- Haskell-Cafe - Haskell.org

Can HXT and Parsec 3 be used in the same library?
by John Millikin 09 Oct '09

09 Oct '09

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I'm writing a library that needs to parse both plain text (with Parsec) and XML (with HXT). HXT's .cabal file specifies that it only works with parsec (>= 2.1 && < 3), but it still builds when I depend on parsec >= 3. It's only during "cabal install" that the following error is displayed: - -------------------------------------------- Resolving dependencies... cabal-1.7.3: cannot configure hxt-8.3.1. It requires parsec >=2.1 && <3 For the dependency on parsec >=2.1 && <3 there are these packages: parsec-2.1.0.0 and parsec-2.1.0.1. However none of them are available. parsec-2.1.0.0 was excluded because dbus-core-0.5 requires parsec >=3.0.0 parsec-2.1.0.1 was excluded because dbus-core-0.5 requires parsec >=3.0.0 - -------------------------------------------- Is there a good solution to this? I was thinking about changing my library to use Parsec 2, but that just pushes the problem to my users, it doesn't really solve it. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkrI3BgACgkQIkiy1+fm1TrmJACghhgKDBJNIIa9OVhrZ6zrSii+ HEYAnjqfi0KiIQSNrhGCi6eN1DQoHHs+ =aODO -----END PGP SIGNATURE-----

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type inference question
by minh thu 09 Oct '09

09 Oct '09

Hi, I'd like to know what are the typing rules used in Haskell (98 is ok). Specifically, I'd like to know what makes let i = \x -> x in (i True, i 1) legal, and not let a = 1 in (a + (1 :: Int), a + (1.0 :: Float)) Is it correct that polymorphic functions can be used polymorphically (in multiple places) while non-functions receive a monomorphic type ? Also, I'd like to know why id id True is permitted but not (\f -> f f True) id Is it possible to change the later expression so it type checks ? Are there any type inference algorithms that would accept it ? I'm asking this because I have implemented a simple type inference algorithm following PLAI and I can infer the type for id id True (because I use fresh type variable names for id whenever I apply it before unification) and have an occur check fail for the later expression, just like in haskell. Thanks, Thu

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Any example of concurrent haskell application?
by Daryoush Mehrtash 09 Oct '09

09 Oct '09

I am trying to learn more about concurrent applications in Haskell by studying an existing a real application source code. I would very much appreciate if you can recommend an application that you feel has done a good job in implementing a real time application in Haskell. Daryoush

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GPipe example and screenshot
by Tobias Bexelius 08 Oct '09

08 Oct '09

I've put a simple GPipe example (including a screenshot) on the haskellwiki now, showing off an animated spinning box. You'll find the page here: http://www.haskell.org/haskellwiki/GPipe Later on, I will add more examples, and also some kind of GPipe-tutorial. Hang in there... Cheers Tobias Bexelius _________________________________________________________________ Windows Live: Gör det enklare för dina vänner att se vad du håller på med på Facebook. http://www.microsoft.com/windows/windowslive/see-it-in-action/social-networ…

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New seed. New random number. Right? Wrong.
by michael rice 08 Oct '09

08 Oct '09

I'm entering a new number to make a different seed for my generator each time through the loop, but my resulting numbers are anything but random. Where am I going wrong? Michael ============================= import System.Random import Control.Monad(when) main = do numberString <- getLine when (not $ null numberString) $ do let seed = read numberString let (randNumber, newGen) = randomR (1,6) (mkStdGen seed) :: (Int, StdGen) in putStrLn $ "Number is " ++ show randNumber main ============================= [michael@localhost ~]$ runhaskell zz.hs 12317 Number is 6 12371 Number is 6 37829 Number is 6 8713 Number is 6 71392 Number is 5 29492 Number is 6 41089 Number is 6 28483 Number is 6 78687 Number is 5 40703 Number is 6 86910 Number is 5 15608 Number is 6 56155 Number is 5 31575 Number is 6 72231 Number is 5 51815 Number is 6 20189 Number is 6 71533 Number is 5 17755 Number is 6 91161 Number is 5 [michael@localhost ~]$

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difference "cabal configure" and "runghc Setup.lhs configure"
by Andrew U. Frank 08 Oct '09

08 Oct '09

i have a strange error, which does not occur when i run runghc Setup.lhs configure but when i use cabal configure and then build, it occurs. the error is Type constructor Control.Exception.Exception used as a class in the instance declaration. (i have imported qualified Control.Exception) two questions: 1. Exception is a class (in Control.Exception) - why the error? 2. why is there a difference between runghc and cabal (using the same cabal file). i attach the cabal file and the two configuratin files produced.

2 1

Generalizing IO
by Floptical Logic 08 Oct '09

08 Oct '09

The code below is a little interactive program that uses some state. It uses StateT with IO to keep state. My question is: what is the best way to generalize this program to work with any IO-like monad/medium? For example, I would like the program to function as it does now using stdin but I would also like it to function over IRC using the Net monad from <http://haskell.org/haskellwiki/Roll_your_own_IRC_bot>. Thanks for any suggestions. -- begin code -- import Control.Monad import Control.Monad.State import Data.List data PD = PD { pdCount :: Int , pdList :: [String] } deriving (Show) type PDState = StateT PD IO main = runStateT loop (PD { pdCount = 0, pdList = [] }) loop :: PDState a loop = forever $ do cmd <- liftIO getLine runCmd cmd runCmd :: String -> PDState () runCmd "Inc" = increment runCmd "PrintCount" = liftIO . print =<< getCount runCmd "PrintList" = liftIO . print =<< getList runCmd str | "Add " `isPrefixOf` str = addToList $ drop 4 str runCmd _ = return () getCount :: PDState Int getCount = pdCount `liftM` get getList :: PDState [String] getList = pdList `liftM` get increment :: PDState () increment = modify $ \st -> st { pdCount = pdCount st + 1 } addToList :: String -> PDState () addToList str = modify $ \st -> st { pdList = pdList st ++ [str]} -- end code --

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Let it be
by michael rice 08 Oct '09

