October 2006 - Haskell-Cafe

split string into n parts
by jim burton 23 Oct '06

23 Oct '06

I want to split a string into 5 parts of equal length, with the last fifth padded if necessary, but can't get it right - here's what I've got - > fifths :: String -> String > fifths s = fifths' "" 0 s > where l = (length s) `div` 5 > fifths' xs c [] = xs ++ (replicate (l-c) 'X') > fifths' xs c (y:ys) = if c == l > then fifths' (xs++[' ',y]) 0 > ys > else fifths' (xs++[y]) (c+1) > ys which, apart from surely being uglier than need be, doesn't work: *Main> fifths "IDOLIKETOBEBESIDETHESEASIDE" "IDOLI KETOBE BESIDE THESEA SIDEXX" *Main> fifths "12345" "1 23 45" Any thoughts? Thanks! This isn't homework BTW, I'm having a go at the ruby quiz puzzles in haskell, which seems to be a nice way to learn. -- View this message in context: http://www.nabble.com/split-string-into-n-parts-tf2496941.html#a6960346 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: type level functions (type of decreasing list)
by oleg＠pobox.com 23 Oct '06

23 Oct '06

Greg Buchholz wrote: > I'm wondering about creating a data structure that has the type of > decreasing numbers. > I can try... > > class Pre a b | a -> b > instance Pre (Succ a) a > instance Pre Zero Zero > > data (Pre a b) => Seq a = Cons a (Seq b) | Nil > > decreasing = Cons three (Cons two (Cons one Nil)) > > ...but that complains about "Not in scope: type variable `b'" One way is to use existentials: > data Seq1 a = forall b. (Pre a b, Show b) => Cons1 a (Seq1 b) | Nil1 > > decreasing1 = Cons1 three (Cons1 two (Cons1 one Nil1)) which perhaps not entirely satisfactory because we have to specify all the needed classes in the definition of Seq1. Perhaps a better -- and a more general alternative -- is to use type-level programming to its fullest. The following defines a list whose elements are constrained to be in the decreasing, increasing, or any other (defined in the future) order. This example shows that Haskell truly has more kinds than it is commonly acknowledged. For more details on constrained lists (list of odd numerals, even numerals, etc), please see the following implementation of Tim Sheard's `Half' example http://pobox.com/~oleg/ftp/Haskell/Half.hs > data Succ a = S a > data Zero = Z > > zero = Z > one = S zero > two = S one > three = S two > > class KIND l a b > > data Cons a b = Cons a b > data Nil = Nil > > > data Increasing = Increasing > instance KIND Increasing a (Succ a) > data Decreasing = Decreasing > instance KIND Decreasing (Succ a) a > -- one can add non-increasing, non-decreasing, etc. > > class ConstrainedL c l > instance ConstrainedL c Nil > instance ConstrainedL c (Cons a Nil) > instance (KIND c a b, ConstrainedL c (Cons b l)) > => ConstrainedL c (Cons a (Cons b l)) > > -- smart constructors. > -- Alternatively, define > as_increasing :: ConstrainedL Increasing l => l -> l > consi :: ConstrainedL Increasing (Cons a l) => a -> l -> Cons a l > consi = Cons > consd :: ConstrainedL Decreasing (Cons a l) => a -> l -> Cons a l > consd = Cons > incr = consi one (consi two (consi three Nil)) > -- No instance for (KIND Increasing (Succ Zero) (Succ (Succ (Succ Zero)))) > -- incr1 = consi one (consi three Nil) > decr = consd three (consd two (consd one Nil)) -- No instance for (KIND Decreasing (Succ (Succ Zero)) (Succ (Succ (Succ Zero)) -- decr1 = consd three (consd two (consd three Nil))

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Re[2]: [Haskell] Pugs gains SMP parallelism support.
by Bulat Ziganshin 23 Oct '06

23 Oct '06

Hello Taral, Monday, October 23, 2006, 1:12:38 PM, you wrote: >> They probably are. However you get the overhead of creating the array >> (when you don't really need O(1) random access) and every thread >> signals the same semaphore which may lead to some congestion which >> could slow things down. > You do need O(1) random access for the writers to put their results in > efficiently. And newArray_ should be faster than N copies of > newEmptyMVar. It is true that I have one congestion point (the > semaphone) instead of N (the mvars). are you sure that writeArray calls may be overlapped? :) they may change some technical fields, such as 'dirty' bit -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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memory, garbage collection and other newbie's issues
by Andrea Rossato 22 Oct '06

