- Haskell-Cafe - Haskell.org

API for reading a big binary file
by Ranjan Bagchi 22 Dec '06

22 Dec '06

Hi -- I'm still learning to use ghc effectively, and I'm trying to use it for more-or-less mundane programming tasks, hopefully in a way which is a lot more elegant than I'd do in Java or Perl. I've got a big [around a gigabyte] binary file, filled with identical binary structures (imagine a C process writing structs). I'd like to process/analyze them efficiently. In C or even Java, i'd memory map the file and extract the data I need. Is there a fast way to do this using ghc? I can extract fields by using a ByteString, but I may not be using it fast enough: I've had to write my own routines to extract ints, longs and doubles. The other option is to write the loading code largely in C, but I'm not clear how to write a data type which maps well to a C struct. Any help / examples would really be appreciated. Thanks, Ranjan

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cabal running multiple preprocessors on one file - suggestion
by Marc Weber 21 Dec '06

21 Dec '06

Am I right that the way to do this now is create your own preprocessor run on files havirg your extionsion? What about naming the source files eg Module.hs.cpp.di.gc.chs ? This would mean first run c2hs, then greencard thin DrIft then cpp and put the results in Module.hs.cpp.di.gc.chs 1) Module.hs.cpp.di.gc 2)` Module.hs.cpp Module.hs pro : you see immediately which preprocessors are to be invoked you don't need write your own preprocessor handler.. con : There are to much possibilities to use doesFileExist? so the way to get the source file is using getDirectoryContents and filter by regex \.hs.* \.lhs.* matches and take longest filename (So that 1) will be perefered over 2) You need a customizable editor to set filetype by regular expression rather by (.gc .hs .lhs .chs ..) What do you think, would this be an enhancement? I'd like to implement this and provide a darcs patch to cabal 1.1.7 Any ideas/ comments on this? Marc

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Haskell Side Effect
by Ashley Yakeley 21 Dec '06

21 Dec '06

Since learning Haskell, I can now count in Spanish! See: "one" in Spanish, "two" in Spanish, "three" in Spanish, "four" in Spanish.. -- Ashley Yakeley

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Timer with Haskell GUI application
by asilovy 21 Dec '06

21 Dec '06

Hi all, I'm still trying to figure out why, in the Graphics.HGL library, is the timer not treated as the other events (mouse clics, key pressing... ? For instance see : http://haskell.org/ghc/docs/latest/html/libraries/HGL/Graphics-HGL-Window.h… As I understand, the timer in a HGL GUI application must be treated by waiting for a tick to occur (rather than doing useful things and being warned with maybeGetWindowEvent if one occurs). This sounds stupid to me and gives rise to non responding situations. Im a misunderstanding something ? Is it a weakness of HGL and does it force me to change all my approach and turn to another library ? Any help would be appreciated. Thanks Alain -- View this message in context: http://www.nabble.com/Timer-with-Haskell-GUI-application-tf2864634.html#a80… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Shrinking the Prelude: The "categorical" approach
by Imam Tashdid ul Alam 21 Dec '06

21 Dec '06

hi guys, I was just wondering if anyone is interested is a "quasi"-project of rewriting the Prelude (only shrinking it as measured by the total number of names imported, read along...) the idea is (just to be substantially different in approach) to develop an "alternate history" of Haskell. take map for example, and fmap, I don't think they should be named different (fmap is ugly, not suggestive, and conceptually the same). mplus could be renamed (++) (they are conceptually the same, mzero looks odd, name it empty, or simply zero) although I think concat should be replaced by msum (or the other way around preferably :) ? msum is a bad name) I am somewhat confused by join. concat seems to match join's type but I don't think the ideas coincide. and so on. in particular, we would want: * no backwards compatibility. the Prelude as is it, is good enough. we are defining a new module. * clean categorical hierarchy of type classes (category theorists wanted!) promoting uniformity. * cleaner names. foldl1 means nothing. absolutely nothing. what's the 1 for? * our Prelude only contains typeclasses, datatypes, and functions of utmost conceptual (and semantic, such as error, undefined and seq) importance, everything else goes to other modules. our Prelude is not going to be a place for "convenient" declarations. * the suffix method of naming (liftM2) is considered messy. we would prefer a seperate module (promoting qualified import). this is a fun project. we will not rewrite the Prelude, we'll merely rename it. I would suggest the name TheOtherPrelude to make our intentions clear. the conveniences (like concatMap) goes to TheOtherPrelude.Extension. I suggest we do it on the wiki. anyone interested just open a page with your name of choice. I am not doing it only because there's no point doing it if no one's interested. cheers, Imam __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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multi parameter type classes for NP problems
by Joshua Ball 20 Dec '06

