- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Processing XML with HXT
by Uwe Schmidt 24 Apr '08

24 Apr '08

Hi Rodrigo, > Just one more question, I didn't find any example describing how to get the > text information of a XML element in the picklers tutorial. So, if the use case > element is described as follwing: > ... here is a complete example for your use case. Take the following XML data in file "uc.xml" ------------------------ <useCaseModel name="firstUseCaseModel"> <useCase> <id>UC_01</id> <name>Opening ...</name> <description>This use case describes how...</description> </useCase> </useCaseModel> ------------------------ and the following picker and test ------------------------ module Main where import Text.XML.HXT.Arrow type Id = String type Name = String type Description = String data UseCaseModel = UCM Name [UseCase] deriving (Show) data UseCase = UseCase Id Name Description deriving (Show) instance XmlPickler UseCase where xpickle = xpUseCase xpUseCaseModel :: PU UseCaseModel xpUseCaseModel = xpElem "useCaseModel" $ xpWrap ( uncurry UCM , \ (UCM n ucs) -> (n, ucs) ) $ xpPair ( xpAttr "name" xpText ) ( xpList xpUseCase ) xpUseCase :: PU UseCase xpUseCase = xpElem "useCase" $ xpWrap ( uncurry3 UseCase , \ (UseCase i n d) -> (i, n, d) ) $ xpTriple (xpElem "id" xpText) (xpElem "name" xpText) (xpElem "description" xpText) main :: IO () main = do runX ( xunpickleDocument xpUseCaseModel [ (a_validate,v_0) , (a_trace, v_1) , (a_remove_whitespace,v_1) -- !!!!!!!!!! throw away whitespace , (a_preserve_comment, v_0) -- !!!!!!!!!! throw away comments ] "uc.xml" >>> processUseCaseModel >>> xpickleDocument xpUseCaseModel [ (a_indent, v_1) ] "new-uc.xml" ) return () -- the dummy for processing the unpickled data processUseCaseModel :: IOSArrow UseCaseModel UseCaseModel processUseCaseModel = arrIO ( \ x -> do {print x ; return x}) ----------------------- This program will print the internal haskell value and writes the data to "new-uc.xml" Cheers Uwe -- Uwe Schmidt Web: http://www.fh-wedel.de/~si/

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Re: [Haskell] ANNOUNCE: Generic Haskell 1.80 (Emerald)
by Thomas van Noort 24 Apr '08

24 Apr '08

Hello, I took the liberty to move this discussion to the Haskell-Cafe mailing list. Adrian Hey wrote: > Thomas van Noort wrote: >> Pleasant programming, > > Hello, > > This looks like good stuff. But having done all this work it seems > a pity not to go the extra mm and cabalise this and make it buildable > on all platforms (at least ghc supported platforms). You are right, cabalizing the Makefiles implies that Generic Haskell can be build on any platform that is supported by GHC. However, this requires a tremendous amount of effort since the Makefiles of Generic Haskell are enormous. > > The reason I'm interested is this may be useful for the GSoC project > I'm mentoring.. > > http://code.google.com/soc/2008/haskell/about.html > > But as things are, I'd be unlikely to consider introducing a dependency > on this. Even if it built "out of the box" with cygwin (which it > doesn't BTW) I don't really think many windows users will be keen to > install cygwin and learn how to use it just so they can build GH. As you already noticed, there is no Windows binary available for the Emerald release. However, there is one for the Coral release, available from: http://www.generic-haskell.org Although this is an old release of Generic Haskell, this release already supports generic types, which is what you need for your project probably. In the user's guide, there is a small example available which defines a generic type to represent tries. The latest release of Generic Haskell supports generic views for generic types. This allows you to define a generic type that generates efficient balanced tries, as you can read in my Master's thesis :) This requires the use of the balanced view on the type level. Unfortunately, this generic view is not implemented in the Generic Haskell compiler... > > Thanks > -- > Adrian Hey Regards, Thomas

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Laziness and Either
by John Goerzen 23 Apr '08

23 Apr '08

Back when I was working on the logic for the bin-packing solver that I added to MissingH (for use with datapacker), I had a design decision to make: do I raise runtime errors with the input using error, or do I use an Either type to return errors? Initially, for simplicity, I just used error. But when I did a simple refactoring to use Either, it occurred to me that this switch likely had a negative impact on laziness. In this particular algorithm, we cannot tell for sure that we have no errors until we have consumed all items of the input list. This is unlike, say, a safe version of "head" where you can tell whether you have an error just by whether you have an empty list or not. In the case of using "error", we can happily process the data assuming everything will be fine, and raise an error if and when it is encountered. By using Either, however, any pattern match on the Left/Right result is going to force the entire input to be evaluated so that we can know whether or not it had any error. Is this analysis sensible? If so, are there better solutions? BTW, here are links to the code I'm talking about: http://git.complete.org/datapacker?a=blob;f=Scan.hs;h=d4e8ac8d6e883f342096a… line 32-43 -- version that uses error http://git.complete.org/datapacker?a=blob;f=Scan.hs;h=acd53739f5d871a3ce7ae… line 32-46 -- version that uses Either http://git.complete.org/datapacker?a=commitdiff;h=0d4e3ee6c4e5c780009ad5c09… -- diff between them -- John

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Re: [Haskell-cafe] Processing XML with HXT
by rodrigo.bonifacio 23 Apr '08

23 Apr '08

Hi Uwe Schmidt, thanks a lot. Just one more question, I didn't find any example describing how to get the text information of a XML element in the picklers tutorial. So, if the use case element is described as follwing: <useCase> <id>UC_01</id> <name>Opening ...</name> <description>This use case describes how...</description> </useCase> How can I set the id, name and desription of a use case? -- use case data type data UseCaseModel = UCM Name [UseCase] data UseCase = UseCase Id Name Description deriving (Show) -- I tried the following picler implementation -- but I think that it is not correct. instance XmlPickler UseCase where xpickle = xpUseCase xpUseCase :: PU UseCase xpUseCase = xpElem "useCase" $ xpWrap ( \ (i, n, d) -> UseCase i n d, \t -> (ucId t, name t, description t)) $ (xpTriple (xpElem "id" xpText) (xpElem "name" xpText) (xpElem "description" xpText)) > Hi Rodrigo, > > > I´m just starting with HXT. My question is, how can I expose a "use case" from the main function below (the XmlPickler for UseCase has been already defined): > > > > main :: IO () > > main = do > > runX ( xunpickleDocument xpUseCase [ (a_validate,v_0) ], "uc.xml" ) > > return () > > > > For example, if I just want to show the use case contents, how can I call "show" for a retrived use case. > > ------------------------------------- > > 1. version > > main :: IO () > main = do > [x] <- runX ( xunpickleDocument xpUseCase [ (a_validate,v_0) ], "uc.xml" ) > print x > return () > > x is processed outside the arrow > > ------------------------------------- > 2. version > > main :: IO () > main = do > runX ( xunpickleDocument xpUseCase [ (a_validate,v_0) ], "uc.xml" > >>> > arrIO print > ) > return () > > x is processed within an arrow by lifting the print function to then arrow level. > > ------------------------------------ > > There is a new wiki page about picklers > http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_X… > > Cheers, > > Uwe > > -- > > Uwe Schmidt > Web: http://www.fh-wedel.de/~si/ > ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

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Storable class?
by Galchin, Vasili 23 Apr '08

23 Apr '08

Hello, Examples of instances of the Storable class please. Thank you, Vasili

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Processing XML with HXT
by rodrigo.bonifacio 23 Apr '08

23 Apr '08

Hi all, I´m just starting with HXT. My question is, how can I expose a "use case" from the main function below (the XmlPickler for UseCase has been already defined): main :: IO () main = do runX ( xunpickleDocument xpUseCase [ (a_validate,v_0) ], "uc.xml" ) return () For example, if I just want to show the use case contents, how can I call "show" for a retrived use case. Thanks in advance. Rodrigo.

