- Haskell-Cafe - Haskell.org

Re: [Haskell-beginners] about the concatenation on a tree
by Thomas Davie 01 Jan '09

01 Jan '09

On 31 Dec 2008, at 16:02, Max cs wrote: > hi all, not sure if there is someone still working during holiday > like me : ) > > I got a little problem in implementing some operations on tree. > > suppose we have a tree date type defined: > > data Tree a = Leaf a | Branch (Tree a) (Tree a) > > I want to do a concatenation on these tree just like the concat on > list. > Anyone has idea on it? or there are some existing implementation? > > Thank you and Happy New Year! > How would you like to concatenate them? Concatonation on lists is easy because there's only one end point to attach the next list to, on a tree though, there are many leaves to attach things to. Here's a few examples though: Attaching to the right most point on the tree (tree data structure modified to store data in branches not leaves here) data Tree a = Leaf | Branch (Tree a) a (Tree a) concatT :: [Tree a] -> Tree a concatT = foldr1 appendT appendT :: Tree a -> Tree a -> Tree a appendT Leaf t = t appendT (Branch l x r) t = Branch l x (appendT r t) Attaching to *all* the leaves on the tree (same modification to the data structure) concatT :: [Tree a] -> Tree a concatT = foldr1 appendT appendT :: Tree a -> Tree a -> Tree a appendT Leaf t = t appendT (Branch l x r) t = Branch (appendT l t) x (appendT r t) merging a list of trees maintaining them as ordered binary trees concatT :: Ord a => [Tree a] -> Tree a concatT = foldr1 unionT unionT :: Ord a => Tree a -> Tree a -> Tree a unionT t = foldrT insertT t foldrT :: (a -> b -> b) -> b -> Tree a -> b foldrT f z Leaf = z foldrT f z (Branch l x r) = f x (foldrT f (foldrT f z r) l) insertT :: Ord a => a -> Tree a -> Tree a insertT x Leaf = Branch Leaf x Leaf insertT x (Branch l y r) | x <= y = Branch (insertT x l) y r | otherwise = Branch l y (insertT x r) Hope that helps. Bob

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Re: Gitit - Encoding
by Arnaud Bailly 31 Dec '08

31 Dec '08

Hi Jeremy and John, Thank you very much Jeremy for your very detailed answer ! I checked in Gitit source and found various occurences of usage of Byte8 strings, which John as confirmed. I switched template.html to advertising encoding of iso-8859-1 and things are working ok. I setup LANG (and LC_ALL) to fr_FR.UTF-8. When I set meta tag with utf-8 encoding, things get messy. John: I checked your url and, oddly enough, characteres appears OK when I manually change page display encoding to latin-1. I came to the possibly obvious conclusion that: -gitit communicated with HApps hence the outside world in latin-1 - talk to git in utf-8 Is this the right picture ? If so, then I have part of the problem solved. Using latin-1 for input/display is annoying only for the € (euro) symbol. Regards, Arnaud BTW, my accents problem solved, I would like to work with RSS feeds so that: 1. a user could put a feed inside a page 2. gitit could produce feeds for changes/adds I have done things like that before in Java but would need some directions to incorporate it in gitit. -- Arnaud Bailly, PhD OQube - Software Engineering web> http://www.oqube.com

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fedora-devshell and Haskell
by Yaakov Nemoy 31 Dec '08

31 Dec '08

Hi Folks, I spent the better part of yesterday working on some tools for doing Fedora packages for Cabal. I've put up a blog report with some preliminary explanations on how to use it, but it might be of value to regular Haskell programmers. The post can be found below. http://loupgaroublond.blogspot.com/2008/12/lazyness-and-cabal-packages.html fedora-devshell's going to be aimed primarily on managing and automating the development of haskell packages for an RPM based distro. It might be useful to other developers too, and i would like to hear if anyone finds it useful. Cheers, and Happy New Year, Yaakov Nemoy

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Reading group for Programming Collective Intelligence
by Creighton Hogg 31 Dec '08

31 Dec '08

Hello Haskellers, For those of your who aren't nose-deep in Real World Haskell at the moment, I'd like to start a small group for the O'Reilly book Programming Collective Intelligence*. I think it might be very instructive to convert the examples & exercises to Haskell. I think it would be good experience for doing neat applications in Haskell & may provide opportunities for adding to Hackage if the book relies on Python libraries that we don't have an equivalent for. If anyone is interested, just e-mail me & I can work on the organization. Cheers, Creighton * http://tinyurl.com/7x4rga

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instance Enum [Char] where ...
by JustinGoguen 31 Dec '08

31 Dec '08

I am having difficulty making [Char] an instance of Enum. fromEnum is easy enough: map fromEnum to each char in the string and take the sum. However, toEnum has no way of knowing what the original string was. For example, running fromEnum on the string "d" will result in 100. But when we pass 100 to toEnum, it does not know if it should treat 100 as "d" or "22" (fromEnum '2' == 50). Source so far: instance Enum [Char] where succ = undefined pred = undefined toEnum n = undefined -- what to do? fromEnum xs = sum $ map fromEnum xs

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Function composition
by Oscar Picasso 31 Dec '08

31 Dec '08

Hi, I can write: *Main> let yes = not . not *Main> :t yes yes :: Bool -> Bool But not: *Main> let isNotEqual = not . (==) <interactive>:1:23: Couldn't match expected type `Bool' against inferred type `a -> Bool' Probable cause: `==' is applied to too few arguments In the second argument of `(.)', namely `(==)' In the expression: not . (==) Why? Oscar

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Re: [Haskell-cafe] represent data sturcture using function
by Ryan Ingram 31 Dec '08

31 Dec '08

On Mon, Dec 29, 2008 at 4:29 AM, <raeck(a)msn.com> wrote: > Would you please give me a complete example of code that I could have more > information > on the idea? Sure, I put up an example at http://ryani.freeshell.org/haskell/gmap.hs class MapKey k where data (:->) k :: * -> * newMap :: (k -> v) -> (k :-> v) fetch :: (k :-> v) -> (k -> v) update :: k -> (v -> v) -> (k :-> v) -> (k :-> v) assign :: k -> v -> (k :-> v) -> (k :-> v) assign k v m = update k (const v) m empty :: v -> (k :-> v) empty = newMap . const with instances & associated data types: instance MapKey () where -- A single value newtype () :-> v = UMap v instance MapKey Bool where -- A value for False and True data Bool :-> v = BMap v v instance (MapKey k1, MapKey k2) => MapKey (k1,k2) where -- A "curried" map newtype (k1,k2) :-> v = PMap (k1 :-> k2 :-> v) instance (MapKey k1, MapKey k2) => MapKey (Either k1 k2) where -- sub-maps for Left k1 and Right k2 data (Either k1 k2 :-> v) = EMap (k1 :-> v) (k2 :-> v) instance MapKey k => MapKey (Maybe k) where -- Now we can build up from existing structures! newtype (Maybe k) :-> v = MaybeM (Either () k :-> v) instance MapKey k => MapKey [k] where -- Value for [] and map for (head:tail) -- -- Note that this includes a recursive ([k] :-> v) map -- in the pair map (k,[k]) :-> v data [k] :-> v = ListM v ((k,[k]) :-> v) instance MapKey Positive where -- We just convert a positive number into -- a list of Bools, then make a map of those newtype Positive :-> v = PosMap ([Bool] :-> v) instance MapKey Integer where -- Now an integer is either negative, zero, or positive. -- So we store a map for negative numbers, a zero value, -- and a map for positive numbers. data Integer :-> v = IntMap (Positive :-> v) v (Positive :-> v) The rest of the class functions are reasonably easy to derive from their type and these data types. -- ryan

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Strange error with HDBC-odbc and MySQL
by Justin Bailey 30 Dec '08

