- Haskell-Cafe - Haskell.org

Yampa integral function
by felipe zapata 17 Apr '13

17 Apr '13

Hello, I'm trying to write some simulator of physical systems using Yampa. The fundamental idea is to integrate the position and velocity, using the following algorithm: 1. get the velocity using the formula, v = v0 + integral ( 0.5 * force * recip mass) where v0 is the previous velocity. 2. calculate the new position as follows, x = x0 + integral (v) and some other steps that are not relevant here. Therefore I wrote the following piece of code: type Pos = Double type Vel = Double type Mass = Double type Force = Double integralCoordSF :: Force -> Mass -> SF (Pos,Vel) (Pos,Vel) integralCoordSF force mass = proc (x0,v0) -> do v <- integralVelSF force mass -< v0 vdt2 <- integral -< v let x = x0 + vdt2 returnA -< (x,v) integralVelSF :: Force -> Mass -> SF Double Double integralVelSF force mass = proc v0 -> do t1 <- integral -< 0.5 * (force/mass) returnA -< v0 + t1 Now, let's the speed (velocity), force and the mass equal 1.0. And the initial potion equal zero. Then using the "embed" function we get that, *Main> embed (integralCoordSF 1.0 1.0) ((0.0,1.0),[(0.1,Nothing),(0.1,Nothing)]) [(0.0,1.0),(0.1,1.05),(0.20500000000000002,1.1)] According to the algorithm, if dt = 0.1 then v = 1.0 + integral (0.5 * 1.0 * recip 1.0) = 1.05 in agreement with the algorithm. But let's see about the position x = 0 + integral (v) Using the formula of step 2, the position should be equal to 0.105, but the result given by embed is 0.1. As a beginner certainly and missing a lot of concepts and details, can someone please explain me what's going on here? Thank you for your help! Felipe Z

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Haskell/functional-language people in Raleigh/Durham, North Carolina, USA?
by Benjamin Redelings 17 Apr '13

17 Apr '13

Hi, I'm curious if there are any other people on this list who are interested in Haskell and functional languages in the Triangle Area, in North Carolina? thanks! -BenRI

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I would like to know how to use the following events handlers : dropTargetOnData, dropTargetOnDrop, dropTargetOnEnter,
by Luc TAESCH 16 Apr '13

