Jeff Polakow 31 Oct '07

31 Oct '07

I forgot to send this reponse to haskell-cafe earlier... Hello, > You mean for the IO monad, right? > Sorry. I meant divergence is unavoidable for any strict Monad, such as IO. However, sequence will always compute over the entire list; if the resulting computation itself is lazy then the result can be inspected lazily. > take 10 $ execWriter $ sequence $ repeat $ tell ([3]::[Int]) > This is a good example. Note that the computation of sequence itself is infinite. snd $ runWriter $ sequence (repeat $ tell [3]) >>= return . take 10 will result in an infinite list, but fst $ runWriter $ sequence (repeat $ tell [3]) >>= return . take 10 will return a 10 element list. -Jeff --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

1 0

Re: [Haskell] Image manipulation
by Dan Piponi 31 Oct '07

31 Oct '07

Jerzy, There is a simple framework for performing filtering operations on images lazily: http://sigfpe.blogspot.com/2006/12/evaluating-cellular-automata-is.html (Scroll down to the 2D example in the comments.) 2D digital filters lend themselves nicely to a comonadic framework though you probably have to drop the comonads for operations other than convolution type things. And you're right, doing this lazily at the pixel level is expensive. However, if we replace pixels with 256x256 blocks, say, where the blocks themselves are strict but the images are lazily made up of blocks, then we have the potential for a nice and efficient image processing library that inherits its laziness directly from Haskell. On 10/30/07, jerzy.karczmarczuk(a)info.unicaen.fr <jerzy.karczmarczuk(a)info.unicaen.fr> wrote: > Dan Piponi adds to a short exchange: > > > jerzy.karczmarczuk: > >> [iso-8859-1] Bj�rn Wikstr�m writes: > >> > >> > Hi! I have lots and lots of images (jpegs) that I would like to manipulate > >> > and shrink (in size). They are around 5 Mb big, so I thought this would > >> > be a good Haskell project since it's a lazy evaluating language. > >> ... > >> I must say that I don't see much use of laziness here. > > > Laziness plays a big role in real world image processing. Typically, > > in applications like Apple's Shake, you build a dataflow > > representation of the image processing operations you wish to perform, > > and the final result is computed lazily so as to reduce the amount of > > computation. For example, if you blur an image, and then zoom in on > > the top left corner, then only the top left corner will be loaded up > > from the original image (assuming your image file format supports > > tiled access). You still work on tiles or scan-lines, rather than > > individual pixels, so the laziness has a 'coarse' granularity. > > > > But I'm not sure if this is what the original poster was talking about. > > I am neither... > Still, Dan, I think that there is quite a difference between incremental > processing of signals, and images, etc., and the *lazy evaluation* of > them. Of course, a stream is consumed as economically as it can, but > not less. If you filter an image (shrinking, so some low-pass MUST be > done), a pixel must be loaded with its neighbourhood, which means *some* > scan lines. > With a JPEG this means that a 8x8 block should be loaded also with its > vicinity. But would you suggest that individual pixel processors should > be lazy? It would be useless, and probably resulting in some penalties. > > So, the laziness of Haskell for me here is less than useful. > Noooow, the lazy *generation* of streams is another story... > Generating music (low level, i.e. sound patterns) through lazy algorithms > is quite interesting. > > Jerzy Karczmarczuk > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

1 0