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Haskell Generic Function Question
by William Gilbert 29 May '09

29 May '09

I am trying to write a function that will covert either an integer or an int into a list containing its digits. ex. toIntegralList 123 -> [1,2,3] I have written the following definition that tries to use read to generically cast a string value to an Integral type that is the same as the Integral passed in: toIntegralList :: (Integral a) => a -> [a] toIntegralList x = map (\c -> read [c] :: a) (show x) I understand it would be very simple to just create two functions, one that converts an Int and one that converts an Integer, however I was wondering if there were any way to accomplish what I am trying to do here. Thanks In Advance, Bryan

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Case Expressions
by Nathan Holden 29 May '09

29 May '09

I've been playing with my musical interval problem from before, and I got a little further, but I came up with a problem. I have two types-- Interval and BasicInterval. Interval has 12 constructors, Unison, MinorSecond, MajorSecond, MinorThird, et cetera. BasicInterval has 8 constructors, BasicUnison, Second, Third, Fourth, and so on. I want to be able to convert between them somewhat; I have function interToBasic, which at the moment looks like: interToBasic :: Interval -> BasicInterval interToBasic a = if (b == 1 || b == 2) then Second else if (b == 3 || b == 4) then Third .. where b = fromEnum a What I wanted to do, and figure is probably doable, but I can't seem to find it, is to say something like case (fromEnum a) of 1 || 2 -> Second 3 || 4 -> Third .. Is this doable? If so, what's the syntax?

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System.Random
by Thomas Friedrich 28 May '09

28 May '09

Hi everyone, Using random numbers in Haskell is not entirely trivial (at least, still not for me) and again I am bagging my head against the Gen-Monad. I'd like to write a kind of bootstrap function sample :: Int -> [a] -> [a] sample n xs = ... that samples uniformly n elements from the list xs. I am not sure how to go about this. Would you try something like sample1 :: StdGen -> Int -> [a] -> [a] and later use this in an IO Monad, something along the lines of: do {g <- mkStdGen; return $ sample g n xs}, or would you write it like sample2 :: Int -> [a] -> Gen [a] and then use a function from QuickCheck like `generate` to get your samples? You see, I don't even know how to start thinking about the problem. If anyone got an idea, I'd be pleased if you could help me. Cheers, Thomas

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ANNOUNCE: Haskell Communities and Activities Report (16th ed., May 2009)
by Janis Voigtlaender 26 May '09

26 May '09

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (16th edition, May 2009) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of October 2009: target deadline for contributions to the November 2009 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in October (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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cabal install readline: Missing HSReadlline.h on Mac OS X Leopard
by man 25 May '09

25 May '09

I got this error installing readline (I really was installing lambdabot, it depends on readline): Configuring readline-1.0.1.0... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking for GNUreadline.framework... checking for readline... yes checking for rl_readline_version... yes checking for rl_begin_undo_group... yes checking for rl_erase_empty_line... yes checking for rl_free_undo_list... yes checking for rl_completion_word_break_hook in -lreadline... yes configure: creating ./config.status config.status: creating config.mk config.status: creating readline.buildinfo config.status: creating include/HsReadlineConfig.h cabal: Missing dependency on a foreign library: * Missing header file: HsReadline.h This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: lambdabot-4.2.2.1 depends on readline-1.0.1.0 which failed to install. readline-1.0.1.0 failed during the configure step. The exception was: exit: ExitFailure 1 This is on Leopard, with GNUreadline installed. As far as I see, HSReadline is an internally generated header in readline-1.0.1.0, how possibly cabal may be missing it? I got nothing with google, only that readline seems to be problematic on OSX. PS: I just installed cabal-install in ~/.cabal, it installs other packages without fuss (gsl-random being an exception, but it may be the GSL part...). Any help welcome! -- happy hacking... man

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Re: [Haskell-beginners] Re: Empty list Exception
by Chicão Fernandes Junior 24 May '09

24 May '09

Thanks everyone, for the answers. I didn't knew the *elem* function. That will make my life easier. For the sake of simplicity, I changed the code to a shorter *userRegistration* function, eliminating the *checkUser* function: userRegistration user list > | elem user list = list > | otherwise = user:list > It did worked fine. I'm glad that I can count with this list in my learning process. Thanks again. -- ____________________________________________________________________________ Francisco Borges "Chicão" Junior .......................................................................................... http://pedepinico.blogspot.com .......................................................................................... "Quem de boa vontade carrega o difícil, também carrega o menos difícil..." Lao Tsé - Tao Te Ching