16 Apr '13

WxHaskell and DragAndDrop I would like to know how to use the following events handlers : dropTargetOnData, dropTargetOnDrop, dropTargetOnEnter, dropTargetOnDragOver….[1] Could you check if my current believes are corrects : From wx Widgets/ wxPython [2] / [3] / [4] it looks like they need to be used to manage DragAndDrog for non trivial examples. they are not actionable via an existing event like "on drag", etc.. I tried and create my own event. but it does not get "activated". [7] besides, from the signatures [1], these look like being activated on DropTarget, unlike other events on Reactive/ Windows/Controls . Is It Correct ? Heinrich created its own events "onText", (in reactive-Banana) but this is on a Control. [6] Could someone confirm these events effectively worked for them in WxHaskell, and maybe hint how to do that [1]: from Graphics.UI.WXCore.Events , line 1933 onwards Set an event handler that is called when the drop target can be filled with data. This function require to use 'dropTargetGetData' in your event handler to fill data. dropTargetOnData :: DropTarget a -> (Point -> DragResult -> IO DragResult) -> IO () ... -- | Set an event handler for an drop command in a drop target. dropTargetOnDrop :: DropTarget a -> (Point -> IO Bool) -> IO () -- | Set an event handler for an enter command in a drop target. dropTargetOnEnter :: DropTarget a -> (Point -> DragResult -> IO DragResult) -> IO () -- | Set an event handler for a drag over command in a drop target. dropTargetOnDragOver :: DropTarget a -> (Point -> DragResult -> IO DragResult) -> IO () -- | Set an event handler for a leave command in a drop target. dropTargetOnLeave :: DropTarget a -> (IO ()) -> IO () [2] : http://docs.wxwidgets.org/2.8/wx_wxdroptarget.html#wxdroptargetondrop [3] : http://wiki.wxpython.org/DragAndDrop [4] : http://www.blog.pythonlibrary.org/2012/06/20/wxpython-introduction-to-drag-… [5] : http://wewantarock.wordpress.com/2011/06/17/how-does-wxhaskell-event-handli… [6] : https://github.com/HeinrichApfelmus/reactive-banana/blob/master/reactive-ba… L 88 [7] : module Main where import Graphics.UI.WX hiding (empty) import Data.Maybe import Control.Monad import Graphics.UI.WX.Events import Graphics.UI.WXCore.WxcClassesMZ --import Graphics.UI.WXCore.WxcClassesAL import Graphics.UI.WXCore.DragAndDrop import Graphics.UI.WXCore.Events main = start dndtest dndtest = do f <- frame [text := "Drag And Drop test"] p <- panel f [] ok <- button p [text := "Ok"] xinput <- textEntry p [text := "here :"] --textEntry yinput <- staticText p [text := "drag me"] zinput <- staticText p [text := "result me"] set f [defaultButton := ok ,layout := container p $ margin 10 $ column 5 [boxed "coordinates" (grid 5 5 [[label "source:", hfill $ widget yinput] ,[label "target(focus first):", hfill $ widget xinput] ,[label "result:", hfill $ widget zinput] ]) ,floatBottomRight $ row 5 [widget ok]] ] set xinput [ on enter := onEnter] set yinput [ ] --------------------------------------------------------- --- meaningful stuff starts here --------------------------------------------------------- -- prepare the drop source : create a DataObject and associate it with the source textdata' <- textDataObjectCreate "text dropped" src <- dropSource textdata' yinput -- prepare the drop target: create a DataObject (placeholder here) and associate it with the target textdata <- textDataObjectCreate ".." drop <- dropTarget xinput textdata set drop [ on onMyDrop := showMeDrop ] ---- <<<< I am expecting this to get fired but no ... -- obj create a new event on drop invoking .. -- and see if it is invoked set yinput [ on drag := onDrag src ] set xinput [ ] ------ <<<< I am expecting the target to react when dropped (Its DroopedTarget i fact) set zinput [ on mouse := showMeE] set ok [ on command := close f ] return () --- this is the custom event, just a setter to fire dropTargetOnDrop. not sure at all this is the correct way. onMyDrop = newEvent "onmyDrop" (\w -> ioError (userError ("attribute '" ++ "onmyDrop" ++ "' is write-only."))) dropTargetOnDrop --dropTargetOnDrop :: DropTarget a -> (Point -> IO Bool) -> IO () --- the rest are jsut helper to see whats going on showMeEo = putStr "showMeEo" showMeDrop p = do putStr "showMeDrop" return True onDrag s p = do -- dragAndDrop :: DropSource a -> DragMode -> (DragResult -> IO ()) -> IO () dragAndDrop s Default (\r -> do {putStr "DnD handler called: "; putStrLn(show r); return ()}) putStrLn "on Drag activated:" showMeE :: EventMouse -> IO () showMeE (MouseMotion point mod) = putStr "" --- discard meaningless Motion event showMeE e = putStrLn $ show e -- onEnter p = putStrLn $ "on Enter:" ++ show p http://stackoverflow.com/questions/15911219/wxhaskell-and-draganddrop-how-t…

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customizing haskeline?
by Evan Laforge 16 Apr '13

16 Apr '13

I tried to colorize a haskeline prompt by putting control characters in it, but line editing was hopelessly confused, presumably because haskeline doesn't understand control characters and thought the prompt was longer than it really was. From looking at Haskeline.promptedInput, it seems like there's no way to set the prompt to a higher level terminal operation since it's hardcoded to String, and there's no way to tell it how many displayed characters the prompt contains. So is there some way to have a fancy prompt that I'm missing? Has anyone done that successfully? This also implies it's impossible for haskeline to cooperate with cursor addressing in general. Or maybe it would be ok as long as I don't do anything fancy on the prompt line. But also seems like you can't set custom keybindings, e.g. bind a control key to a "history menu" like shells can do, which is my real goal here. I guess I'm interested if anyone has been able to use haskeline for anything more complicated than a prompt with the hardcoded default behaviour. My impression is that it's basically non-extensible, but maybe there's some hook I'm missing. By extension it seems impossible to colorize a ghci prompt, and indeed embedding control characters directly in :set prompty leads to the same problems.

