- Haskell-Cafe - Haskell.org

Parsing bytestream
by C K Kashyap 15 Nov '10

15 Nov '10

Hi, I've been trying to implement the RFB (VNC) protocol and I was exploring a neat way of parsing the protocol bytestream. The protocol specifies that the client first sends a byte that identifies the command then a sequence of word8, word16, word32 and paddings. I've tried to capture that logic with the function below. commandFormat :: Word8 -> [Int] -- 0 for padding bytes commandFormat c | c == setPixelFormat = [0,0,0,1,1,1,1,2,2,2,1,1,1,0,0,0] | c == setEncodings = [0,2] | c == framebufferUpdateRequest = [0,2,2,2,2] | c == keyEvent = [1,2,4] | c == pointerEvent = [1,2,2] | c == clientCutText = [0,0,0,4] | otherwise = [] What I am struggling with is a function like this - parseCommand :: Word8 -> ByteStream -> [Int] parseCommand command byteStream = ... I want the function to use the commandFormat above and return me a list of integers -- for example - if the command is keyEvent and the bytestream is "<1> <0> <2> <0> <0> <0> <3>" The output should be -> [1,2,3]!!! -> That way, I can process it easily. I'd appreciate some help on this very much. -- Regards, Kashyap

5 13

Curious data family bug
by Michael Snoyman 15 Nov '10

15 Nov '10

Hey all, While trying to get a commit pushed for Yesod[1], Alexander Dunlap pointed out one of his programs didn't work with the new code. After some investigation, I was able to reproduce the bug with the following code snippet: {-# LANGUAGE TypeFamilies #-} data family Foo a data Bar = Bar data instance Foo Bar = Bar1 | Bar2 | Bar3 | Bar4 | Bar5 | Bar6 | Bar7 | Bar8 | Bar9 deriving Eq This produces: Couldn't match expected type `Main.R:FooBar' against inferred type `Foo Bar' NB: `Foo' is a type function In the first argument of `Main.$con2tag_R:FooBar', namely `a' In the expression: (Main.$con2tag_R:FooBar a) In the expression: case (Main.$con2tag_R:FooBar a) of { a# -> case (Main.$con2tag_R:FooBar b) of { b# -> (a# GHC.Prim.==# b#) } } The especially strange thing about this bug is that it only occurs when there are more than 8 constructors; if I remove Bar9, everything seems to work. Does anyone have experience with this occuring? Michael [1] http://docs.yesodweb.com/blog/please-break-yesod/

3 3

date parsing and formatting
by Alex Baranosky 15 Nov '10

15 Nov '10

I've been working with Haskell's Date.Time modules to parse a date like 12-4-1999 or 1-31-1999. I tried: parseDay :: String -> Day parseDay s = readTime defaultTimeLocale "%m%d%Y" s And I think it wants my months and days to have exactly two digits instead of 1 or 2... What's the proper way to do this? Also, I'd like to print out my Day in this format: 12/4/1999 what's the Haskell way to? Thanks for the help. Alex

2 1

Encoding problem (?)
by llama＠ruomad.net 15 Nov '10

15 Nov '10

Hello, I have this strange problem, with the same program (simple) behaving differently when run from console and from WinGHCi. Platform is Windows XP. Haskell Platform 2.0.0. The program : import IO import Data.Maybe tfind s = lookup (head s) $ zip "\232\222" "12" main = do h <- readFile "g:\\CODE\\rlib\\test.txt" putStrLn h print $ tfind h The file test.txt only contains a "è" (should be \232). Encoding is ANSI. I compile the pgm from WinGHCi : Prelude> :! ghc --make "ftest2.hs" and run it from WinGHCi : Prelude> :! ftest2.exe è Just '1' Now if I use the _same_ compiled program from cmd line : g:\CODE\rlib>ftest2.exe è Just '2' Is there any plausible explanation ? Bruno

