- Beginners - Haskell.org

Applicability of FP unchanging things to real world changing things
by Glurk 17 Dec '09

17 Dec '09

I come from an OO background, but have recently started to see the light and think that functional programming has a lot to offer in real world, general software development - not just some small specific niche. Having the OO background certainly has limited the way I view software development, and I guess in learning something new, I have a tendency to compare it with what I do know, looking for similarities and differences. So, perhaps that's why I am having difficulty in seeing how some functional programming ideas apply. What's got me a little puzzled at the moment is non mutable data. On the one hand, I think it's great that I can mostly forget about errors caused by my variables having unexpected values, and that I can reason about my program much more easily... However, on the other hand, I wonder how it ties in with real world systems that do in fact do deal with objects that change state. For example, if I was building a library system - I might have book objects, and if a book gets borrowed, there remains the same book object, but it now just has a different state - from "on shelf" say, to "borrowed". Now, in Haskell, I might have a function to lend a book :- lend book1 and this is going to return a new book, with the appropriate status change.. ..but what of the old book ? I don't want 2 book objects floating around.. ...to my way of thinking, it's really not a new book, it really is a change of state. How do people see this kind of situation ? Is it just the way of looking at it... ? Do I just accept that I toss out my old library object, and have a new one, which happens to be the same as the old one, except for the "new" book which is now borrowed ? Do I really need to get over thinking of objects in the first place, even though it's what I see exist in the real world ? Is there where we part company with non mutability and venture into the monads where we do have side effects ? Any thoughts appreciated ! :) Thanks !

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What about scratch pad for each library
by Marc Weber 13 Dec '09

13 Dec '09

You can read about the complete proposal here: http://haskell.org/haskellwiki/Hackage_wiki_page_per_project_discussion Benefits: If something doesn't work for you should contact the author/ maintainer or the mailinglist. But if you find a workaround you can put it online immediately. You should remove it again after having discussed the issue with maintainers. However they may be on holidays.. So if you've had minor trouble getting started with a package and if you think that some small examples would have helped you in the past you should register at the haskellwiki and write about your experiences so that I get a understanding of whether this idea improves your experience or makes it worse. Of course not all wiki pages will be maintained. So it's very likely that you find outdated information. But I personally think that one tip is worth 10 outdated wiki pages or such (?) Additionally are you interested in helping removing outdated content when you find such ? You'll find a link to the haskell-cafe mailinglist thread on the wiki page as well. Hopefully most wiki pages are empty or contain links to the project pages only because there are no issues and you understand everything immediately :) Thank you for your feedback Marc Weber

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Problem implementing a stack
by Jacob Faren 13 Dec '09

13 Dec '09

Hello. I'm implement data structures (a stack, here) to get familiar with Haskell. It's not going very well. I have the following code... -- test.hs data Stack a = Top (Stack a) | Layers [a] deriving (Show) stack :: Stack a stack = Top (Layers []) push :: Stack a -> a -> Stack a push (Top (Layers ls)) value = Top (Layers (value:ls)) And I run the following in ghci... Prelude> :l test [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> push stack 4 Top (Layers [4]) *Main> let st = stack *Main> push st 4 ^CInterrupted. The output of the last command never resolves. I have to kill it. Might anyone have an idea why the push function is failing with st, but not with stack? I know there are other stack implementations that work, but I really want to know what's going on here. Thanks

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How to create a monad instance
by Marco De Oliveira 13 Dec '09

13 Dec '09

Hi, Is it possible to create an instance of Monad for IOException? Each time a try I stay blocked by IO monad. with the definition: data IOException a = IOExceptionCons (IO (Exception a)) data Exception a = SuccessCons (Maybe Warning) a | ErrorCons Error data Warning = WarningCons1 | WarningCons2 data Error = ErrorCons1 | ErrorCons2 Regards

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Printing the bits of an Int|Double
by M Xyz 11 Dec '09

11 Dec '09

I've been trying to play around with binary data, but I haven't made much progress trying to print the bits of a Double. With help from #haskell I've made it this far: -- Printing the bits of an Int main = do putStrLn $ showIntAtBase 2 (chr . (48+)) z "" -- 103 = 1100111, after bit shifting 11001 z = shiftR (103 :: Int64) 2 This is as far as I got with Doubles: import Data.Binary.IEEE754 import qualified Data.ByteString.Lazy as BS main = do BS.putStrLn $ runPut $ putFloat64be 4.123 Instead of playing with ByteStrings, is there just a way to fill an Int64 with the bits of a Double (Similar to Java's long = Double.doubleToLongBits(double))?

