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How to translate Repa 2 program to efficient Repa 3 code?
by Michael Serra 26 May '12

26 May '12

Hi Haskellers, I've posted this question (and my code) to stack overflow as well ( http://stackoverflow.com/questions/10747079/what-are-the-key-differences-be…) so if anyone here has the answer, I'll post it to that site for the world's reference. Using the Repa 2 API, I have written some simple image convolution tests which run more than fast enough. The trick to getting good performance was to call 'force' after every array transformation. I can't quite figure out the analogous thing to do with Repa 3 - at stackoverflow you can see my Repa 3 code, which runs correctly but very slowly. It is not clear to me how exactly the monadic "computeP" functions in Repa 3 are intended to be used - I have several calls to 'force' in my Repa 2 code, but only 1 call to computeP in the Repa 3 version. I've read the excellent "Numeric Haskell" Repa tutorial by Don S, but it doesn't cover Repa 3. My thanks in advance!

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gona to join in haskell-cafe
by Dante.py 26 May '12

26 May '12

Hello,friends! I come from China, wishing to enjoy haskell with you. -- 张晔 Dante.py 中山大学09级数学与应用数学专业个人主页：http://dantepy.yslsg.org/

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Correspondence between libraries and modules
by Alvaro Gutierrez 25 May '12

25 May '12

Hi there, I've only dabbled in Haskell, so please excuse my ignorance: why isn't there a 1-to-1 mapping between libraries and modules? As I understand it, a library can provide any number of unrelated modules, and conversely, a single module could be provided by more than one library. I can see how this affords library authors more flexibility, but at a cost: there is no longer a single, unified view of the library universe. (The alternative would be for every module to be its own, hermetic library.) So I'm very interested in the rationale behind that aspect of the library system. Thanks! Alvaro

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Template Haskell antiquotation in user-defined quasiquoters
by Sam Lindley 25 May '12

25 May '12

Template Haskell supports antiquotation for built-in quasiquotes, e.g.: [| \x -> x + $([|3 * 4|]) |] However, as far as I can tell, there is no way of supporting antiquotation in user-defined quasiquoters, because the only way to specify a new quasiquoter is through a quoteExp function of type String -> Q Exp. Of course, it is perfectly possible to write a parser for some fragment of Haskell inside your quoteExp function, but that seems crazy given that Template Haskell or rather GHC already implements a parser for the whole language. I know about Language.Haskell.Exts.Parser in haskell-src-exts, which provides parseExp :: String -> ParseResult Exp, but that Exp is a different type to the one provided by Template Haskell.<http://hackage.haskell.org/packages/archive/haskell-src-exts/1.9.0/doc/html…> I'm also aware of Dominic Orchard's syntax-trees package, which supports converting between the two representations using a cunning hack that pretty-prints the haskell-src-exts representation to a string and uses Template Haskell to parse it back. Is there a saner way of simulating antiquotation in user-defined quasiquoters? Sam -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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webcam library on github
by . 25 May '12

25 May '12

Hello Cafe, since I haven't found anything like that, I wrote a small library [1] to read from a webcam in Haskell, using V4L on Linux. It uses the v4l2 package and repa for images. Is anyone interested in contributing to that, or giving some hints on how it could be made faster (I am not sure if I use repa correctly for best performance at all times), or just in emitting any other constructive comments? If yes, please give me a shout. Cheers, Christian PS I would put the whole thing on hackage, but my cabal currently doesn't want to do an sdist. [1] https://github.com/cgo/hsimage

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Typed TemplateHaskell?
by Ilya Portnov 25 May '12

