February 2009 - Haskell-Cafe

Efficient string output
by Ketil Malde 09 Feb '09

09 Feb '09

Hi, I'm currently working on a program that parses a large binary file and produces various textual outputs extracted from it. Simple enough. But: since we're talking large amounts of data, I'd like to have reasonable performance. Reading the binary file is very efficient thanks to Data.Binary. However, output is a different matter. Currently, my code looks something like: summarize :: Foo -> ByteString summarize f = let f1 = accessor f f2 = expression f : in B.concat [f1,pack "\t",pack (show f2),...] which isn't particularly elegant, and builds a temporary ByteString that usually only get passed to B.putStrLn. I can suffer the inelegance were it only fast - but this ends up taking the better part of the execution time. I tried to use lazy ByteStrings, the theory being that the components that already are (strict) ByteStrings could be recycled as chunks. I also tried to push the output down into the function (summarize :: Foo -> IO ()), but both of these were actuall slower. Since I surely can't be the first person that needs to output tab-separated text, I'd be grateful if somebody could point me in the right direction. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Formal semantics for Haskell?
by Gregg Reynolds 09 Feb '09

09 Feb '09

ML has a formal definition[1]; why not Haskell? Would this be a Good Thing, or a Waste Of Time? The Report strikes me as a hybrid of formal and informal. I guess that's not a problem so far, but there aren't that many implementations; if we had dozens, how could we verify conformance? A formal semantics would be useful, but it would also be Fun to use Category Theory notation in a language definition. Such a task would be way beyond my abilities, but I have come up with an idea for a formal semantics of IO and other non-deterministic elements of the language that I think is kind of interesting. It's inspired by Category Theory and the Z specification language. See my (brief) blog article<http://syntax.wikidot.com/blog:3> . Actually, I'm in a state of rather intense euphoria about it, so a bucket of cold water realism over my head might be a Good Thing. Then I could get some sleep instead of obsessing about category theory and Haskell. :) I propose any formal definition include the following warning, modeled on Knuth's warning about MetaFont: WARNING: Haskell can be hazardous to your other interests. Once you get hooked, you will develop intense feelings about language design; semantic models will intrude on the program texts you read. And you will perpetually be thinking of improvements to the programs that you see everywhere, including those of your own design. Thanks, gregg [1] The Definition of Standard ML (Revised); a preview is on Google Books

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Error in building profiling
by Marco Túlio Gontijo e Silva 09 Feb '09

09 Feb '09

Hello, I'm trying to package ghc-paths for debian, and I got to this: $ ./setup configure --enable-library-profiling --disable-library-vanilla Configuring ghc-paths-0.1.0.5... $ ./setup build Preprocessing library ghc-paths-0.1.0.5... Building ghc-paths-0.1.0.5... /usr/bin/ar: creating dist/build/libHSghc-paths-0.1.0.5_p.a /usr/bin/ld: dist/build/GHC/Paths.o: No such file: No such file or directory I thought this could be a problem with this package, but I've tried with mtl, and I got the same result: /usr/bin/ld: dist/build/Control/Monad/Cont.o: No such file: No such file or directory I'm using ghc6 6.10.1+dfsg1-5 and binutils 2.19-1~exp1. Greetings. -- marcot http://marcot.iaaeee.org/

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Data.Map: Enumerating ordered subset of keys
by Jared Updike 09 Feb '09

09 Feb '09

I would like to enumerate a subset of keys in a Map satisfying \ key >= key1 && key <= key2 but in the expected, reasonable amount of time (e.g. < O(log(n)) + O(m) for n total keys and m keys in the subset). (Or key > key1 and/or key < key2 or some such combination). Is there an appropriate idiom or combination of library functions to accomplish this, short of digging into the code for Data.Map and writing such a function for a forked version of Data.Map? For example I could try something like a Set.split of a Set.split of Map.keysSet of my original map, but will laziness magically do what I really want? which is to walk down the tree to key1 (or the nearest key > key1) and enumerate keys in order until key2 is reached? Data.Map almost looks like what I need if I can do this. Jared.