08 Oct '09

>From Learn You a Haskell ("Let it be" section): 1. cylinder :: (RealFloat a) => a -> a -> a 2. cylinder r h = 3. let sideArea = 2 * pi * r * h 4. topArea = pi * r ^2 5. in sideArea + 2 * topArea =================== What's the proper indentation for LET so these problems (below) don't arise? I thought LET and IN should be aligned in the same column. Also, isn't a LET expression an "expression." Michael ============== This works: import System.Random main = do gen <- getStdGen let (randNumber, newGen) = randomR (1,6) gen :: (Int, StdGen) in putStrLn $ "Number is " ++ show randNumber ============== This works: import System.Random main = do gen <- getStdGen let (randNumber, newGen) = randomR (1,6) gen :: (Int, StdGen) putStrLn $ "Number is " ++ show randNumber ============== This doesn't: import System.Random main = do gen <- getStdGen let (randNumber, newGen) = randomR (1,6) gen :: (Int, StdGen) in putStrLn $ "Number is " ++ show randNumber [michael@localhost ~]$ runhaskell zz.hs zz.hs:4:2: The last statement in a 'do' construct must be an expression

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Test.QuickCheck: generate
by Michael Mossey 08 Oct '09

08 Oct '09

In Test.QuickCheck, the type of 'generate' is generate :: Int -> StdGen -> Gen a -> a I can't find docs that explain what the Int does. Some docs are here: <http://www.haskell.org/ghc/docs/latest/html/libraries/QuickCheck/Test-Quick…>

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On DSLs - one last time
by Günther Schmidt 08 Oct '09

08 Oct '09

Hi all, I'd like to summarize my impressions on the DSL issue, gathered from the responses to my posts. It is absolutely amazing, various people have shown here that it is possible to design languages within haskell that are almost or just as powerful and safe as haskell itself. It is probably even possible to create an EDSL within an EDSL within haskell. Amazing! Uhm... ... but in that case I really wonder why I should not use Haskell directly then. Because out of the EDSL I can clearly see the "E" and I can see the "L" but having slight problems figuring out where the "D" and the "S" come in. In short, yes it's great that you can create a language within Haskell so powerful that it's gone full circle and is general purpose again, but what good is that? The efforts I've seen here where efforts to create a rich and powerful syntax, but aren't we supposed to focus on expressing semantics specific to a problem in a DSL rather? I'm not trying to put the efforts and achievements of the people who have done such amazing work down, but what exactly is it that I missed? Günther

2 1

random question
by Michael Mossey 08 Oct '09

08 Oct '09

My thread about randomness got hijacked so I need to restate my remaining question here. Is it acceptable to write pure routines that use but do not return generators, and then call several of them from an IO monad with a generator obtained by several calls to newStdGen? shuffle :: RandomGen g => g -> [a] -> [a] shuffle = ... foo :: [a] -> [a] -> IO () foo xs ys = do g1 <- newStdGen print $ shuffle g1 xs g2 <- newStdGen print $ shuffle g2 ys Does this kind of thing exhibit good pseudorandomness? Thanks, Mike

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#include path
by Sean McLaughlin 08 Oct '09

08 Oct '09

Hi, I'm trying to compile some code using Cabal. One of the files has a CPP directive #include "undefined.h" The file undefined.h is in the same directory as the file with the directive. If I use the full path name, cabal can compile it. However, if I use the relative path, it complains about not being able to find the file. I tried adding it to extra-source-files, but that didn't help. I see that Hackage packages like Agda use this trick, so I know it's possible. How can I get these paths worked out correctly? There isn't much comment in the Cabal docs. Thanks, Sean

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Cabal, sdist and flags
by Magnus Therning 08 Oct '09

08 Oct '09

A while ago I moved a project away from using a mix of Cabal and make to build (Cabal to build the library and make to build the tests) to using only Cabal. I added a flag and then made a construct like this: flag BuildTests Description: Build unit and quickcheck tests. Default: False ... executable tests if flag(BuildTests) main-is: Test.hs build-depends: test-framework, test-framework-hunit, HUnit, test-framework-quickcheck2, QuickCheck >= 2.1.0.0 hs-source-dirs: test-src src else main-is: Test.hs buildable: False Interestingly that does work for things like 'configure' and 'build', but it doesn't work for 'sdist': % ./Setup.hs sdist Distribution quality errors: Building source dist for dataenc-0.13.0.1... Setup.hs: Test.hsTest.hs doesn't exist To make this work I had to move the 'main-is' outside of the if-else: executable tests main-is: Test.hs hs-source-dirs: test-src src if flag(BuildTests) build-depends: test-framework, test-framework-hunit, HUnit, test-framework-quickcheck2, QuickCheck >= 2.1.0.0 else buildable: False I found this surprising, so I'm tempted to call it a bug. A quick search didn't tell me whether this is well-known already, hence the email. Would you consider it a bug? Has it been raised already? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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What *is* a DSL?
by oleg＠okmij.org 08 Oct '09

08 Oct '09

Perhaps it would be appropriate to point out the IFIP conference on exactly that topic, DSL. The conference took place in July, here is the permanent record: http://dsl09.blogspot.com/ with pointers to the slides and the discussions. The panel discussion has debated that very question, what exactly is DSL. http://dsl09.blogspot.com/2009/07/panel.html As you can see, the agreement could not be reached on what is Domain. There will be another DSL conference, in Australia. Perhaps all the Haskell-Cafe debaters may want to participate.

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ANNOUNCE: vty-ui 0.2
by Jonathan Daugherty 08 Oct '09

08 Oct '09

I'm happy to announce the release of vty-ui 0.2. Get it from Hackage: http://hackage.haskell.org/package/vty-ui Or get the source with darcs: http://repos.codevine.org/vty-ui This version of vty-ui is everything you hoped it would be: fewer bugs, more widget types, cleaner code, better abstractions, and more generality. If you've written any applications to use vty-ui, only a few of your type signatures will need to change. Here's a summary of changes: General * Unify HBorder and VBorder types (now called Border) * Unify HFill and VFill types (now called Fill) * Unify HBox and VBox types (now called Box) * Box layout: layout documentation, edge case bug fixes * Widget class: refactor growthPolicy into growHorizontal and growVertical * Add Graphics.Vty.Widgets.Composed for high-level widget compositions * Add Graphics.Vty.Widgets.All convenience module to export everything in the library Lists * List items may now be of any widget type! * Add resize, pageUp, pageDown, scrollBy functions and expose other list functionality * Generalize scrolling API * Add convenience API for using lists of Text widgets * Support internal identifiers for list items * Always consume scrollWindowSize lines of space even if the list is smaller New widgets * WrappedText: automatically wraps text in available space; try resizing the demo. * Borders module: one-row/one-column ASCII borders between boxed widgets; ASCII art box container * Demo: use crazy boxed list items to show off border boxes and generic widget support in Lists! * Demo: support PageUp/PageDown to demonstrate List paging API Enjoy! -- Jonathan Daugherty