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(flawed?) benchmark : sort
by Krzysztof Skrzętnicki 23 Apr '08

23 Apr '08

Hi I was playing with various versions of sorting algorithms. I know it's very easy to create flawed benchmark and I don't claim those are good ones. However, it really seems strange to me, that sort - library function - is actually the worse measured function. I can hardly belive it, and I'd rather say I have made a mistake preparing it. The overall winner seems to be qsort_iv - which is nothing less but old sort replaced by mergesort now. Any clues? Regards Christopher Skrzętnicki --- cut here --- [Tener@laptener haskell]$ ghc -O2 --make qsort.hs && ./qsort +RTS -sstderr -RTS > /dev/null [1 of 1] Compiling Main ( qsort.hs, qsort.o ) Linking qsort ... ./qsort +RTS -sstderr (1.0,"iv") (1.1896770400256864,"v") (1.3091609772011856,"treeSort") (1.592515715933153,"vii") (1.5953543402198838,"vi") (1.5961286512637272,"iii") (1.8175480563244177,"i") (1.8771604568641642,"ii") (2.453160634439497,"mergeSort") (2.6627090768870216,"sort") 26,094,674,624 bytes allocated in the heap 12,716,656,224 bytes copied during GC (scavenged) 2,021,104,592 bytes copied during GC (not scavenged) 107,225,088 bytes maximum residency (140 sample(s)) 49773 collections in generation 0 ( 21.76s) 140 collections in generation 1 ( 23.61s) 305 Mb total memory in use INIT time 0.00s ( 0.00s elapsed) MUT time 20.39s ( 20.74s elapsed) GC time 45.37s ( 46.22s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 65.76s ( 66.96s elapsed) %GC time 69.0% (69.0% elapsed) Alloc rate 1,279,723,644 bytes per MUT second Productivity 31.0% of total user, 30.5% of total elapsed --- cut here --- {-# OPTIONS_GHC -O2 #-} module Main where import System.CPUTime import System.IO import System.Environment import System.Random import Data.List( partition, sort ) data Tree a = Node (Tree a) a (Tree a) | Leaf qsort_i [] = [] qsort_i (x:xs) = qsort_i (filter (< x) xs) ++ [x] ++ qsort_i (filter (>= x) xs) qsort_ii [] = [] qsort_ii (x:xs) = let (ls,gt) = partition (< x) xs in qsort_ii ls ++ [x] ++ qsort_ii gt qsort_iii xs = qsort_iii' [] xs qsort_iii' acc [] = acc qsort_iii' acc (x:xs) = let (ls,gt) = partition (< x) xs in let acc' = (x:(qsort_iii' acc gt)) in qsort_iii' acc' ls qsort_v [] = [] qsort_v (x:xs) = let (xlt, xgt ) = foldl (\ (lt,gt) el -> case compare x el of GT -> (el:lt, gt) _ -> (lt, el:gt) ) ([],[]) xs in qsort_v xlt ++ [x] ++ qsort_v xgt -- zmodyfikowany i qsort_vi [] = [] qsort_vi (x:xs) = qsort_vi (filter (\y-> compare x y == GT) xs) ++ [x] ++ qsort_vi (filter (\y-> compare x y /= GT) xs) -- zmodyfikowany iii qsort_vii xs = qsort_vii' [] xs qsort_vii' acc [] = acc qsort_vii' acc (x:xs) = let (ls,gt) = partition (\y-> compare x y == GT) xs in let acc' = (x:(qsort_vii' acc gt)) in qsort_vii' acc' ls -- qsort is stable and does not concatenate. qsort_iv xs = qsort_iv' (compare) xs [] qsort_iv' _ [] r = r qsort_iv' _ [x] r = x:r qsort_iv' cmp (x:xs) r = qpart cmp x xs [] [] r -- qpart partitions and sorts the sublists qpart cmp x [] rlt rge r = -- rlt and rge are in reverse order and must be sorted with an -- anti-stable sorting rqsort_iv' cmp rlt (x:rqsort_iv' cmp rge r) qpart cmp x (y:ys) rlt rge r = case cmp x y of GT -> qpart cmp x ys (y:rlt) rge r _ -> qpart cmp x ys rlt (y:rge) r -- rqsort is as qsort but anti-stable, i.e. reverses equal elements rqsort_iv' _ [] r = r rqsort_iv' _ [x] r = x:r rqsort_iv' cmp (x:xs) r = rqpart cmp x xs [] [] r rqpart cmp x [] rle rgt r = qsort_iv' cmp rle (x:qsort_iv' cmp rgt r) rqpart cmp x (y:ys) rle rgt r = case cmp y x of GT -> rqpart cmp x ys rle (y:rgt) r _ -> rqpart cmp x ys (y:rle) rgt r -- code by Orcus -- Zadanie 9 - merge sort mergeSort :: Ord a => [a] -> [a] mergeSort [] = [] mergeSort [x] = [x] mergeSort xs = let(l, r) = splitAt (length xs `quot` 2) xs in mergeSortP (mergeSort l) (mergeSort r) -- funkcja pomocnicza scalajÄ…ca dwie listy uporzÄ…dkowane w jednÄ… mergeSortP :: Ord a => [a] -> [a] -> [a] mergeSortP xs [] = xs mergeSortP [] ys = ys mergeSortP (x:xs) (y:ys) | x <= y = x:(mergeSortP xs (y:ys)) | otherwise = y:(mergeSortP (x:xs) ys) -- Zadanie 10 - tree sort treeSort :: Ord a => [a] -> [a] -- pointless po raz drugi treeSort = (treeSortInorder . treeSortToTree) treeSortToTree :: Ord a => [a] -> Tree a treeSortToTree [] = Leaf treeSortToTree (x:xs) = let (xlt, xgt) = foldl (\ (lt,gt) el -> case compare x el of GT -> (el:lt, gt) _ -> (lt, el:gt) ) ([],[]) xs in Node (treeSortToTree xlt) x (treeSortToTree xgt) treeSortInorder :: Ord a => Tree a -> [a] treeSortInorder Leaf = [] treeSortInorder (Node l x r) = (treeSortInorder l) ++ [x] ++ (treeSortInorder r) -- end code by Orcus -- big_number = 1000000 :: Int main = do gen <- getStdGen let xs' = randomRs (1::Int, big_number) gen xs <- return (take big_number xs') t1 <- getCPUTime print (qsort_i xs) -- i t2 <- getCPUTime print (qsort_ii xs) -- ii t3 <- getCPUTime print (qsort_iii xs) -- iii t4 <- getCPUTime print (qsort_iv xs) -- iv t5 <- getCPUTime print (qsort_v xs) -- v t6 <- getCPUTime print (qsort_vi xs) -- vi t7 <- getCPUTime print (qsort_vii xs) -- vii t8 <- getCPUTime print (sort xs) -- sort t9 <- getCPUTime print (mergeSort xs) -- mergeSort t10 <- getCPUTime print (treeSort xs) -- treeSort t11 <- getCPUTime let getTimes xs = zipWith (-) (tail xs) xs let timers = [t1,t2,t3,t4,t5,t6,t7,t8,t9,t10,t11] let times = getTimes timers let table = zip times ["i","ii","iii","iv", "v", "vi", "vii", "sort","mergeSort","treeSort"] let sorted = sort table let scaled = map (\(x,n) -> (((fromIntegral x / (fromIntegral $ fst (head sorted)))::Double),n)) sorted let toShow = concatMap (\x-> show x ++ "\n") scaled hPutStr stderr toShow main_small = do gen <- getStdGen let xs' = randomRs (1::Int, 100000) gen xs <- return (take big_number xs') t1 <- getCPUTime print (qsort_iv xs) -- iv t2 <- getCPUTime print (sort xs) -- sort t3 <- getCPUTime print (mergeSort xs) -- mergeSort t4 <- getCPUTime print (treeSort xs) -- treeSort t5 <- getCPUTime let getTimes xs = zipWith (-) (tail xs) xs let timers = [t1,t2,t3,t4,t5] let times = getTimes timers let table = zip times ["iv", "sort","mergeSort","treeSort"] let sorted = sort table let scaled = map (\(x,n) -> (((fromIntegral x / (fromIntegral $ fst (head sorted)))::Double),n)) sorted let toShow = concatMap (\x-> show x ++ "\n") scaled hPutStr stderr toShow hPrint stderr times --- cut here ---