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scope for variables
by legajid 11 Dec '09

11 Dec '09

Hello, i wrote the following (very simplified) program : word :: [Char] word="initial" letters=['a'..'z'] myloop = do myloop2 myloop2 = do putStrLn word putStrLn letters main = do putStrLn "Enter the word" word <- enter_word myloop enter_word= do return("abcdefghij") "word" is initially binded to the value "initial" myloop, calling myloop2, displays this value "Improving" my program, i add "main" that enters a new value for "word" (here a fix one, ideally a getLine) To display this new value, i must call myloop with this parameter, which is then passed to myloop2. Is there a way to make a "global binding ?", thus changing the value of "word", so that i don't have to modify the code for myloop and myloop2 (now being passed a parameter), that worked fine. This would make the program modular and easy to modify, without modifying all existing functions. Or is it non-sense in Haskell ? Thanks, Didier.

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Re: [Haskell-beginners] Is there a function to determine if a date isa business day?
by Gaius Hammond 11 Dec '09

11 Dec '09

You can buy this data from companies like Reuters. They collate it by having someone phone every embassy every year. Cheers, G ------Original Message------ From: Deniz Dogan Sender: beginners-bounces(a)haskell.org To: Hong Yang Cc: beginners(a)haskell.org Subject: Re: [Haskell-beginners] Is there a function to determine if a date isa business day? Sent: 11 Dec 2009 09:35 2009/12/11 Hong Yang <hyangfji(a)gmail.com>: > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > I don't know, but wouldn't it be nice if there was some package dealing with "cultures"? Something along the lines of: data Culture = Culture { country :: Country, language :: Language, isHoliday :: (Day -> Bool), firstDayOfWeek :: Day, ... } -- Deniz Dogan _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners ------------------

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Is there a function to determine if a date is a business day?
by Hong Yang 11 Dec '09

11 Dec '09

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text file to data type
by Brent Pedersen 09 Dec '09

09 Dec '09

hi, i have files with lots (millions of rows) of data like this: chr6 chr10 96.96 3392 101 2 79030508 79033899 4160024 4163413 0.0 5894 chr6 chr10 93.19 4098 228 13 117152751 117156826 11355389 11359457 0.0 5886 chr6 chr10 95.82 3445 130 5 112422073 112425513 7785396 7788830 0.0 5666 and i'd like to read it into a type like this: data Blast = Blast { query :: S.ByteString , subject :: S.ByteString , hitlen :: Int , mismatch :: Int , gaps :: Int , qstart :: Int , qstop :: Int , sstart :: Int , sstop :: Int , pctid :: Double , evalue :: Double , bitscore :: Double } deriving (Show) where each of those fields corresponds to a column in the file. in python, i do something like: >>> line = [fn(col) for fn, col in zip([str, str, int, int, int, int, int, int, int, float, float], sline.split("\t")] what's a fast, simple way to do this in haskell? is it something like: instance Read Blast where readsPrec s = ????? any pointers on where to look for simple examples of this type of parsing would be much appreciated. thanks, -brentp

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using the current time
by Torsten Otto 08 Dec '09

08 Dec '09

Hi! We are trying to build a chatbot that can among many other things tell the partner what the current time is. Now obviously current time and pure functional programming don't mix well, but Haskell CAN do it. We have played around with it for a while and have figured out quite a bit, but the final icing is missing on the cake. It would be cool to have a dictionary like so > wissen = [ Einfache Begrüßung -> Frage nach dem Wohlbefinden > (["hallo", "hi", "hey"], "Hi, wie geht's?"), > (["guten tag", "tag"], "Wie geht es dir?")] (yes, the bot speaks German!) that could include a line like > (["Uhr","Zeit","spät"], time) where time is a function evaluating to a String including the current time. Once we have the time, we can get it translated into German by > formatiereZeit :: ClockTime -> String > formatiereZeit t > | (head(words (calendarTimeToString (toUTCTime t))) == "Mon") = "Montag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Tue") = "Dienstag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Wed") = "Mittwoch" > | (head(words (calendarTimeToString (toUTCTime t))) == "Thu") = "Donnerstag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Fri") = "Freitag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Sat") = "Samstag" > | (head(words (calendarTimeToString (toUTCTime t))) == "Sun") = "Sonntag" > | otherwise ="keine Uhr heute" which results in a string. Now how do we access the system time in a useful way? > time = formatiereZeit (NOW WHAT?) Would the current time step out of the IO monad, please? Regards, Torsten

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