1 0

dsl and gui toolkit
by Andrew U. Frank 07 Oct '09

07 Oct '09

thanks for the connection to the gui runner - it is interesting and worthwile, but a different approach. my approach is strictly declarative and does not describe sequences of process. it describes screens and links operations to buttons. then it takes inputs from the screen and runs the operations and posts the result on the screen. some operations open new screens etc. i started with the idea that ontologies describe the semantics of data in a static way; this should be (nearly) enough to create the user interface - which is all about semantics of the data. of course, you have to include in the ontology the semantics of the operations (as class with functions). the implementation is close to a reactive framework - adding a representation of all what is visible on the screen (the universer of discourse) in a format usable by the operations. then you have only to connect this 'universe of discourse' to the programs which operate on the data and post their reaction in the UoD and map the UoD to the screen the details are messy as i attempted to construct the internal data structures etc. as automatically as possible and with minimal designer input I did not use template haskell, yet, but will need to (i think). i am happy to share it with interested parties and hear the comments and perhaps somebody can help me with using templates to reduce the . it is IMHO not yet ready for posting in a library - but this is certainly the goal.

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New to Haskell - List Comprehension Question
by Steven1990 07 Oct '09

07 Oct '09

Hi, I'm currently learning Haskell, and I've been trying to work out a function for the following problem for a couple of days now. I want to use a list comprehension method to change the first letter of a string to upper case, and the rest of the string to lower case. Eg: "heLLo" -> "Hello" As I'm trying to learn this, I would appreciate hints rather than the explicit solution if possible? I'm sure I'm close to a solution, I must be missing something though. Driving me crazy! My attempts are something similar to this: upperCase :: String -> String upperCase xs = [toUpper(x):toLower(xs) | x <- xs] I think 'toLower' expects a single character rather than the list which is one place I'm going wrong? Thanks in advance, Steven -- View this message in context: http://www.nabble.com/New-to-Haskell---List-Comprehension-Question-tp257941… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Finally tagless - stuck with implementation of "lam"
by Günther Schmidt 07 Oct '09

07 Oct '09

Hi all, I'm playing around with finally tagless. Here is the class for my Syntax: class HOAS repr where lam :: (repr a -> repr b) -> repr (a -> b) app :: repr (a -> b) -> repr a -> repr b fix :: (repr a -> repr a) -> repr a let_ :: repr a -> (repr a -> repr b) -> repr b int :: Int -> repr Int add :: repr Int -> repr Int -> repr Int sub :: repr Int -> repr Int -> repr Int mul :: repr Int -> repr Int -> repr Int and here is one instance of that class for semantics. newtype I a = I { unI :: IO a } instance HOAS I where app e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 return $ e1' e2') int i = I (putStrLn ("setting an integer: " ++ show i) >> return i) add e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 putStrLn (printf "adding %d with %d" e1' e2') return $ e1' + e2') sub e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 putStrLn (printf "subtracting %d from %d" e1' e2') return $ e1' - e2') mul e1 e2 = I (do e1' <- unI e1 e2' <- unI e2 putStrLn (printf "multiplying %d with %d" e1' e2') return $ e1' * e2') I'm stuck with the "lam" method, for 2 days now I've been trying to get it right, but failed. Is there a possibility that it isn't even possible? Günther

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Re: [Haskell] ANNOUNCE: GPipe-1.0.0: A functional graphics API for programmable GPUs
by Petr Pudlak 07 Oct '09

07 Oct '09

Hi Tobias, (I'm completely new to GPU programming, so my question may be completely stupid or unrelated. Please be patient :-).) Some time ago I needed to perform some large-scale computations (searching for first-order logic models) and a friend told me that GPUs can be used to perform many simple computations in parallel. Could GPipe be used for such a task? I.e. to program some non-graphical, parallelized algorithm, which could be run on a GPU cluster? Thanks for your answer, Petr On Sun, Oct 04, 2009 at 08:32:56PM +0200, Tobias Bexelius wrote: > I'm proud to announce the first release of GPipe-1.0.0: A functional graphics > API for programmable GPUs. > > GPipe models the entire graphics pipeline in a purely functional, immutable > and typesafe way. It is built on top of the programmable pipeline (i.e. > non-fixed function) of OpenGL 2.1 and uses features such as vertex buffer > objects (VBO's), texture objects and GLSL shader code synthetisation to create > fast graphics programs. Buffers, textures and shaders are cached internally to > ensure fast framerate, and GPipe is also capable of managing multiple windows > and contexts. By creating your own instances of GPipes classes, it's possible > to use additional datatypes on the GPU. > > You'll need full OpenGL 2.1 support, including GLSL 1.20 to use GPipe. Thanks > to OpenGLRaw, you may still build GPipe programs on machines lacking this > support. > > The package, including full documentation, can be found at: > http://hackage.haskell.org/package/GPipe-1.0.0 > > Of course, you may also install it with: > cabal install gpipe > > > Cheers! > Tobias Bexelius > > ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ > kolla in resten av Windows LiveT. Inte bara e-post - Windows LiveT är mycket > mer än din inkorg. Mer än bara meddelanden

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better way to do this?
by Michael Mossey 07 Oct '09

07 Oct '09

I'm looking for a hint to write the following code with less redundancy. I have a constructor called BoxBounds, and I want to make one with random values. randomBox :: IO BoxBounds randomBox = do x <- getStdRandom (randomR (-10,10)) y <- getStdRandom (randomR (-70,70)) t <- getStdRandom (randomR (5,10)) b <- getStdRandom (randomR (5,10)) l <- getStdRandom (randomR (5,10)) r <- getStdRandom (randomR (5,10)) return (BoxBounds x y l r t b)

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Network.Curl and posting XML data
by Erik de Castro Lopo 07 Oct '09

07 Oct '09

Hi all, I need to do a HTTP post of XML data (Content-type == "text/xml") to a HTTP (eventually HTTPS) server and to retrieve both the response code and some returned XML data. I have had an extensive read of the haskell-curl docs: http://hackage.haskell.org/packages/archive/curl/1.3.5/doc/html/Network-Cur… as well as playing around with some code, but haskell-curl doesn't seem to support this. I've tried curlPost which along with not returning a response also insists on encoding my XML data as application/x-www-form-urlencoded which simply doesn't work. Anyone ben able to get something like this to work? Are there any alternatives to haskell-curl which will work for my application? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Dutch HUG: meeting next week (October 12th) in Utrecht
by Tom Lokhorst 07 Oct '09

