May 2005 - Haskell-Cafe

NumberTheory library
by Bo Herlin 15 May '05

15 May '05

Hi I've been running Hugs for a couple of weeks now and I'm slowly starting to get the Haskell idea, and I certanly like it! But there are functions that I cant find and that I assume others before me must have missed and then perhaps also implemented them. Is there any standard library with functions like: binomial isCatalan nthCatalan nextCatalan isPrime nthPrime nextPrime factorize isFibonacci nthFibonacci nextFibonacci and possibly/hopefully other essential functions as well? (and of course better naming...) /Bo

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Re: Should inet_ntoa Be Pure?
by Dominic Steinitz 14 May '05

14 May '05

On Monday 09 May 2005 11:45 am, you wrote: > On 07 May 2005 11:59, Dominic Steinitz wrote: > > Does anyone know why these are in the IO monad? Aren't they pure > > functions converting between dotted-decimal strings and a 32-bit > > network byte ordered binary value? > > > > Dominic. > > http://www.haskell.org/ghc/docs/latest/html/libraries/network/Network.So > cket.html#v%3Ainet_addr > > http://www.haskell.org/ghc/docs/latest/html/libraries/network/Network.So > cket.html#v%3Ainet_ntoa > > Clearly it should be pure. To avoid breaking the interface, we could > add: > > showHostAddress :: HostAddress -> String > > and deprecate inet_ntoa? > > Cheers, > Simon I haven't followed the thread on this carefully but as a pragmatic approach, how about: hostAddress :: HostAddress -> (Word8,Word8,Word8,Word8) inetAddress :: (Word8,Word8,Word8,Word8) -> HostAddress then there shouldn't be any errors to deal with as all the addresses are valid in both representations. And if you want to show the address you can always do: showHostAddress x = concat $ intersperse "." (map show [first x, second x, third x, fourth x]) where first (a,_,_,_) = a second (_,b,_,_) = b third (_,_,c,_) = c fourth (_,_,_,d) = d Dominic.

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resolving missing class instances @ compile time
by Greg Buchholz 13 May '05

13 May '05

Here's a little quirk I ran into recently. While making a little vector data type in class Num (code below), I didn't implement an instance of "fromInteger" (thinking I didn't need it). Well as you can probably guess, it turns out I did need it, and subsequently got a run time exception. Which surprised me a little, since it seems like it could have been caught at compile time. (Although I did ignore a warning). This has probably been answered a thousand times, but could someone point me in the direction of documentation explaining why it compiled? Thanks, Greg Buchholz --uncomment out "fromInteger" to get a working program instance Num Vec where (V a b c) + (V x y z) = (V (a+x) (b+y) (c+z)) (V a b c) - (V x y z) = (V (a-x) (b-y) (c-z)) --fromInteger 0 = V 0.0 0.0 0.0 instance Eq Vec where (V a b c) == (V x y z) = (a==x) && (b==y) && (c==z) data Vec = V !Double !Double !Double deriving Show main = print $ sum [(V 1.0 2.0 3.0), (V 4.0 5.0 6.0)]

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ANNOUNCE: Haskell Communities & Activities Report (8th ed., May 2005)
by Andres Loeh 13 May '05

13 May '05

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (8th edition, May 2005) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in several formats: PDF, for screenreading as well as printing, HTML, for those of you who prefer not to deal with plugins or external viewers, and PostScript, for those of you who have nice quick printers that do not grok PDF. 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 we did. If you haven't 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 well 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 we 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 2005: target deadline for contributions to the November 2005 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 the middle of October (you could point us to them as well, and we 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! Andres Loeh <hcar at haskell.org> <andres at cs.uu.nl> -- Haskell Communities and Activities Report (http://haskell.org/communities)

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rank 2 polymorphism example
by Scott J. 12 May '05

12 May '05

Hi, can someone give me an example of an arbitrary rank polymorphism function? Thanks Jan

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subRegex bug?
by S. Alexander Jacobson 11 May '05

