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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 25 May '09

25 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 25 May '09

25 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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HaL4: Haskell-Treffen in Halle/Saale, 12. Juni 2009 - Programm und Anmeldung
by unknown＠example.com 20 May '09

20 May '09

Kindly excuse the German noise, please ... Liebe Haskell-Freunde! Das Programm fuer unser Haskell-Treffen steht jetzt so gut wie fest und man kann sich online anmelden. --------------------------------------------- HaL4 : Haskell - Tutorial + Workshop + Party am Freitag, dem 12. Juni 2009, in Halle/Saale --------------------------------------------- Das traditionsreiche HaL-Treffen bietet eine gute Mischung von Haskell-bezogenen Themen aus Forschung, Anwendung und Lehre mit vielen Möglichkeiten zu Diskussion und Unterhaltung bei der anschließenden Party. Der Workshop wird in diesem Jahr ergänzt durch Tutorien für Haskell-Ein- und Umsteiger. Diesmal findet das Treffen in Halle/Saale im Institut für Informatik der Martin-Luther-Universität Halle-Wittenberg statt. Wir bieten: 10 Uhr: Entwurfsmuster vom Kopf auf die Fuesse gestellt: data, fold und laziness statt Kompositum, Visitor, Iterator. Johannes Waldmann, HTWK Leipzig. 11 Uhr: CAL/openquark: (openquark.org) Haskell + Java => CAL, eine praktische Einfuehrung (Eclipse als CAL-IDE, Excel lesen, typsicheres SQL = LINQ fuer Java?) Alf Richter, iba Consulting, Leipzig 12 Uhr: Wie man das Semikolon ueberlaedt: Code-Strukturierung und -Wiederverwendung durch Monaden 15 Uhr: Hayoo! Haskell API Search Timo Huebel, FH Wedel 16 Uhr: Haskell als reine Spezifikationssprache Baltasar Trancon y Widemann, Universit Bayreuth 17 Uhr: Funktional-logische Programmierung mit Curry Jan Christiansen, Universitaet Kiel 19 - 22 Uhr: Grillparty Wir freuen uns auf rege Teilnahme sowie spannende Vorträge mit heißen Diskussionen und bitten um Anmeldung bis zum 31. Mai. Weitere Informationen auf http://www.iba-cg.de/hal4.html Beste Gruesse Henning Thielemann

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"computation", "action"
by Michael P Mossey 20 May '09

20 May '09

I've seen the following language used: - "computations" - "actions" - "effects" - "context" (For example, a functor is a context, and fmap applies a function to an object in that context without changing the context.) One words that seem to come up in Haskell are "algorithm" and "procedure." For example, a monad is sometimes called a "computation" or an "effect", but I've not seen it called an "algorithm." Can someone elaborate on these terms and why they are used? What is the difference between a computation and an algorithm? The word "effect" has a fairly obvious meaning (like an IO side-effect) but I suspect there's more to it. Why is the word "context" important, and when it is more appropriate than "container"? thanks, Mike

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Intervals?
by Nathan Holden 20 May '09

20 May '09

I have been trying to optimize some research I was doing in my spare time. To spare complexity, it involves music-- I have a class, defining each note in an octave, and I want to write a function that will convert between two notes and an interval-- so, for instance: Foo As {- A sharp-- can't use # -} B = 1 because in the standard scale, A# and B are one semitone apart. My question, however, boils primarily down to this: Is there a way to make Foo :: Note -> Note -> Int Foo a b = a-b work?

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Re: [Haskell-beginners] Intervals?
by Nathan Holden 20 May '09

20 May '09

Yeah, it would. But I always write code like that, it feels ungainly... writing super-long, case-by-case instance declarations (So that I can add in one little caveat at the end, or something), and stuff. Makes me feel like I must be dumb! On Tue, May 19, 2009 at 2:54 PM, Derek Gladding <derek(a)ebollocks.net> wrote: > Nathan Holden wrote: > >> [snip] >> My question, however, boils primarily down to this: Is there a way to make >> >> Foo :: Note -> Note -> Int >> Foo a b = a-b >> >> work? >> > > Would a function > > Bar :: Note -> Int > > make writing Foo easier? > > - Derek >

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