November 2010 - Haskell-Cafe

http://www.haskell.org/ is down !
by Pasqualino "Titto" Assini 15 Nov '10

15 Nov '10

Hi, is haskell.org being updated or, as I fear, Haskell's HQ has been overrun by a mob of PHPers ? If so, I am ready to fight ! titto P.S. Just need to find my Excalibur, oh god, the wife just sent it to the Dry Cleaner. -- Pasqualino "Titto" Assini, Ph.D. http://quid2.org/

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Re: [Haskell-cafe] Concurrent with monads
by John Lato 15 Nov '10

15 Nov '10

> > From: Jiansen He <jiansenhe(a)googlemail.com> > > Hi cafe, > > I wounder if it is possible to tell a haskell system that two computations > with side effects could be executed concurrently. > > Here is an affected example: > > Suppose two people want to compare their age, but do not want to leak their > personal information. The following program reads one person's age after > another then send back compared result. > > age :: IO () > age = do > i <- readIntFrom a > j <- readIntFrom b > writeTo a (i-j) > writeTo b (j-i) > > How can I express the fact that two readings could be carried out in any > order? The best approach is probably to fork a thread for each concurrent evaluation. There are a few packages on Hackage that can help with this; I've used threads[1] and spawn[2] successfully. If the "readIntFrom" function is meant to be responding to a client, a threaded model may make sense organizationally, but for this to be a performance benefit there needs to be sufficient work for each thread to carry out. Haskell threads are lighter than Posix threads, but there's still overhead involved. There is an alternative I hesitate to mention because it involves some voodoo, but Roman Leshchinskiy came up with a clever technique that might be appropriate if you want the behavior of "par" but for IO[3]. [1] http://hackage.haskell.org/package/threads [2] http://hackage.haskell.org/package/spawn [3] http://unlines.wordpress.com/2010/04/21/sparking-imperatives/

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Opportunity: Spring semester internship on Haskell project at Intel
by Ryan Newton 15 Nov '10

15 Nov '10

Dear Haskellers, We're looking for outstanding candidates for an internship in Spring 2011. The internship will be in a suburb of Boston (Hudson, MA). Graduate students and talented undergraduates are welcome to apply, but time is a bit short. We are a small research group run directly by the CTO<http://www.intel.com/pressroom/kits/bios/plowney.htm>of Intel's Developer Products Division. Our group works on high-level parallel programming tools<http://software.intel.com/en-us/articles/intel-concurrent-collections-for-c…>(including for Haskell<http://software.intel.com/en-us/blogs/2010/05/27/announcing-intel-concurren…>) and applies binary translation <http://www.pintool.org/> to various problems; also we collaborate closely with Intel's various developer tools teams (Cilk <http://supertech.csail.mit.edu/cilk/>, TBB<http://www.threadingbuildingblocks.org/>, etc). We are looking for someone to work on a new, self-contained Haskell project to create an eDSL that targets the ArBB virtual machine (VM) for vectorization. One possible solution is to adapt existing projects that target CUDA (e.g. accelerate <http://hackage.haskell.org/package/accelerate>and Nikola <%20http://www.eecs.harvard.edu/%7Emainland/projects/nikola/>). Array Building Blocks (ArBB)<http://software.intel.com/en-us/articles/intel-array-building-blocks/>is an interesting and ambitious project that attempts to bring metaprogramming and eDSLs to the masses. It's based on research by Michael Mccool at Waterloo that he used to found a company, RapidMind<http://en.wikipedia.org/wiki/RapidMind>, which was later bought by Intel. The basic idea is that the VM abstracts over the thread and vectorization capabilities of CPUs, GPUs, and other manycore chips<http://www.intel.com/pressroom/archive/releases/20100531comp.htm>. The VM defines a restricted compute language and provides a managed (yes, garbage collected) environment. Using the normal eDSL and metaprogramming tricks the user writes a portable program in a host language streams ASTs to the VM at runtime. These vector programs include the usual aggregate array operations and high level transformations (map, fold, etc), and the VM can perform fusion/deforestation optimizations while JITting vector codes. The VM API for generating programs is a simple C API<http://software.intel.com/sites/products/documentation/arbb/arbb_manual_win…>that can be called from anywhere. The ArBB team is anxious to demonstrate language frontends for other languages, and Haskell is a good candidate. The official requisition will be available at the Intel Jobs site shortly, but in the meantime if you are interested, please forward your CV to me at ryan.r.newton(a)intel.com . Cheers, -Ryan LOCATION: Hudson, MA DURATION: 3-6 Months, flexible START: Some flexibility, ~January, 2011 EXPERIENCE: Expert Haskell/GHC programmer. Interest in research a plus. LOCATION NOTE: With a car it is possible to live in Cambridge or Boston and reverse-commute to our location. (Working offsite two days a week is possible.)

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Re: [Haskell] intent-typing
by Max Rabkin 15 Nov '10

15 Nov '10

I still don't understand what intent typing is, but this particular problem is discussed (with a type-based, statically checked solution) at http://blog.moertel.com/articles/2006/10/18/a-type-based-solution-to-the-st… --Max On Mon, Nov 15, 2010 at 17:17, Marcus Sundman <sundman(a)iki.fi> wrote: > Hi, how would one go about implementing (or using if it's supported > out-of-the-box) intent-typing* for haskell? > > *) Intent-typing is compiler/verifier/parser enforced typing similar to > "apps hungarian" notation. E.g., say I have a webapp with fields for a > user's first name and last name, which I at some point show on another > webpage that the user loads, which is a typical XSS vulnerability situation. > So I have 2 functions, getParam(...) and setParam(...), which return the > value of a user-provided parameter and gives a parameter to the template > displayed to the user, respectively. I want to somehow flag the > getParam(...)-function as returning a user-provided string (say, > "unsafe-string"), and the setParam(...)-function as requiring a safely > encoded string (say, "safe-string"), so that if I sometimes forget to run > the user-provided string through some anti-xss-encoding thingy (which would > take an "unsafe-string" and return a "safe-string"), and don't (unit-)test > this particular aspect of said code, then some verifier thingy (preferably a > static verifier) would notice this and show me a warning. > > Cheers, > Marcus > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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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

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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/

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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

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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

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[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

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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

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