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Empty list Exception
by Chicão Fernandes Junior 24 May '09

24 May '09

Hello! I'm starting programming in haskell and I got some problems running in GHCi (version 6.8.2) a module I've wrote. The following ... --The first string is the name and the second is the user ID, just for test > type User = (String, String, Integer) > > users = [("Marcelo Castro", "1234", 2), > ("Joao Vicente Claudino", "1235", 2), > ("Wilson Brandao", "1236", 2), > ("Humberto Teixeira", "1237", 2), > ("Luiz Gonzaga", "1238", 2), > ("Severino Dias", "1239", 2), > ("Hermeto Pascoal", "1234", 2)] > > checkUser :: User -> [User] -> Bool > checkUser user userList > = if (head (filter (\userParse -> userParse == user) userList)) == > user > then True > else False > > > userRegistration :: User -> [User] -> [User] > userRegistration user list > | checkUser user list = list > | not(checkUser user list) = (user : list) > | otherwise = [] > -- ____________________________________________________________________________ Francisco Borges "Chicão" Junior .......................................................................................... http://pedepinico.blogspot.com .......................................................................................... "Quem de boa vontade carrega o difícil, também carrega o menos difícil..." Lao Tsé - Tao Te Ching

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QuickCheck
by Thomas Friedrich 24 May '09

24 May '09

Hallo everyone, I am currently playing around with the QuickCheck library. I came across the Test.QuickCheck.Batch module a couple of times. I don't seem to have this module installed? Where can I get it and how do I install it? I also would like to invoke the following in ghci. generate 10 (System.Random.mkStdGen 1) arbitrary :: [Int] However, the function `generate` doesn't get imported when importing QuickCheck, and I just cannot find out which module this one would be in. I came to the conclusion that `generate` is just not the best word for a google search :( Cheers, Thomas

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Catching Network Connection Error
by aditya siram 23 May '09

23 May '09

Hi all, I am trying to Network.HTTP to connect to a website and catch the error if the network is down. Here's what I have so far: data Errors = DocumentParseError | WebsiteUnreachableError | ISBNNotFoundError respHTML' :: ISBN -> IO (Either Errors (IO String)) respHTML' isbn = do rsp <- simpleHTTP (getRequest $ " http://isbndb.com/api/books.xml?results=details&access_key=" ++ key ++ "&index1=isbn&value1=" ++ isbn :: Request_String) return $ Right $ getResponseBody rsp `E.catch` (\(e :: E.SomeException ) -> return $ Left $ WebsiteUnreachableError) I get the following error when I load it into GHCI: Couldn't match expected type `[Char]' against inferred type `Either Errors b' Expected type: IO String Inferred type: IO (Either Errors b) In the expression: return $ Left $ WebsiteUnreachableError In the second argument of `E.catch', namely `(\ (e :: E.SomeException) -> return $ Left $ WebsiteUnreachableError)' Failed, modules loaded: none. Control.Exception.catch has the following signature : catch :: forall a. IO a -> (IOError -> IO a) -> IO a I would expect that the handler function needs to return an IO (Either Errors (IO String)). Why is it trying to return an IO String?

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Searching Maybe lists
by aditya siram 23 May '09

23 May '09

Hi all, I would like to define a function that takes a list and a function that evaluates each member of the list to a Maybe value and output the first element in the list that evaluates to 'Just y', or 'Nothing' once the list has been completely processed. So something like: findMaybe :: [a] -> (a -> Maybe b) -> Maybe b The problem is that I don't want it to go through the entire list, but short-circuit when it hits a 'Just ...'. So far I have: orMaybe :: Maybe a -> Maybe a -> Maybe a orMaybe m1 m2 = case (m1,m2) of (_, Just a) -> Just a (Just a, _) -> Just a _ -> Nothing findMaybe :: [a] -> (a -> Maybe b) -> Maybe b findMaybe as f = foldr (\a sofar -> sofar `orMaybe` (f a)) Nothing as 'findMaybe', as far as I can tell, traverses the entire input list which is undesirable for long lists. How can I fix it? Curiously, the regular 'Data.List.find' function that applies a Boolean predicate to each member of the list also seems to first traverse the entire list using 'filter' and then grabs the head of the result. Thanks ... -deech

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