07 Oct '09

I'd like to invite functional programmers in The Netherlands to the Dutch Haskell User Group [1, 2] meeting next Monday at 19:00. Each meeting I see new faces, so that's why I continue to send these invitations to several mailing lists; it's working! At this meeting we'll have two talks. Andres Löh will talk about lhs2TeX, a tool for typesetting Haskell code using LaTeX. Yaakov Nemoy will talk about the status of Haskell packaging in Fedora and how to do it yourself. Like last month, the meeting will be in the Booth Hall [3] of the Utrecht University Library. There's free car parking [4] at the library, and it's easily accessible by bike and bus [5]. Unfortunately, I won't be able to attend, but I hope you will! - Tom Lokhorst [1]: http://www.haskell.org/haskellwiki/Dutch_HUG [2]: http://groups.google.com/group/dutch-hug [3]: http://www.uu.nl/EN/library/contact/university_library/zaalverhuur/Pages/de… [4]: http://www.uu.nl/EN/library/contact/university_library/Parkeren/Pages/defau… [5]: http://www.uu.nl/EN/library/contact/university_library/plattegrondenrou/Pag…

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Creating an alias for a function
by michael rice 07 Oct '09

07 Oct '09

How do I create an alias for a function, like giving CAR the same functionality as HEAD. I know I can do it by creating a definition (see below), but is there a better way, like Scheme's (define head car) car :: [a] -> a car x = head x The reason for doing this is to more closely mirror legacy code. Michael

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RETURNED MAIL: SEE TRANSCRIPT FOR DETAILS
by svens＠gmx.de 07 Oct '09

07 Oct '09

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Fwd: [Haskell-cafe] I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
by Alberto G. Corona 07 Oct '09

07 Oct '09

May be because consciousness is relatively new and thus, not optimized. Sequentiallity is somehow related with lack of information and lack or resources. There is nothing more sequential than a Turing machine. The Von Newman architecture is designed to make as much as possible with a few more resources. The older parts of the brain are fast and parallel because they have a long history of optimizations by natural selection., The new cortex still struggles, step by step, to deduce new information from the available internal and external information at the pace they arrive. 2009/10/1 Tom Tobin <korpios(a)korpios.com> On Thu, Oct 1, 2009 at 3:26 AM, Andrew Coppin > <andrewcoppin(a)btinternet.com> wrote: > > It might be a better argument to say that human thinking is fundamentally > > sequential; parallel computers have been around for a little while now... > > Perhaps *conscious* human thinking is sequential — yet our brains are > massively parallel processors, and have been around for quite a long > time. ;-) > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Fast times at Inferable-but-not-Checkable High.
by Joe Fredette 07 Oct '09

07 Oct '09

So, I've been fiddling with an utterly random idea. What if I had a class: class Hom a b where data Rep a b hm :: Rep a b -> b im :: a -> Rep a b That is, all types that have some conversion between them (an isomorphism originally, then I thought homomorphism, now I'm not sure what the hell I'm talking about, so I've just stuck with Hom...) which preserves some sense of "structure" and "content", for instance data CouldBe a = Only a | Nada is _obviously_ the same type as `Maybe`. However, it can't be used transparently where `Maybe` can be used. That is, shouldn't I be able to define a 1-1, onto function `phi :: CouldBe a -> Maybe a` and as such another `pho :: Maybe a -> CouldBe a` such that phi . pho = pho . phi = phum ... errr. `id`? Hom a b represents one end of that (specifically `hm . im = phi`, and `hm . im` for the instance `Hom b a` would `pho`), then I could, instead of writing a type which expects maybe, simply expects anything _equivalent_ to a maybe, eg safeHead :: Hom (Maybe a) b => [c] -> Rep a (Maybe c) safeHead [] = im Nothing safeHead (x:_) = im (Just x) Though- I think this is a little bit off in terms of how it should work, perhaps this is my problem, but the idea is that instead of returning a Maybe, it returns something which can be converted _from_ a maybe. That is, a "generic" type like: data X1 a = X a | Y which is the "form" of any "maybe-like" type. In one sense, I'm almost trying to be polymorphic over the constructors of a given type, kindof, sortof, if you squint your eyes just right and try not to think too much. My problem comes when I try to do this: hom = hm . im eq x y = hom x == hom y Which, I reason, ought to satisfy the type: eq :: (Hom a b, Eq b) => a -> a -> Bool this assumes that hom defines a equality-preserving conversion. However, the type it infers is: eq :: (Hom a b, Hom a1 b, Eq b) => a -> a1 -> Bool Now, this makes sense, all my signature is is a special case of this type. Interesting consequence, this eq could compare a `Maybe a` and `CouldBe a` in a sensical way. Which is a nice benefit. However, here's where it gets weird, If I try to _provide_ this signature (the second, more general one, or the first, specific one, it doesn't matter), GHC gives me the following error: Iso.hs:29:10: Could not deduce (Hom a b) from the context (Hom a b1, Eq b1) arising from a use of `hom' at Iso.hs:29:10-14 Possible fix: add (Hom a b) to the context of the type signature for `eq' In the first argument of `(==)', namely `(hom x)' In the expression: (hom x) == (hom y) In the definition of `eq': eq x y = (hom x) == (hom y) Failed, modules loaded: none. for the latter and a similar one (but for each of the `Hom a b`, `Hom a1 b` cases. Punchline is this, I'm trying to write a generic equality that works for any two types with the same constructor-structure, when I try to write said function, GHC can infer but not Check (it seems, I'm not sure if that's the correct lingo) the type... My question is twofold. 1. Is this whole idea even possible? I mean- is there some limitation I'm going to run into, I have run into problems when trying to do: instance (Hom a b, Eq b) => Eq a where blah wrt UndecideableInstances. But it seems so _obvious_ what that means, I don't think I fully understand what's going on -- that is, whether I'm misunderstanding how class contexts work here, and why this (seemingly obvious) signature is undecidable, or whether I'm just mistaken in my type signature all together. (My goal is to say that anything which can be converted freely to and from something that is an instance of the Eq class, must preserve the equality relationships. 2. Why is GHC unable to check the types, but infer them. I always understood inferring to be the "hard" part of the problem, and checking to be the "easy" part. Is my intuition wrong? I've posted the code here[1] for your perusal. It's not particularly important that I get this solved, it's just a random idea I wanted to explore and thought the notion might appeal to some other Haskeller's around there who like to have self-documenting datatypes, but hate having to rewrite lots of simple utility functions as penance for our documentarian sins. /Joe

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Fast times as Inferable-but-not-Checkable High -- Link to Source
by Joe Fredette 07 Oct '09