22 Oct '06

Hi! I'm a newbie and, as a learning experience, I'm writing a feed reader with hscurses and hxt. For the present time the feed reader just reads a Liferea cache but, as you can imagine, I'm running into the usual newbie problems of memory consumption and garbage collection, probably (I'm not sure) related to strictness/laziness. Even though I spent a couple of hours search the mailing list archives, I did not come up with something I can relate to, so I'll try to explain my problem. The feed reader, that should be compatible with Liferea, takes an opml (1.0) file, that stores information on folders and subscribed feeds. It uses it as the major component of the state of a ST monad, after adding some attributes used by the reader UI. The UI, that uses the widget library of hscurses and is derived from the Contact Manager example, will just display this opml file, and every UI event (collapsing/expanding of folders, displaying feeds, tagging, flagging, and so on) is just an XML transformation of this opml state component. So, when the feed reader boots, only the layout of folders and subscribed feeds is presented to the user. When the user selects a feed do be displayed, the cached file containing up to 100 saved posts, is read and transformed into a data type (called Feed, obviously). After that this data type is transformed into an opml (xml) tree, that is inserted as a child in the appropriate place of the opml state component. Moreover the parent element of the opml state component (which holds the original information of the subscribed feed) is edited for adding general feed information (such as last update, feed's attributes, and so on) retrieved by reading the file. When the user collapses the feed, the added opml chunk is deleted from the state component (but not the added information to the parent of this chunk). Now, I would expect that after the opml chunk is deleted all the memory allocated for reading the cached file would be garbage collected. This is not happening, so, every time you open (or reopen) a feed, the used memory of the feed reader increases, and never decreases. After profiling I've seen that the problem is occurring in the function that reads the cached file: loadFeed :: String -> IO [Feed] readFeed id = do [a] <- runX $ readDocument [(a_validate, v_0)] (cachePath ++ id) return $ runLA toFeed a What this function does is reading the file with: h <- openFile ... hGetContents h and applying some XML filters to get the Feed type populated with the needed information. I tried making the function strict with $!. I tried using fps. It doesn't change this behaviour, obviously. Now, I know that this is a typical newbie problem: could you give me some direction on how to debug this problem and possibly find a solution? Or just some hints on the kind of problem I'm facing: is it related to strictness/laziness, or it's just that I did not understand a single bit of how garbage collection works in Haskell? Thanks for your kind attention. If I'll be able to solve this problem probably I'll be also able to share the feed reader (I know that you are not going to answer after reading this....;-) regards, Andrea

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Exact Real Arithmetic
by Henning Thielemann 22 Oct '06

22 Oct '06

On http://www.haskell.org/hawiki/ExactRealArithmetic there is a module by David Lester mentioned, with a link to http://www.cs.man.ac.uk/arch/dlester/exact.html which does not work, and I also did not get a reply from the author. Does someone of you have this module?

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[Haskell-cafe] Re: Strictness, order of IO operations: NewCGI & HDBC
by oleg＠pobox.com 22 Oct '06

22 Oct '06

Tim Smith wrote: > Has anyone found out how to lift bracket into another monad? Yes, please see the thread `Re: Control.Exceptions and MonadIO' staring at http://www.haskell.org/pipermail/haskell-cafe/2006-April/015444.html There is also a Haskell' ticket: http://hackage.haskell.org/trac/haskell-prime/ticket/110 We have used this CatchableMonadIO in takusen and the other project, with success. One wish is to have HUnit work in this CatchableMonadIO rather than just IO (currently Alistair Bayley has to maintain an adjusted version of HUnit for that purpose). Regarding the issue of proper disconnections from the database server: it was the thrust for the development of Takusen is to statically prevent errors like forgetting to disconnect from the database or to use the database connection after disconnecting (as well as related issues of using a cursor after closing it, or forgetting to close the cursor or to dispose the resultset). As to performance: Takusen can read and process 2 million rows from the database, in the same space it takes to fetch and process 10 rows. In fact, doing this is one of the performance tests.

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Re: Best way to write endsWith (RESOLVED)
by John Ky 22 Oct '06

22 Oct '06

Hi, Thanks all. I learnt quite a few things: 1. There was already an equivalent builtin function. 2. That the best function argument order is the least surprising one. 3. That I can choose my preferred function order by changing the name of the function. 4. That the most efficient way of doing something in Haskell can be so short. 5. Operators really are awesome. -John On 10/22/06, John Ky <newhoggy(a)gmail.com> wrote: > > Hello, > > I have this function here: > > > endsWith :: Eq a => [a] -> [a] -> Bool > > endsWith suffix list > > | lengthDifference < 0 = False > > | otherwise = (drop lengthDifference list) == suffix > > where lengthDifference = (length list) - (length suffix) > > Would this be the preferred function argument order? Or is the reverse > (ie. endsWith list suffix) better? > > I like being able to say "abc" `endsWith` "c", but I also like to be able > to say map (endsWith 't') ["cat", dog"] but I can't have both. > > By the way, is there a better way to write this function to be clearer and > more efficient? > > Thanks > > -John > >

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Best way to write endsWith
by John Ky 21 Oct '06

21 Oct '06

Hello, I have this function here: > endsWith :: Eq a => [a] -> [a] -> Bool > endsWith suffix list > | lengthDifference < 0 = False > | otherwise = (drop lengthDifference list) == suffix > where lengthDifference = (length list) - (length suffix) Would this be the preferred function argument order? Or is the reverse (ie. endsWith list suffix) better? I like being able to say "abc" `endsWith` "c", but I also like to be able to say map (endsWith 't') ["cat", dog"] but I can't have both. By the way, is there a better way to write this function to be clearer and more efficient? Thanks -John

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Re: Data.List.join
by Stefan Holdermans 21 Oct '06

21 Oct '06

Samuel wrote: >> But wait! There is also a join in Control.Monad! Josef replied: > Good point. But I don't really see that as a problem. Don't you think > the two 'join's can co-exist? I do think it's a problem, for list are also monads. Instantiating the monadic join with [] we'd wind up with both join :: [[a]] -> [a] and join :: [a] -> [[a]] -> [a] which to me seems just a little too confusing. Cheers, Stefan

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List comparisons and permutation group code
by Mikael Johansson 21 Oct '06

21 Oct '06

Comparing the code for permutationgropus at http://www.polyomino.f2s.com/david/haskell/codeindex.html with my own thoughts on the matter, I discover the one line to figure out whether a specific list represents the identity: isIdentity (PL xs) = all (\(i,j) -> i==j) (zip [1..] xs) Is there any sort of benefit to be won by using this construction instead of isIdentity (PL xs) = xs == [1..(length xs)] and if so, what? Best, -- Mikael Johansson | To see the world in a grain of sand mikael(a)johanssons.org | And heaven in a wild flower http://www.mikael.johanssons.org | To hold infinity in the palm of your hand | And eternity for an hour

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