June 2006 - Haskell-Cafe

cabal-get and HXT?
by Greg Fitzgerald 20 Jun '06

20 Jun '06

I'd like to use a tool to automatically install Haskell XML Toolkit and its dependencies. What are my options? How's cabal-get coming? Thanks, Greg

3 2

RE:[Haskell-cafe] A function using List and Tuple
by jotajota 19 Jun '06

19 Jun '06

Hi On 6/19/06, Sara Kenedy <sarakenedy(a)gmail.com> wrote: > Hello, > > I want to creat a function sumList as below examples: > sumList [("s1",2),("s2",4),("s3",3),("s4",2)] = ("#", 1/2 + 1/4 + 1/3 + 1/2) > sumList [("s1",2),("s2",4),("s3",3) = ("#", 1/2 + 1/4 + 1/3) > > I attempted it as following: > > sumList :: (Fractional a) => [(String,a)] -> (String, a) > sumList [] = ??? > sumList (x:xs) = ("#", 1/(snd x) + 1/snd(sumList xs)) > > I know this function cannot give the correct answer (even if I define > sumList []), but I did not find the right way. > If anyone can give me a suggestion, I really appereciate for that. Thanks. > Yo can use the foldr function: sumList a = ("#",foldr suma 0 a ) where suma (s,x) y = 1/x + y Juan Jose

1 0

Parsec and destructuring HTML content
by Joel Reymont 19 Jun '06

19 Jun '06

Has anyone explored destructuring HTML with Parsec? Any other ideas on how to best do this? I'm looking to scrape bits of information from more or less unstructured HTML pages. I'm looking to structure, tag and classify the content afterwards. I think that developing HTML scrapers requires short tweak-compile- run cycles and is probably best done in Perl, Python, Ruby, i.e. dynamic languages but I wonder if someone has found otherwise. Thanks, Joel -- http://wagerlabs.com/

4 3

A function using List and Tuple
by Sara Kenedy 19 Jun '06

19 Jun '06

Hello, I want to creat a function sumList as below examples: sumList [("s1",2),("s2",4),("s3",3),("s4",2)] = ("#", 1/2 + 1/4 + 1/3 + 1/2) sumList [("s1",2),("s2",4),("s3",3) = ("#", 1/2 + 1/4 + 1/3) I attempted it as following: sumList :: (Fractional a) => [(String,a)] -> (String, a) sumList [] = ??? sumList (x:xs) = ("#", 1/(snd x) + 1/snd(sumList xs)) I know this function cannot give the correct answer (even if I define sumList []), but I did not find the right way. If anyone can give me a suggestion, I really appereciate for that. Thanks. S.

2 1

Fibonacci numbers generator in Haskell
by Vladimir Portnykh 18 Jun '06

18 Jun '06

Fibonacci numbers implementations in Haskell one of the classical examples. An example I found is the following: fibs :: [Int] fibs = 0 : 1 : [ a + b | (a, b) <- zip fibs (tail fibs)] To get the k-th number you do the following: Result = fibs !! k It is elegant but creates a list of all Fibonacci numbers less than k-th, and the code is not very readable :). I wrote my own Fibonacci numbers generator: fib :: Int -> [Int] fib 0 = [0,0] fib 1 = [1,0] fib n = [sum prevFib, head prevFib] where a = fib (n - 1) To get the k-th number you do the following: result = head (fib k) It does not generate full list of Fibonacci numbers, but keeps only 2 previous numbers, and has only one recursive call. Because the list always has only 2 elements using the functions head and sum is a bit overkill. Can we do better? _________________________________________________________________ Are you using the latest version of MSN Messenger? Download MSN Messenger 7.5 today! http://join.msn.com/messenger/overview

9 14

take the keywords from a string
by Sara Kenedy 18 Jun '06

18 Jun '06

Hi everybody, I have a function: takeKeyword :: String -> [String] This function will take the input as a string and return a list of keywords taken from the input string and they are elements of ListOfKeywords. The order of the result list is sequenced as: the last keyword found is set as the first element of the list, and so on. For example, ListOfKeywords = ["expand, "limit", "diff"] takeKeyword :: String -> [String] takeKeyword limit(x + expand((x+3)^2), x=3) = ["expand", "limit"] takeKeyword limit(x -7, x= 3) =["limit"] If you have any suggestions for that, please share with me. Thanks in advance. S.