07 Oct '09

Sorry, I forgot to actually include the link to the source: http://lowlymath.net/Iso.hs /Joe

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Fwd: [Haskell-cafe] Num instances for 2-dimensional types
by Alberto G. Corona 07 Oct '09

07 Oct '09

Why? real numbers, complex nuimbers, n-dimensional spaces have well defined + and * operations (vectorial product in the latter case). even algebraic expressions like: data Expr = Var String | Number Integer | Sin Expr | Cos Expr can be instances of Num and express certain simplification rules in the definition. instance Num Expr where ... (Sin x) * (Cos x) = (Sin (2 * x))/2 ... I started to develop a extensible symbolic math manipulation around the idea of using symbolic expressions as instances of Num and other basic classes. But I leaved it out due to lack of time. By the way I attach it just in case anyone want to get some idea from it: module DynAlgebra where data Dyn= forall a.(Num a, Eq a, Show a, Ord a, Read a, Simplify a)=> Dyn a instance Show Dyn where show (Dyn a)= show a instance Eq Dyn where (Dyn a) == (Dyn b)= solve a== solve b class Simplify a where simplify :: a -> a solve :: a -> Polonomial Expr1 data Polonomial e = e :/ e | e :+ e | e :* e deriving Show data Expr1= Var String | I Integer | Pi | E deriving Show -- Integer :/ Integer for exact rational arithmetic data Expr= Polinomial Expr | Formula Dyn deriving (Show, Eq) instance Num (Polonomial Expr1) where fromInteger = I (+) (I a) (I b)= I (a+b) (+) e1 e2 = e1 :+ e2 (*) (I a) (I b)= I (a*b) (*) e1 e2 = (:*) e1 e2 abs (I x) | x>=0 = I x | otherwise = I (-x) signum (I x) | x >= 0 = 1 | otherwise= -1 instance Ord Polonomial where compare (I a) (I b)= compare a b compare ( x :+ z) y | x < y && z >0 = LT | x > y && z >0 = GT compare y ( x :+ z) | y > x && z >0 = LT | y > x && z >0 = GT instance Eq Polonomial where (I a) == (I b) = a == b (I a :+ I b)== ( I c :+ I d)= a + b== c + d (I a :* I b)== ( I c :* I d)= a * b== c * d (a :+ b)== (c :+ d)= a==c && b == d (a :* b)== (c :* d)= a== c && b == d (a :/ b)== (c :/ d)= a * d == b * c exp1 == exp2 = simplify exp1== simplify exp2 instance Simplify (Polonomial Expr1) where solve x = simplify x simplify ( (I x) :/ (I y))= case quotRem x y of (q,0) -> I q (q,r) -> let m= mcd y r in (I(x `div` m)) :/ (I(y `div` m)) simplify ((I a) :+ (I b))= I (a + b) simplify ((I a) :* (I b))= I (a * b) simplify ((a :* b) :+ (c :* d)) | a == c = simplify $ a * (b :+d) simplify (exp1 :+ exp2) = simplify exp1 :+ simplify exp2 simplify (exp1 :* exp2) = simplify exp1 :* simplify exp2 simplify expr= expr mcd x y= case mod x y of 0 -> y t -> mcd y t subst:: Polonomial -> [(String, Polonomial )] -> Polonomial subst exp l= subs1 exp where subs1 (Var v)= case lookup v l of Nothing -> Var v Just e -> e subs1 (e1 :+ e2) = ((subs1 e1) :+ (subs1 e2)) subs1 (e1 :* e2) = ((subs1 e1) :* (subs1 e2)) subs1 (e1 :/ e2) = ((subs1 e1) :/ (subs1 e2)) subst e= e f x= x :* x main= print $ solve $ 2 :+1 2009/10/5 Miguel Mitrofanov <miguelimo38(a)yandex.ru> > > Sönke Hahn wrote: > > I used to implement >> >> fromInteger n = (r, r) where r = fromInteger n >> >> , but thinking about it, >> fromInteger n = (fromInteger n, 0) >> >> seems very reasonable, too. >> > > Stop pretending something is a number when it's not. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Cabal - how do I remove a package?
by Paul Moore 06 Oct '09

06 Oct '09

As it says in the subject, how do I remove a package that I installed (with the --user flag) via cabal? Come to that, how do I list the packages I've installed via cabal? cabal list --installed includes all the GHC standard library (which I didn't install) and it doesn't seem to support a --user flag to list per-user installed packages. FWIW, I'm on Windows 7. Thanks, Paul.

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ANNOUNCE: Graphalyze-0.8.0.0 and SourceGraph-0.5.5.0
by Ivan Lazar Miljenovic 06 Oct '09

06 Oct '09

To keep Joe Fredette happy [1] this time, I've combined these two announcements into one. [1] http://sequence.complete.org/hwn/20091003 I'm pleased to announce the latest versions of Graphalyze [2] and SourceGraph [3]. The only change to Graphalyze has been the addition of Legend support (which necessitated a slight change in the data structures used, hence the large version bump) but SourceGraph contains more substantial changes. [2] http://hackage.haskell.org/package/Graphalyze-0.8.0.0 [3] http://hackage.haskell.org/package/SourceGraph-0.5.5.0 Changes to SourceGraph include: * A legend so that you know what all the fancy symbols, etc. actually mean. * Class instances now drawn correctly. * Better colour support. * Clean up printing of cliques, etc. I've also put some sample SourceGraph reports online at http://code.haskell.org/~ivanm/Sample_SourceGraph/SampleReports.html so that you can see what kind of results it produces. Enjoy! -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Haskell Platform - changing the global install dir
by Paul Moore 06 Oct '09

06 Oct '09

I've installed the Haskell Platform to a non-default location (D:\Apps\Haskell) to avoid some of the Win7 funniness over writing to C:\Program Files. However, even though I've done this, the cabal global install directory is still C:\Program Files\haskell. This has been added to my PATH and is used by default. Is there a way I could have specified that I want the global install directory in D:\Apps\Haskell? I guess I could hack my cabal\config file (and presumably change PATH) but that seems a bit ugly. Is the use of Program Files even when the install was to somewhere else a bug? It's not a big deal - I can easily stick to user installs for now. Thanks, Paul.