30 Dec '08

I have run into a weird bug with HDBC-odbc that only occurs on my unix system. When I execute a select which returns more than 435 rows twice in the same process, the second execution fails with this error message: SQL error: SqlError {seState = "[\"HYT00\"]", seNativeError = -1, seErrorMsg = "connectODBC/sqlDriverConnect: [\"1045: [unixODBC][MySQL][ODBC 3.51 Driver]Access denied for user 'dummy'@'localhost' (using password: NO)\"]"} I think HDBC-odbc is at fault because the same query works fine from ruby. In both cases the unixODBC library is used. isql (the unixODBC command line utility) also has no problem. I have these software packages installed: ghc 6.8.3 HDBC 1.1.5 HDBC-odbc 1.1.4.4 unixODBC - 2.2.11 MySQL - 5.0.38 Linux - 2.6.16-xenU The program below demonstrates the problem. It does require a table with 500+ rows. On most production servers, information_schema.columns will usually do the trick. \begin{code} import System.Environment import Database.HDBC import Database.HDBC.ODBC import System.IO (hPutStrLn, stderr) import qualified Data.ByteString.Lazy.Char8 as L -- Execute with DNS name as only argument. E.g.: -- ./TestMain localdb main = do [dsn] <- getArgs testMatches dsn testMatches dsn testMatches dsn = do let handler e = do hPutStrLn stderr ("SQL error: " ++ show e) -- Execute a statement and return all the rows getAllRows conn sql = do stmt <- prepare conn sql _ <- execute stmt [] fetchAllRows' stmt handleSql handler $ do conn <- connectODBC $ "dsn=" ++ dsn -- 435 rows works fine, but not 445. rows <- getAllRows conn "select 1 from information_schema.columns limit 445" let evaledRows = filter (\[id] -> (fromSql id) /= (-1 :: Int)) rows putStrLn $ "Got " ++ show (length evaledRows) \end{code}

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Re: [Haskell] hsc2hs can't find gcc under windows
by Duncan Coutts 30 Dec '08

30 Dec '08

On Thu, 2008-12-25 at 15:17 +0900, Curt Sampson wrote: > I'm trying to upgrade from ghc 6.8.3 to 6.10.1. Unfortunately, there > seem to be problems with the Windows version. (I've installed it on > two different machines, both Windows XP, and they've both had the same > problem. Both worked fine with 6.8.3.) > > When I run hsc2hs, on an hsc file, I get the error, "Can't find gcc". > > Just as an experiment, I tried adding c:\ghc\ghc-6.10.1 to my path. That > lets it find gcc, but then it can't find cc1. If I go even further down > this road and add c:\ghc\ghc-6.10.1\gcc-lib to my path, it can find cc1, > but then I get a stream of errors about not being able to find header > files, such as HsFFI.h. > > Does anybody have any clues here to help me? I can provide code, or even > access to the machine, if needed. So here's the story: Up until ghc-6.10, ghc shipped a fork of hsc2hs. The forked version used ghc as the C compiler rather than gcc. This had the advantage that it would know about ghc packages and the include files shipped with packages. The disadvantage was that it was a fork and behaved differently than using hsc2hs with hugs. It also meant that real C compiler options had to be double-escaped to get hsc2hs to pass them to ghc and get ghc to pass them to gcc. In ghc-6.10, ghc ships the normal hsc2hs. What we did not notice before 6.10.1 was released was that calling hsc2hs on its own now does not work without passing additional flags. We never noticed because few people actually do call hsc2hs directly, it is typically used via Cabal. Cabal tells it where gcc lives and what include dirs to use. So that's why at the moment in the Windows installer, hsc2hs cannot even find gcc, because gcc is not on the %PATH% and hsc2hs does not know about ghc's special bundled version of gcc. It also does not know about ghc's include directories. It's clear we'll need to think a bit more about how to get hsc2hs to be both a compiler neutral tool where we can share a single instance between multiple compilers and also have it work "out of the box". > We're also happy to pay for support, if someone out there is offering > commercial support for ghc. We offer commercial support through Well-Typed: http://www.well-typed.com/ Duncan

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IORef vs TVar performance: 6 seconds versus 4 minutes
by Jim Snow 30 Dec '08

30 Dec '08

I decided to try to implement a graph algorithm using STM. Each node in the graph has a set of TVar-protected lists of the nodes it links to and the nodes that link to it. Also, there is a global TVar-protected Data.Map that contains all the nodes in the graph, indexed by name (which is polymorphic): data Node k r = Node { fwdPos :: TVar [Node k r], -- forward links (nodes we like) fwdNeg :: TVar [Node k r], -- we allow "negative" links, too (nodes we don't like) revPos :: TVar [Node k r], -- backlinks (nodes that like us) revNeg :: TVar [Node k r], -- negative back links (nodes that don't like us) currRep :: r, -- extra user-defined data name :: k -- node's unique identifier } deriving Show data Network k r = Network { node :: TVar (M.Map k (Node k r)), -- map of nodes by name trusted :: TVar [Node k r] -- a list of nodes we need to iterate over occasionally } deriving Show I tried loading a datafile of about 20,000 nodes into the graph in one big transaction, and found that it takes about 4 minutes. This seemed rather slow, so I replaced all the TVars with IORefs (and substituted STM with IO in the type signatures), and the same operation with the new version took about 6 seconds! This is all with one thread, so there should be no contention for the TVars. Is there something about STM that makes it scale worse than linearly wrt the number of mutations in a transaction? Above performance numbers are for ghc-6.10.1. With ghc-6.8.3, the STM version takes more than 9 minutes. According to profiling, one of my trouble spots is this function, which just adds an entry onto a TVar [a]: stmcons :: k -> TVar [k] -> STM () stmcons x tv = do xs <- readTVar tv writeTVar tv (x:xs) This seems like it ought to be pretty innocuous, unless the whole list is getting evaluated each time I cons a new entry, or if readTVar or writeTVar are much more expensive than they appear. -jim

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Timing difference
by Aai 30 Dec '08

30 Dec '08

Programming for Rosetta Codes task http://www.rosettacode.org/wiki/Probabilistic_Choice I observed a remarkable (that's to say: for me) difference in timing using lambda function in the one case and point free in the other. Timing was measured in GHCi. Compiled there's no difference! Using lambda ============ ... labels = ["aleph", "beth", "gimel", "daleth", "he", "waw", "zayin", "heth" ] piv n = take n . flip (++) (repeat ' ') main = do g <- newStdGen let rs :: [Float] rs = take 1000000 $ randomRs(0,1) g ps, sps :: [Float] ps = ap (++) (return. (-) 1 .sum) $ map recip [5.0..11.0] sps = scanl1 (+) ps ix = map ((/1000000.0).fromIntegral.length). group.sort $ map (\x -> fromJust $ findIndex not $ map (x>) sps) rs mapM_ (\(l,s,c)-> putStrLn $ (piv 6 l) ++ " " ++ (piv 12 $ show $ s) ++ " " ++ ((piv 12 $ show $ c) )) $ zip3 labels ps ix Using point free ================ replace $ map (\x -> fromJust $ findIndex not $ map (x>) sps) rs by $ map (fromJust. findIndex not. flip map sps. (>)) rs lambda: ======= *Main> main ... (18.24 secs, 2524489600 bytes) point free: =========== *Main> main ... (12.15 secs, 2470893260 bytes) Should I program using point free as much as possible? :-) Thanks =@@i

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ANN: Control.Monad.IfElse 0.85
by Jeff Heard 30 Dec '08

30 Dec '08

Updated Control.Monad.IfElse to include Wren Thornton's Control.Monad.Extras, unlessM, ncond (unless-else chaining), ncondM, and more binary conditional operators.