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semi-closed handles
by Abhay Parvate 23 Apr '08

23 Apr '08

Hello, In describing the Handle type, the GHC documentation says (in the System.IO documentation): GHC note: a Handle will be automatically closed when the garbage collector detects that it has become unreferenced by the program. However, relying on this behaviour is not generally recommended: the garbage collector is unpredictable. If possible, use explicit an explicit hClose to close Handles when they are no longer required. GHC does not currently attempt to free up file descriptors when they have run out, it is your responsibility to ensure that this doesn't happen. But one cannot call hClose on Handles on which something like hGetContents has been called; it just terminates the character list at the point till which it has already read. Further the manual says that hGetContents puts the handle in the semi-closed state, and further, A semi-closed handle becomes closed: - if hClose is applied to it; - if an I/O error occurs when reading an item from the handle; - or once the entire contents of the handle has been read. So do I safely assume here, according to the third point above, that it's fine if I do not call hClose explicitly as far as I am consuming all the contents returned by hGetContents? Thanks, Abhay

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Anyone mind proofing a short monad transformers explanation?
by Jack Kelly 22 Apr '08

22 Apr '08

I sent this to a friend who asked me about monad transformers and he reckons it should be saved somewhere. If anyone feels up to giving it a quick fact-check, that'd be great. I'm not sure what to do with it, though. It doesn't seem to be made of the right stuff for a tutorial but I'm not sure what to do with it. --- SNIP --- Basically it's like making a double, triple, quadruple, ... monad by wrapping around existing monads that provide wanted functionality. You have an innermost monad (usually Identity or IO but you can use any monad). You then wrap monad transformers around this monad to make bigger, better monads. Concrete example: Suppose I was writing a server. Each client thread must be of type IO () (that's because forkIO :: IO () -> IO ThreadID)). Suppose also that this server has some configuration that (in an imperative program) would be global that the clients all need to query. > data Config = Config Foo Bar Baz One way of doing this is to use currying and making all the client threads of type Config -> IO (). Not too nice because any functions they call have to be passed the Config parameter manually. The Reader monad solves this problem but we've already got one monad. We need to wrap IO in a ReaderT. The type constructor for ReaderT is ReaderT r m a, with r the shared environment to read from, m the inner monad and a the return type. Our client_func becomes: > client_func :: ReaderT Config IO () We can then use the ask, asks and local functions as if Reader was the only Monad: (these examples are inside do blocks) > p <- asks port (Assuming some function port :: Config -> Int or similar.) To do stuff in IO (or in the general case any inner monad) the liftIO function is used to make an IO function work in this wrapped space: (given h :: Handle, the client's handle) > liftIO $ hPutStrLn h "You lose" > liftIO $ hFlush h IO is once again special. For other inner monads, the lift function does the same thing. Note also that IO has no transformer and must therefore always be the innermost monad. This is all well and good, but the client_func now has type ReaderT Config IO () and forkIO needs a function of type IO (). The escape function for Reader is runReader :: Reader r a -> r -> a and similarly for ReaderT the escape function is runReaderT :: ReaderT r m a -> r -> m a: (Given some c :: Config that's been assembled from config files or the like) > forkIO (runReaderT client_func c) Will do the trick. Monad transformers are like onions. They make you cry but you learn to appreciate them. Like onions, they're also made of layers. Each layer is the functionality of a new monad, you lift monadic functions to get into the inner monads and you have transformerised functions to unwrap each layer. They're also like a present in that regard: in this example we unwrapped the outer wrapping paper to get to the present: an object of type IO (), which lets us make haskell do something.

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GSoC accepted projects
by Malcolm Wallace 22 Apr '08

22 Apr '08

Haskell.org is pleased to announce that we received 7 funded student slots from Google for this year's Summer of Code. Of the 30 proposals we received, the mentoring group chose the following 7: * GHC API Improvements by Thomas Schilling, mentored by Simon Marlow * Dynamically Loaded Plugins for the Glasgow Haskell Compiler by Maximilian Conroy Bolingbroke, mentored by Sean Seefried * Haskell API Search as an interface to Hackage by Neil Mitchell, mentored by Niklas Broberg * Language.C, a standalone parser/pretty printer library for C99 by Benedikt Huber, mentored by Iavor S. Diatchki * Cabal 'make-like' dependency framework by Andrea Vezzosi, mentored by Duncan Coutts * #1560 Efficient maps using generalised tries by Jamie Brandon, mentored by Adrian Charles Hey * Data parallel physics engine by Roman Cheplyaka, mentored by Manuel M. T. Chakravarty Slightly fuller details at http://code.google.com/soc/2008/haskell/about.html Congratulations to these students! Before coding begins officially at the end of May, there is a "community bonding period", where the students and community are encouraged to interact and further refine the work that will be done. To those students that did not get funding: do not be disheartened. Almost all of the proposals were good, and we had some difficult choices to make. What's more, since this is Open Source, you can continue to work on your projects anyway! Many of them will be useful to the community. In fact, Google have said that if any GSoC applicant manages (in the judgement of the mentors) to complete their project by the end of the summer, despite not receiving a funded place, they will send you a goodie-bag of Google swag (including a standard Google T-shirt, not the special GSoC one). Regards, Malcolm

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Re: [Haskell] ANNOUNCE: Galois web libraries for Haskell released
by Henrique Ferreiro García 22 Apr '08

22 Apr '08

> * feed > Interfacing with RSS (v 0.9x, 2.x, 1.0) and Atom feeds Great! I had planned to make a feed reader using gtk2hs and this was exactly what I was waiting for. I have just seen that it is released as BSD3. Anyone knows if is there any problem in making use of it to develop a GPLv3 application? -- Henrique Ferreiro García <hferreiro(a)udc.es>

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Wrong Answer Computing Graph Dominators
by Denis Bueno 22 Apr '08

22 Apr '08

I'm using the Data.Graph.Inductive.Query.Dominators library (http://haskell.org/ghc/docs/latest/html/libraries/fgl/Data-Graph-Inductive-…) with GHC 6.8.2. The library is a bit spare on comments, so I may or may not be using it correctly. My goal is to compute the set of nodes that dominate two different nodes of a DAG, with respect to one initial node. My approach is: import qualified Data.Graph.Inductive.Graph as Graph import qualified Data.Graph.Inductive.Query.Dominators as Dom uips = intersect domConfl domAssigned :: [Graph.Node] -- my goal domConfl = fromJust $ lookup conflNode domFromLastd -- dominators of first node domAssigned = fromJust $ lookup (negate conflNode) domFromLastd -- dominators of second node domFromLastd = Dom.dom conflGraph lastd -- source node I compute the dominators individually and my answer is their intersection. When I call the dom function, my assumption is that I need to pass it the source node, and it tells me, for any node x to which there is a path from the source, the list of nodes for which *any path* from source to x must touch, i.e., the list of dominators of x. The DAG in question is attached as a postscript file, which is generated by `dot -Tps` using the output of the Graphviz module directly from the graph object. The nodes for which I'm computing dominators are labeled -2 and 2 (that is, conflNode = 2). The problem is that I think the answer is wrong. In particular I think the set of dominators for -2 is {20, -2}, and the set for 2 is {20, 2}. Their intersection is {20}, which is what I expect. But what I am getting is: --> uips = [-9,20] --> domConfl = [2,-9,20] --> domAssigned = [-2,-9,-12,20] --> lastd = 20 But -9 is not a dominator for 2, since 20,-7,8,6,2 is a path from 20 to 2 that does not touch 9. -9 is also not a dominator for -2, since 20,-7,8,6,-2 is a path from 20 to -2 not touching 9. Am I missing something? I've simplified the code above slightly from the original code in order to ignore some irrelevancies. The original code is for computing a learned clause in a SAT solver. The code in the state that will reproduce the error above is available via a git clone: git clone http://fid.stygian.net/dpllsat Build via cabal, configuring with --disable-optimization in order to enable assertions, then run: ./dist/build/dsat/dsat ./tests/problems/uf20/uf20-0226.cnf The graph should be saved in conflict.dot and with `dot -Tps -o conflict.ps conflict.dot` you should be able to turn it into the ps file attached. The problematic code referenced above starts on line 759. -- Denis