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I/O Haskell question
by Maria Boghiu 06 Oct '09

06 Oct '09

Hey. I'm trying to configure Xmonad, the window manager. for this purpose, I'm trying to read the *workspaces* variable in the code below from file. As you can see, I do workspaces = readWS where * readWS = do l <- readFile "~/bla" return l* I get an error saying I am mismatching types IO [String] and [String]. But I thought that once one does * l <- readFile "~/bla"*, l is the a string or, in this case, a String list. If I run that line in prelude it seems to be working fine, prints the content of the file "~/bla" on screen (though I'm not sure if as a String or as a list of Strings, i.e. [String]) Can anyone help please? I've been struggling with this for days... 94 main = do 95 xmonad 96 $ defaultConfig { 97 manageHook = manageDocks <+> myManageHook <+> manageHook defaultConfig, 98 layoutHook = avoidStruts $ layoutHook defaultConfig, 99 logHook = dynamicLogWithPP $ conkyPP "", 100 terminal = "gnome-terminal", 101 keys = myKeys, 102 workspaces = readWS, 103 mouseBindings = myMouseBindings, 104 focusedBorderColor = "#008E00", 105 borderWidth = 3 106 --modMask = mod1Mask, --This is to rebind mod key 107 } `additionalKeys`

6 5

Curried function terminology
by michael rice 06 Oct '09

06 Oct '09

This is from Learn You A Haskell: ========== "Curried functions Every function in Haskell officially only takes one parameter. So how is it possible that we defined and used several functions that take more than one parameter so far? Well, it's a clever trick! All the functions that accepted several parameters so far have been curried functions. What does that mean? You'll understand it best on an example. Let's take our good friend, the max function. It looks like it takes two parameters and returns the one that's bigger. Doing max 4 5 first creates a function that takes a parameter and returns either 4 or that parameter, depending on which is bigger. Then, 5 IS APPLIED TO THAT FUNCTION and that function produces our desired result. What really happens when we do multThree 3 5 9 or ((multThree 3) 5) 9? First, 3 is applied to multThree, because they're separated by a space. That creates a function that takes one parameter and returns a function. So then 5 IS APPLIED TO THAT, which creates a function that will take a parameter and multiply it by 15. 9 IS APPLIED TO THAT FUNCTION and the result is 135 or something." ======= The language (in CAPS) in the above two paragraphs seems to be backwards. In the first paragraph, since functions are conventionally "applied" to parameters shouldn't it read something like THE PARTIALLY APPLIED FUNCTION IS THEN APPLIED TO the 5? Or is the terminology different for Haskell, as opposed to say, Scheme or just about any other language? Michael

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Mutable data within a (non-IO) monad
by Gregory Crosswhite 05 Oct '09

05 Oct '09

Hey everyone, I am thinking about creating a particular data structure with an immutable and mutable version. The key of my problem is that I would like the user to be able to work with a mutable version of the data within a non-IO monad and then get an immutable value at the end, allowing them to do stateful-like computation in pure code. The key is that I need to enforce the ordering within the monad, but I can't do this using a state-transformer since the state doesn't actually change; instead, the data structure is internally modified, but the original pointer is kept to it. There are many data structures which already have this kind of functionality, such as STRefs and MArrays, however I am having trouble distilling from the source code what the best "trick" is to accomplish mutability of this form within a monad. There seem to be at least a couple of strategies: using "type-threading" with GHC's State# type, and using the ST monad with a rank-2 qualifier over the type of the state. Do you all have any thoughts on ways to solve this problem, and/or a high-level explanation of what is going behind the scenes with STRefs and MArrays? Thanks! - Greg

4 4

dsl and gui toolkit
by Andrew U. Frank 05 Oct '09

05 Oct '09

writing a gui is a mess (independent of wx or gtk) - too much detail is shown and not enough abstraction is done. haskell can help. i have written an experimental way of producing the GUI automatically with a description of the semantics of the types and operations involved (a la ontology, evnetually comparable what protege produces). the input is a descriptionof the entity ypes, the fields used, the functional dependencies between the fiels, plus the operations used. the division in screens and their layout. the rest ist automatic. the result is a GUI (with preferably gtk but i had also a wx version running). the ideas were inspired by eliot conal's work and wxgeneric, which seemed for administrative applications either too restricted or to specific. if somebody wants to try it out for his application, please write frank(a)geoinfo.tuwien.ac.at (there is not much documentation and the code is not yet completely clean - testing by somebody else would be very valuable!) andrew

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Designing a DSL?
by Günther Schmidt 05 Oct '09

05 Oct '09

Hi, there are numerous examples on how to implement a DSL, but I haven't been able to figure out how to "design" one. I mean I have a pretty good idea of the problem domain, I've coded it over and over again until I got it right. Now I'd like to express that part as a DSL instead of "hard coding" it as before so I can be more prepared if I need to change or amend it. Ideally I'd only have to change the interpreter, right? And that I find to be the really tricky part, how do I *design* a DSL? Günther

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System.Random.Mersenne - no randomR variants
by Paul Moore 05 Oct '09

05 Oct '09

I notice that System.Random.Mersenne has no randomR variants. Is there a reason for this, or is it just an oversight? It makes porting code from System.Random a little more complicated than it needs to be. I know it's possible to write a function mapping a Double (or other value, as appropriate) to a range, effectively replacing randomR, but it's fiddly to get the edge cases right. A standard implementation would be helpful. Paul

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suggestions for doing date/time values with Data.Generics ?
by Thomas Hartman 05 Oct '09

