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Re: HXT: desperatedly trying to "concat"
by Steffen Schuldenzucker 02 Apr '09

02 Apr '09

Hello again. I finally got it myself. It was just a matter of parentheses: See http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3229 for the corrected version. Looks like what I was trying to do is not expressable via just an arrow, one needs a function mapping the input arrow to a new one. I'm gonna make a big "WATCH YOUR PARENTHESES!" poster... Yeah, with some arrows on it... Thanks for reading, anyway. Steffen ________________________________________________________________________ Neu bei WEB.DE: Kostenlose maxdome Movie-FLAT! https://register.maxdome.de/xml/order/LpWebDe?ac=OM.MD.MD008K15726T7073a

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Funny type signature question
by Michael Snoyman 02 Apr '09

02 Apr '09

I've butted into this problem multiple times, so I thought it's finally time to get a good solution. I don't even have the terminology to describe the issue, so I'll just post the code I'm annoyed with and hope someone understands what I mean. import Control.Monad.Random import System.Random data Marital = Single | Married | Divorced deriving (Enum, Bounded, Show) randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a randomEnum = do let minb = minBound maxb = maxBound randVal <- getRandomR (fromEnum minb, fromEnum maxb) return $ head [toEnum randVal, minb, maxb] -- if I do the obvious thing (return $ toEnum randVal) I get funny errors main = do stdGen <- newStdGen let marital = evalRand randomEnum stdGen :: Marital putStrLn $ "Random marital status: " ++ show marital Any help is appreciated. Thanks! Michael

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ANN: Buster 0.99.1, a library for application orchestration that is not FRP
by Jeff Heard 02 Apr '09

02 Apr '09

Read more about it on its webpage: http://vs.renci.org/jeff/buster Yes, it’s to solve a particular problem. And yes, this is a rough draft of an explanation of how it works. I’ve not even really solidified the vocabulary yet, but I have this module which couches a large, abstract, interactive (both with the user and the system), multicomponent application in terms of a bus, inputs, behaviours, and events. * Time is continuous and infinite. * An event is a static, discrete item associated with a particular time. * The bus is the discrete view of event in time at an instant. * A widget is an IO action that assigns events to a particular time based only upon sampling the outside world (other events and behaviours are irrelevant to it). e.g. a Gtk Button is a widget, a readable network socket is an widget, the mouse is an widget, the keyboard is an widget, a multitouch gesture engine is a widget. * A behaviour is a continuous item — it exists for the entire program and for all times — which maps events on the bus to other events on the bus. It is an IO action as well — where widgets only sample the outside world and are in a sense read only, behaviours encapsulate reading and writing.

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Character I/O
by Andrew Coppin 02 Apr '09

02 Apr '09

I ran the following program: >module Main where > >main = do > putStrLn "Key" > c <- getChar > print c > main Unfortunately, it doesn't do what I was hoping. Specifically, it only returns any data when I hit [return]. It also seems to be giving me a command history somehow. Special keys (e.g., arrow keys, backspace, escape) don't get through to the application. I'm trying to write a program that uses individual keys as commands. What's the correct way to get this to work?

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HXT: desperatedly trying to "concat"
by Steffen Schuldenzucker 02 Apr '09

02 Apr '09

Hi. I've got a problem with the Haskell XML Toolkit (hxt). I want to write a little app that performs REST requests from a certain (rather simple) XML format. A example procedure Call looks like testData defined below. What I'd like to do is to transform this xml tree into a GET variable string using an XmlArrow. The task sounds easy, and it has to be easy, but I've been sitting here for about a day now, staring at my code. It looks like this (the transformation should be done by the arrow "mkGetStr"): -- Rest.hs -- This is also on HPaste: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3210#a3210 {-# LANGUAGE NoMonomorphismRestriction #-} module Rest where import Text.XML.HXT.Arrow import Data.List getParamsA = hasName "param" >>> getChildren >>> isElem >>> (getName &&& (getChildren >>> getText)) >>> arr2 mkGetPair mkMethodStr = ("method=" ++) mkGetPair k v = k ++ "=" ++ v getMethodA = hasName "method" >>> getChildren >>> getText >>> arr mkMethodStr mkGetStr = isElem >>> (getMethodA <+> getParamsA) >>. intercalate "&" -- Try it with: runX (testData >>> mkGetStr) >>= print testData = xread <<< constA ( "<method>my.Method</method>" ++ "<param>" ++ "<foo_arg>Foo</foo_arg>" ++ "<bar_arg>Bar</bar_arg>" ++ "</param>" ) -- End of Rest.hs What I get out of it is this (in ghci): *Rest> runX (testData >>> mkGetStr) >>= print "method=my.Methodfoo_arg=Foo&bar_arg=Bar" There is an "&" missing right after "method=my.Method"! Why? I've tried many variants of this and they give me either this or a similar result or multiple results (what I don't want either). I'd be really happy if someone could save my day and help me with this issue. Thanks in advance, Steffen ________________________________________________________________________ Neu bei WEB.DE: Kostenlose maxdome Movie-FLAT! https://register.maxdome.de/xml/order/LpWebDe?ac=OM.MD.MD008K15726T7073a

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SciMark2 benchmark
by Jon Harrop 02 Apr '09

02 Apr '09

Has anyone ported the SciMark2 benchmark suite to Haskell? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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ZipList monad, anyone?
by Patai Gergely 02 Apr '09

