September 2009 - Beginners

Anyone use haskell + cassandra?
by Joe Van Dyk 28 Sep '09

28 Sep '09

I'm thinking of using Haskell + Cassandra + Rails to build a horizontally-scalable web app. Haskell + Cassandra would power the webservice that Rails communicates with. Has anyone done anything like that? -- Joe Van Dyk http://fixieconsulting.com

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Better way to retrieve data strucures from sql-db?
by Roman Naumann 28 Sep '09

28 Sep '09

The following function converts the data read from an sql database to a player data structure. toPlayer :: [SqlValue] -> Player toPlayer sx = Player { plID = fromSql (sx!!0), plAccount = fromSql (sx!!1), plForename = fromSql (sx!!2), plSurename = fromSql (sx!!3), plPos_x = fromSql (sx!!4), plPos_y = fromSql (sx!!5) } It appears to me, that I have to write this boilerplate code every time I introduce a new data structure that has to be read from the sql database. Can you think of a way to automate this? Thanks for your replies, Roman Naumann

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Haskell type system
by informationen 27 Sep '09

27 Sep '09

Hi, i am trying to understand the Haskell type system. I thought i understood it quite well until i encountered the three following exercises. As you can see, i have the answers already. But i don't understand, why they are correct. Could anybody tell me a good place where i could learn how to answers these kind of questions correctly or could give me some explanations, why these answers are correct? Any help is highly appreciated. Kind regards Chris Two functions f and g with the following signatures are given: f :: a -> a g :: b -> c -> b A) Give the type of the following expressions: 1) g [False] True :: 2) g [] True :: 3) g (f True) :: 4) g f :: Answers: 1 [Boolean] 2) [a] 3) c -> Bool 4) c -> (a -> a)1) g f 1 B) Which of the following statements is correct? 2) g (f 1) is type correct 3) g . (f 1) is type correct 4) g . f 1 is type correct 5) (g . f) 1 is type correct 6) none of the expressions is correct Answers: 1,2 and 5 are correct. C) A function h is given as: h p x = p (f x). Which of the following statements is correct. 1) h :: a -> b -> a -> b 2) h :: (a -> a) -> a -> a 3) h :: (a -> b) -> a -> b 4) h is equivalent to h' with h' p = p . f 5) h is equivalent to h' with h' p = p f 5) h is equivalent to h' with h' p x = p f x Answers: (I am not sure, if i remember correctly, but 3) and 4) should be correct.)

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proxy
by Hong Yang 24 Sep '09

24 Sep '09

Hi, I was using Finance.Quote.Yahoo, but got an error: user error (openTCPConnection: host lookup failure for " download.finance.yahoo.com"). I think it is due to I being behind a proxy. I then imported Network.Browser, ran setProxy, and tried to fetch data using Finance.Quote.Yahoo.getQuote. I still got the error. So I suggest the module author incorporate proxy setting inside the module. Thanks, Hong

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Question regarding infering the type of a function
by MoC 21 Sep '09

21 Sep '09

Hi everybody, today somebody on IRC (#haskell on Freenode) jokingly submitted the following to lambdabot: > (.) (.) It turned out that (.) (.) :: (a1 -> b -> c) -> a1 -> (a -> b) -> a -> c. I got interested in why that is so, but unfortunately I have not been able to figure this out by myself yet. I know that partial application is used, but don't know to what it would expand to and why. Could someone please explain? (I asked on the channel, but nobody did answer so I thought I'd try my luck here.) Regards, MoC

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Vaccum Cairo Errors
by aditya siram 21 Sep '09

21 Sep '09

Hi all, I am trying out the awesome Vacuum Cairo package on GHC 6.10.3 but I am getting some interesting errors. The following works in GHCI: > view [1 .. 3] But the following does not work in GHCI: > let x = [1 .. 3] > view x or, > x <- return [1 .. 3] > view x For the last two I get: context: "(:)|0" -> >>> {"1|1", <<< "(:)|2"} ghc: ../../src/xcb_lock.c:77: _XGetXCBBuffer: Assertion `((int) ((xcb_req) - (dpy->request)) >= 0)' failed. Aborted and GHCI quits. Any ideas? thanks ... deech