11 May '05

Am I misunderstanding the regex docs? *MyMod> subRegex (mkRegex "\\." ) "foo.bar" "blah" "foo*** Exception: Text.Regex.Posix.regcomp: error in pattern -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Re: Haskell-Cafe Digest, Vol 21, Issue 27
by Stijn De Saeger 11 May '05

11 May '05

Quinn Dunkan <qdunkan(a)gmail.com> wrote : > ... It is very natural to > write in a somewhat functional style, especially in regards to > sequence processing: higher order functions and listcomps provide the > processing and its built in generators and iterator protocol provide > some of the benefits of laziness. Its elementary pattern matching > encourages you to return as many values from a function as you need > and use zip() (another haskell steal) to iterate over parallel > sequences (all pattern matches are irrefutable, though). I'm confused... Python has pattern matching ? s.

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RE: [Haskell-cafe] Tables in Haskell (data model /business app pr ototyping)
by Bayley, Alistair 11 May '05

11 May '05

> From: Rene de Visser [mailto:rene_de_visser@hotmail.com] > > >What do you mean by "external" database? If you just mean "on another > > I would like to write my views and constraints in Haskell, > and not in a > seperate database language. > > i.e. I want to use Haskell also as declarative active > database language to > define the business logic and rules. This would seem to me > to be a lot > easier when the database is written in Haskell itself and > part of the same > program. > > Rene. Well, in that case, HaskellDB is probably about as good as it's gonna get. Were you looking for something similar to an existing system that you're familiar with? (Datalog? Kliesli?) Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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and what about AliceML?
by fred 11 May '05

11 May '05

Hi, As long as this group seems to tolerate "What about Haskell and language X?" has anyone had any experience with AliceML? My initial impression is that it might be a nice tool for teaching fundamental concepts. ---fred

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Type classes and definite types
by Bryn Keller 11 May '05

11 May '05

Hi folks, I set out to perform a (seemingly) simple experiment last night. I thought I'd try to get used to the HSQL library by writing a little app that would take an SQL query on the command line, run it, and print the results. So far, this seems to be impossible. I'd like to know if there's a way around this problem, or if this is actually insurmountable (short of generating and compiling new Haskell programs for each query). The HSQL library works quite well for queries whose types you know in advance, as in printRow stmt = do (id :: Int) <- getFieldValue stmt "ID" (code :: String) <- getFieldValue stmt "Code" (name :: String) <- getFieldValue stmt "Name" putStrLn (unwords [show id, show code, show name]) however, it doesn't work at all if you can't specify the types at compile time: printRow' (names,types,nulls) stmt = do values <- mapM (getFieldValue stmt) names putStrLn $ unwords (map show values) The problem is that getFieldValue returns a value of type (SqlBind a) => a. That is, there's no type information associated with this return value other than it's a valid SQL value. There are no operations in the SqlBind class, it's just a marker as near as I can tell. So when you call getFieldValue, the exact type has to be fixed in some way (by annotations in this example). If the type is not fixed, it can't be used, as shown by the fact that printRow' won't compile. I think this is just the same problem we have with other type classes, e.g., (show (read s)), not anything new. The question is, what can be done about it? Is there anything I, the user can do about it, or can only the library authors solve this problem? Is there something we can change in Haskell to make this sort of thing possible? Thanks, Bryn import Database.HSQL import Database.HSQL.ODBC import System.Environment (getArgs) main = do (connString:sql:_) <- getArgs conn <- connect connString "" "" putStrLn "Connected" stmt <- query conn sql putStrLn $ "Ran query " ++ sql let info = unzip3 $ getFieldsTypes stmt putStrLn (show info) forEachRow' printRow stmt printRow stmt = do (id :: Int) <- getFieldValue stmt "ID" (code :: String) <- getFieldValue stmt "Code" (name :: String) <- getFieldValue stmt "Name" putStrLn (unwords [show id, show code, show name]) printRow' (names,types,nulls) stmt = do values <- mapM (getFieldValue stmt) names putStrLn $ unwords (map show values)

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