July 2006 - Haskell-Cafe

XML transformation difficulties
by Greg Fitzgerald 17 Jul '06

17 Jul '06

I'm trying to think of a way to translate this input, to the output below: Input: <test> <foo name="a"> <wahoo>A</wahoo> </foo> <foo name="b"> <wahoo>B</wahoo> </foo> <foo name="c"> <wahoo>C</wahoo> </foo> <group> <bar ref="b"/> <bar ref="a"/> </group> </test> Output: <test> <group> <wahoo>B</wahoo> <wahoo>A</wahoo> </group> </test> That is, anywhere there is a 'bar', replace it with the contents of the 'foo' it references. I'm having a difficult time representing this with HXT's Arrow API because the value of the 'ref' attribute is the output of an arrow, but I need it to be just a plain string so that I could use it as an input parameter to the 'hasAttrValue' function. A similar problem, using 'processTopDown', once I traverse to a 'bar' node, I need to then traverse the root again to find the 'foo', but I'm in the context of the 'bar' node, not the root. My ears are open to solutions with HaXML or Scrap Your XML-plate, or anything else. Thanks, Greg

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announce: (preliminary) dbus haskell bindings
by Evan Martin 17 Jul '06

17 Jul '06

"D-Bus is a message bus system, a simple way for applications to talk to one another." [1] It's particularly popular on free software desktops (Gnome, KDE). HDBus wraps the DBus APIs so your Haskell code can broadcast messages and make calls to services. For example, on my recent Ubuntu system, these bindings are sufficient to pop up balloon-tip-style notifications via a daemon I wasn't previously aware of. Very preliminary code, so please let me know if you find it useful. (Also, I don't know where it's appropriate to announce this, so please direct me to other mailing lists.) Home page: http://neugierig.org/software/hdbus/ Haddock: http://neugierig.org/software/hdbus/doc/ Repository browser: http://neugierig.org/software/darcs/browse/?r=hdbus;a=summary [1] http://www.freedesktop.org/wiki/Software/dbus

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Re: [Haskell-cafe] RE: ANN: System.FilePath 0.9
by Brian Smith 17 Jul '06

17 Jul '06

Hi Neil, On 7/17/06, Neil Mitchell <ndmitchell(a)gmail.com> wrote: > > Hi Brian, > > You sent this email just to me, and not to the list. If you indended > to send to the list then feel free to forward my bits on to the list. > > > > I know that FilePath is defined by Haskell '98 as a String and so it > cannot > > be changed. So, perhaps a new type or class should be created for this > > library (hereafter "GoodPath," although I am not suggesting that is the > best > > name). > The problem is people will have to marshal their data into this > GoodPath, and marshal it out again. When people can shortcut that > marshalling, as the current readFile/writeFile definitions ensure they > can, they will. At that point you loose all safety because people will > abuse it. I disagree. It would be trivial to create a new module that exported new definitions of file IO actions that operated on "GoodPath" instead of "FilePath," transparently delegating to the original readFile/writeFile/etc. until they could be removed in the future. This would also support the "SuperFilePath" idea you mentioned. Another thing I thought of would be a "canonicalPath" IO action (canonicalPath :: FilePath -> IO FilePath) that returns a FilePath that implements case-preserving-case-insensitive matching. For example, if there is a file named "Hello There.txt" in C:\, then (canonicalPath "c:\hello there.txt ") would give "C:\Hello There.txt"). I think that the xxxDrive functions should only be exported from System.FilePath.Windows and no System.FilePath since it is unclear as to how they should be used effectively by cross-platform software. - Brian

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process simulation in functional style
by Alexander Vodomerov 17 Jul '06

17 Jul '06

Hello! I'm writing a program that simulates multiple processes. The processes may send/receive messages, do some work, or sleep for some amount of time. I have seen that many such things can be expressed in Haskell in very elegant manner using it functional or lazy properties. For example, client/server interaction may be expressed via inifinite lists as shown in "Gentle Introduction to Haskell". Another way of process simulation is describied in http://citeseer.ist.psu.edu/harcourt95extensible.html, where simple and concise CCS interperter is constructed. I've tried to apply the idea of infinite lists or CCS-style processes, but fail. The main reason is that: 1) messages are asynchronous (it may be received and handled while process are sleeping, without waking them) 2) in some cases received message may wake up process 3) all activity in system is ordered by time 4) there are >2 process and during simulations new processes may be created I've no idea how to implement 1, 2 in CCS interpeter. The approach of infinite lists seems to have problems with 3, 4. Have somebody any ideas how this can be solved in simple and concise way? With best regards, Alexander. PS. Currently I have some code (see below), but it is VERY UGLY. The main drawback is that is very "imperative". It employs notion of "Global state". It doesn't use features of Haskell and can be rewritten in ML or C++ without changes. Another flaws is that it is very unextensible, and all processes must have the same state. -- example code latency = 0.001 type Message = String type MsgM = WriterT [(Int, Message)] IO -- process may send messages -- process states next id = (id + 1) `mod` 3 type State = (Int, Int, Double) -- proc. number, counter, interval do_step :: State -> MsgM (Double, State) --- do something and then sleep do_step (id, cnt, delay) = do tell [(next id, "ping " ++ show id ++ " cnt " ++ show cnt)] return (delay, (id, cnt + 1, delay)) handle_message :: Message -> State -> MsgM State handle_message msg (id, cnt, delay) = do -- liftIO $ putStrLn $ show id ++ " received msg " ++ msg if msg !! 0 == 'p' then tell [(next id, "reply " ++ show id ++ " to " ++ msg)] else return () return (id, cnt, delay) -- global event queue data Event = MsgRecv Message | Work deriving Show type EventQueue = [(Double, Int, Event)] compare_event (t1, n1, e1) (t2, n2, e2) = compare t1 t2 type EventM = WriterT EventQueue IO queue_event :: Int -> Event -> Double -> EventM () queue_event dest ev time = tell [(time, dest, ev)] type FullState = Map.Map Int State handle_event :: Int -> Double -> Event -> FullState -> EventM FullState handle_event procnum time ev fullstate = do let localstate = (fullstate Map.! procnum) case ev of MsgRecv msg -> do (nstate, messages) <- lift $ runWriterT (handle_message msg localstate) sequence_ $ map (\(dst, msg) -> queue_event dst (MsgRecv msg) (time + latency)) messages return $ Map.insert procnum nstate fullstate Work -> do ((pause, nstate), messages) <- lift $ runWriterT (do_step localstate) sequence_ $ map (\(dst, msg) -> queue_event dst (MsgRecv msg) (time + latency)) messages queue_event procnum Work (time + pause) return $ Map.insert procnum nstate fullstate run_queue :: FullState -> EventQueue -> IO () run_queue st eventqueue = case eventqueue of [] -> return () (time, dest, ev) : rest -> do putStrLn $ "processing event " ++ (showFFloat (Just 3) time) "" ++ " at procnum " ++ show dest ++ " " ++ show ev (nst, nev) <- runWriterT (handle_event dest time ev st) let nqueue = foldl (\res -> \e -> insertBy compare_event e res) rest nev run_queue nst nqueue init_state = Map.fromList [(0, (0, 0, 0.3)), (1, (1, 0, 0.4)), (2, (2, 0, 0.5))] main = run_queue init_state [(0, 0, Work), (0, 1, Work)]