20 Dec '06

Hi all, For my own study, I've been playing around with various NP complete problems. Previuosly I was doing so in Java, but because I want to learn Haskell, I'm trying to port the algorithms. In Java, I had an abstract class called AbstractNPProblem which looked like this: public abstract class AbstractNPProblem implements NPProblem { public abstract boolean validates(Certificate c); public abstract List<Certificate> certificates(); public boolean decide() { for (Certificate c : certificates()) { if (validates(c)) { return true; } } return false; } } This has one problem, however: it is slightly dynamically typed. Anybody that implements the verify method must cast the object c to a particular type (could be a bitmask, a list of integers, a subgraph, etc.) I'd like to get rid of this problem in porting to Haskell. Here is how I am trying to solve the problem, using multi-parameter type classes. class NPProblem inst cert where validates :: cert -> inst -> Bool certificates :: inst -> [cert] decide :: inst -> Bool decide i = any (\x -> x `validates` i) $ certificates i Unfortunately, ghc throws the following type error: NPProblem.hs:5:45 Could not deduce (NPProblem inst a) from the context (NPProblem inst cert) arising from use of `certificates' at NPProblem.hs:5:45-58 Possible fix: add (NPProblem inst a) to the class or instance method `decide' In the second argument of `($)', namely `certificates i' In the expression: (any (\ x -> x `validates` i)) $ (certificates i) In the definition of `decide': decide i = (any (\ x -> x `validates` i)) $ (certificates i) Could somebody explain what is wrong with my intuitive approach? Also, is multi parameter type classes the way to go, or is there a better way?

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Origins of (x:xs)?
by Toby Hutton 20 Dec '06

20 Dec '06

Hi, This may have been asked before, sorry if so. I've wondered where the convention of pattern matching a list to (x:xs) came from? I've read a couple of old papers recently which let me believe it may have started back in the '70s with Miranda and its ilk. Does anyone know why (x:xs)? Is xs meant to be a synonym for 'excess'? Yours curiously, Toby.

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YAHT "bug"
by Johan Grönqvist 20 Dec '06

20 Dec '06

Hi, As noted on e.g. http://paste.lisp.org/display/30002, there seems to be a misprint in YAHT. The code on page 124 for law3a does not use g, and uses y without introducing it. The page says this was corrected in the wikibook, but it seems not to have been fixed in the darcs repository at haskell.org. I found no mentioning on the YAHT page of ways to change this, so sending a mail to this list seemed simplest to me. Thanks for all good tutorials! Johan

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#haskell irc channel reaches 300 users
by dons＠cse.unsw.edu.au 20 Dec '06

20 Dec '06

A small announcement :) 5 years after its inception, under the guiding hand of Shae Erisson (aka shapr), the #haskell IRC channel[1] on freenode has finally reached 300 users! We reached 200 users in January, 250 users for the first time in August, and didn't expect hit to reach 300 for a while. Then last night: 12:50 lambdabot> Maximum users seen in #haskell: 305 This puts the channel at around position 15 of the 4000 freenode channels. For comparision, a sample of the state of the other language communities: #perl 484 #php 474 #c 381 #c++ 379 #ruby 368 > #haskell 305 #python 303 #lisp 214 #java 213 #javascript 156 #scheme 112 #lua 72 #ocaml 56 #sed 35 #d 33 You can see the growth of the channel over here: http://www.cse.unsw.edu.au/~dons/irc/nick-activity.png If you've not dropped by the channel yet, feel free to come and chat, and toss around some lambdas! :) Cheers, Don 1. http://www.haskell.org/haskellwiki/IRC_channel

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FD problem in GHC 6.6
by Robert 20 Dec '06

20 Dec '06

Fellow Haskellers, I have a package that uses some light typeclass hackery to automaticly build parsing algorithms based on the type of a function. I was recently informed that my package doesn't compile on GHC 6.6 due to the new restrictions on FD resolution; in particular I have instance declarations which fail the coverage condition. I can use undecidable instances to make the package compile again, but I'd prefer not to if I can avoid it. Are there any easy tricks I can use to make this work? I only sort-of understand the new restrictions, so I'm at something of a loss as to how to work around them. The problem is fairly isolated in my code: the affected section begins at line 140 in http://www.eecs.tufts.edu/~rdocki01/shellac/src/System/Console/Shell/Comman… I've reproduced the class and a few of the instance declarations here: class CommandFunction f st | f -> st where parseCommand :: String -> f -> CommandParser st commandSyntax :: f -> [Doc] instance CommandFunction (Sh st ()) st where parseCommand wbc m str = -- list monad do (x,[]) <- runRegex (maybeSpaceBefore (Epsilon (CompleteParse m))) str return x commandSyntax _ = [] instance CommandFunction r st => CommandFunction (Int -> r) st where parseCommand = doParseCommand Nothing intRegex id commandSyntax f = text (show intRegex) : commandSyntax (f undefined) instance CommandFunction r st => CommandFunction (Integer -> r) st where parseCommand = doParseCommand Nothing intRegex id commandSyntax f = text (show intRegex) : commandSyntax (f undefined) -- Robert robdockins(a)fastmail.fm -- http://www.fastmail.fm - IMAP accessible web-mail

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