4 8

Sets symmetric difference
by Jenny678 17 Jun '06

17 Jun '06

Hello, Know Somebody the code for symmetric difference (Sets) examples >symmetric_difference [1..8][5..7] >[1,2,3,4,8] >symmetric_ difference [1..8][] >[1,2,3,4,5,6,7,8] >symmetric_ difference [1..12][10..20] >[1,2,3,4,5,6,7,8,9,13,14,15,16,17,18,19,20] Thanks for Help -- View this message in context: http://www.nabble.com/Sets-symmetric-difference-t1805165.html#a4919646 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

2 1

SMTP, HTTP, Telnet
by Lyle Kopnicky 17 Jun '06

17 Jun '06

Hi all, Anybody know of some good Haskell libraries providing: * an SMTP client, * an HTTP client, * or a Telnet client? There's a significant amount to these protocols, over and above the socket layer. Thanks, Lyle Kopnicky

2 1

RE: why isn't everything an instance of Data?
by Ralf Lammel 16 Jun '06

16 Jun '06

Hi Frederik, Cc Haskell-café as proposed by Frederik, > Why isn't everything an instance of Data? > > It seems natural to give every type which is defined via 'data' an > automatic Data instance. This would make implementing many things much > simpler. Certain types such as functions would only be able to have > "stub" instances, for instance not allowing traversal of constructor > arguments, but this would no more hurt the user than not having such > instances defined in the first place. To clarify, what I am suggesting > is that Data no longer be a class but an interface - one would be able > to call 'toConstr' on any type. [Ralf Lammel] I get your overall question but let me ask about one detail. When you say "an interface", what does this mean in the Haskell sense (where normally a type-class is like an OO interface *anyhow*)? > Is there a fear that this would lead to "sloppy programming"? [Ralf Lammel] I agree that it could be convenient to assume gfoldl and all that as true primitives. Personally, I wouldn't necessarily like that: Haskell's type system seems to be designed to (mostly) distinguish parametrically polymorphic functions from the rest; just as much as it (mostly) makes explicit any effects involved in a computation. Also, how would these primitives need to be implemented? We would need a whole lot of type information in the sense of reflection to be around at run-time. > Also, it would be nice if it were possible to have the compiler infer, > in a function such as > > cast :: a -> Maybe b > > that 'a' and 'b' are instances of the same classes. However, unless > I'm mistaken this would require 'cast' to have special status with the > compiler (or for there to be some new syntax allowing one to indicate > that two type variables implement the same classes). [Ralf Lammel] There are several problems here. First, I doubt that we want to establish "that 'a' and 'b' are instances of the same classes". That sounds like a too special case. I guess we would want to establish that a given type instantiates a given type class. Second, our new cast operation (which would indeed be very much like an OO down-cast) would need to be a special primitive indeed, including special syntax. If we had first-class type-classes, special syntax is not needed, but we would still need a primitive for the actual cast. Third, we need to come up with some fairly tricky means of getting the dictionary for a type "out of the blue" at run-time. This would be sort of possible (certainly for monomorphic types) if we had the type of the value/expression around at run-time, so that we can use it to look up a dictionary from a global pool. However, that type is not available without extra effort. To summarize, if this was the question, this expectation seems to lead pretty far away from what we have in Haskell and likely to get anywhere soon -- as far as I can tell. > But without such > inferences, the Generics stuff has limited usefulness - so much in > Haskell is based on classes, but the only class one can use in the > argument of say 'everywhere' is Data itself: > > everywhere :: (forall a . Data a => a -> a) -> forall a . Data a => a -> a [Ralf Lammel] There are two stories here: a) Any type-specific case can make any amount of type-class assumptions even in the original SYB1 style. This gets you pretty far in practice. b) If you want to write generic functions whose set of types is characterized by a designated type class, then you need to use SYB3 style. HTH, Ralf PS: Some related stuff ... - I uploaded a new SYB paper recently: http://www.cwi.nl/~ralf/syx/ - We also updated the SYB site to include other people's SYB papers: http://www.cs.vu.nl/boilerplate - See Pablo Nogueira's paper "Context-Parametric Polykinded Types" for some interesting GH-related issues on type classes; http://www.cs.nott.ac.uk/~pni/Papers/ - Finally, I plan to use my new blog for Haskell-related stuff as well: http://blogs.msdn.com/ralflammel/

1 0

putStrLn
by Jenny678 16 Jun '06

16 Jun '06

Hello, Can somebody help me I want to work with putStrLn main n = putStrLn "*" for example How must I define the code that: >main 5 >***** Thanks for any help -- View this message in context: http://www.nabble.com/putStrLn-t1799896.html#a4905456 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

3 2