October 2008 - Haskell-Cafe

Re: [Haskell] Probably a trivial thing for people knowing Haskell
by Paul Johnson 19 Oct '08

19 Oct '08

Friedrich wrote: > Paul Johnson <paul(a)cogito.org.uk> writes: > >> [...] Because file reading is lazy, >> each line is only read when it is to be processed, and then gets >> reaped by the garbage collector. So it all runs in constant memory. >> > Would you mind to elaborate a bit about it. What's so terrible to open > one file after the other, reading it line by line and close the file > thereafter. > Its not wrong, its just more work. Also from a structural point of view its better to separate the code that reads the files from the code that processes the text. The conventional way forces you to mix them. >> (By the way, putting in the top level type declarations helps a lot >> when you make a mistake.) >> > Well I have my problems with that. Probably it comes from using > Languages like Ruby and my special dislike of "typing things" comes > especially from Java, C++ (well C is not "innocent" in that regard > also. > > OK, its a matter of personal preference (although it really does help anyone else reading your code). However I find that if I leave out the top level definitions then type error messages at compile time are much harder to figure out, especially in a big program. So I find that the extra typing pays in the long run. :-/ Paul.

1 0

package question/problem
by Galchin, Vasili 19 Oct '08

19 Oct '08

Hello, I am trying to "cabal install" HSQL. I am using ghc 6.8.2. I get the following error about a non-visible/hidden package (old-time-1.0.0.0): vigalchin@ubuntu:~$ cabal install hsql Resolving dependencies... 'hsql-1.7' is cached. [1 of 1] Compiling Main ( Setup.lhs, dist/setup/Main.o ) Linking dist/setup/setup ... Configuring hsql-1.7... Warning: No 'build-type' specified. If you do not need a custom Setup.hs or ./configure script then use 'build-type: Simple'. Preprocessing library hsql-1.7... Building hsql-1.7... Database/HSQL.hsc:66:7: Could not find module `System.Time': it is a member of package old-time-1.0.0.0, which is hidden cabal: Error: some packages failed to install: hsql-1.7 failed during the building phase. The exception was: exit: ExitFailure 1 vigalchin@ubuntu:~$ I don't see (old-time-1.0.0.0) with parens around it so I am confused: vigalchin@ubuntu:~$ ghc-pkg list /usr/local/lib/ghc-6.8.2/package.conf: Cabal-1.2.3.0, GLUT-2.1.1.1, HUnit-1.2.0.0, OpenGL-2.2.1.1, QuickCheck-1.1.0.0, array-0.1.0.0, base-3.0.1.0, bytestring-0.9.0.1, cgi-3001.1.5.1, containers-0.1.0.1, directory-1.0.0.0, fgl-5.4.1.1, filepath-1.1.0.0, (ghc-6.8.2), haskell-src-1.0.1.1, haskell98-1.0.1.0, hpc-0.5.0.0, html-1.0.1.1, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, parallel-1.0.0.0, parsec-2.1.0.0, pretty-1.0.0.0, process-1.0.0.0, random-1.0.0.0, readline-1.0.1.0, regex-base-0.72.0.1, regex-compat-0.71.0.1, regex-posix-0.72.0.2, rts-1.0, stm-2.1.1.0, template-haskell-2.2.0.0, time-1.1.2.0, unix-2.3.0.0, xhtml-3000.0.2.1 /home/vigalchin/.ghc/i386-linux-6.8.2/package.conf: Cabal-1.4.0.1, GLFW-0.3, HDBC-1.1.5, HTTP-3001.0.4, MonadRandom-0.1.1, Stream-0.2.6, Takusen-0.8.3, X11-1.4.2, arrows-0.4, binary-0.4.2, category-extras-0.53.5, chp-1.1.0, dlist-0.4.1, event-list-0.0.7, ipc-0.0.3, midi-0.0.6, mtl-1.1.0.1, network-bytestring-0.1.1.2, non-negative-0.0.3, posix-realtime-0.0.0.1, posix-realtime-0.0.0.2, probability-0.2.1, quantum-arrow-0.0.4, unix-2.4.0.0, zlib-0.4.0.4 ?? What is the history of old-time package? Regards, Vasili

6 7

Detecting unused read handles? (was: File handles and pipes)
by Stephen Hicks 19 Oct '08

