August 2010 - Haskell-Cafe

Question about rpc design
by lysgaard 06 Aug '10

06 Aug '10

Hi, I'm tinkering with making a DHT im my spare time this summer. For this i need to do some unusual rpc calls. You see in a dht a query (rpc) is first sent from you (node "a") to a node "b" which probably doesn't know the answer. If it still does know the answer it returns the value of the query to node "a". This gives the latency: 2*latency(a,b) On the other hand, if i t doesn't it sends a new rpc to a node "c" more probable to know the ansver. Then the latency is: latency(a,b) + 2*latency(b,c) + latency(b,a) While what I would like is this: latency(a,b) + latency(b,c) + latency(c,a) you see that for many forwards this adds considerable latency and also many more calls than nessesary. Is this way of sending messages already known? Does it have a name? And if not, how hard do you think it would be to hack some rpc library to implement this? -- View this message in context: http://old.nabble.com/Question-about-rpc-design-tp29356441p29356441.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Wildly off-topic: de Boor's algorithm
by Andrew Coppin 06 Aug '10

06 Aug '10

Given a suitable definition for Vector2 (i.e., a 2D vector with the appropriate classes), it is delightfully trivial to implement de Casteljau's algorithm: de_Casteljau :: Scalar -> [Vector2] -> [[Vector2]] de_Casteljau t [p] = [[p]] de_Casteljau t ps = ps : de_Casteljau t (zipWith (line t) ps (tail ps)) line :: Scalar -> Vector2 -> Vector2 -> Vector2 line t a b = (1-t) *| a + t *| b Now if you want to compute a given point along a Bezier spline, you can do bezier :: Scalar -> [Vector2] -> Vector2 bezier t ps = head $ last $ de_Casteljau t ps You can chop one in half with split :: Scalar -> [Vector2] -> ([Vector2], [Vector2]) split t ps = let pss = de_Casteljau t ps in (map head pss, map last pss) And any other beautiful incantations. Now, de Boor's algorithm is a generalisation of de Casteljau's algorithm. It draws B-splines instead of Bezier-splines (since B-splines generalise Bezier-splines). But I think I may have ACTUALLY MELTED MY BRAIN attempting to comprehend it. Can anybody supply a straightforward Haskell implementation?

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what's the best environment for haskell work?
by Rustom Mody 06 Aug '10

06 Aug '10

Do most people who work with haskell use emacs/vi/eclipse or something else?? Personal Note: I used gofer some 15 years ago. At that time I hacked up a emacs mode (I did not know of any then) along with some changes to gofer to have gofer inside emacs rather than vi inside gofer. Things have got more exciting now -- just trying to catch up!! [Note: My preferrred/default OS is debian-squeeze]

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Re: using Network.CGI
by Anders Kaseorg 06 Aug '10

06 Aug '10

On Thu, 5 Aug 2010, Andrew U. Frank wrote: > which is clearly not producing the second part of the code, namely > output a greeting. That’s what it’s supposed to output. Have you considered typing in a name and submitting the form? It works fine for me: <http://andersk.mit.edu/greet.cgi>. > i try to use the Network.CGI package from darcs. (Note also that the current version is maintained in Git, not Darcs, but that’s a minor detail. See http://hackage.haskell.org/package/cgi .) Anders

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Re: [Haskell-cafe] subversion with haskell
by Yuras Shumovich 06 Aug '10

06 Aug '10

Hi, Try the next: % env EXTRA_CPPFLAGS="-I/usr/local/include/subversion-1" \ EXTRA_LDFLAGS="-L/usr/local/lib" \ runhaskell Setup.hs configure % runhaskell Setup.hs build % runhaskell Setup.hs install (and read the installation instructions included into the tarball :) ) 2010/8/5 Andrew U. Frank <frank22(a)geoinfo.tuwien.ac.at>: > i found the file in usr/include/subversion-1 (i use an ubuntu (debian) > installation form). but the cabal installer (i.e. the configure script) > does not find the file. > > can you give me a hint where it could search and where i could copy the > file to. i tried to read the configure script, but cannot see, where it > searches. i am not familiar with cpp and do not understand what the hint > 'extra cpp flags' could mean. > > thanks for your help! > > andrew >