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represent data sturcture using function
by Raeck chiu 30 Dec '08

30 Dec '08

Merry Christmas! Hope everybody is enjoying the Christmas! I am doing some readings and I found something seems to be interesting. People sometime will try to represent a quantity-regard-only data structure (such a order-regadless List) using functions instead of ta concrete data structure (like the haskell build-in []), any particular reason for this? For example, > data Sex = Male | Female > classA :: Sex -> Int > classA Male = 100 > classA Female = 200 when I should prefer the above solution instead of using a concrete data structure such as [] to represent the classA members? It seems to be very difficult to change the number of Male or Female if a concrete data structure is not used. Is it possible change the number of Male in classA when represent classA using function? Thank you very much! Best wishes, Raeck _________________________________________________________________ It’s the same Hotmail®. If by “same” you mean up to 70% faster. http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_broad1_…

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Maintaining laziness
by Henning Thielemann 30 Dec '08

30 Dec '08

In case someone cares - after some battles with functions that are less lazy than expected, I have written a tutorial on how to get functions lazy and how to test, whether they are actually lazy: http://www.haskell.org/haskellwiki/Maintaining_laziness I found it especially enlightening, that one can avoid a 'force' function in a lazy parser by making the possibility of a parser failure explicit in its type. I.e. a parser that cannot fail, need no 'force'. (I learnt that in polyparse the 'force' function is hidden in 'apply': http://hackage.haskell.org/packages/archive/polyparse/1.1/doc/html/Text-Par… )

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Data.List.sort, not so good?
by Adrian Neumann 30 Dec '08

30 Dec '08

Hello Haskell-Cafe, lately I've been playing around with sorting. I discovered that Data.List.sort is not as optimized I thought. At least not for random lists. I think we should consider adding a Data.List.Sort package to hackage (Similar to Data.List.Split) that offers a wide variety of sorting algorithms. I don't consider myself to be a very advanced Haskell programmer, but I could come up with a Mergesort that beats List.sort, time- and spacewise. > http://hpaste.org/13403 On my machine List.sort takes ~10 sec, mergeSort 7, qs 4 (compiled with -O2). List.sort eats too much ram to sort 20.000.000 ints, my algorithms don't. QuickCheck says my implementations are correct. I suppose List.sort does clever things for nonrandom lists, that I didn't consider? I had a look at the source, but I couldn't find anything obvious. Adrian

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Trouble with the ST monad
by Andre Nathan 30 Dec '08

30 Dec '08

Hello, I'm trying to write a function that would take an STArray and and shuffle its elements. I'm having trouble with the ST monad, though, and couldn't find out how fix this issue. The problem happens when I use runST to extract the shuffled array from the ST monad. I'm getting the following error: Inferred type is less polymorphic than expected Quantified type variable `s' is mentioned in the environment: a :: STArray s Int a The full code is at http://hpaste.org/13240#a1 Any help would be appreciated. Thanks, Andre

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Using classes for a heterogeneous graph structure
by Jeremy Bowers 30 Dec '08

30 Dec '08

I'm trying to use a class to create a heterogeneous graph structure. Part of this involves holding things in a Data.Map identified only as "instances of some class", but then I can't figure out how to actually use these things. What is mystifying me is exemplified in the following code: ----------------- module Sample where import qualified Data.Map as Map class Thing thing where set_int :: thing -> Integer -> thing -- for wrapper wrapper :: thing -> ThingWrapper wrapper thing = ThingWrapper thing instance Thing Integer where set_int me i = i -- in the generic case, this actually does something type ThingsByString = (Thing thing) => Map.Map Integer thing update :: Integer -> Integer -> ThingsByString -> ThingsByString update key value map = let (Just thing) = Map.lookup key map in Map.insert key (set_int thing value) map test = let my_map = Map.empty::ThingsByString map1 = Map.insert 0 1 my_map map2 = update 0 8 map1 in map1 data ThingWrapper = forall t. (Thing t) => ThingWrapper t instance Thing ThingWrapper where set_int (ThingWrapper thing) i = wrapper $ set_int thing i wrapper thing_wrapper = thing_wrapper ----------------- This gives an error message (in GHC 6.8 w/ -fglasgow-exts , and yeah, that's deprecated, I'm still in dev): Sample.hs:27:16: Inferred type is less polymorphic than expected Quantified type variable `thing' is mentioned in the environment: map1 :: Map.Map Integer thing (bound at Sample.hs:26:9) my_map :: Map.Map Integer thing (bound at Sample.hs:25:9) In the third argument of `update', namely `map1' In the expression: update 0 8 map1 In the definition of `map2': map2 = update 0 8 map1 Now, I sort of understand this message. What I don't understand is how to do what I want to do; I've rearranged things in a number of ways but this comes up every which way. It seems like once I have one of these "things" there's absolutely nothing I can do to it (or absolutely no way to do what update does, which is mutate it using only the defined class interface without ever knowing what's in there). What's annoying is that at least as a human, what I want seems well-defined to me. Moreover, as shown in the code above, I can create a "wrapper" structure, and if I do that and manipulate only the wrapper (not shown in this example), everything works as I'd like. (If that's the answer, that's OK with me, but it seems klunky, wasteful, and amenable to being automatically done by the compiler as I understand it, and I expect there's a reason it isn't.) My question is: How can I have this heterogeneous graph structure and actually be able to manipulate it solely through a class interface? Or, is it impossible? (I'm open to "you're doing it entirely wrong and you should do X", but in that case I'd like to know what using classes to specify the interface a node should conform to is the wrong thing, because it sure *seems* right, based on what the interface is and how classes are documented. And it would need to work for a heterogeneous graph structure, not my very-simplified example above.) ---------------------------------- Barracuda Networks makes the best spam firewalls and web filters. www.barracudanetworks.com

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RE: Haskell Performance
by Simon Peyton-Jones 29 Dec '08

29 Dec '08

Dear Dave | My question to you is, if you were asked to make a recommendation on | which programming language to use for a program which would run on a | multiple core processor, but also required adequate memory and running | time performance, would you lean towards or away from Haskell? What | other factors would you consider? In other words, was I correct in my | initial opinion that Haskell was more concerned about code expression | rather than performance? Thanks for your email. I'm ccing the Haskell Cafe, which is full of people with practical experience (many more than I) of using Haskell for real applications. I don't think it's possible to give a categorical yes/no answer to your question. Yes, Haskell is designed primarily for high-level expressiveness rather than performance. This is good for programmers, but it cuts both ways for implementation. On the one hand, the programmer is taking less control, and the implementation may not be as clever as a programmer could be (the quicksort example you cite is a good example). On the other hand, the high-level-ness gives the opportunity for the implementation to do stuff that the programmer could not feasibly do by hand; the vectorisation transformation in nested data parallelism is a good example (http://research.microsoft.com/~simonpj/papers/ndp/index.htm) Long term, my bet is that the second trend will win. If you want to write parallel programs, it makes sense to start with a language that is by-default parallel, rather than by-default sequential. But that does not mean that you get parallel performance without tears. Far from it: parallelism is a complex, multi-faceted beast (concurrency, communication, locality, granularity...). So the short-term answer might be different to the long term one. That said, GHC comes out very competitively in the (flawed, but available) language shootout http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=all. So performance is not at all bad. Simon | -----Original Message----- | From: Dave Roberts [mailto:dave.a.roberts@gmail.com] | Sent: 15 December 2008 14:42 | To: Simon Peyton-Jones | Subject: Haskell Performance | | Dear Mr. Peyton Jones, | | I recently watched a video of you giving a talk about type driven | testing in Haskell. | | http://video.google.com/videoplay?docid=- | 4991530385753299192&ei=PWFGSaazDJPuqAL779n1DA&q=simon+peyton+jones+testing&emb=1 | | I found it to be a very informative and entertaining presentation | about Haskell. About halfway through the video you examine the | performance of the n-shell problem in Haskell versus C++. You say the | functional solution to the problem is 100x faster than the C++ | implementation. You continue to examine reasons why, citing that set | operations in a purely functional language can make assumptions that | side-effect laden languages cannot. For example creating a union of | two sets does not need to allocate a new set in memory because it can | reuse large portions of the original sets. You continue to show that | purely functional languages lend themselves to parallelism better than | imperative languages because state is not being changed by default. | | Taking this into consideration, the Haskell website has a section | titled, When C is better. | | http://www.haskell.org/haskellwiki/Introduction#When_C_is_better | | (I suspect you have already read this if not wrote it yourself) This | section looks at the quick sort example presented in the wiki and | claims that if written in an imperative language, the quick sort can | have time and memory requirement advantages over the Haskell version. | I read the haskellwiki long before viewing the video and at the time I | branded Haskell as a language which stresses expression over | performance. | | My question to you is, if you were asked to make a recommendation on | which programming language to use for a program which would run on a | multiple core processor, but also required adequate memory and running | time performance, would you lean towards or away from Haskell? What | other factors would you consider? In other words, was I correct in my | initial opinion that Haskell was more concerned about code expression | rather than performance? What you suggest in the video seems to be to | the contrary. | |

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Incremental trasnformations (not Haskell topic)
by frantisek kocun 29 Dec '08