19 Oct '08

On Sun, Oct 19, 2008 at 1:44 AM, Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> wrote: > On 2008 Oct 19, at 1:37, Stephen Hicks wrote: >> >> I'm trying to understand how to get pipes working in Haskell, in >> particular with the revamped System.Process (though I've tried similar >> experiments with System.Posix.IO and Control.Concurrent). >> Specifically I'm trying to concatenate the output of two system calls >> into the input of a third. The following code does not get the job >> done: > > Pipes are perhaps a bit misnamed: if you want to combine the output of two > pipes and funnel it into a third you can't simply "plumb them together", you > need to provide code which reads from the output pipes and writes into the > input pipe. If you don't care about order, forkIO a thread for each output > pipe which reads from the output pipe it's passed and writes to the input > pipe. If order is significant, use mapM/forM to run the output-to-input > code on each handle in turn. Thanks a lot - that seems to work very well, and even scales to large amounts of data nicely (and quickly, with Lazy ByteStrings). I've got one more question now. Suppose I want to do the same thing on the other side, with two processes *receiving* the data. Is there a way to tell whether the first process wants input, and if not, wait for the second process to do anything? That is, suppose I have something like > do (Just inh1, _, _, p1) <- createProcess (shell "echo 1") { std_in = CreatePipe } > -- wait for p1, possibly feeding it some input? > (Just inh2, _, _, p2) <- createProcess (shell "cat") { std_in = CreatePipe } Is there a way to figure out that the "echo 1" process never wanted any input, and therefore not give it any? I looked through all of System.IO and everything seemed to indicate that inh1 was open, even after the process ended. The only indication otherwise was that "hflush inh1" failed with "resource vanished". I guess what I'm asking is I want to "wait until the process p1 is waiting for input and/or terminates", and if it's the latter, move on to the next process in line, before actually doing any hPutStr into the read handles. Is that possible? Thanks, steve

2 1

external sort on hackage Re: [Haskell-cafe] External Sort and unsafeInterleaveIO
by Thomas Hartman 19 Oct '08

19 Oct '08

External sort is on hackage at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/external-sort Ben, I forgot to correct your name, but I will fix it soon. Thomas. 2008/10/11 Ben <midfield(a)gmail.com>: > Fine with me, except my last name is "Lee" not "Midfield." Thanks for > doing this. > > Ben "Midfield" Lee > > On Sat, Oct 11, 2008 at 7:52 AM, Thomas Hartman <tphyahoo(a)gmail.com> wrote: >> I kinda-sorta half-cabalized it at >> >> darcs get http://darcsdump.dreamhosters.com/external-sort (untested >> via cabal install but mostly done) >> >> As soon as my project gets approved I'll put it up on hackage. >> >> If Ben wants it under his account at hackage of course I'll defer to him. >> >> Thomas. >> >> 2007/7/18 Donald Bruce Stewart <dons(a)cse.unsw.edu.au>: >>> midfield: >>>> hi -- >>>> >>>> thanks for the useful comments! i will definitely go through them >>>> carefully. unfortunately for this code (but fortunately for me) i >>>> defend my dissertation on monday so i'm a little distracted right >>>> now..... >>>> >>>> i'm more than happy to donate this code or whatever improvements >>>> happen to it. actually, hGetContentsWithCursor seems like a candidate >>>> for inclusion with Data.ByteStrings or Data.Binary or something -- it >>>> seems like it might find other uses. (i think you liked that bit of >>>> code because i ripped it off of you guys! it's very short hamming >>> >>> Can't fault that style ;) >>> >>>> distance from the original.) anyhow, all that will have to wait a >>>> couple weeks or so. also i've never cabalized anything so i may come >>>> begging for help. >>> >>> We have a tutorial for that, luckily: >>> >>> http://haskell.org/haskellwiki/How_to_write_a_Haskell_program >>> >>> And a tool to automate it, mkcabal, so should be fairly straightforward. >>> >>>> >>>> at some point i thought i saw how to do recursive external sort, to >>>> keep memory usage truly constant, but with my current lack of sleep i >>>> have lost that illusion. i'm also curious about the performance >>>> characteristics of this vs Prelude sort vs the version using the >>>> tournament mergesort apfelmus suggested. i need to find a computer >>>> with a lot more RAM than my weakling laptop. finally, it would be >>>> good to be able to have the blocksize controlled by Kb of RAM rather >>>> than # of elements, not sure how to get that information. >>>> >>>> ultimately this was part of my project to write lucene for haskell. i >>>> think with this out of the way, plus all the Data.Binary / ByteString >>>> goodness, it shouldn't take too long. keep writing good libraries for >>>> me! >>>> >>> >>> Great. Good to see things working. >>> >>> -- Don >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> >