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Hoogle
by Galchin, Vasili 22 Apr '08

22 Apr '08

Hello, I tried d to use Hoogle to find openFd's signature and more importantly FileMode. I found FileMode which is a type synonym with CMode. I don't understand what are the "values" for FileMode (so I can call openFd). ?? Thanks, Vasili

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Functional board games
by Dougal Stanton 22 Apr '08

22 Apr '08

I'm having a go at making a functional board game (the back-end logic for one, at least) and as with all good projects it raises lots of questions. But I'll keep it to one this time. Does anyone know of functional-style implementations of chess/draughts/go/anything else that might give me ideas? I am writing a game of Thud (yes, from the Terry Pratchett book...) but I don't hold much hope of their being a functional-style Thud game already in existence! Cheers, D. -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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Help with associated types
by Emil Axelsson 21 Apr '08

21 Apr '08

Hello! I'm trying to rewrite some FD classes to use associated types instead. The Port class is for type structures whose leaves have the same type: class Port p where type Leaf p type Struct p toList :: p -> [Leaf p] fromList :: [Leaf p] -> p (Leaf p) gives the leaf type, and (Struct p) gives a canonical representation of the structure regardless of leaf type. Here we just instantiate two leaf types: instance Port Int where type Leaf Int = Int type Struct Int = () toList a = [a] fromList [a] = a instance Port Bool where type Leaf Bool = Bool type Struct Bool = () toList a = [a] fromList [a] = a There's also a function for mapping over ports: mapPort :: ( Port pa , Port pb , Struct pa ~ Struct pb ) => (Leaf pa -> Leaf pb) -> (pa -> pb) mapPort f = fromList . map f . toList The equality constraint makes sure that we're mapping between equal structures. When I try to run this, I get: *Main> mapPort even (5::Int) <interactive>:1:8: No instance for (Integral (Leaf Int)) ... because as it stands, mapPort is not able to infer (pb = Bool) from (Struct pb = ()) and (Leaf pb = Bool). What's the easiest way to encode that pb can be inferred from (Struct pb) and (Leaf pb)? Thanks, / Emil PS. I used to have a class class SameStruct pa a pb b | pa -> a, pa b -> pb, pb -> b, pb a -> pa In the example above, we'd have pa=Int and b==Bool, so the second dependeny would infer pb=Bool.

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Announce: bytestring 0.9.1.0
by Don Stewart 21 Apr '08

21 Apr '08

Hey all, I'm pleased to announce a new major release of bytestring, the efficient string library for Haskell, suitable for high-performance scenarios. This release is primarily an (incremental) performance improvement release, though with some notable significant improvements, along with long term test coverage and quality control changes. Highlights: * a long term performance bug with Ord instances, involving very small strings, and Data.Map has been squashed. * everything's a little faster -- shootout problems showed a 1-5% speedup just by switching to the new library. Thanks goes to the Hac4 Haskell Hackathon organisers, in Gothenburg, Sweden, where the majority of this work to create this release took place. Key changes: * Data.Map short key performance greatly improved: - 'words Map' running time: 6.310s bytestring 0.9.0.1 1.071s bytestring 0.9.1.0 * Uses cheaper unsafeDupablePerformIO for allocation. - tail recursive tight loops (fixes obscure stack overflow) * Generally faster: - Shootout sum-file: 1.218s to 1.190s - Shooout fasta: 9.210s to 8.811s * 4-5x faster small substring search (breakSubstring/findSubstring/isInfixOf). * Extensive QuickCheck coverage reporting and improvements: - http://code.haskell.org/~dons/tests/bytestring/hpc_index.html Get the code: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring Note that if you upgrade to the new bytestring release, older packages built against previous releases will still require the old bytestring package. For best results, rebuild any bytestring-depending packages against the new library only. Cheers, Don

5 6

letting go of file handles and Data.Binary
by Ben 21 Apr '08

21 Apr '08

hi all -- using binary 0.4.1 on ghc 6.8.2, vista 64 sp1. consider the following program: import System.Directory import Data.Binary main = do let dat = [1..10]::[Int] fname = "foo.dat" encodeFile fname dat dat2 <- decodeFile fname print (dat == dat2) removeFile fname this throws a permission denied exception, presumably because the file is still open when the removeFile is called. i've grovelled the source of Data.Bytestring.Lazy and all but i can't seem to understand the "right" way to make this work. note that i've forced the evaluation of dat2 and presumably therefore the filehandle is at least half-closed. any suggestions? take care, Ben

4 9

ANN: HXQ: An XQuery Compiler/Interpreter for Haskell
by Fegaras, Leonidas 21 Apr '08

21 Apr '08

Dear Haskell cafe members, I would like to announce the release of an XQuery compiler/interpreter for Haskell, called HXQ. You can get more info and download it at: http://lambda.uta.edu/HXQ/ HXQ is a fast and space-efficient translator from XQuery (the standard query language for XML) to embedded Haskell code. The translation is based on Haskell templates. HXQ takes full advantage of Haskell's lazy evaluation to keep in memory only those parts of XML data needed at each point of evaluation, thus performing stream-based evaluation for forward queries (queries that do not contain backward steps). This results to an implementation that is as fast and space-efficient as any stream-based implementation based on SAX filters or finite state machines. Furthermore, the coding is far simpler and extensible since its based on XML trees, rather than SAX events. For example, a complex XQuery against the DBLP bibliography (420MB XML) runs in 39 seconds on my laptop (using 18MB of max heap space in ghc). To contrast this, Qexo, which compiles XQueries to Java bytecode, took 1 minute 17 seconds (using no less than 1400MB of heap space). Also XQilla, which is written in C++, took 1 minute and 10 secs (using 1150MB heap). Current Status: HXQ supports most essential XQuery features, although some system functions are missing (but are easy to add). HXQ has little support for backward step axes, such as /.. (parent). Some, but not all, parent axis steps are removed using optimization rules; all others cause a compilation error. Finally, HXQ does not comply to the XQuery semantics that requires duplicate elimination and sorting by document order for every XPath step, which is very expensive and unnecessary in most cases. Please email me any suggestions for improvements or extensions you'd like to see. My plan is to use it as a base for various XQuery projects, such as adding SQL connectivity, as well as native storage and indexing. Best, Leonidas Fegaras U. of Texas at Arlington http://lambda.uta.edu/

1 0

Parallel weirdness
by Andrew Coppin 21 Apr '08

21 Apr '08

OK, so just for fun, I decided to try implementing a parallel merge sort using the seq and par combinators. My plan was to generate some psuedo-random data and time how long it takes to sort it. To try to account for lazy evaluation, what the program actually does is this: 1. Write the input data to disk without any sorting. (This ought to force it to be fully evaluated.) 2. Sort and save the data to disk 8 times. (So I can average the runtimes.) This is done with two data sets - one with 1 million items, and another with 2 million rows. Each data set is run through both the purely sequential algorithm and a simple parallel one. (Split the list in half, merge-sort each half in parallel, and then merge them.) The results of this little benchmark utterly defy comprehension. Allow me to enumerate: Weird thing #1: The first time you sort the data, it takes a few seconds. The other 7 times, it takes a split second - roughly 100x faster. Wuh? Weird thing #2: The parallel version runs *faster* than the sequential one in all cases - even with SMP disabled! (We're only talking a few percent faster, but still.) Weird thing #3: Adding the "-threaded" compiler option makes *everything* run a few percent faster. Even with only 1 OS thread. Weird thing #4: Adding "-N2" makes *everything* slow down a few percent. In particular, Task Manager shows only one CPU core in use. Adding more than 2 OS threads makes everything slow down even further - but that's hardly surprising. Can anybody explain any of this behaviour? I have no idea what I'm benchmarking, but it certainly doesn't appear to be the performance of a parallel merge sort!