29 Dec '08

Hi, this is not a Haskell question but I'm not sure if anyone in java community can answer this. And I'm sure every Haskell programmer know OO programing. I'm programming one big boring db application and making a MDD tool to model and generate such applications. Many times object have computed attributes: E.g. value attribute of class Invoice is sum of values of all associated classes Item (Invoice 1 <>---> * Item) So value attribute of invoice is computed every time it is demanded. So far easy.Sometimes it is necessary not to compute it when it is needed but to have it precomputed (computed it every time, item is added/removed to invoice or item value is changed). Contract of attribute value is the same but it is not obvious from code if you program it in event-based as if you program it with expression which is computed every time. Event-based=no big picture. Expression=big picture but little performance. Question: Is there any way how to transform the OO transformation (invoice value is a sum of values of items) to incremental transformation (update invoice value, when item is created/deleted/changed)? Is there any product doing it. I am not interesting in product itself I want only see how it works? Is there any science paper on this? I call this not a function but a OO transformation because it can be much more complicated. I know it is not possible for all the functions (transformations), you have to do undo step, but it is possible for that what we use. One more complicated example: Provider provides money to receiver. It provides money with special code. Receiver sees all the money from all the providers grouped by the code and summed. Every summed group on the code has a subgroup summed on the provider. Provider A provides to R 10 on code "ABC" Provider A provides to Q 10 on code "ABC" Provider A provides to R 5 on code "ABC" Provider A provides to R 20 on code "DEF" Provider B provides to R 30 on code "ABC" Reciever R - recieves 45 on code "ABC", 15 from A, 30 from B - recieves 20 on code "DEF", 20 from A Reciever Q - recieves 10 on code "ABC", 10 from A This example can be easily programmed declarative way in Scala the way that everytime everything is computed. But have no clue if it can be transformed automatically to incremental way. Hope you understand what I mean. Thanks for your help and opinions Fero

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Cross compiling from Linux to windows?
by Erik de Castro Lopo 29 Dec '08

29 Dec '08

Hi all, I've googled for this but haven't really found an answer yet. Is it possible to build (or better yet download) a GHC compiler that runs on Linux and generates windows binaries? I already use the MinGW C compiler to generate windows binaries from C code and I run my test suite under WINE. This has been working very well for me for some time and now I'd like to do the same for Haskell code. Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- Those who do not understand Unix are condemned to reinvent it, poorly. -- Henry Spencer

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Data.ByteString join
by Galchin, Vasili 29 Dec '08

29 Dec '08

Hello, http://cvs.haskell.org/Hugs/pages/libraries/base/Data-ByteString.html but vigalchin@ubuntu:~$ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m +Data.ByteString Prelude Data.ByteString> :t join <interactive>:1:0: Not in scope: `join' Prelude Data.ByteString> Why no "join" function? Regards, Vasili

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Transactional container for storing anonymous deletable objects
by John Ky 29 Dec '08

29 Dec '08

Hi, I need a container data structure for storing anonymous objects - most likely things that have not value such as STM (), but probably other things as well. This will allow me to later on, iterate over the container and process those objects. Additionally I have the requirement that I need to be able to remove individual objects from this container. It needs to be linear time. main = do container <- nil key1 <- cons 1 container key2 <- cons 2 container key3 <- cons 3 container key4 <- cons 4 container key5 <- cons 5 container unlink key3 container unlink key2 container unlink key4 container list <- toList container putStrLn (show list) The above should give: [1, 5] What should I use? Thanks -John

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Request for feedback: Understanding Haskell Monads
by Ertugrul Soeylemez 29 Dec '08

29 Dec '08

Hello fellow Haskellers, In the last few weeks I have written a comprehensive tutorial about Haskell monads [1], and I was hoping to get some constructive feedback. I'd appreciate any well meant criticism. [1] http://ertes.de/articles/monads.html Greets, Ertugrul. -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://blog.ertes.de/

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generating random lists until some predicate holds
by Nicholas O. Andrews 29 Dec '08

29 Dec '08

Hi all, What's the best way to implement the following Python code in Haskell? It is purposefully written in a functional style (and as a result will kill your recursion stack every other run). # begin Python from random import * def genList (): return [randint(0,9) for x in range(10)] def randWhile (predicate): result = genList () if predicate(result): return result else: return randWhile (predicate) def allEven (list): return reduce(lambda x,y: x and y, [x%2 == 0 for x in list]) print randWhile (allEven) # End Python Thanks!

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Fram
by Haakon Meland Eriksen 29 Dec '08

29 Dec '08

Dear Haskell community, It was kindly suggested to me by John Peterson that I might reach you here and tell you a little bit about Fram, a small Mediawiki extension. Fram lets you intersperse your explanation with actual code, and then Fram extracts and concatenates the code, giving you a file with code for download. This means anyone with access to Fram can improve your code, either by explaining it or by enhancing the code itself. It is released under the GNU GPL version 3 at no cost. Fram is now at version 0.2, and equals the Mediawiki extension RawFile 0.2, written by Belgian developer Philippe Teuwen. Phil and I are collaborating to improve Fram into a fully fledged literate programming tool like "noweb" written by Norman Ramsey. It was originally Ramsey who suggested to me that Fram may be interesting to the Haskell community wiki, so here we are. If this sounds useful, please have a look at http://far.no/fram/ or go below the main-deck into the cabins to explore the articles http://far.no/fram/index.php?title=Fram http://far.no/fram/index.php?title=Test http://far.no/fram/index.php?title=Usbpix and if these don't make sense, have a look at our help page http://far.no/fram/index.php?title=Help:Contents If you are curious about the name, "Fram" is also the name of a famous ship used by Norwegian polar explorers in the last century. It means "Forward". Yours sincerely, Haakon Meland Eriksen

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Missing Network Functions
by John Van Enk 29 Dec '08

29 Dec '08

While working on a project, I discovered that hton[sl] and ntoh[sl] are missing from the networking libraries. It appears there was some justification for this, but I've fallen upon a few instances where things were *not* working as expected. (Specifically when newtype'ing around HostAddress.) As such, I'm putting together a few extra functions that I think should exist in Network somewhere. I'm calling the package Network.Util (but am open to changes). Currently this only has: - htons - htonl - ntohs - ntohl Before I drop this on hackage: 1. Is there a reason I shouldn't do this? 2. Are there other suggestions for missing functions? (preferably cross-platform suggestions) Thanks all. /jve

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chroot a single thread
by Jeremy Shaw 29 Dec '08

29 Dec '08

Hello, I am working on an application where I would like to chroot a thread, but I am not seeing a way to do it. I already have code which can run an IO action inside a chroot. The type signature is: fchroot :: FilePath -> IO a -> IO a The first argument is the new root, the second argument is the action to run with that root. You can see the implementation here if you are curious: http://src.seereason.com/build-env/Chroot.hs The problem with that function is that chroot affects the root of the whole process. In a single-threaded program this is (possibly) ok, because the original root is restored after the IO action completes (though with unsafeInterleaveIO, perhaps bad stuff will happen). In a multithreaded program it can be disasterous. As far as I know, the only solution would be to implement a function like fchroot which looks more like: pchroot :: FilePath -> IO () -> IO ExitCode Here the IO action would be forked off and run in a whole new process so that changing its root does not affect other 'threads'. Of course, you also can't use any of the nifty Haskell intra-thread communication stuff. Basically you can pass some values in and get back an exit code. However, I am not even sure how to implement a useful version of pchroot. In theory, I can just use forkProcess, but, the Giant Warning for forkProcess indicates that this will not be very useful in practice: forkProcess comes with a giant warning: since any other running threads are not copied into the child process, it's easy to go wrong: e.g. by accessing some shared resource that was held by another thread in the parent. Another example is the I/O manager thread: since the I/O manager isn't running in the child, attempting to do any Handle-based I/O will deadlock. Anyone have an ideas ? Thanks! - jeremy

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Re: Re[2]: [Haskell-cafe] Data.ByteString join
by Galchin, Vasili 28 Dec '08

28 Dec '08

ooops ... .my bad ... so I guess Hoogle is the way to go?? On Sun, Dec 28, 2008 at 5:39 AM, Bulat Ziganshin <bulat.ziganshin(a)gmail.com>wrote: > Hello Vasili, > > Sunday, December 28, 2008, 1:59:43 PM, you wrote: > > > http://cvs.haskell.org/Hugs/pages/libraries/base/Data-ByteString.html > > look carefully - it's hugs docs > > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > >

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Re: Missing Network Functions
by John Van Enk 28 Dec '08