25 May '12

Hello Haskell world. One well-known missfeature of TemplateHaskell is it's «untypedness». I mean, that if you have say an expression (Exp or Q Exp), you do not know it's type (does it represent String or Int or whatever). And GHC does not know too. So, one could easily construct bad-typed expressions using TH, and GHC will complain only on generated code, not on TH code. Such a missfeature was a reasoned decision, because adding some typing into TH will add much complexity to compiler and to language. As far as can I see, using features of last GHC one could write typed TH library relatively easily, and saving backwards compatibility. For example, now we have Q monad and Exp type in "template-haskell" package. Let's imagine some new package, say "typed-template-haskell", with new TQ monad and new polymorphic type Exp :: * -> *. Using last GHC's features, one will easily write something like "expr :: Exp String", which will mean that "expr" represents a string expression. And we will need a new function, say runTQ :: TQ a -> Q a (or some more complicated type), which will turn TypedTemplateHaskell's constructs into plain TH. One question will be on quotations and antiquotations: they are implemented to work untyped TH constructs, how to implement compatibility with new typed TH? We could add some support of them to GHC... Or, maybe define a phantom type (say, Any), and some conversion functions, say untyped :: Language.Haskell.TH.Syntax.Exp -> Language.Haskell.TH.Typed.Syntax.Exp Any. Main idea is: seems new GHC enables us to implement "typed TH" without modifying GHC itself, only by writing a library. Is it so? Any further thoughts? With best regards, Ilya Portnov.

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"Generic figures and their glueings" book ..... please help
by Vasili I. Galchin 24 May '12

24 May '12

Hello, I think the above book by Reyes, Reyes and Zolfaghari is quite intriguing. At the same time I can't get my mind around the notions of "generic figures" and "change of figures"! The first seems to be objects in the base category and the latter morphisms in the base category. Whoever has read this book and understands it please give me some pointers. Sorry the vague call for help. Kind regards, Vasili

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Re: [Haskell-cafe] Can Haskell outperform C++?
by Roman Werpachowski 24 May '12

24 May '12

> From: "Richard O'Keefe" <ok(a)cs.otago.ac.nz> > Subject: Re: [Haskell-cafe] Can Haskell outperform C++? > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: <5F6605A2-DFE0-4AEA-9987-3B07DEF34C42(a)cs.otago.ac.nz> > Content-Type: text/plain; charset=us-ascii > > On 17/05/2012, at 2:04 PM, Gregg Lebovitz wrote: > >> Richard, >> >> Thank you. This is an example of what I had in mind when I talked about changing the playing field. Do you have a slide deck for this lecture that you would be willing to share with me? I am very interested in learning more. > > > No slide deck required. The task is "generating alternating permutations". > > Method 1: generate permutations using a backtracking search; > when a permutation is generated, check if it is alternating. > > Method 2: use the same backtracking search, but only allow extensions > that preserve the property of being an alternating permutation. Gregg, what makes Method 2 so much harder than Method 1 to implement in C or C++? Regards, RW

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Re: [Haskell-cafe] Converting wiki pages into pdf
by do_not_reply_to_this_address＠t-online.de 24 May '12

24 May '12

I invested an enormous amount of time into this problem. Accordingly I got a very well working solution. http://de.wikibooks.org/wiki/Benutzer:Dirk_Huenniger/wb2pdf http://en.wikibooks.org/wiki/File:Haskell.pdf I am happy If you find it useful. Yours Dirk Hünniger > Thu, 08 Sep 2011 05:36:44 -0700 > Hello all > I am trying to write a Haskell program which download html pages from > wikipedia including images and convert them into pdf . I wrote a > small script > > import Network.HTTP > import Data.Maybe > import Data.List > > main = do > x<- getLine > htmlpage<- getResponseBody =<< simpleHTTP ( getRequest x ) -- > open url > --print.words $ htmlpage > let ind_1 = fromJust . ( \n -> findIndex ( n `isPrefixOf`) . > tails $ htmlpage ) $ "" > ind_2 = fromJust . ( \n -> findIndex ( n `isPrefixOf`) . > tails $ htmlpage ) $ "" > tmphtml = drop ind_1 $ take ind_2 htmlpage > writeFile "down.html" tmphtml > > and its working fine except some symbols are not rendering as it > should be. Could some one please suggest me how to accomplish this > task. > > Thank you > Mukesh Tiwari >

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Formalisation for types of monads
by Yves Parès 24 May '12

24 May '12

When explaining monads to beginners (having an imperative background), I found myself to say that there is *roughly* three "groups" of monads (because they're always worried about their cost, i.e. their incidental complexity): - Function-oriented monads (e.g. State, Reader, Cont) - Reductible data-oriented monads (e.g. Maybe, Identity, Writer) - Irreductible data-oriented monads (e.g. List, free monads) (which, as I understood, have a (>>=) operation that has a quadratic complexity if not used properly) Are there others "groups" I'm missing and is there terms to formalize them?

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