7 22

C++ interface with Haskell
by Miguel Lordelo 20 Apr '08

20 Apr '08

Hi all, Well...somehow I'm a beginner in Haskell. But actually my interest in Haskell will increase if it is possible to call a haskell function in C++. Something like GreenCard ( http://www.haskell.org/greencard/ ) simplifying the task of interfacing Haskell programs to external libraries (usually). But is there also a task to interface a foreign language with Haskell, but calling Haskell functions. Or c2hs which is an interface generator that simplifies the development of Haskell bindings to C libraries. I want to know this, because in my company some guys are doing some testing with Frotran and MatLab and I want to show them the power of haskell and the software which we are using is implemented in C++ (there is the reason to make Haskel -> C++). I read somewhere that the only way for C++ calling a haskell function is to create a binding between Haskell and C and from C to C++, but a easy "Hello World" example was not there. Unfortunatelly I couldn't found anything usefull, like an complete example, or how to compile the code from haskell to C to C++. Can sombody help me, please :P Chears, Miguel Lordelo.

7 14

Problems with "Bananas"
by Creighton Hogg 20 Apr '08

20 Apr '08

Hi, This isn't about Haskell per se, but I was reading the old Meijer et al. paper "Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire" & I think there's a notational pun that's really confusing me. On page 12 we have the CataEval equation (|phi|) . in = phi . (|phi|)_L Now, the subscript L & the following example of cons lists implies that L is a functor in this equation, yet the line immediately after this equation says that "(CataEval) states how to evaluate an application of (|phi|) to an arbitrary element of L...." so then that makes it sound like the L here is the fixed point of some functor F, not the functor itself. I'm sure I'm just being dumb, but this is really bugging me. Thanks, Creighton Hogg

2 1

Strange message from GHC
by Chris Smith 20 Apr '08

20 Apr '08

I'm running into this in some code I wrote. What does it mean? It says to look at -fspec-constr-count, but that flag doesn't seem to be in the GHC documentation. This isn't critical; the code still seems to work fine. It just makes the build uglier. Thanks. Message below. SpecConstr: too many specialisations for one function (see -fspec-constr- count): Function: main:Expr.simplifyTerm{v rqC} [lid] Specialisations: [([sc_s2wb{v} [lid], sc_s2wc{v} [lid], sc_s2wd{v} [lid], sc_s2we{v} [lid], sc_s2wf{v} [lid], sc_s2wg{v} [lid]], [main:Expr.Graph{v r51} [gid] sc_s2wg{v} [lid] sc_s2wf{v} [lid] sc_s2we{v} [lid] sc_s2wd{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Edge{v r4Q} [gid] sc_s2wc{v} [lid]) sc_s2wb{v} [lid]]), ([sc_s2wh{v} [lid], sc_s2wi{v} [lid], sc_s2wj{v} [lid], sc_s2wk{v} [lid], sc_s2wl{v} [lid], sc_s2wm{v} [lid]], [main:Expr.Graph{v r51} [gid] sc_s2wm{v} [lid] sc_s2wl{v} [lid] sc_s2wk{v} [lid] sc_s2wj{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Ghost{v r4P} [gid] sc_s2wi{v} [lid]) sc_s2wh{v} [lid]]), ([sc_s2wR{v} [lid], sc_s2wS{v} [lid], sc_s2wT{v} [lid]], [sc_s2wT{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Vertex{v r4R} [gid] sc_s2wS{v} [lid]) sc_s2wR{v} [lid]]), ([sc_s2wU{v} [lid], sc_s2wV{v} [lid], sc_s2wW{v} [lid]], [sc_s2wW{v} [lid], base:GHC.Base.:{(w) v 65} [gid] @ main:Expr.Gen{tc r53} (main:Expr.Vertex{v r4R} [gid] (base:GHC.Base.I#{(w) v 6d} [gid] sc_s2wV{v} [lid])) sc_s2wU{v} [lid]])]

2 1

"publishing" a Haskell package
by Galchin, Vasili 20 Apr '08

20 Apr '08

Hello, In the past someone sent me a URL on how to formally "publish" Haskell work for public review and consumption. Does anyone have his link? Regards, Vasili

2 1

Re. intro to FDs - thanks!
by Alexis Hazell 19 Apr '08

19 Apr '08

Thanks to all who helped me with my query about functional dependencies - much appreciated. And special thanks to Bulat for reminding me that the GHC user guide is more than just a user guide. :-) Alexis.

1 0

HTTP and file upload
by Adam Smyczek 19 Apr '08

19 Apr '08

Is form based file upload supported in HTTP module (HTTP-3001.0.4)? Adam

3 3

RFC: A standardized interface between web servers and applications or frameworks (ala WSGI)
by Johan Tibell 19 Apr '08

19 Apr '08

Good day hackers, The Python community have been successful in standardizing an interface between web server and applications or frameworks resulting in users having more control over their web stack by being able to pick frameworks independently from web servers, and vice versa. I propose we try to do the same for Haskell. I've written half a draft for a Haskell version of Python's PEP 333 [1]. If you're interested in taking part in this effort please read through the Python spec first (as it is way more complete and you can understand this proposal better by reading it, I've skipped some important issues in my first draft) and then go read the Haskell spec [2]. I'm particularly interesting in feedback regarding: * Doing in this way won't work as it violates HTTP/CGI spec part X, Y and Z (the Python spec takes lots of things from the CGI spec including naming and semantics). * My server/framework could never provide/be run under this interface. * This interface has bad performance by design. * Using a different set of data types would work better. The spec needs to be extended to cover all the corners of HTTP. Some parts need to be motivated better. It is easier for me to motivate things if people would tell me what parts are badly motivated. Note: I'm open to a complete rewrite if needed. I'm not wedded to the current design and/or wording. In fact parts of the wording is borrowed from the Python spec. The parts with bad grammar are all mine. 1. http://www.python.org/dev/peps/pep-0333/ 2. http://www.haskell.org/haskellwiki/WebApplicationInterface -- Johan

8 9

[ANN] cabal-rpm 0.4
by Bryan O'Sullivan 19 Apr '08

19 Apr '08

I've just uploaded version 0.4 of cabal-rpm to Hackage. This is a program that generates an RPM package from a Cabal package. RPM is the package format used by several major Linux distributions. New in this version are support for GHC 6.8.2 and the Cabal 1.2 release series. Download: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-rpm-0.4 Source: darcs get http://darcs.serpentine.com/cabal-rpm <b

1 0

ANNOUNCE: cpuid 0.2 - Binding for the cpuid machine instruction
by Martin Grabmueller 18 Apr '08