28 Dec '08

I want to group them with Network functions because traditionally these specific functions *are* in networking packages. On *nix systems, the prototypes reside in <netinet/in.h>. On Windows they reside in <winsock2.h> and one must link against libws2_32. The package I have locally supports all platforms. It just does a foreign import of the function (and uses a preprocessor definition to pick the calling convention). One could make an argument to add these sorts of functions to Word and Int packages. /jve On Sat, Dec 27, 2008 at 7:14 PM, Chris Kuklewicz < haskell(a)list.mightyreason.com> wrote: > Are these going to be available on Windows or just Posix systems? > > And these are NOT performing network communication. They are bit > manipulations. > > I could imagine putting them in/under the Data.Word or Data.Bits more than > Network.* > > And they should also be made to work with the corresponding Foreign.C.Types > of CShort, etc. > > > John Van Enk wrote: > >> While working on a project, I discovered that hton[sl] and ntoh[sl] are >> missing from the networking libraries. It appears there was some >> justification for this, but I've fallen upon a few instances where things >> were *not* working as expected. (Specifically when newtype'ing around >> HostAddress.) >> >> As such, I'm putting together a few extra functions that I think should >> exist in Network somewhere. I'm calling the package Network.Util (but am >> open to changes). >> >> Currently this only has: >> >> * htons >> * htonl >> * ntohs >> * ntohl >> >> Before I drop this on hackage: >> >> 1. Is there a reason I shouldn't do this? >> 2. Are there other suggestions for missing functions? (preferably >> cross-platform suggestions) >> >> Thanks all. >> >> /jve >> >> >> ------------------------------------------------------------------------ >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Re: [Haskell-cafe] ByteString typechecking issues....
by Galchin, Vasili 28 Dec '08

28 Dec '08

Not in scope: `Data.ByteString.join' when Data.ByteString.join $ encode (buildHeader ss) -- ByteString -- [] (map encode (buildEntries (sequenceListExtract ss))) -- [ByteString] ?? Thanks, guys On Sat, Dec 27, 2008 at 3:13 AM, Eugene Kirpichov <ekirpichov(a)gmail.com>wrote: > I think Luke meant that you forgot to qualify the import for join, and > the compiler guessed that you are meaning the monad one, thus the > error. > > 2008/12/27 Galchin, Vasili <vigalchin(a)gmail.com>: > > Hi Luke, > > > > join :: ByteString -> [ByteString] -> ByteString ??? > > > > Vasili > > > > On Sat, Dec 27, 2008 at 1:58 AM, Luke Palmer <lrpalmer(a)gmail.com> wrote: > >> > >> 2008/12/26 Galchin, Vasili <vigalchin(a)gmail.com> > >>> > >>> Hello, > >>> > >>> I have a ByteString -> [ByteString] -> ByteString situation, i.e. > >>> concatenation ..... > >>> > >>> > >>> -- marshall into ByteString representation > >>> join > >>> (encode (buildHeader > >>> ss)) -- ByteString > >>> (map encode (buildEntries (sequenceListExtract > >>> ss))) -- [ByteString] > >>> > >>> > >>> I get the following typecheck error which is vexing me .... > >>> > >>> Couldn't match expected type `t -> t -> B.ByteString' > >>> against inferred type `B.ByteString' > >>> > >>> ??? > >> > >> join is not a function in Data.ByteString. By the error I'm guessing > >> you're getting the join from Control.Monad, instantiated to (->). > >> You are looking for "concat"; i.e. > >> concat $ > >> encode (buildHeader ss) : > >> -- ByteString > >> map encode (buildEntries (sequenceListExtract ss)) > >> -- [ByteString] > >> > >> (Control.Monad.join does end up meaning"concat" when working on lists of > >> lists, but it does not generalize to lists of other things). > >> Luke > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > >

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Re: [Haskell-cafe] Re: [Haskell] ANN: HLint 1.0
by Claus Reinke 28 Dec '08

28 Dec '08

>> You were asking about getting the output of ':show modules' into a >> variable 'x', so that you can process it further. ':redir x :show modules' >> should do just that. There is another example command for implementing >> ':edit' this way (by now a native ghci command). > > I think I'm seeing your meaning. So that brings me up to this: > > let hlint _ = return $ unlines [":redir hlintvar1 :show modules", "let > hlintvar2 = map (fst . break (==',') . drop 2 . snd . break (== '(')) > $ lines hlintvar1", ":! hlint (concat $ intersperse \" \" hlintvar2"] > :def hlint hlint > > This doesn't work. The issue is that :! is weird; for it to work, one > need to pass each argument as a separate string, and it won't evaluate > a variable. It isn't just ':!', quoting/variable interpretation is generally rather uncomfortable in GHCi scripting (so much so that I originally submitted output redirection as a patch before figuring out that it could be done without patching GHCi - that surprise find was the motivation for posting my findings as an email). Have you tried reading the mini tutorial that I keep mentioning and which the "using GHCi" page is pointing to? Here's the direct link: http://www.haskell.org/pipermail/haskell-cafe/2007-September/032260.html The discussion is rather brief, but that tutorial has several examples that need to work around issues like this, ranging from simple but tedious construct-the-command-string to extra levels of ':cmd' in order to get extra levels of interpretation (when you need to construct a command string from a variable that will be bound via a constructed command string (see the definitions of ':find', ':le' or ':b(rowse)' - the latter is an example of using the info from ':show modules'). Claus

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Don't make 'show*' functions
by Thomas DuBuisson 28 Dec '08

28 Dec '08

Hello cafe, This is just a small thought, but its been bugging me. We have these things called type classes for a reason (I like to think). When making a new data type 'Data', it is not productive to avoid type classes such as 'Show' and export a 'showData' function. Examples of what I'm talking about include showHtml, showTrie, showInstalledPackageInfo... I know the default derivation (and thus generally accepted) instance of Show isn't pretty, but that just means to me that we need either more methods within the Show type class or start using the prettyclass package more. If the problem is an API issue lets fix Pretty or Show. But this show* stuff should disappear in the long run. Tom

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Typeclass question
by Andrew Wagner 28 Dec '08

28 Dec '08

I'm sure there's a way to do this, but it's escaping me at present. I want to do something like this: data Foo = Bar a => Foo a Bool ... That is, I want to create a new type, Foo, whose constructor takes both a Boolean and a value of a type of class Bar.

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ANNOUNCE: citeproc-hs-0.2
by Andrea Rossato 28 Dec '08

28 Dec '08

Hello, I'm happy to announce the release of citeproc-hs-0.2. WHAT'S NEW 1. Added support for citation collapsing options. This fills the last major gap to a usable CSL implementation; 2. added a wrapper around hs-bibutils[1] (disabled by default). By installing hs-bibutils and linking citeproc-hs to it with the '-fbibutils' configuration flag, it is possible to read all the bibliographic databases supported by bibutils[2]; 3. added some haddock API documentation: http://code.haskell.org/citeproc-hs/docs/ 4. simplified and started to document the internals of the library. DOWNLOADS citeproc-hs can be downloaded from Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/citeproc-hs ABOUT citeproc-hs is a Haskell implementation of the Citation Style Language. It adds to pandoc, the famous Haskell text processing tool, a Bibtex like citation and bibliographic formatting and generation facility. The Citation Style Language (CSL) is an XML language for specifying citation and bibliographic formatting, similar in principle to BibTeX .bst files or the binary style files in commercial products like Endnote or Reference Manager. CSL is used by Zotero for bibliographic style formatting, and a huge number of CSL styles have been developed by the Zotero community. More information, with installation instructions, can be found here: http://code.haskell.org/citeproc-hs/ Hope you'll enjoy, Andrea Rossato [1] http://code.haskell.org/hs-bibutils/ [2] http://www.scripps.edu/~cdputnam/software/bibutils/

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ANNOUNCE: hs-bibutils-0.1
by Andrea Rossato 28 Dec '08