1 0

haskell & facebook
by Jason Dusek 19 Oct '08

19 Oct '08

Has anyone taken a stab at Haskell FaceBook bindings? -- _jsn

3 2

HDBC or HSQL
by George Moschovitis 19 Oct '08

19 Oct '08

Dear devs, I am a Haskell newbie and I would like to hear your suggestions regarding a Database conectivity library: HSQL or HDBC ? which one is better / more actively supported? thanks in advance, -g. -- http://phidz.com http://blog.gmosx.com http://cull.gr http://www.joy.gr http://nitroproject.org

15 25

Re: [Haskell-cafe] File handles and pipes
by Donn Cave 19 Oct '08

19 Oct '08

Quoth "Stephen Hicks" <sdh33(a)cornell.edu>: > In general, given a handful of system calls (and/or > read/write handles), is there a way to combine them all together? Well, sure - if I understand what you mean. Not "Handles", in the sense of a Haskell library construct, but UNIX pipes and processes are reasonably simple to work with in a general purpose programming language. I'm sorry to say the following isn't real code, since GHC doesn't run on the platform I'm using to write this mail, but I hope it will serve to demonstrate the principles using bare UNIX system calls, and that it doesn't contain any serious errors. forkProcess $ do -- pipe tail (pa0, pa1) <- createPipe -- Command exec'd in this fork reads from this pipe, -- so dup 0 (input) from pipe input fd. Both ends of -- pipe will be inherited by child forks. dupTo pa0 0 forkProcess $ do -- first pipe head -- Command exec'd in this fork writes to parent's pipe. dupTo pa1 1 -- Close parent's pipe so fds won't be inherited by -- next fork. closeFd pa0 closeFd pa1 forkProcess $ execFile "/bin/df" ... -- second pipe head execFile "/bin/df" ... -- Close own pipe write end, so child process exit(s) will cause EOF closeFd pa1 execFile "/usr/bin/sort" ... Key points: Input, output and error are file descriptors 0, 1 and 2, by definition (pronounced "unit 0", etc.) dupTo (UNIX system call dup2()) duplexes an I/O stream, so "dupTo p0 0" sets up a pipe for a standard UNIX command that reads from unit 0. Reads from the pipe's read fd will either return data or block, until exit of the *last* process with the pipe's write end open. Hence the importance of closing the write end of your own pipe, and not letting pipe fds "leak" into child processes. Donn Cave, donn(a)avvanta.com

1 0

Template Haskell and unsafePerformIO
by Achim Schneider 18 Oct '08

18 Oct '08

Shouldn't be there some kind of way to run IO code once at the beginning of template processing? Something like a pair of templateInit :: IO a and templateData :: a where the a of both functions is the same and the latter is automagically generated by TH. Right now, I have to either use unsafePerformIO or runIO and IORef's to load a file only once. Both solutions aren't necessarily pretty. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.

1 0

Arch Haskell News: Oct 18 2008
by Don Stewart 18 Oct '08

18 Oct '08

= News about Haskell on Arch Linux = Arch Linux now has 639 Haskell packages in AUR. That’s an increase of 15 new Haskell packages in the last 7 days. Growth appears to be holding steady at just over 2 new packages a day on Hackage this month. Noteworthy * haskell-gnuplot-0.2: “2D and 3D plots using gnuplot” * alex-2.3: “Alex is a tool for generating lexical analysers in Haskell” * happy-1.18.1: “Happy is a parser generator for Haskell” * haskell-tfp-0.1: “Type-level programming library using type families” Full details on the site. http://archhaskell.wordpress.com/2008/10/18/arch-haskell-news-oct-18-2008/

1 0

Linux binary dist problems
by Leif Warner 18 Oct '08

18 Oct '08

I had the same problem with the 6.8.3 install on Dreamhost, "unable to determine current path" or something. Ended up using 6.8.2 install, and had to crack open the rpm of the old readline version they linked, put it in $HOME/lib, and set $LD_RUN_PATH to point to that. http://www.haskell.org/ghc/dist/6.8.2/ghc-6.8.2-i386-unknown-linux.tar.bz2 And there is actually an Eclipse plugin for Haskell: http://eclipsefp.sourceforge.net/ Not that I break it out for work. -Leif Warner

2 1