18 Apr '08

Hello fellow Haskellers, I have just uploaded my new package cpuid to Hackage. Description: This module provides the function 'cpuid' for accessing information about the currently running IA-32 processor. Both a function for calling the 'cpuid' instruction directly, and some convenience functions for common uses are provided. This package is only portable to IA-32 machines. Home page: http://uebb.cs.tu-berlin.de/~magr/projects/cpuid/doc/ Hackage page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cpuid-0.2 I'm looking forward to comments, improvements and bug fixes. I hope it's useful to you! Happy Haskell Hacking, Martin

1 0

GC'ing file handles and other resources
by Conal Elliott 18 Apr '08

18 Apr '08

Are Haskell folks satisfied with the practical necessity of imperatively & explicitly reclaiming resources such as file handles, fonts & brushes, video memory chunks, etc? Doesn't explicit freeing of these resources have the same modularity and correctness problems as explicit freeing of system memory (C/C++ programming)? I wrote a lovely purely functional graphics library that used video memory to lazily compute and cache infinite-resolution images, and I found that I don't know how to get my finalizers to run anytime soon after video memory chunks become inaccessible. Explicit freeing isn't an option, since the interface is functional, not imperative (IO). I guess I'm wondering a few things: * Are Haskell programmers generally content with imperative and bug-friendly interfaces involving explicit freeing/closing of resources? * Do people assume that these resources (or handling them frugally) aren't useful in purely functional interfaces? * Are there fundamental reasons why GC algorithms cannot usefully apply to resources like video memory, file descriptors, etc? * Are there resource management techniques that have the flexibility, efficiency, and accuracy of GC that I could be using for these other resources? Thanks, - Conal 2008/4/14 Abhay Parvate <abhay.parvate(a)gmail.com>: > Hello, > > In describing the Handle type, the GHC documentation says (in the > System.IO documentation): > > GHC note: a Handle will be automatically closed when the garbage collector > detects that it has become unreferenced by the program. However, relying on > this behaviour is not generally recommended: the garbage collector is > unpredictable. If possible, use explicit an explicit hClose to close Handles > when they are no longer required. GHC does not currently attempt to free up > file descriptors when they have run out, it is your responsibility to ensure > that this doesn't happen. > > But one cannot call hClose on Handles on which something like hGetContents > has been called; it just terminates the character list at the point till > which it has already read. Further the manual says that hGetContents puts > the handle in the semi-closed state, and further, > > A semi-closed handle becomes closed: > > - if hClose is applied to it; > - if an I/O error occurs when reading an item from the handle; > - or once the entire contents of the handle has been read. > > So do I safely assume here, according to the third point above, that it's > fine if I do not call hClose explicitly as far as I am consuming all the > contents returned by hGetContents? > > Thanks, > Abhay > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

5 7

Separate a string into a list of strings
by Sara Kenedy 18 Apr '08

18 Apr '08

Hi all, I want to write a function to separate a string into a list of strings separated by commas. Example: separate :: String -> [String] separate "Haskell, Haskell, and Haskell" = ["Haskell", "Haskell", "and Haskell"] If anyone has some ideas, please share with me. Thanks. S.

14 35

incompatibility issues in installing wxHaskell 0.10.3 with GHCi 6.8.2 on Windows XP Pro
by Benjamin L. Russell 18 Apr '08

18 Apr '08

Perhaps this is not the best mailing list for this issue, but since this problem involves a library for Haskell and is not a bug of GHC proper, I thought this would be a better place than the Glasgow-haskell-bugs mailing list.... I tried installing wxHaskell 0.10.3 with GHCi 6.8.2 already installed on Windows XP Professional Service Pack 2, but every time I run the wxhaskell-register.bat script to install the library, the following error message appears (I am installing on the Japanese version of Windows XP at work, so some of the error messages are in Japanese, which I shall translate accordingly below): ----- error message begins immediately after this line ----- 1 ÂÌt@CðRs[µÜµ½B R}hÌ\¶ªëÁÄ¢Ü·B R}hÌ\¶ªëÁÄ¢Ü·B - register packages Reading package info from "C:\\Documents and Settings\\Benjamin\\My Documents\\H askell\\Libraries\\wxHaskell\\wxhaskell-0.10.3\\bin\\wxcore.pkg" ... done. WARNING: unversioned dependencies are deprecated, and will NOT be accepted by GH C 6.10: base haskell98 ghc-pkg: C:\Documents doesn't exist or isn't a directory (use --force to overrid e) Reading package info from "C:\\Documents and Settings\\Benjamin\\My Documents\\H askell\\Libraries\\wxHaskell\\wxhaskell-0.10.3\\bin\\wx.pkg" ... done. ghc-pkg: dependency wxcore-0.10.3 doesn't exist (use --force to override) error: Unable to register the package using "ghc-pkg". Maybe you have an incompatible version of ghc installed? ±s·éÉÍ½©L[ðµÄ¾³¢ . . . ----- error message ends immediately before this line ----- The first paragraph above means the following: " 1 file copied. Syntax error in command. Syntax error in command." The final line above means the following: "Press any key to continue...." However, there do not seem to be any errors in my unmodified wxhaskell-register.bat file, quoted below: ----- text of wxhaskell-register.bat file begins immediately after this line ----- @SET wxclibname=wxc-msw2.6.4-0.10.3 @SET generate=yes @ECHO OFF IF "%1"=="" GOTO noarg IF "%1"=="--help" GOTO help IF "%1"=="-?" GOTO help IF "%1"=="/?" GOTO help GOTO installarg :installarg SET installdir=%1 GOTO check :noarg COPY /B /Y setcd setcd.bat CD .. CD >> bin\setcd.bat CD bin CALL setcd.bat DEL setcd.bat SET installdir=%CURDIR% GOTO check :check IF "%generate%"=="yes" GOTO generate GOTO checkfile :generate IF NOT EXIST %installdir%\bin\wx-partial.pkg GOTO notfound COPY /Y %installdir%\bin\wx-partial.pkg %installdir%\bin\wx.pkg ECHO import-dirs:%installdir%\lib\imports >> %installdir%\bin\wx.pkg ECHO library-dirs:%installdir%\lib >> %installdir%\bin\wx.pkg COPY /Y %installdir%\bin\wxcore-partial.pkg %installdir%\bin\wxcore.pkg ECHO import-dirs:%installdir%\lib\imports >> %installdir%\bin\wxcore.pkg ECHO library-dirs:%installdir%\lib >> %installdir%\bin\wxcore.pkg GOTO checkfile :checkfile IF NOT EXIST "%installdir%\bin\wx.pkg" GOTO notfound :install ECHO - register packages SET wxhlibdir=%installdir%\lib ghc-pkg update "%installdir%\bin\wxcore.pkg" ghc-pkg update "%installdir%\bin\wx.pkg" IF ERRORLEVEL 1 GOTO regerror ECHO. IF "%OS%"=="Windows_NT" GOTO copyNT ECHO - copy %wxclibname%.dll to the windows system directory (%WINDIR%\system) COPY %installdir%\lib\%wxclibname%.dll %WINDIR%\system\%wxclibname%.dll GOTO checkcopy :copyNT ECHO - copy %wxclibname%.dll to the windows system directory (%WINDIR%\system32) COPY %installdir%\lib\%wxclibname%.dll %WINDIR%\system32\%wxclibname%.dll :checkcopy IF ERRORLEVEL 1 GOTO copyerror ECHO - done! GOTO end :regerror ECHO error: ECHO Unable to register the package using "ghc-pkg". ECHO Maybe you have an incompatible version of ghc installed? ECHO. goto end :copyerror ECHO error: ECHO Unable to copy dynamic link libraries to the system folder. ECHO This is not a real problem but you will have to add the wxHaskell ECHO library directory to your PATH environment variable. ECHO. ECHO The library directory is: %installdir%\lib ECHO. goto end :notfound ECHO error: ECHO Unable to find the wxHaskell packages (bin\wx.pkg) ECHO Maybe the installation directory is not properly specified? ECHO. GOTO help :help ECHO usage: ECHO wxhaskell-register [installdir] ECHO. ECHO (or just double click on the batch file from the explorer) ECHO. GOTO end :end PAUSE ----- text of wxhaskell-register.bat file ends immediately before this line ----- Any ideas? Has anybody else encountered this bug? Benjamin L. Russell

