- Haskell-Cafe - Haskell.org

puzzling polymorphism behavior (7.0.3 windows)
by gladstein＠gladstein.com 16 Mar '12

16 Mar '12

Why does the following program compile and produce the results it does? It seems like 3 and "x" got interpreted as the same type, (). Thanks in advance for your help. import Data.IORef import System.IO.Unsafe cell = unsafePerformIO $ newIORef [] push i = modifyIORef cell (++ [i]) main = do push 3 push "x" readIORef cell >>= return *Main> :browse cell :: GHC.IORef.IORef [a] push :: a -> IO () main :: IO [a] *Main> main [(),()]

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Trying to use more than one array in runSTUArray
by Juan Miguel Vilar 15 Mar '12

15 Mar '12

Hello, café: I am trying to use more than one array with runSTUArray but I don't seem to be able to understand how it works. My first try is this: test1 n = runSTUArray $ do a <- newArray (1, n) (2::Int) b <- newArray (1, n) (3::Int) forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b but it does not work. However, when I write test2 n = runSTUArray $ do let createArray v n = newArray (1, n) (v::Int) a <- createArray 2 n b <- createArray 0 n forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b everything is fine although I expected the two versions to be equivalent. To further complicate matters, the following createArray v n = newArray (1, n) (v::Int) test3 n = runSTUArray $ do a <- createArray 2 n b <- createArray 3 n forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b does not work either. Where can I find an explanation for this behaviour? Furthermore, what I am after is to use two arrays with different types (Int and Bool), is it possible? Thanks in advance, Juan Miguel -- Juan Miguel Vilar Torres Profesor titular de universidad Vicedirector de la ESTCE para ITIG e ITIS Departamento de Lenguajes y Sistemas Informáticos Escuela Superior de Tecnología y Ciencias Experimentales Universitat Jaume I Av. de Vicent Sos Baynat s/n 12071 Castelló de la Plana (Spain) Tel: +34 964 72 8365 Fax: +34 964 72 8435 jvilar(a)lsi.uji.es

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HXT: how to get sibling element
by Никитин Лев 15 Mar '12

15 Mar '12

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Functional programming podcast
by Christopher Done 15 Mar '12

15 Mar '12

Show of hands, who would be interested in working on a podcast weekly or biweekly and what would you like to provide? Light banter is an acceptable answer. Some points that might be covered on such a podcast might be: * Latest FP conferences/hackathons/etc * Competitions * Interesting papers (new, or old and dug up) * Interesting blog posts or stackoverflow questions * New and interesting libraries/tech released * Developments in the communities, funding, business developments * Interviews with prominent FP chappies This bleeds over a little with Haskell Weekly News, but it's more like FP weekly "interesting stuff", and therefore there would be a lot more news to pack in. It would be something to listen to while you're walking to work with your ipod. On 21 February 2012 15:55, Mats Rauhala <mats.rauhala(a)gmail.com> wrote: > On 15:15 Tue 21 Feb , Christopher Done wrote: >> I recently thought it would be Pretty Cool to make an FP podcast, sort >> of a spoken Haskell Weekly News but covering all FP, blogs, packages, >> conferences, papers, standards, mailing lists, even stackoverflow, >> whatever's interesting in FP. We could use Gtalk or Mumble (both quite >> high quality audio) to conduct it and cut it up in audacity. > > I for one would be an interested listener

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HXT: how to get sibling element
by Никитин Лев 15 Mar '12

15 Mar '12

Hello, haskellers. Suppose we have this xml doc (maybe, little stupid): <div> <span>Some story</span> <span>Description</span>: This story about... <span>Author</span>: Tom Smith </div> In the end I whant to get list: [("Title", "Some story"), ("Description","This story about..."), ("Author", "Tom Smith")], or, maybe this: Book "Some story" [("description","This story about..."), ("Author", "Tom Smith")] (Book = Book String [(String, String)]. First span is a special case then others and I undestand how to process it: =============== import Text.XML.HXT.Core import Text.XML.HXT.Curl import Text.XML.HXT.HTTP pageURL = "http://localhost/test.xml" main = do r <- runX (configSysVars [withCanonicalize no, withValidate no, withTrace 0, withParseHTML no] >>> readDocument [withErrors no, withWarnings no, withHTTP []] pageURL >>> getChildren >>> isElem >>> hasName "div" >>> listA (getChildren >>> hasName "span") >>> getTitle <+> getSections) putStrLn "Статьи:" putStr "<" mapM_ putStr $ map (\i -> (fst i) ++ ": " ++ (snd i) ++ "| ") r putStrLn ">" getTitle = arr head >>> getChildren >>> getText >>> arr trim >>> arr ("Title",) getSections = arr tail >>> unlistA >>> ((getChildren >>> getText >>> arr trim) &&& (getChildren >>> getText >>> arr trim)) ltrim [] = [] ltrim (' ':x) = ltrim x ltrim ('\n':x) = ltrim x ltrim ('\r':x) = ltrim x ltrim ('\t':x) = ltrim x ltrim x = x rtrim = reverse . ltrim . reverse trim = ltrim . rtrim =================== And I' get list: [("Title", "Some story"), ("Description","Description"), ("Author", "Author")] (Maybe, there is a better way to get this list?) But I cannot find a way to get text that followes some span. I suppose that I have to use function from Data.Tree.NavigatableTree.XPathAxis, but I don't "puzzle out" how to do it. Please, help me.