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interface files
by Luca Padovani 21 Sep '09

21 Sep '09

I'd like to be able to extract the "interface" of a Haskell module, where the interface should basically contain the list of values defined within the module along with their type, as well as the types defined in the module. It seems to me like the only way of doing this is with GHC and the option --show-iface, which dumps the .hi file into some readable format. However, the dumped interface seems to contain far more information than the one I'm looking for (like annotations to the data types explaining whether they are recursive, etc.), and am not sure whether the grammar of this format is documented anywhere (other than the actual source of GHC, I mean). Is there any other convenient utility to automatically extract this kind of "interfaces"? [*] Thanks in advance. Luca

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Maybe, Either
by Michael Mossey 18 Sep '09

18 Sep '09

I want to use 'lookup' inside an Either String monad. So I want to write something like eitherLookup :: Eq a => String -> a -> [(a,b)] -> Either String b eitherLookup s x ps = case lookup x ps of Just y -> Right y Nothing -> Left s Is there such a function existing? Thanks, Mike

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type class question
by Ben 18 Sep '09

18 Sep '09

dear haskellers -- this email is an (il)literate haskell file. suppose i have class of computations a -> State s b. for concreteness, let's say i'm writing a library of on-line statistical summary functions, like > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances #-} > > module Foo where > > import Control.Monad > import Control.Monad.State > import Control.Monad.State.Class > > data RunningAverageState = S Double Int > > runningAverage :: Double -> State RunningAverageState Double > runningAverage v = do > S sum count <- get > let nsum = sum + v > ncount = count + 1 > put $ S nsum ncount > return $ nsum / (fromIntegral ncount) > > test = take 10 $ evalState (mapM runningAverage [1..]) $ S 0 0 test -> [1.0,1.5,2.0,2.5,3.0,3.5,4.0,4.5,5.0,5.5] here "on-line" means that we may be taking data from an intermittant external source, e.g. a data generator IO [Double], say, and want to be able to feed the summarizer datum one-by-one, and produce intermediate summaries. also we may want to be able to serialize our computation state (with Data.Binary, say) so that we can resume data collection and summarization later. naturally i want to create some common higher order operations on these primitives, like applying them to a stream of data, or combining them in some way. it seems that one would want some kind of type class to define a common interface to them. > class (MonadState s m) => Summarizer s m | m -> s where > initialState :: s > runOne :: Double -> m Double > where initialize puts some intial state into the system, and runOne collects and summarizes the next piece of data. an instance for runningAverage would look like > instance Summarizer RunningAverageState (State RunningAverageState) where > initialState = S 0 0 > runOne = runningAverage but how would i use this, e.g. > --summarizeMany vs = last $ evalState (mapM runOne vs) initialState does not compile. 1) what am i doing wrong? what are the right type class and instance declarations? 2) is there a better way of expressing this kind of "on-line" calculation, perhaps in pure (non-monadic) functions? best, ben

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Check out my photos on Facebook
by Renick Bell 18 Sep '09

18 Sep '09

Hi beginners(a)haskell.org, I set up a Facebook profile where I can post my pictures, videos and events and I want to add you as a friend so you can see it. First, you need to join Facebook! Once you join, you can also create your own profile. Thanks, Renick To sign up for Facebook, follow the link below: http://www.facebook.com/p.php?i=100000244637353&k=Z6E3Y5U6QX413GGJPJ53RTSVZ… beginners(a)haskell.org was invited to join Facebook by Renick Bell. If you do not wish to receive this type of email from Facebook in the future, please click on the link below to unsubscribe. http://www.facebook.com/o.php?k=7d4325&u=1810302595&mid=11da955G6be70683G0G8 Facebook's offices are located at 1601 S. California Ave., Palo Alto, CA 94304.

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