1 1

Shim: finding modules
by Graham Fawcett 18 Apr '08

18 Apr '08

Hi folks, I'm a newbie, so please forgive any terminological mistakes. I've been using Shim in Emacs with great success, but there's one issue I've encountered, and I don't know if it's configuration problem or something fundamental. Consider a module 'App' and submodules 'App.Front' and 'App.Back'. When editing code in App that depends on the submodules, Shim works well at resolving the location of the submodules, and validates my code. But when editing App.Front, which depends on App.Back, Shim fails to validate -- my working directory is App.Front, and Shim doesn't seem able to find the location of App.Back. I'd like to be able to tell Shim that 'App' is the root of my project, and to locate modules from that root. Is this possible? On a related note, it seems that Shim is moribund (though others have forked it, mostly for Vim support?). Given its utility, I'm surprised by this. Do you have other support tools that serve a similar role? (I know of haskell-mode, but the GHC API integration seems to be the feature that sets Shim apart.) Thanks, Graham

4 6

Announcing DarcsWatch (with link)
by Joachim Breitner 18 Apr '08

18 Apr '08

Hi, sorry for the spam, but what I forgot in the first mail: You can find DarcsWatch at: http://darcswatch.nomeata.de/ A lot of haskell-related projects use the darcs version control system. Darcs has the nice feature that you can easily submit a patch by e-mail, usually sent to the project maintainer or a mailing list. What has bothered me in the past was that I had to manually check whether my patch was applied yet. So this week, I wrote DarcsWatch. DarcsWatch receives submitted patches and compares them to the repositories it knows about, whether the patch is missing there or not. You either send your patches to DarcsWeb directly when submitting them, using CC, or DarcsWatch is subscribed to the project mailing list itself, which is the case for the darcs-devel and xmonad lists. Other than that, not much is to do, and you’ll find a nice overview of your patches with diffs and download links. For an example, see: http://darcswatch.nomeata.de/user_mail@joachim-breitner.de.html If you want to use DarcsWatch for your contributions to other projects, or for your own projects, have a look the documentation at http://darcs.nomeata.de/darcswatch/documentation.html There are probably still bugs around, and the problem of marking patches as obsolete is not really solved yet, so if you have ideas about it, mail me or come to #darcs on freenode. Patches are always welcome, you can get the source from DarcsWatch’s repository, found at http://darcs.nomeata.de/darcswatch/ Enjoy, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata(a)joachim-breitner.de

1 0

Ann: Emping 0.5 in Hackage
by Hans van Thiel 18 Apr '08

18 Apr '08

Emping is a utility which derives the shortest rules from a table of rules. For example, in a list of 8000 different mushrooms, it finds 21 single predicates that determine the mushroom is poisonous, and 23 that it is edible. But it also finds all combinations of two, three and more predicates that determine this property. Each shorter rule is a generalization of an original one. So, if r1 covers {a,b,c} and r2 {a,b}, then r2 implies r1 (r1 entails r2). The shorter rules are partially ordered. Thanks to the functional graphs library, which comes with ghc, the reduced rules are now displayed as graphs which can be viewed with a Graphviz viewer. Also new in 0.5 is that the reduced rules, and the reduced rules in an equivalence class, are ordered by length. Lastly, the GUI has been improved. Testing has also shown some bugs and issues, most importantly when the data contains ambiguous rules. Ambiguities are rules with the same antecedent, but a different consequent. You can test for them, but if the user turns the checks off and run Emping without correcting the data, the program could hang, in a special case. See http://home.telfort.nl/sp969709/emp/empug.html for more info and a few screenshots. Thank you all, Hans van Thiel

1 0

export question (GHC.Handle)
by Graham Fawcett 18 Apr '08

18 Apr '08

I notice in the source for GHC.Handle that certain functions (e.g. fdToHandle_stat) are in the export list, but are not actually exported (at least, it seems you cannot import them). What mechanism causes these functions to be "hidden", and are they still accessible in some way? Graham

3 5

Announcing DarcsWatch
by Joachim Breitner 18 Apr '08

18 Apr '08

Hi, (Hi again to those on haskell-cafe, but things have changed since the other announcement) A lot of haskell-related projects use the darcs version control system. Darcs has the nice feature that you can easily submit a patch by e-mail, usually sent to the project maintainer or a mailing list. What has bothered me in the past was that I had to manually check whether my patch was applied yet. So this week, I wrote DarcsWatch. DarcsWatch receives submitted patches and compares them to the repositories it knows about, whether the patch is missing there or not. You either send your patches to DarcsWeb directly when submitting them, using CC, or DarcsWatch is subscribed to the project mailing list itself, which is the case for the darcs-devel and xmonad lists. Other than that, not much is to do, and you’ll find a nice overview of your patches with diffs and download links. For an example, see: http://darcswatch.nomeata.de/user_mail@joachim-breitner.de.html If you want to use DarcsWatch for your contributions to other projects, or for your own projects, have a look the documentation at http://darcs.nomeata.de/darcswatch/documentation.html There are probably still bugs around, and the problem of marking patches as obsolete is not really solved yet, so if you have ideas about it, mail me or come to #darcs on freenode. Patches are always welcome, you can get the source from DarcsWatch’s repository, found at http://darcs.nomeata.de/darcswatch/ Enjoy, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata(a)joachim-breitner.de

1 0

Fwd: [Haskell-cafe] Hackage Haddock fails where local succeeds
by David Waern 18 Apr '08

18 Apr '08

2008/4/17, John Goerzen <jgoerzen(a)complete.org>: > On Thu April 17 2008 7:01:18 am Ross Paterson wrote: > > On Thu, Apr 17, 2008 at 06:45:40AM -0500, John Goerzen wrote: > > > My local haddock, 0.8, parses this file fine. Is Hackage running and > > > older version? Could it be upgraded? Or is there something else going > > > on here? > > > > No, it's running haddock 2.1.0, which understands all the GHC > > extensions, but is more picky about comments. > > > OK. I have been able to replicate this behavior with Haddock 2.0.0.0, which > is the latest according to www.haskell.org/haddock. Where did 2.1.0 come > from? > > Although I can replicate the behavior, I can't explain it. My reading of the > Haddock docs leads me to believe I have done the correct thing. Is this a > bug in Haddock or am I missing something? It's a bug. Comments on type synonyms were missed when Haddock support was added to the GHC parser. This might be fixed in future versions of GHC. David

1 0

Hackage Haddock fails where local succeeds
by John Goerzen 17 Apr '08

17 Apr '08

Hi folks, Over at http://hackage.haskell.org/packages/archive/MissingH/1.0.1.2/logs/failure/g… the build has failed in the Hackage step with this output: src/Data/BinPacking.hs:78:32: parse error on input `-- ^ The sizes of bins' haddock: Failed to check module: Data.BinPacking That section of code looks like this: {- | The primary type for bin-packing functions. These functions take a list of size of bins. If every bin is the same size, you can pass @(repeat binSize)@ to pass an infinite list of bins if the same size. Any surplus bins will simply be ignored. -} type BinPacker = (Num size, Ord size, Show size, Show obj) => [size] -- ^ The sizes of bins -> [(size, obj)] -- ^ The sizes and objects -> Either (BinPackerError size obj) [[(size, obj)]] -- ^ Either error or results (the last line got artificially wrapped in my mail client) My local haddock, 0.8, parses this file fine. Is Hackage running and older version? Could it be upgraded? Or is there something else going on here? -- John

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Intro to functional dependencies in Haskell?
by Alexis Hazell 17 Apr '08

17 Apr '08

Hi all, i'm having some trouble 'getting' functional dependencies in the Haskell context (although my understanding of them in the context of relational database theory isn't that great either). Could someone please point me to an introduction / tutorial in this regard? Thanks! Alexis.