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Unix gurus, help me please
by Bulat Ziganshin 17 Jul '06

17 Jul '06

Hello gurus :) i got message about problems compiling Streams 0.2.1 library on Unix systems: - I include file io.h, but this particular system has "sys/io.h" instead. I think that i should solve this problem by including HsBase.h which should include proper io.h on any system supported. One more question is whether this HsBase.h will be available on non-GHC platforms, including Hugs, yhc, jhc? - Michael Stone (which reported all these problems) suggests to use #!/usr/bin/env runhaskell instead of #!/usr/bin/runhaskell at the start of Setup.hs file. Is that really better? - mingw supports _commit operation which (as i guess) saves to disk buffers of given file. Is that true that regular unixes has fsync function to perform the same action? (and what the hell MS don't use the same name??) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: Breaking cycles in a directed graph.
by C Rodrigues 16 Jul '06

16 Jul '06

I ran across this problem a while ago, working on identifying edges in dependence cycles for software pipelining. The problem you want to solve is known as the "minimum feedback arc set" problem and is NP-hard. In my case, I could use other knowledge about the problem domain to find a good approximate solution. I'll refer you to the Wikipedia article: http://en.wikipedia.org/wiki/Feedback_arc_set I hope that helps.

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Haskell performance in heavy numerical computations
by Joel Reymont 16 Jul '06

16 Jul '06

Is anyone using Haskell for heavy numerical computations? Could you share your experience? My app will be mostly about running computations over huge amounts of stock data (time series) so I'm thinking I might be better of with OCaml. Thanks, Joel -- http://wagerlabs.com/

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packaging modules with internal-only functions
by Evan Martin 15 Jul '06

15 Jul '06

I'm writing an FFI bindings module, and one of the pieces from the library I want to wrap is their "Error" type. > module Error where To the user of the module, I'd like to only expose the nicely wrapped interface: > data Error = Error String String > instance Typeable Error where ... -- for throwDyn But within the module's implementation, I need to share some other, internal-only functions: > type ErrorTag = () > type ErrorP = Ptr ErrorTag > withErrorP :: (ErrorP -> IO a) -> IO a Where withErrorP potentially throwDyns an Error, so it needs the first module. What is the proper way to divide these up in terms of package exports? The best options I can figure out are either to split these into multiple modules, or to have my "Error" module export them and mark them not to be documented by haddock...

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Windows PowerShell "Monad"
by Chad Scherrer 15 Jul '06

15 Jul '06

Yep, that's its codename. Now, I'm not much of a Windows person. Is the name just a weird coincidence, or does it have anything to do with monads as we know them? http://en.wikipedia.org/wiki/MSH_(shell) -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: Why is Day and Month bounded?
by Johan Holmquist 15 Jul '06

15 Jul '06

I'll answer my own post to elaborate: If Day (and Month) where NOT instances of Bounded, the following would be possible: [Monday .. Sunday] => should return [Monday, Tuesday ... Saturday, Sunday] => but returns [] [Saturday .. Tuesday] => should return [Saturday, Sunday, Monday, Tuesday] => but returns [] succ Saturday => should return Sunday => but yields an exception It would feel natural to be able to get the desired results above. The Haskell98 report stipulates: --->8--- For any type that is an instance of class Bounded as well as Enum, the following should hold: The calls succ maxBound and pred minBound should result in a runtime error. --->8--- So, the standard library System.Time respects that and yields exception for succ Saturday. But if Day was not an instance of Bounded should it not be legal for succ Saturday to return Sunday? Now I cannot see the advantage of letting Day (and Month) be instances of Bounded. The only thing we get out of that is access to functions maxBound and minBound, but those do not seem useful to me for either Day or Month. /Johan

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