28 Dec '08

Hello, I'm happy to announce the first release of hs-bibutils, the Haskell bindings to Chris Putnam's bibutils.[1] Bibutils is a library and a set of bibliographic utilities to interconvert between various bibliography database formats using a common MODS-format XML intermediate. hs-bibutils is meant to provide to citeproc-hs, and indirectly to pandoc, the ability to parse all the bibliographic database formats supported by bibutils. citeproc-hs[2] is a Haskell implementation of the Citation Style Language. It adds to pandoc a Bibtex like citation and bibliographic formatting and generation facility. DOWNLOADS hs-bibutils can be downloaded from Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hs-bibutils The project home page can be found here: http://code.haskell.org/hs-bibutils/ INSTALLATION AND KNOWN ISSUES bibutils builds, without installing it, a static library to be used to compile the various converting utilities. But in order to use the library with pandoc (or ghci) it is necessary to create and install a shared object before building and installing hs-bibutils. hs-bibutils provides a patch to the bibutils building scripts to enable the shared object creation and installation. Alternatively a patched version of the latest release of bibutils can be downloaded from the project home page, where all the needed information for the installation are provided: http://code.haskell.org/hs-bibutils/#installation I was able to test the patched building scripts only on a Linux machine with a x86 system. Please report me any success or failure on other systems/architectures. BUG REPORTS To submit bug reports you can either contact me directly or use the citeproc-hs bug tracking system available at the following address: http://code.google.com/p/citeproc-hs/issues Hope you'll enjoy, Andrea Rossato [1] http://www.scripps.edu/~cdputnam/software/bibutils/ [2] http://code.haskell.org/citeproc-hs/

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ByteString typechecking issues....
by Galchin, Vasili 27 Dec '08

27 Dec '08

Hello, I have a ByteString -> [ByteString] -> ByteString situation, i.e. concatenation ..... -- marshall into ByteString representation join (encode (buildHeader ss)) -- ByteString (map encode (buildEntries (sequenceListExtract ss))) -- [ByteString] I get the following typecheck error which is vexing me .... Couldn't match expected type `t -> t -> B.ByteString' against inferred type `B.ByteString' ??? Thanks, Vasili

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Type classes
by Oscar Picasso 27 Dec '08

27 Dec '08

>From Real World Haskell: data JValue = JString String | JNumber Double | JBool Bool | JNull | JObject [(String, JValue)] | JArray [JValue] deriving (Eq, Ord, Show) .... type JSONError = String class JSON a where toJValue :: a -> JValue fromJValue :: JValue -> Either JSONError a instance JSON JValue where toJValue = id -- Really ? fromJValue = Right Now, instead of applying a constructor like JNumber to a value to wrap it, > we apply the toJValue function. If we change a value's type, the compiler > will choose a suitable implementation of toJValue to use with it. Actually it does not work. And I don't see how it could with this toJValue implementation. Is it possible to make it work by providing another implementation? Oscar

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Haskell Weekly News: Issue 98 - December 25, 2008
by Brent Yorgey 27 Dec '08