05 Oct '09

I want to patch the time package, which I darcs got head on. Goal is to be able to do newtype deriving for UTCTime which I reckon is the base time value, for use in happstack applications state. basically, need instances for UTCTime, for Data and Typeable. ********** on #haskell: <patch-tag> I just darcs got time package and attempted to install. cabal install didn't work because missing configure fle. there's a .ac file there, so I tried first autoconf, then cabal install. configure file got created, but failed with config.status: error: cannot find input file: include/HsTimeConfig.h.in. now what? <patch-tag> my goal is to patch time package so can do data generics deriving. <Berengal> patch-tag, use System.Time instead? * Berengal shrugs and sighs [12:20] <patch-tag> could try that... *** o-regalia (n%3Do-regali(a)host-0-130.mimpvbg.clients.pavlovmedia.com) has quit: "Lost terminal" <Berengal> It already has a Serializeable instance defined in one of the happstack modules, so there's probably a Typeable and Data instance somewhere as well <Berengal> patch-tag, unfortunately, UTCTime has a component of a hidden datatype which isn't an instance of Typeable or Data, and since it's not exported it's impossible to create the instance as well <Berengal> If it were exported, it'd be pretty easy <patch-tag> hw can I use UTCTime, deriving Typeable and Data? (want to use UTCTime in happstack). <patch-tag> http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=4278#a4278 <patch-tag> I guess I should have said I'd like to use newtype deriving rather than define instance myself if that's possible. <Berengal> patch-tag, that's possible, but could break other things. The correct solution is to patch the existing package, but that takes time <Berengal> patch-tag, Day, or something. Can't remember off the top of my head *** paolino (n=paolino@87.7.161.162) has quit: "Leaving." <ski> Philonous : if you can't implement both operations (satisfying laws), though, it is probably better to make a new class [12:09] <patch-tag> Berengal: a package is hidden if when you do :info in ghci, it comes out as fully qualified, right? <patch-tag> pity also that newtype deriving doesn't give you a nicer error message like "deriving choked at type blah" <Berengal> patch-tag, it comes fully qualified if it's not imported, so you'll have to try to import it to see if it's really hidden [12:10] ********* Seems like the right thing to do is patch the time package anyway. File Edit Options Buffers Tools Haskell Help {-# LANGUAGE DeriveDataTypeable #-} module TypeableTime where import Data.Time import Data.Data import Data.Generics import Data.Typeable -- doesn't work --data MyUTCTime = MyUTCTime UTCTime -- deriving (Typeable,Data) -- should I work up to UTCTime by doing this...? data MyDay = MyDay {toMyDayModifiedJulianDay :: Integer} deriving (Typeable,Data) newtype MyString = MyString String deriving (Read,Show,Ord,Eq,Typeable,Data) -- should I just modify time package? is this easier?

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Re: [Haskell-cafe] Num instances for 2-dimensional types
by Brad Larsen 05 Oct '09

05 Oct '09

On Mon, Oct 5, 2009 at 10:36 AM, Miguel Mitrofanov <miguelimo38(a)yandex.ru> wrote: [...] > Of course, it's OK to call anything "numbers" provided that you stated > explicitly what exactly you would mean by that. But then you have to drop > all kind of stuff mathematicians developed for the usual notion of numbers. > In the same way, you shouldn't use the "Num" class for your "numbers". > > On the other hand, people can (ab)use the "Num" class as they wish, and it's > their business until they ask a question about it somewhere outside - which > makes the business not only theirs. [...] The Num class has `negate' as part of its definition. Natural numbers are numbers, but I don't believe there is any sensible definition of `negate' for them. Haskell 98's numeric hierarchy combines many operations which should be separate. As further evidence, every bit of Haskell I have seen that does symbolic manipulation of numeric expressions either leaves out instances that would make the syntax more convenient, or else defines partial instances because certain class functions have no sensible definition for symbolic expressions. Sincerely, Brad

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apply function arguments in a list
by Michael Mossey 05 Oct '09

05 Oct '09

If I have a list containing the arguments I want to give to a function, is there a general way to supply those arguments in a compact syntax? In other words, I could have args = [1,2,3] f x y z = ... I would write t = f (args!!0) (args!!1) (args!!2) but there may be a neater, more general syntax. I tried writing a fold with $, but I don't think that works because the type of each step is different. I.e. f :: a -> a -> a -> a f x :: a -> a -> a f x y :: a -> a f x y z :: a This would seem to preclude a general way of writing a function that supplies arguments from a list.

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ICFP videos now available
by Wouter Swierstra 05 Oct '09

05 Oct '09

I am happy to announce that videos of all talks at ICFP and some of the associated workshops this year have made available online: http://www.vimeo.com/user2191865/albums I'm sure you'll join me in thanking Malcolm Wallace for the time and effort he put into making this possible. Thank you Malcolm! Wouter

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Is c2hs able to generate FunPtr rather than just the function?
by Rafal Kolanski 05 Oct '09

05 Oct '09

Hi Everyone, I'm trying to figure out an elegant way of letting the Haskell garbage collector deal with structures allocated from C. So I have the C pair: fluid_event_t* new_fluid_event(void); void delete_fluid_event(fluid_event_t* evt); I've handled the new_fluid_event thing nicely, ForeignPtr and all. Now if I want to add a finalizer to the ForeignPtr via newForeignPtr, and for that I need a FunPtr. So what I need is: foreign import ccall safe "fluidsynth.h &delete_fluid_event" deleteFluidEvent'' :: FunPtr (((Ptr (Event)) -> (IO ()))) (if I use that as a finalizer, everything works nicely) But the closest I can get to generating something similar with c2hs is: {# fun delete_fluid_event as ^ {fromEvent `Event'} -> `()' #} which results in: foreign import ccall safe "Event.chs.h delete_fluid_event" deleteFluidEvent'_ :: ((Ptr (Event)) -> (IO ())) Is there a neat way to have c2hs generate the FunPtr version for me? Sincerely, Rafal Kolanski.

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ANN: TxtSushi 0.4.0
by Keith Sheppard 05 Oct '09

05 Oct '09

Hello Cafe, I've just uploaded TxtSushi 0.4.0 to hackage. TxtSushi is a collection of command line utilities for processing comma-separated and tab-delimited files. I posted details on my blog (along with an advertisement to see if others are interested in hacking TxtSushi): http://blog.keithsheppard.name/2009/10/txtsushi-040.html Best Regards Keith -- keithsheppard.name

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Fwd: [Haskell-cafe] killer app sought
by Alberto G. Corona 04 Oct '09

04 Oct '09

Yes, Maybe The piece of the web that desperately need a boost in performance, declarativeness, safety, static typing threading, modularity etc etc etc is the Web Browser. 2009/10/4 John A. De Goes <john(a)n-brain.net> > With few exceptions, no such thing as a killer server-side app. > > The Web 3.0 paradigm is simple: all work except sharing and persistence of > data is done on the client. > > Regards, > > John A. De Goes > N-Brain, Inc. > The Evolution of Collaboration > > http://www.n-brain.net | 877-376-2724 x 101 > > > On Oct 3, 2009, at 9:08 PM, Mark Wotton wrote: > > Hi, >> >> I've been writing a little binding from Ruby to Haskell called Hubris ( >> http://github.com/mwotton/Hubris) which I think has some potential both >> for making Haskell web apps easier to write, and also for bringing the more >> adventurous Ruby programmers into the Haskell community. Code-wise it's >> coming along nicely, and once 6.12 is out it'll run without modifications at >> least on Linux (remains to be seen how long it'll take to get the Mac >> patches into shape). My real problem is marketing: I need a killer app that >> shows it's easy either to >> >> 1. wrap a kickarse Haskell library in a convenient Ruby web app shell >> 2. speed up a poorly performing Ruby web app >> >> I've been badgering the Ruby guys in Sydney that I know on the second >> point, but either none of them have performance problems, or none of them >> want to admit it. The first is entirely possible - if you only attack the >> subset of problems where your runtime is dominated by the database and >> network latency, language performance is moot. Conversely, if that's your >> worldview, the other problems that could be attacked won't ever come to mind >> (to monstrously abuse the Sapir-Whorf hypothesis). >> >> So, I'm asking you guys. What are some really nice Haskell libraries or >> apps that could benefit from being shown off in one of the plethora of >> slick, mature web frameworks that exist in Ruby? Manuel Chakravarty >> suggested something with vector operations in order to take advantage of his >> 'accelerate' library (once it gets a GPU backend, of course), and more >> generally, something taking advantage of Haskell's support for multicore >> would be cool. (The standard edition of Ruby is still unicore, I believe.) >> >> Parenthetically yours, >> Mark >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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killer app sought
by Mark Wotton 04 Oct '09