2 1

[ANN] shpider-0.1.1
by John Morrice 14 Nov '10

14 Nov '10

Shpider is a web automation library for Haskell. It allows you to quickly write crawlers, and for simple cases ( like following links ) even without reading the page source. Shpider is on hackage at http://hackage.haskell.org/package/shpider Shpider development repository is now on github at https://github.com/elginer/shpider Shpider 0.1.1 now compiles on ghc-6.12, with the latest version of tagsoup-parsec. Thanks to Noriyuki OHKAWA for a patch! John Morrice

1 0

how do i resolve these screwy cabal errors
by Anatoly Yakovenko 14 Nov '10

14 Nov '10

package derive-2.3.0.2-ad85bd58710fede3f840a467e84c403e is unusable due to missing or recursive dependencies: haskell-src-exts-1.9.4-e0f8c55bea9fc97376aa3598dfdca6d6 package derive-2.4.1-415d44d2f93198aab5dff67866c17b64 is unusable due to missing or recursive dependencies: haskell-src-exts-1.9.4-e0f8c55bea9fc97376aa3598dfdca6d6 what does that mean? i reinstalled haskell-src-exts and derive, doesn't seem to make any difference Linking dist/build/derive/derive ... Installing library in /home/anatolyy/.cabal/lib/derive-2.4.1/ghc-6.12.3 Installing executable(s) in /home/anatolyy/.cabal/bin Registering derive-2.4.1... /home/anatolyy/.cabal/lib/haskell-src-exts-1.9.4/ghc-6.12.3 Registering haskell-src-exts-1.9.4... so i cant import Data.DeriveTH

1 1

Compiler constraints in cabal
by Reiner Pope 13 Nov '10

13 Nov '10

Hi, I have a library, hmatrix-static, on Hackage. Version 0.3 (the current version) compiles with ghc-6.12. Let's say I want to upgrade my library using new features in ghc-7.0, and then release these upgrades as version 0.4. Is there any way to state in my cabal file that this new version will no longer compile under ghc-6.12? The reason I would like to state this is so that a user with ghc-6.12 can do 'cabal install hmatrix-static' (or do a cabal install of a program depending on hmatrix-static) and see that cabal will install version 0.3 rather than attempt to install version 0.4 and fail. Thanks for your help. Reiner

7 10

Type-Directed Name Resolution for Records?
by John Smith 13 Nov '10

13 Nov '10

Two significant points which have emerged from the TDNR thread are: -The wiki page combines two orthogonal proposals: type-directed name resolution, which requires no special syntax, and the x.f syntax, which does not require TDNR. -Implementing both proposals may be a desired fix for the record system, but their use in the rest of the language is more controversial. I would therefore like to propose that TDNR (both parts thereof) be implemented for record accessors. In the fullness of time, the community may come to some consensus regarding the implementation of either or both of these features in the rest of the language.

1 0

xml packages
by Permjacov Evgeniy 13 Nov '10

13 Nov '10

First question. As I saw in sources, both hxt and haxml uses [Char]'s. this is very inefficient. I want to know, does any effective parser for haskell, written in haskell, exists. Efficient means using ByteString to store strings and possibly building representations that shares one string for all similiary named elements. If there is no, is anyone interested in writing one? Second question. I'd like to have a package for read-write-edit SVG graphics. I think, xslt (both transformations and formatting object) are interesting too. Is anyone considering writing such packages?

4 3

a problem about Regex in Real World Haskell
by wolf python london 13 Nov '10

13 Nov '10

hi ,folks , I'm a newbie of haskell and learn haskell using the textbook "Real World Haskell". in the page 201, ghci> "good food" =~ ".ood" :: [String] my output is here https://gist.github.com/675024 it doesn't match what the textbook gives . And I found the ebook of Real World Haskell doesn't match it ,either(just like my output ) . I was wondering How I can fix this problem about Regex. much thanks ~ -- ________________________ wolf python london(WPL) Do as you soul should do ! ________________________

2 2