27 Dec '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20081225 Issue 98 - December 25, 2008 --------------------------------------------------------------------------- Welcome to issue 98 of HWN, a newsletter covering developments in the [1]Haskell community. Happy holidays! An exciting HWN for you this week, including a number of cool new libraries, the [2]public release of [3]Cryptol, a [4]Haskell logo contest, and the [5]second most awesome GHC bug ever (see augustss's quote at the end of the Quotes section for the most awesome GHC bug ever). Announcements Hieroglyph 0.85. Jeff Heard [6]announced that the Thingie library has been renamed [7]Hieroglyph, and now has support for displaying images on the Cairo canvas. Cryptol now freely available. Don Stewart [8]announced that [9]Cryptol, the language of cryptography, is [10]now available to the public! Cryptol is a domain specific language for the design, implementation and verification of cryptographic algorithms, developed over the past decade by Galois for the United States National Security Agency. It has been used successfully in a number of projects, and is also in use at Rockwell Collins, Inc. Cryptol is implemented in Haskell. Control.Monad.IfElse. Jeff Heard [11]announced the [12]Control.Monad.IfElse module, which provides useful anaphoric and monadic versions of if-else and when. llvm-0.4.0.1. Lennart Augustsson [13]announced version 0.4.0.1 of the [14]release that is quite incompatible with the old 0.0.2 release.) Haskell LLVM bindings. LLVM is a virtual machine and the bindings allow you to generate code for this virtual machine. This code can then be executed by a JIT or written to a file for further processing by the LLVM tools. bytestring-trie 0.1.0. wren ng thornton [15]announced the release of [16]bytestring-trie 0.1.0, an efficient finite map from (byte)strings to values. The implementation is based on big-endian patricia trees, like Data.IntMap. RWH book club. Don Stewart [17]announced that Matt Podwysocki has set up a [18]Real World Haskell book club, a mailing list on google groups with already some 200 members discussing typical new user Haskell questions. Feel free to join if you like talking about Haskell, or teaching new users. Thingie-0.80. Jeff Heard [19]announced the release of [20]Thingie, a library for creating 2D visualizations in a purely functional manner. It supports static visualizations and animation, and like most vis libraries, can probably do games as well as simple viz graphics. typehash version 1.3. Lennart Augustsson [21]announced the release of the [22]typehash library, which allows you to produce a unique identifier (a cryptographic hash) for a type. This is useful if you save values of some type to a file (text, binary, whatever format you wish) and then when you read it back in again you want to verify that the type you want to read is the one you actually wrote. The library also supports type codes, which encode the complete structure of a type and can be used for finer comparison than just equality. uvector-algorithms 0.1. Dan Doel [23]announced the release of [24]uvector-algorithms, a library of algorithms (mostly sorting) for the mutable arrays defined in uvector. It has several varieties of sorting, including introsort (quicksort which falls back on heapsort in bad cases), heapsort, a simple top- down merge sort and a radix sort. Also exposed are the operations that allow you to use the arrays as heaps and a combinator for safely using these mutable array algorithms to sort immutable arrays. All algorithms have been painstakingly profiled and optimized. Data.List.Split. Brent Yorgey [25]announced the release of [26]Data.List.Split, which provides a wide range of strategies and a unified combinator framework for splitting lists with respect to some sort of delimiter. Hoogle with more libraries. Neil Mitchell [27]announced that [28]Hoogle will now search lots of the libraries present on hackage! HLint 1.0. Neil Mitchell [29]announced the re-release of [30]HLint, a tool for making suggestions to improve your Haskell code. Previously this tool was called Dr Haskell and depended on a working installation of Yhc; now it depends on GHC 6.10.1. rangemin-1.0. Louis Wasserman [31]announced the release of [32]rangemin, a library for efficiently preprocessing an array to find minimum elements of subranges of the array in constant time. Discussion length of module name affecting performance??. Daniel Gorin [33]reported a GHC bug where in certain cases, changing the name of a module to something longer results in a 2x-3x performance hit! Strange but true. Time for a new logo?. Don Stewart [34]proposed a competition to produce a new Haskell logo! Submissions should go on the [35]wiki page; the deadline for logo submissions is December 31. Pattern combinators. Andrew Wagner [36]started a thread turning a [37]paper on pattern-matching in Haskell into actual code for hackage. Coroutines. Ryan Ingram [38]posted some interesting code showing how to implement coroutines with session types. Type wildcards. Eyal Lotem [39]proposed a 'type wildcards' extension to the language. Blog noise [40]Haskell news from the [41]blogosphere. * Galois, Inc: [42]Cryptol, the language of cryptography, now available. * Dan Piponi (sigfpe): [43]The Mother of all Monads. * Real-World Haskell: [44]Pat Eyler Interviews the Real World Haskell Team. * Philip Wadler: [45]Unsafe. * Conrad Parker: [46]Release: HOgg 0.4.1. * ezekiel smithburg: [47]fast string appending/concatenation in haskell. * Brent Yorgey: [48]Data.List.Split. * Alson Kemp: [49]Turbinado: Implementing a poor-man's wiki. * Luke Palmer: [50]Reactive spaces. * Conal Elliott: [51]Smarter termination for thread racing. * Real-World Haskell: [52]RWH on Twitter. * >>> Muharem Hrnjadovic: [53]My new favourite book. * Alson Kemp: [54]Turbinado update. * Philip Wadler: [55]Type Safe Pattern Combinators, by Morten Rhiger. * Osfameron: [56]Crossword puzzles in Haskell. * Brent Yorgey: [57]QuickCheck rocks my socks. * >>> Matt Youell: [58]If programming languages were martial arts. * >>> Sebastian Fischer: [59]Haskell idioms I did not understand before hacking them on my own. * Alson Kemp: [60]Thinking about Haskell: You Know Lazy Evaluation; You Just Don't Know It. * >>> Brian McCallister: [61]Real World Haskell, for Jon. * Ivan Lazar Miljenovic: [62]RWH Arrives Down Under!. * >>> Thomas ten Cate: [63]XMonad with Ubuntu, dvorak, Pidgin and Skype. * >>> Jeremy Frens: [64]PE Problem #3 in All Languages. * Jeremy Shaw: [65]Data Migration with HApps-Data. * Chris Done: [66]Haskell Formlets: Composable web form construction and validation. * Real World Haskell: [67]Haskell around the world. * Dougal Stanton: [68]A brief look at fingertrees. Quotes of the Week * luqui: no!!! I was building a joke, but then I sent it before I thought of one. * roconnor: We put up a clothes line and made a turing machine by hand at a party once ... that is the sort of parties I go to. * cjs: In what other language could I have learned so much about Win32 programming (summary: basically, the Windows 3.0 API but with all sorts of hacks to deal with having more than one thread in the system), and come out having *enjoyed* myself? Praise to the Lord! * PaulJohnson: A paradox of the Haskell world is that, while the language is Vulcan, the community around it is dominated by Warm Fuzziness. Clearly the two are not mutually exclusive. * Botje: Caleskell even has unsafeSolveHaltingProblem? * Taejo: * Taejo needs to write Sitar Hero in Yampa * dons: it is safer for incompetent people to be working in Haskell than C++. * mpeter: the quality of my code increased drastically when i realized i should stop telling the computer to do things which were stupid. * byorgey: <Cale> RandomT/Random are effectively state monads. (in fact, they're thin candy shells around StateT/State.) <byorgey> "newtype: melts in the compiler, not in your hands" * quicksilver: [on classes having the same name as constructs in other languages] it's like having a laxative called "after dinner mint", and people being upset when they were looking for something nice to eat after dinner. * quicksilver: #haskell is a loquacracy! * quicksilver: It's also the same thing as the Yoneda lemma. That's the thing about maths. Everything is actually the same. * hugo: i feel like i was drugged with imperative programming, and now im in rehab. * chrisdone: yo dawg we heard you like haskell so we installed a lambdabot in your ghci so you can monad while you monad * augustss: ghc had a bug once where it deleted the source file if it had a type error. Quite sensible, I think. About the Haskell Weekly News New editions are posted to [69]the Haskell mailing list as well as to [70]the Haskell Sequence and [71]Planet Haskell. [72]RSS is also available, and headlines appear on [73]haskell.org. To help create new editions of this newsletter, please see the information on [74]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [75]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.galois.com/blog/2008/12/24/cryptol-the-language-of-cryptography-… 3. http://www.cryptol.net/ 4. http://haskell.org/haskellwiki/Haskell_logos/New_logo_ideas 5. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/16037 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49528 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hieroglyph 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49516 9. http://www.cryptol.net/ 10. http://www.galois.com/blog/2008/12/24/cryptol-the-language-of-cryptography-… 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49365 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/IfElse 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49340 14. file://localhost/home/brent/hacking/hwn/(This 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49234 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49168 18. http://groups.google.com/group/real-world-haskell-book-club 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49147 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Thingie 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/48902 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/typehash 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/48889 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uvector%2Dalgori… 25. http://article.gmane.org/gmane.comp.lang.haskell.general/16700 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/split 27. http://article.gmane.org/gmane.comp.lang.haskell.general/16698 28. http://haskell.org/hoogle 29. http://article.gmane.org/gmane.comp.lang.haskell.general/16692 30. http://www-users.cs.york.ac.uk/~ndm/hlint/ 31. http://article.gmane.org/gmane.comp.lang.haskell.general/16686 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rangemin 33. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/16037 34. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/48892 35. http://haskell.org/haskellwiki/Haskell_logos/New_logo_ideas 36. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/49267 37. http://www.itu.dk/people/mir/typesafepatterns.pdf 38. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/49108 39. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/48987 40. http://planet.haskell.org/ 41. http://haskell.org/haskellwiki/Blog_articles 42. http://www.galois.com/blog/2008/12/24/cryptol-the-language-of-cryptography-… 43. http://sigfpe.blogspot.com/2008/12/mother-of-all-monads.html 44. http://www.realworldhaskell.org/blog/2008/12/24/pat-eyler-interviews-the-re… 45. http://wadler.blogspot.com/2008/12/unsafe.html 46. http://blog.kfish.org/2008/12/release-hogg-041.html 47. http://tehgeekmeister.wordpress.com/2008/12/22/fast-string-appendingconcate… 48. http://byorgey.wordpress.com/2008/12/21/datalistsplit/ 49. http://www.alsonkemp.com/haskell/turbinado-implementing-a-poor-mans-wiki/ 50. file://localhost/home/brent/hacking/hwn/20081225.html 51. http://conal.net/blog/posts/smarter-termination-for-thread-racing/ 52. http://www.realworldhaskell.org/blog/2008/12/18/rwh-on-twitter/ 53. http://muharem.wordpress.com/2008/12/18/my-favourite-book/ 54. http://www.alsonkemp.com/haskell/turbinado-update/ 55. http://wadler.blogspot.com/2008/12/type-safe-pattern-combinators.html 56. http://greenokapi.net/blog/2008/12/18/crossword-puzzles-in-haskell/ 57. http://byorgey.wordpress.com/2008/12/16/quickcheck-rocks-my-socks/ 58. http://www.youell.com/matt/writing/?p=46 59. http://www-ps.informatik.uni-kiel.de/~sebf/projects/haskell-idioms-i-did-no… 60. http://www.alsonkemp.com/?p=110 61. http://kasparov.skife.org/blog/src/haskell/real-world-haskell-for-jon.html 62. http://ivanmiljenovic.wordpress.com/?p=30 63. http://typethinker.blogspot.com/2008/12/xmonad-with-ubuntu-dvorak-pidgin-an… 64. http://jdfrens.blogspot.com/2008/12/pe-problem-3-in-all-languages.html 65. http://nhlab.blogspot.com/2008/12/data-migration-with-happs-data.html 66. http://chrisdone.com/blog/2008/12/14/Haskell-Formlets:-Composable-web-form-… 67. http://www.realworldhaskell.org/blog/?p=154 68. http://www.dougalstanton.net/blog/index.php/2008/12/12/a-brief-look-at-fing… 69. http://www.haskell.org/mailman/listinfo/haskell 70. http://sequence.complete.org/ 71. http://planet.haskell.org/ 72. http://sequence.complete.org/node/feed 73. http://haskell.org/ 74. http://haskell.org/haskellwiki/HWN 75. http://code.haskell.org/~byorgey/code/hwn/

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How does one use Text.Regex.Base.RegexLike?
by Lyle Kopnicky 26 Dec '08

26 Dec '08

I'm trying to migrate code from using the old Text.Regex to the new Text.Regex.Base. But, I'm getting type errors. I can't even create a regex. Looking at the docs, it seems like this should print "bcd": import Data.Array import Text.Regex.Base import Text.Regex.Posix rx = makeRegex "a(.*)A" Just (_, mt, _) = matchOnceText rx "abcdA" main = putStrLn (fst (mt ! 0)) But I get an error: src\regex.hs:5:5: No instance for (RegexMaker regex compOpt execOpt [Char]) arising from a use of `makeRegex' at src\regex.hs:5:5-22 Possible fix: add an instance declaration for (RegexMaker regex compOpt execOpt [Char]) In the expression: makeRegex "a(.*)A" In the definition of `rx': rx = makeRegex "a(.*)A" src\regex.hs:7:18: No instance for (RegexLike regex [Char]) arising from a use of `matchOnceText' at src\regex.hs:7:18-41 Possible fix: add an instance declaration for (RegexLike regex [Char]) In the expression: matchOnceText rx "abcdA" In a pattern binding: Just (_, mt, _) = matchOnceText rx "abcdA" Why does it say there is no instance? Isn't the instance imported by Text.Regex.Posix? Why in the world is it so complicated just to get a matched substring out of the text? Is there an easier way? Thanks, Lyle

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forkIO on multicore
by Paul Keir 26 Dec '08

26 Dec '08

Hi all, I'm seeing no performance increase with a simple coarse-grained 2-thread code using Control.Concurrent. I compile with: > hc conc.hs -o conc --make -threaded and I run with > time ./conc +RTS -N2 But using either "-N1" or "-N2", the program runs in about 1.8secs. (I'd prefer a longer running thread task, but my fib function currently runs out of memory). Anyway, my program is below, and I'm using GHC version 6.8.2 on a 2-core Pentium D. Can anyone help? module Main where import Control.Concurrent fibs = 0 : 1 : zipWith (+) fibs (tail fibs) heavytask m = putMVar m (fibs !! 100000) main = do ms <- sequence $ replicate 2 newEmptyMVar mapM_ (forkIO . heavytask) $ tail ms heavytask $ head ms ms' <- mapM takeMVar ms mapM_ print ms' Regards, Paul

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Hackage packages for indexing by Google
by Henning Thielemann 26 Dec '08

26 Dec '08

When testing the ShowMeta parallel web: http://hackage.haskell.org.showmeta.parallelnetz.de/packages/archive/pkg-li… I found, that all HackageDB pages forbid crawling and indexing. Actually, the content of this page is dynamic - but it changes only incrementally. It would be very worth when I search for "markov chain haskell" with Google, that I will be directed to the according package page of HackageDB.