04 Oct '09

Hi, I've been writing a little binding from Ruby to Haskell called Hubris (http://github.com/mwotton/Hubris ) which I think has some potential both for making Haskell web apps easier to write, and also for bringing the more adventurous Ruby programmers into the Haskell community. Code-wise it's coming along nicely, and once 6.12 is out it'll run without modifications at least on Linux (remains to be seen how long it'll take to get the Mac patches into shape). My real problem is marketing: I need a killer app that shows it's easy either to 1. wrap a kickarse Haskell library in a convenient Ruby web app shell 2. speed up a poorly performing Ruby web app I've been badgering the Ruby guys in Sydney that I know on the second point, but either none of them have performance problems, or none of them want to admit it. The first is entirely possible - if you only attack the subset of problems where your runtime is dominated by the database and network latency, language performance is moot. Conversely, if that's your worldview, the other problems that could be attacked won't ever come to mind (to monstrously abuse the Sapir-Whorf hypothesis). So, I'm asking you guys. What are some really nice Haskell libraries or apps that could benefit from being shown off in one of the plethora of slick, mature web frameworks that exist in Ruby? Manuel Chakravarty suggested something with vector operations in order to take advantage of his 'accelerate' library (once it gets a GPU backend, of course), and more generally, something taking advantage of Haskell's support for multicore would be cool. (The standard edition of Ruby is still unicore, I believe.) Parenthetically yours, Mark

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Where do these warnings come from?
by Colin Paul Adams 04 Oct '09

04 Oct '09

Does anyone recognize which module/function would emit the following warnings? WARNING: nonstandard use of escape in a string literal LINE 11: '# Pandoc\r\n\r\nPandoc is a program for converting ... ^ HINT: Use the escape string syntax for escapes, e.g., E'\r\n'. -- Colin Adams Preston Lancashire

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Need some help with Alex macros
by Niklas Broberg 04 Oct '09

04 Oct '09

Hi all, I'm going crazy trying to get Alex to do what I want. I have the following regexp macros: @octEscape = [0123]? $octdig{1,2} @hexEscape = 'u' $hexdig{4} @charEscape = '\\' (@octEscape | @hexEscape | b | t | n | f | r | \" | \') and the following rules: \' (. # [\'\\] | @charEscape) \' { \p s -> CharTok (readCharTok s) } \" (. # [\"\\] | @charEscape)* \" { \p s -> StringTok (readStringTok s) } I've tried a lot of variations on where to put the backslashes and quotes to get it to do what I want, what you see above is just one variation, but no matter what I try I can't get it to work. > alexScanTokens $ show '\n' *** Exception: lexical error Can anyone help shed some light on this for me? Why can't I get alex to recognize escapes inside character literals? Cheers, /Niklas

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ANNOUNCE: SourceGraph 0.5.2.0
by Ivan Lazar Miljenovic 04 Oct '09

04 Oct '09

Yes, this is yet another SourceGraph announcement, this time for version 0.5.2.0 [1]. [1] http://hackage.haskell.org/package/SourceGraph-0.5.2.0 The changes in this release are as follows: * Shift overall analysis to the top and per-module analysis to the end, as suggested by Duncan Coutts. * Graph drawing fixups: instances are now drawn correctly, and the module graph now has modules located in the correct directory. * Improve some graph drawing aspects (node margins, colours for module graphs, etc.). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Fwd: [Haskell-cafe] Haskell for Physicists
by Alberto G. Corona 03 Oct '09

03 Oct '09

I´m a physicist, so I think they would be attracted by something like Haskell: high level physics modelling at Fortran speeds Haskell: mathematics beyond numerical calculus 2009/9/30 <edgar(a)ymonad.com> Hi, > > I will give a seminar to physicists at USP (Universidade de São Paulo, > Brazil) university and they asked me for a good title, something that can > attract physicists. Anyone has some suggestions? (Will be > a seminar about the use of Haskell to substitute C or Fortran > in a lot of tasks, and how it can be used in some problems instead of > Matlab, Mathematica, etc.) > > Thanks, > Edgar > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Alex problem, help needed!!!!!!!!! ASAP :)
by María Laura 03 Oct '09

03 Oct '09

Dear list, I am trying to make a compiler and we are having a hard time getting Alex to work. We have succeded to work out Alex using older version, but with the the 2.2 version we keep getting this error and we havent been able to figure it out. So this is our tokens definition: { module Lexico where import Alex } %wrapper "posn" $digit = 0-9 -- digits $alpha = [a-zA-Z] -- alphabetic characters tokens :- $white+ ; --Los espacios en blanco los omito \/\/.* ; --Lo que venga después de dos barras omito \/\*.*\*\/ ; --Lo que está entre las barras de comentario omito $digit+ { \p s -> TokenEntero p (read s) } \' [$alpha $digit \_]* \' { \p s -> TokenString p (read s)} [$digit]+\.[$digit]+ { \p s -> TokenDouble p (read s) } $alpha [$alpha $digit \_]* { \p s -> TokenVar p (read s) } And when we call the alexScanTokens "hello" we get this error: [TokenVar (AlexPn 0 1 1) "*** Exception: Prelude.read: no parse So we are concerned about the "TokenVar p (read s)" . Would that be the way to read a string? We just get this error with TokenVar and TokenString, whenever we call the function like: alexScanTokens 19 , for instance we dont get any problems and the token 19 is recognized ok. Please, any ideas on what am i doing wrong???????? regards, L _________________________________________________________________ Keep your friends updated—even when you’re not signed in. http://www.microsoft.com/middleeast/windows/windowslive/see-it-in-action/so…

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