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Mutually recursive types?
by Ron Alford 17 Apr '08

17 Apr '08

Here's the setup: I have a series of problems that use various logical connectives. The problem is that they're not all the same. So instead of creating one giant datatype (or duplicating much code), I'd like to assemble them like toy blocks. I've boiled down an example here: data LogicalConnective a = Not a | And [a] | Or [a] data BasicGoal a = Atomic String [Term] | Empty | Logical (LogicalConnective a) deriving (Show, Eq) data PreferenceGoal1 = Basic1 PreferenceGoal1 | Prefer1 PreferenceGoal1 This works OK, but PreferenceGoal1 is a dead end. I can't combine it with other connectives. So I try: data PreferenceGoal2 a = Basic2 (PreferenceGoal2 a) | Prefer2 (PreferenceGoal2 a) And this works fine, but seems impossible to explicitly type (ie, there is nothing to substitute for 'a' in a type declaration). Or am I wrong? Also, it could be that this is just an ugly way to represent things (it does require a huge number of constructors). Any suggestions? -Ron

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ANN: datapacker 1.0.0
by John Goerzen 17 Apr '08

17 Apr '08

Hi, I'm pleased to announce the first release of datapacker. datapacker is a tool to pack files into a minimum number of CDs, DVDs, or any other arbitrary bin. It groups file by size. It is designed to group files such that they fill fixed-size containers (called "bins") using the minimum number of containers. This is useful, for instance, if you want to archive a number of files to CD or DVD, and want to organize them such that you use the minimum possible number of CDs or DVDs. In many cases, datapacker executes almost instantaneously. Of particular note, the hardlink action can be used to effectively copy data into bins without having to actually copy the data at all. datapacker is a tool in the traditional Unix style; it can be used in pipes and call other tools. More information, including downloads and a full manual, can be found at: http://software.complete.org/datapacker datapacker is also available on Hackage.

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Module for parsing C (pre-processed header files only)?
by Magnus Therning 17 Apr '08

17 Apr '08

Is there such a beast out there? If this were a list on some OO language I'd ask for something that created an AST and then offered an API based around a visitor pattern. The pre-processor (platform specific most likely, but could be cpphs of course) would be run prior to the tool I'm considering writing. After that I only need access to declarations of C functions and definitions of types (structs, enums, other typedefs). I had a quick look at the code in c2hs yesterday, but found it a little hard to make heads or tails of it without some guidance, so I couldn't tell whether it would be suitable. Would it? Any help or pointers appreciated. /M

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Searching for possible PhD supervisors/projects
by Ivan Miljenovic 16 Apr '08

16 Apr '08

Next year, I'm thinking of going overseas (I'm currently in Australia) to do a PhD. Preferably, I'd like to do something in the area of computational combinatorics using Haskell. Does anyone know of any particular unis/supervisors I should be looking at/talking to about this? -- Ivan Lazar Miljenovic

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Funny State monad dependency
by Hans Aberg 16 Apr '08

16 Apr '08

When I load the State module in Hugs, then I can define the function f below, but I do not immediately see exactly what function "return" returns. Explanation welcome. For example: > f [2..4] [6..9] [6,7,8,9,6,7,8,9,6,7,8,9] That is, it just repeats the second argument as many times as the length of the second argument. Hans Aberg -------- import Control.Monad.State f :: Monad a => a b -> a c -> a c f x y = x >>= (return y) --------

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monadic debugging
by Galchin, Vasili 16 Apr '08

16 Apr '08

Hello, I have an Linux executable of my Haskell library and test case. I see there are several debuggers, e.g. Buddha, Hat, etc. Which debugger is currently preferred for monadic (imperative) code? Thanks. Vasili

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Installing HaXml
by rodrigo.bonifacio 15 Apr '08

15 Apr '08

Hi all, I've tried to install HaXml as explained in the documentation: > runhaskell Setup.hs configure However, I get as response: dyld: Library not loaded: GNUreadline.framework/Versions/A/GNUreadline Referenced from: /usr/local/bin/runhaskell Reason: image not found Trace/BPT trap Any idea about this problem? Some comments about my configuration: Mac/OS Darwin 8.11.1 GHC-6.6 Hugs - Version September 2006 Thanks in advance, Rodrigo.

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Re: [Haskell-cafe] Module for parsing C (pre-processed header files only)?
by Magnus Therning 15 Apr '08

15 Apr '08

On Tue, Apr 15, 2008 at 4:12 PM, Brian Sniffen <bts(a)evenmere.org> wrote: > 2008/4/15 Magnus Therning <magnus(a)therning.org>: > > Is there such a beast out there? > > Well, there's CIL (http://manju.cs.berkeley.edu/cil/) an OCaml > library attacking the same problem. It has a very positive > reputation. Interesting answer on a Haskell list ;) I've been toying with the idea of looking at OCaml though, so this might be just the opportunity I was waiting for... /M

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Help understanding sharing
by Patrick Surry 15 Apr '08

15 Apr '08

I'm new to Haskell and trying to get a better understanding of sharing (and ultimately memoization). I've read SOE and various of the tutorials, as well as browsing around the wiki and old mailing lists. Most of the examples of memoization seem to revolve around Fibonacci, and are based either on the fact that a list defined within the function will get shared between calls, or on doing some 'unsafeIO' (which I haven't dug too deeply into.) I've read various discussions that explain why function calls are generally not automatically memoized (e.g. f x gets recalculated rather than looked up based on the prior result). The rationale for that (big space leak and no guarantee of improved performance) makes sense. (Though I did like one poster's suggestion of a compiler pragma that hints that a particular function should be memoized.) I've seen other discussions that suggest that lists are always shared while in scope (so the fibs trick works). But is that just a feature of the standard compilers, or is it somewhere mandated in the Hakell spec (I don't see anything obvious in the Haskell Report tho haven't read it cover to cover)? The wiki page http://www.haskell.org/haskellwiki/Performance/Strictness says laziness == non-strictness + sharing but again nowhere gives a set of rules that guarantees what will be shared and what won't. I was hoping I might find it here: http://www.haskell.org/haskellwiki/Sharing but no such luck. Or are there no guarantees and you just have to know how your particular compiler works?? Cheers, Patrick ________________________________________________________________________ DISCLAIMER: This e-mail is intended only for the addressee named above. As this e-mail may contain confidential or privileged information, if you are not the named addressee, you are not authorised to retain, read, copy or disseminate this message or any part of it. If you received this email in error, please notify the sender and delete the message from your computer.

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retrospective on 'seq' -> 'unsafeSeq' ?
by Henning Thielemann 15 Apr '08

15 Apr '08

When reading the section "10.3 Controlling Evaluation Order" in "History of Haskell" I thought that the example that justified the 'seq' to be unrestricted polymorphic was mainly a debugging problem. I wondered if the better solution would have been to provide an 'unsafeSeq' which has no type restriction but must be absent from production code just like 'trace'. With 'unsafeSeq' one could nicely play around until time and space behaviour is as wanted and then one can replace all 'unsafeSeq's by 'seq' while adding type constraints accordingly. (Analogously there could be an unsafeShow that allows showing offending values in an 'error' without adding a Show constraint to the type signature.)

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Monad proof
by Rafael C. de Almeida 15 Apr '08

15 Apr '08

Hello, I was studying Monads and I was trying to think about new Monads I could define. So, a question poped into my mind: how is proof regarding the 3 Monad laws handled? I know that, aside from testing all the possible values (and there can be a lot of them), there's no general way to prove it. Nonetheless, I think that it would be insightful to see how people write those proofs for their monads -- specially for new user monads. Is there some article or some notes on proving that Monads are implemented correctly? []'s Rafael

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