May 2012 - Haskell-Cafe

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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question about linking with sqlite3
by Canhua 24 May '12

24 May '12

Hi, I made a cabal package that use HDBC-sqlite3 on Windows. When cabal install it, I got an error from ld.exe: cannot find -lsqlite3. How can I tell ld.exe where sqlite3.dll is plcaed? Thanks.

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Parallel cooperative multithreading?
by Benjamin Ylvisaker 24 May '12

24 May '12

Has anyone ever worked on implementing something like this in Haskell? http://www.cs.hmc.edu/~stone/papers/ocm-unpublished.pdf The outline of the idea: - Concurrent programming is really hard with the popular frameworks today. - For most purposes parallel programming is hard, in some part because it requires concurrent programming. Of course there are attempts to do non-concurrent parallel programming, but I hope it's not too controversial to say that such frameworks are still on the fringe. - Cooperative concurrency is way easier than preemptive concurrency because between invocations of pause/yield/wait, sequential reasoning works. - Historically, cooperative concurrency only worked on a single processors, because running threads in parallel would break the atomicity of sequential blocks (between invocations of p/y/w). - Researchers have been poring tons of effort into efficiently running blocks of code atomically. - Hey, we can do parallel cooperative multithreading! The paper discusses implementations in Lua, C++ and C, but I think Haskell could be an awesome substrate for such a framework. Has anyone thought about this? Ben

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Re: [Haskell-cafe] [Haskell-beginners] Most C++ compilers will not optimize x^2.0 as x*x but instead will do an expensive ...
by Tom Murphy 23 May '12

23 May '12

Just to play devil's advocate, if you look back at the list, KC has written a lot of helpful and informative messages in the past. Tom On 5/23/12, Bardur Arantsson <spam(a)scientician.net> wrote: > On 05/24/2012 04:31 AM, Brandon Allbery wrote: >> On Wed, May 23, 2012 at 10:19 PM, Bardur Arantsson >> <spam(a)scientician.net>wrote: >> >>> This has come up before -- this KC person probably has a broken mail >>> client which doesn't set appropriate References headers. >>> >> >> That, however, ignores the rest of it; the lack of references in this >> case >> forms a pattern with the other things I noted, in that a conversation is >> apparently being held in the form of single observations emitted at the >> point of observation instead of being collected and presented *as* a >> conversation. >> > > Right. I was actually just about to respond to (only) KC in person, but > perhaps unwisely, decided to "hijack" your response to add a little > explanation for everyone. > > You are of course right that not quoting context and just randomly > spewing out small bits of text is not really suitable for a mailing list. > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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