02 Apr '09

Does ZipList have any useful monad instance? The thought came up while thinking about higher order dataflows and an ArrowApply interface for Yampa. As a ZipList can be thought of as a function with a discrete domain, I figured its monadic form could be analogous to the reader monad, hence: instance Monad ZipList where return = ZipList . repeat ZipList xs >>= f = ZipList $ zipWith ((!!) . getZipList . f) xs [0..] Correct me if I'm wrong, but it seems to me that f always returning an infinite list is a sufficient condition for the monad laws to hold. However, this is painfully inefficient. I assume that bringing FRP-like switching and some clever data structure (which optimises stateless streams for starters) into the picture can potentially lead to constant-time access at least in the case of sequential sampling. Is there any recent work going in this direction? Gergely -- http://www.fastmail.fm - One of many happy users: http://www.fastmail.fm/docs/quotes.html

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Re: [Haskell-beginners] Re: making translation from imperative code
by Michael Mossey 02 Apr '09

02 Apr '09

Thanks very much for the help... I will look at this over the next couple of days. Your code actually addresses a different problem, the one of merging separates lists of timed events. I do need to write code to do that eventually, so I will try to understand what you have written here. However, the original problem concerns visual layout, which actually takes place *after* creating a merged list. In layout, items do have times associated with them, but also take up physical space. Different items takes up different amounts of space, and at any given "time," there may be items on all the staves or just some of them. I will try to come up with (1) a more succinct explanation of the problem (with textual graphics as a visual aid) (2) a more succinct algorithm. For example, you are right that I'm mixing concerns. The system layout can terminate for two reasons: (1) reached the end of the score (2) reached the right edge of the page. There might be a way to simplify the loop or fold so that these concerns look more unified. -Mike PS a question below: Heinrich Apfelmus wrote: > Michael Mossey wrote: >> Heinrich Apfelmus wrote: >>> Can you elaborate on what exactly the algorithm is doing? Does it just >>> emit notes/chords/symbols at given positions or does it also try to >>> arrange them nicely? And most importantly, "where" does it emit them to, >>> i.e. what's the resulting data structure? >>> >>> So far, the problem looks like a basic fold to me. >> Here is some Haskell code that explains the problem in >> more detail. >> [...] > > Thanks for the elaboration. > > I think the code doesn't separate concerns very well; mixing information > about widths and times, page size and the recursion itself into one big > gnarl. > > Also, there is one important issue, namely returning a special value > like -1 as error code in > >> tryAgain state = >> case scoreNextTime score (time state) of >> -1 -> indicateNoMoreChunks state >> t -> layoutSystem' (setTime state t) > > Don't do this, use Maybe instead > > tryAgain state = case scoreNextTime score (time state) of > Nothing -> indicateNoMoreChunks state > Just t -> layoutSystem' (state { time = t }) > > where Nothing indicates failure and Just success. > > > Back to the gnarl in general, I still don't have a good grasp on the > problem domain, which is key to structuring the algorithm. Therefore, > I'll expand on toy model and you tell me how it differs from the real thing. > > The model is this: we are given several lists of notes (f.i. a piano > part and a vocal line) where each note is annotated with the time it is > to be played at. We abstract away the fact that we are dealing with > musical notes and simply consider a list of *events* > > type Time = Integer > type Events a = [(Time, a)] > > with the invariant that the timestamps are (strictly) increasing: > > valid :: Events a -> Bool > valid xs = all $ zipWith (\(t1,_) (t2,_) -> t1 < t2) xs (drop 1 xs) > > Now, the toy task is to merge several lists of similar events into one > big list that is ordered by time as well. > > merge :: [Events a] -> Events [a] > > Since some events may now occur simultaneously, the events of the > results are actually lists of "primitive" events. > > One possibility for implementing merge is to start with a function to > merge two event lists > > merge2 :: Events [a] -> Events [a] -> Events [a] > merge2 [] ys = ys > merge2 xs [] = xs > merge2 xs@((tx,x):xt) ys@((ty,y):yt) = case compare tx ty of > LT -> (tx,x ) : merge2 xt ys > EQ -> (tx,x++y) : merge2 xt yt > GT -> (ty, y) : merge2 xs yt > > and to apply it several times > > merge = foldr merge2 [] . map lift > where lift = map $ \(t,x) -> (t,[x]) > > > Another possibility is to simply concatenate everything first and then > sort by time > > merge = map (\((t,x):xs) -> (t,x:map snd xs)) > . groupBy ((==) `on` fst) > . sortBy (comparing fst) > . concat > > > The code above can be made more readable by choosing nice names like > > time = fst > event = snd > > or avoiding pairs altogether and implementing these names as record > fields. Also, the (&&&) combinator from Control.Arrow is very handy. > > merge = map (time . head &&& map event) > . groupBy ((==) `on` time) > . sortBy (comparing time) > . concat > > > I hope this gives you a few ideas to think about. How does this toy > model differ from the real thing? > > > Regards, > apfelmus > > > PS: If some parts of my example code give you trouble, it's probably > fastest to ask around on the #haskell IRC channel. > > -- > http://apfelmus.nfshost.com > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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GSoC proposal
by Csaba Hruska 02 Apr '09

02 Apr '09

Hi! I've submitted my proposal on gsoc portal. (3D Rendering Engine) What's your opinion?

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Leksah patch release 0.4.4.1
by Jürgen Nicklisch-Franken 02 Apr '09

02 Apr '09

I've pushed a patch with a minor fix for the recurring query to open a file: * Doesn't attempt a background build, when there is no open package. * Doesn't keep bugging the user if the session they select does not have a valid package * Improved title on package open dialog * Updated the manual for better Ubuntu installation description Please note that Leksah doesn't work with GHC version 6.10.2 currently, because gtk2hs has problems with the changes on finalizers. Hope they can fix this soon. Jürgen

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