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Why do "unsafe" foreign calls block other threads?
by Gregory Crosswhite 05 Aug '10

05 Aug '10

Hey everyone, Could someone explain to me the logic behind having "unsafe" calls block other threads from executing? It seems to me that if anything it would make more sense for "safe" calls to block other threads since the call can call back into the Haskell runtime, as opposed to "unsafe" calls which (by assertion) will never call back into Haskell and therefore should be safer to run in parallel with other threads. What am I missing here? Cheers, Greg

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network.cgi - incomplete translation of code to javascript
by Andrew U. Frank 05 Aug '10

05 Aug '10

i try to use the Network.CGI package from darcs. my testing uses the example code from the webpage 'practical web programming in haskell' http://www.haskell.org/haskellwiki/Practical_web_programming_in_Haskell#Get… specifically the example with code: import Network.CGI import Text.XHtml inputForm = form << [paragraph << ("My name is " +++ textfield "name"), submit "" "Submit"] greet n = paragraph << ("Hello " ++ n ++ "!") page t b = header << thetitle << t +++ body << b cgiMain = do mn <- getInput "name" let x = maybe inputForm greet mn output $ renderHtml $ page "Input example" x main = runCGI $ handleErrors cgiMain -- Get the value of an input variable, for example from a form. -- If the variable has multiple values, the first one is returned. -- getInput :: String -> CGI (Maybe String) this runs (runghc) and produces the output: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" ><head ><title >Input example</title ></head ><body ><form ><p >My name is <input type="text" name="name" id="name" /></p ><input type="submit" value="Submit" /></form ></body ></html > which is clearly not producing the second part of the code, namely output a greeting. the same happens with the next example, using cookies, where from two options for pattern matching only the first is include in the javascript. i assume that i do something wrong - but what? any help is appreciated! andrew

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jswm - how to manipulate strings?
by Andrew U. Frank 05 Aug '10

05 Aug '10

i try to use jswm to build some web applications, but i got quickly stuck. I used the example programs provided and tried to change them. starting with the calculator, i would like to use the string that the user puts in and apply a function to it (say transform it to capitals, later i want to do more complex operations). now i cannot find a way to apply a function to the type returned from the input: get'value :: (Monad mn, CHTMLTextAreaElement this) => Expression this -> mn (Expression String) how can i apply some function to the string in the input : mn (Expression String) ? or how can i get the string out such that i can change it and later put it back into an expression? help is appreciated - there seems to be only a small piece missing.. andrew

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Haskell Platform OPENGL dependencies
by aditya siram 05 Aug '10

05 Aug '10

Hi folks, I just installed the latest Haskell Platform on a fresh Ubuntu Lucid machine and I had to install the following packages to satisfy Open GL: libgmp3-dev,libgl1-mesa-dev, libglu1-mesa-dev, freeglut3-dev Just thought you might want to document that on the Haskell Platform page. -deech

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ANN: Elerea 2.x.x + question about AOP in Haskell
by Patai Gergely 05 Aug '10

05 Aug '10

Hello all, I silently updated Elerea [1] to version 2 about half a month ago, which deprecated the old interface in favour of the 'experimental' branch of version 1. The most important change besides this swap is the addition of the clocked variant of the library, which allows the programmer to easily freeze parts of the data-flow network, a feature similar to the clock system of Lucid Synchrone. Also, as of version 2.1.0 (today's update), the parametric variant was revised and simplified, which is reflected in the types of the combinators. Feel free to play with the new features! On to my question. There are three main variants of the library: a basic one and two others that extend it in some way. The extensions are orthogonal -- on the level of denotational semantics they could be represented as monad transformers (at least I strongly believe so, but I haven't verified yet). This means that there should be a fourth variant that combines the two extensions. Also, it might easily be the case that there will be more independent extensions in the future. This smells like a perfect opportunity to turn to aspect oriented programming techniques, but unfortunately I couldn't find a sensible way to exploit the similarities (of which there are plenty!) and share the core logic of the library across these slightly distinct variants. So the question: how does one go about doing aspect oriented programming in Haskell? Any nice examples in the wild? Gergely [1] http://hackage.haskell.org/package/elerea -- http://www.fastmail.fm - Does exactly what it says on the tin

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