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Re: [Haskell-cafe] Unable to call function from DLL using FFI
by rajendra prasad 15 Mar '12

15 Mar '12

Hi, Thanks for very quick reply Claude. Your suggestion resolved the issue. Using extern "C" worked for me. Thank you very much. Thanks to all. Regards, Rajendra > > ---------- Forwarded message ---------- > From: Claude Heiland-Allen <claude(a)goto10.org> > Date: Wed, Mar 14, 2012 at 7:43 PM > Subject: Re: [Haskell-cafe] Unable to call function from DLL using FFI > To: haskell-cafe(a)haskell.org > > > On 14/03/12 14:01, rajendra prasad wrote: > >> My c++ code(HelloWorld.cpp) looks like this: >> > > Try adding extern "C" { ... } to use the C ABI instead of a C++ ABI (which > usually features symbol name mangling to add type information, among other > things). (This may not solve the entire problem, but is an important > puzzle-piece). > > > __declspec(dllexport) int doubleMe(int a) >> { >> return 2*a; >> } >> > > > Claude > > ______________________________**_________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> > >

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Mapping string to a function
by Haisheng Wu 15 Mar '12

15 Mar '12

Hi there, Do you have any comments / suggestions for the following scenario? I have two list and a function over list testdata :: [Int] testdata2 :: [Int] f testdata = map g testdata What I like to do is choosing what test data via command line arguments. i.e. test.hs testdata2 will run against testdata2 I could make it using pattern match between argument and data definition but it is annoy. code here: https://github.com/freizl/dive-into-haskell/blob/master/sandbox/one-in-arit… I'm wondering it can be done simply in haskell. Thanks a lot. -Haisheng

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A new home for haskell-mode
by Johan Tibell 15 Mar '12

15 Mar '12

Hi all, After a discussion with Philip Weaver and a few other interested parties in the community we've moved Philip's repo from https://github.com/pheaver/haskell-mode to the new official location at: https://github.com/haskell/haskell-mode You can file bugs/feature requests at: https://github.com/haskell/haskell-mode/issues Hosting the repo under the Haskell GitHub organization gives us a few benefits: * It's easier to find. (We'll update all the references out there.) * We can add co-maintainers, to offload Philip and make sure pull requests get merged in a timely manner. * The repo doesn't have to move if Philip no longer has time to maintain haskell-mode some time in the future. Cheers, Johan

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using mutable data structures in pure functions
by E R 15 Mar '12

15 Mar '12

Consider the following idea for implementing pure functions: A pure function can allocate and modify memory as long as a) it never returns a reference to the memory or b) it never again modifies the memory once it returns (i.e. it returns an immutable object). This is based on the idea of "value objects" in object-oriented languages - the only mutation of a value object occurs in the constructor. Once the constructor is finished initializing the object it becomes immutable. My first question is whether or not this is accurate or does it need some qualifications / fixing up. Secondly, where does this idea fit into the Haskell philosophy? For example, consider the definition of Data.List.nub: nub l = nub' l [] where nub' [] _ = [] nub' (x:xs) ls | x `elem` ls = nub' xs ls | otherwise = x : nub' xs (x:ls) Clearly the memory allocated to ls never escapes nub', so it seems that ls could be replaced with a mutable data structure (with an eye towards improving performance in special cases). For another example, consider Data.Map.fromList. I kind of expected fromList to build up the map using a mutable data structure and then "seal it up" before returning it, but it seems to call the same insert that one would call to add to the map after it has been constructed. So, again, what is the Haskell philosophy towards using mutable data structures in pure functions? Is it: 1. leave it to the compiler to find these kinds of opportunities 2. just use the immutable data structures - after all they are just as good (at least asymptotically) 3. you don't want to use mutable data structures because of _____ 4. it does happen, and some examples are ______ Thanks, ER

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[ANNOUNCE] JuicyPixels 1.1 - image loading library (maintenance release)
by Vincent Berthoux 14 Mar '12

14 Mar '12

Hello list, This is a maintenance release without new functionality, the central change is the switch from Haskell Array to Data.Vector (to ease it's use for OpenGL texture loading). Juicy.Pixels 1.1 change log : - Fixing automatic bitmap decoding, the decodeImage function wasn't trying to decode bitmap - Converting Haskell array types to Data.Vector (thanks to Jason Dagit) - Some tiny documentation enhancement Hackage : http://hackage.haskell.org/package/JuicyPixels GitHub : https://github.com/Twinside/Juicy.Pixels Thanks Vincent Berthoux

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