May 2009 - Beginners - Haskell.org

Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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A try on a bank machine algorithm...
by Bernhard Lehnert 01 Jun '09

01 Jun '09

Hi, in a Python Forum someone came up with his homework. Just beginning to learn Haskell I thougt I give it a try. I came up with a working function that I present here to ask you, if I am doing this right or how I could do better. The job was: A cash machine has to compute the division of bank notes for any given amount of money. According to German wikipedia a common algorithm is to start giving one 5$ bill, one 10$ and so on as long as possible (I call this uphill). After that the maximum possible amount of large bank notes is given... Thus every customer gets some small bills and still the number of bills is limited. My answer is a function called 'paymixture' which first calls a function 'uphill' and afterwards a function 'downhill', each of which realizes one part of the above mentioned algorithm. The downhill function might also be usefull apart from the rest which is why indentation ist different... Thanks a lot, Bernhard ------------------------------------------------------------- paymixture :: (Ord t, Num t)=> t->[t]->[t] -- usage: paymixture (sum to pay) (list of possible bills) -- list of bills has to be sorted, like in -- 'paymixture 175 [5,10,20,50,100]' -- last element of the returned list is what could not be paid. paymixture n list = init(up) ++ down where up = uphill n list down = downhill (last(up)) (reverse list) uphill :: (Ord t, Num t)=> t->[t]->[t] uphill n [] = [n] uphill 0 _ = [0] uphill n (x:xs) = if n>=x then x : uphill (n-x) xs else uphill n xs downhill :: (Ord t, Num t)=> t ->[t]->[t] -- usage: downhill (sum to pay) (list of possible bills) -- list if bills has to be reverse sorted, like e. g. -- 'downhill 175 [200, 100, 50, 20, 10,5]' -- last elemt of return value is the rest that could -- not be paid. downhill n supply | (n<last supply) = [n] | otherwise = max : downhill (n-max) supply where max = head([x|x<-supply, x<=n])

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Maybe a and Maybe t
by Ivan Uemlianin 01 Jun '09

01 Jun '09

Dear All I have just started learning Haskell, using the O'Reilly book Real World Haskell [1]. I'm finding it a very exciting language, and I'm working my way painstakingly through the book (I'm up to the exercises at the end of Chapter 3). I have a query based on some code from the book. This question has been asked on the book web site [2], but no answer. I have also Googled about and scanned the documentation. So, here is some code (quoted from [3]): two implementations of a function to return the second element of a list. I have commented out the type signatures. -- file: rwh_examples2/ch03/MySecond.hs -- safeSecond :: [a] -> Maybe a safeSecond [] = Nothing safeSecond xs = if null (tail xs) then Nothing else Just (head (tail xs)) -- tidySecond :: [a] -> Maybe a tidySecond (_:x:_) = Just x tidySecond _ = Nothing My query concerns the inferred types of these functions --- the types inferred when the type signatures are commented out. Here's a ghci session: Prelude> :load seconds.hs [1 of 1] Compiling Main ( seconds.hs, interpreted ) Ok, modules loaded: Main. *Main> *Main> :type safeSecond safeSecond :: [a] -> Maybe a *Main> *Main> :type tidySecond tidySecond :: [t] -> Maybe t *Main> Why is safeSecond type [a] -> Maybe a, and tidySecond type [t] -> Maybe t? I mean why does one use "a" and the other "t"? What does it mean? If I change the order of the two functions in the source file, safeSecond still uses "a" and tidySecond still uses "t". Can anyone help, or point me to the right place in the documentation? In the meantime, if I find out from elsewhere I'll report back here. Thanks and best wishes Ivan [1] http://book.realworldhaskell.org/ [2] http://book.realworldhaskell.org/read/defining-types-streamlining-functions… [3] http://book.realworldhaskell.org/read/defining-types-streamlining-functions… -- ============================================================ Ivan A. Uemlianin Speech Technology Research and Development ivan(a)llaisdy.com www.llaisdy.com llaisdy.wordpress.com www.linkedin.com/in/ivanuemlianin "Froh, froh! Wie seine Sonnen, seine Sonnen fliegen" (Schiller, Beethoven) ============================================================

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Re: [Haskell-cafe] can someone point me to more help about Database.HDBC.ODBC?
by Michael Mossey 30 May '09

30 May '09

This is on Windows, by the way. Do you know the equivalent file names and locations? What is DSN? Thanks, Mike Евгений Тарасов wrote: > Usually you should pass only DSN name in the string, if you used InnoDB > database in MySQL. > > For example, /etc/odbc.ini file: > ; > ; odbc.ini configuration for MyODBC and MyODBC 3.51 Drivers > ; > [.....] > > [Default] > Driver = MySQL > Description = MySQL ODBC 3.51 Driver DSN > Server = localhost > Port = > User = root > Password = > Database = test > Option = 3 > Socket = > > [......] > > Next step, you can test a connection with command: > $ isql Default > +---------------------------------------+ > | Connected! | > | | > | sql-statement | > | help [tablename] | > | quit | > | | > +---------------------------------------+ > SQL> > > And finally, the haskell code: > > conn <- connectODBC "DSN=Default;" > >

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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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