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Re: Haskell and Automation
by Guenther Schmidt 09 Feb '09

09 Feb '09

Dear Michael, sorry too, for not telling you sooner that meanwhile I found out how to get it working, I hope you did not spend too much time trying on your end. It had to do with the context in which these components are initialized. Once you switch to LocalProcess (as opposed to AnyProcess) in coCreateInstance everything works fine and rather smoothly. The defaults in createObject do not work for every kind of object that you do want to instantiate, certainly not Excel. Of course having an old, but still fairly current manual also helps. Good luck with your projects. Günther Michael Snoyman schrieb: > Gunther, > > Sorry for not getting back to you on your previous e-mail, I've been > swamped. > > The reason I didn't put up an example was specifically because of the > error you mention. I was hoping to get a chance to use the COM > bindings in Python to see if I got the same thing. For the life of me > I can't figure out how to get started with Excel and Haskell. Sorry I > can't be more useful at this time. > > Michael > > On Fri, Feb 6, 2009 at 6:56 PM, G?uenther Schmidt <redcom(a)fedoms.com > <mailto:redcom@fedoms.com>> wrote: > > Hi Michael, > > for some reason createObject "Excel.Application" throws an > "Interface not supported" error. > > Can you help me out here? > > Günther > >

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FFI binding to CGAL?
by Peter Verswyvelen 09 Feb '09

09 Feb '09

I was wandering of someone already made a FFI binding to http://www.cgal.org or something similar that does computational geometry?

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Does readFile "/proc/mounts" hang for you?
by David Fox 09 Feb '09

09 Feb '09

I posted a bug about this (http://hackage.haskell.org/trac/ghc/ticket/2971) but its so odd I had to ask here. Using ghc 6.10.1, both readFile "/proc/mounts" and Data.ByteString.Lazy.Char8.readFile "/proc/mounts" hang on an amd64 machine running Linux. Also, Data.ByteString.readFile "/proc/mounts" returns the empty string. Is this behavior present for others? On i386?

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The Haskell re-branding exercise
by Gwern Branwen 09 Feb '09

09 Feb '09

On Sat, Feb 7, 2009 at 1:34 PM, Don Stewart <dons(a)galois.com> wrote: > Oh, we had a long discussion about the need for condorcet voting, > not a system like the reddit which is prone to abuse. > > Also, it would be good to have the images inline. Perfect, please meet better. Better, perfect. Now get along you two! Since January 1st, we could've had hundreds or thousands of votes and easily compensated for any abuse. -- gwern

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threadDelay
by Immanuel Litzroth 09 Feb '09

09 Feb '09

Am I correct in assuming this program should run 100 secs? >import Control.Concurrent >main = do > threadDelay 100000 Why do I get the folling result then? ghc -threaded Main.hs -o delay time ./delay real 0m0.104s user 0m0.001s sys 0m0.002s Thanks in advance for all your wonderful comments, Immanuel

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Switching from Mercurial to Darcs
by Peter Verswyvelen 09 Feb '09

09 Feb '09

I'm trying to switch from HG to Darcs, but I'm failing miserably in getting the same productivity. I often do bad checkins with Darcs, merely because the amount of information I get from Darcs is overwhelming and I don't have time to read the details of the large set of commands. With Mercurial, here's my basic workflow: 1) hg st lists just the filenames of changed, deleted, new files (with a char prefix to show what kind of change it is). I only want to see the filenames, not the lines/content. If I want to see changes in the content, I use hg diff using a visual differ. My brain is not capable of making sense of the textual diff. 2) modify .hgignore to make sure only these files are added that are part of the project. I want this process to be automated, I don't want to check each time which files to add, since this is error prone. 3) hg addrem this adds new files and removes deleted files from local repos. forgetting to add files is a common problem, and is really tricky since no record is made of these files, so if after a couple of versions if a developer finds out a file was missing, the history is useless since you can't reconstruct the old content of that local file anymore, and often it's impossible to give the local file to the other developers since it might be changed. I actually would like to have an option that automatically adds/deletes files on each commit, as it is easier to delete a file after it is checked in, than it is to reconstruct an old version from a local file you forgot to add. 4) hg commit -m "message" this commits my changes locally. I always do this before pulling since then I'm sure my changes are saved in the case a merge goes wrong. 5) hg pull -u this pulls in changes, and updates and merges the files, using a visual merger of my preference. 6) hg commit -m "merged" hg push only needed if I did a merge and want to push that merge. 7) back to 1) It would be nice to know how to do this using darcs, or if a better way exists using darcs, that would be handy too :)

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