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ANN: Hieroglyph 0.85
by Jeff Heard 26 Dec '08

26 Dec '08

I have changed the name of Thingie to Hieroglyph and added support for displaying images on the Cairo canvas. I'm still working on using template haskell to derive the basic UIState class, but I don't understand template haskell all that well, so it's taking some time. Hopefully I can get some examples of the library up over the christmas holiday. -- Jeff

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Re: [Haskell-cafe] Coroutines
by Ryan Ingram 25 Dec '08

25 Dec '08

On Thu, Dec 18, 2008 at 2:00 AM, Nicolas Pouillard <nicolas.pouillard(a)gmail.com> wrote: > I don't see why one would need session types, channels... to express that. > I maybe need a more complicated coroutines (ruby) example that would require > using this system. OK, how would you type these routines in Haskell? def simple yield "hello" yield 1 yield (lambda { |x| x + 1 }) end def useSimple state = 0 result = nil simple { |x| if (state == 0) then result = x else if (state == 1) then result += (x * 4).toString else if (state == 2) then result += x.call(10).toString state = state + 1 } result end I know it's a bit contrived, but you get the idea. In Haskell using Control.Coroutine: simple :: forall rest. Session (String :!: Int :!: (Int -> Int) :!: rest) rest () simple = do put "hello" put 1 put (\x -> x + 1) useSimple :: forall rest. Session (String :?: Int :?: (Int -> Int) :?: rest) rest String useSimple = do string <- get int <- get func <- get return (string ++ show (int * 4) ++ show (func 10)) result :: String result = snd $ connects simple useSimple -- result = "hello411"

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Stupid question #374: why is MaybeT not in the standard library?
by Brian Hurt 25 Dec '08

25 Dec '08

I wrote my own implementation of MaybeT (which was a usefull exercise), but a quick google showed: http://www.haskell.org/haskellwiki/New_monads/MaybeT But I'm wondering why it's not in the standard library. The standards committee just hasn't gotten around to it yet? Or was there some discussion of this in the past on some (public) maillist, that my admittedly shallow googling failed to uncover, that someone could point me at? Thanks. Brian

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ANN: Control.Monad.IfElse
by Jeff Heard 25 Dec '08

25 Dec '08

Provides useful anaphoric and monadic versions of if-else and when, as well as infix operators for the actions to allow things like this: -- Allow the clause to be in m a whenM (Gtk.widgetIsFocus win) $ do this that -- anaphoric if. If the condition is nonempty, pass the conditional value to the "then" clause, otherwise do an action that requires no parameter. aif (lookup name list) (\file -> Right `liftM` hGetLine file) (do f <- openFile name l <- hGetLine file return $ Right (f,l)) -- infix version of Control.Monad.When (x>5) >>? putStrLn x -- infix version of anaphoric when lookup name list >>=? \value -> putStrLn value -- infix version of anaphoric whenM Gtk.widgetIntersect win rect >>=>>? \intersectingRectangle' -> Gtk.invalidate intersectingRectangle'

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ANNOUNCE: Cryptol now freely available
by Don Stewart 25 Dec '08

25 Dec '08

Galois is pleased to announce that Cryptol, the language of cryptography, is now available to the public! Cryptol is a domain specific language for the design, implementation and verification of cryptographic algorithms, developed over the past decade by Galois for the United States National Security Agency. It has been used successfully in a number of projects, and is also in use at Rockwell Collins, Inc. Cryptol is implemented in Haskell. Domain-specific languages (DSLs) allow subject-matter experts to design solutions in using familiar concepts and constructs. Cryptol, as a DSL, allows domain experts in cryptography to design and implement cryptographic algorithms with a high degree of assurance in the correctness of their design, and at the same time, producing a high performance implementation of their algorithms. In this release, Galois has made a implementation of the Cryptol language available free of charge for non-commercial uses. More information on the blog: http://www.galois.com/blog/2008/12/24/cryptol-the-language-of-cryptography-… And at http://www.cryptol.net Enjoy!

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When using functional dependencies to combine...
by Martijn van Steenbergen 25 Dec '08

25 Dec '08

Good evening everyone, My program reads: > module Boom where > > import Control.Monad.State > > type SucParser s = StateT [s] [] > > newtype WithUnit s a = WithUnit (SucParser s (a, ())) > > foo :: SucParser s [s] > foo = get > > bar :: WithUnit s [s] > bar = WithUnit get The compiler complains: Boom.hs:13:0: Couldn't match expected type `([s], ())' against inferred type `[s]' When using functional dependencies to combine MonadState s (StateT s m), arising from the instance declaration at <no location info> MonadState ([s], ()) (StateT [s] []), arising from a use of `get' at Boom.hs:13:15-17 When generalising the type(s) for `bar' I'm wondering if I'm making a silly mistake or if there's something less trivial going on here. Could someone please explain the error and give a hint on how to fix it? Thanks much. :-) Martijn.

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Comparing on multiple criteria
by Martijn van Steenbergen 25 Dec '08

25 Dec '08

Hello all, Data.Ord has a handy function called comparing, and its documentation shows an example of its use. But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this: compareTuple = mconcat [comparing fst, comparing snd] The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!) To reverse the order of a criterion or a set of criteria, flip can be used: compareTuple' = mconcat [comparing fst, flip $ comparing snd] I think it would be really neat if these two use cases were also described in comparing's documentation. Who is the right person to ask this of? Thanks in advance, Martijn.

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The Applicative Functor Monad
by Jeremy Shaw 25 Dec '08

25 Dec '08

Hello, I want to make a Monad which is almost exactly like the Writer monad, except instead of using mappend to glue Monoids together, it uses <*> to glue applicative functors together. Here is the code: import Control.Applicative import Data.Monoid -- * Sample Implementation of the Writer Monad data Writer w a = Writer { runWriter :: (w, a) } instance (Monoid w) => Monad (Writer w) where return a = Writer (mempty, a) (>>=) = bindWriter bindWriter :: (Monoid w) => Writer w a -> (a -> Writer w b) -> Writer w b bindWriter (Writer (w,a)) f = let (Writer (w', b)) = f a in Writer (w `mappend` w', b) -- * Sample Implementation of the Applicative Functor Monad data AF af a = AF { runAF :: (af, a) } bindAF :: (Applicative f) => AF (f (a -> b)) x -> (x -> AF (f a) y) -> AF (f b) y bindAF (AF (f, x)) g = let (AF (a, y)) = g x in AF (f <*> a, y) -- instance (Applicative f) => Monad (AF (f ... As you can see, the similarity is striking. Alas, AF and bindAF do not quite have the right type signatures to be used for an instance of the Monad class. Is there some clever work-around I missing? (Aside from, -fno-implicit-prelude). Thanks! - jeremy

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shortest paths, prioritiy queues
by Johannes Waldmann 25 Dec '08

25 Dec '08

Hello. I am looking for a single-source shortest-path implementation (Dijkstra's algorithm, cf. Chapter 25 in Cormen,Leiserson,Rivest). An efficient implementation would use Binomial or Fibonacci heaps (Chap.s 20 + 21). The problem seems to be "(fib-)heap-decrease-key" which assumes that we have, with the key, a pointer to its position in the heap, because that is where there will be a destructive update. How is this dealt with in Okasaki's book + library? - Thanks, J.W.

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