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trying to understand out of memory exceptions
by Anatoly Yakovenko 16 Apr '13

16 Apr '13

-- So why does this code run out of memory? import Control.DeepSeq import System.IO import qualified Data.ByteString.Char8 as BS scanl' :: NFData a => (a -> b -> a) -> a -> [b] -> [a] scanl' f q ls = q : (case ls of [] -> [] x:xs -> let q' = f q x in q' `deepseq` scanl' f q' xs) main = do file <- openBinaryFile "/dev/zero" ReadMode chars <- BS.hGetContents file let rv = drop 100000000000 $ scanl' (+) 0 $ map fromEnum $ BS.unpack chars print (head rv) -- my scanl' implementation seems to do the right thing, because main = print $ last $ scanl' (+) (0::Int) [0..] -- runs without blowing up. so am i creating a some thunk here? or is hGetContents storing values? any way to get the exception handler to print a trace of what caused the allocation?

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Re: [Haskell-cafe] trying to understand out of memory exceptions
by Anatoly Yakovenko 16 Apr '13

16 Apr '13

unfortunately read file tries to get the file size readFile :: FilePath -> IO ByteStringreadFile f = bracket (openFile f ReadMode) hClose (\h -> hFileSize h >>= hGet h . fromIntegral) which wont work on a special file, like a socket. which is what i am trying to simulate here. On Tue, Apr 16, 2013 at 11:28 AM, Clark Gaebel <cg.wowus.cg(a)gmail.com>wrote: > See the comment for hGetContents: > > "This function reads chunks at a time, doubling the chunksize on each > read. The final buffer is then realloced to the appropriate size. For files > > half of available memory, this may lead to memory exhaustion. Consider > using readFile<http://hackage.haskell.org/packages/archive/bytestring/0.9.2.1/doc/html/Dat…> in > this case." > > > http://hackage.haskell.org/packages/archive/bytestring/0.9.2.1/doc/html/Dat… > > Maybe try lazy bytestrings? > > - Clark > > On Tuesday, April 16, 2013, Anatoly Yakovenko wrote: > >> -- So why does this code run out of memory? >> >> import Control.DeepSeq >> import System.IO >> import qualified Data.ByteString.Char8 as BS >> >> scanl' :: NFData a => (a -> b -> a) -> a -> [b] -> [a] >> scanl' f q ls = q : (case ls of >> [] -> [] >> x:xs -> let q' = f q x >> in q' `deepseq` scanl' f q' xs) >> >> >> main = do >> file <- openBinaryFile "/dev/zero" ReadMode >> chars <- BS.hGetContents file >> let rv = drop 100000000000 $ scanl' (+) 0 $ map fromEnum $ BS.unpack >> chars >> print (head rv) >> >> -- my scanl' implementation seems to do the right thing, because >> >> main = print $ last $ scanl' (+) (0::Int) [0..] >> >> -- runs without blowing up. so am i creating a some thunk here? or is >> hGetContents storing values? any way to get the exception handler to print >> a trace of what caused the allocation? >> >>

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ANN: rematch, an library for composable assertions with human readable failure messages
by Tom Crayford 16 Apr '13

16 Apr '13

I kept on running into this thing where I was calling error in quickcheck to get good error messages about the things I was comparing. In Java land, this stuff is handled by Hamcrest: a library for composable assertions with good error messages. This library is basically a port of hamcrest's core api, but I've been very pleased with how it turned out. I've been using this in tests for production code for a month or so now, and I'm very pleased with it. Running a matcher (in this example in an hunit test) looks like this: expect [1] (is [1]) The core API is very simple: data Matcher a = Matcher { match :: a -> Bool -- ^ A function that returns True if the matcher should pass, False if it should fail , description :: String -- ^ A description of the matcher (usually of its success conditions) , describeMismatch :: a -> String -- ^ A description to be shown if the match fails. } This means you can add/write your own matchers happily, which occasionally means you can write *very* nice test code (here's an example of using a custom matcher for checking the state of an "issue" in a hypothetical issue tracking app): expect latestIssue (hasState Resolved) -- I removed the supporting code to make this assertion actually run, -- this email is already pretty long. There are numerous matchers (and functions for creating matchers) in the rematch library, including some composition functions that provide good failure messages. There are some shims to hook rematch into the common haskell test frameworks (specifically hunit and quickcheck). The two libraries are up on hackage: http://hackage.haskell.org/package/rematch http://hackage.haskell.org/package/rematch-text The code is all up on github: http://github.com/tcrayford/rematch I get rather frustrated when my tests give bad failure explanations, and using rematch goes a long way to fix that. Lastly, rematch is pretty isolated from test frameworks/etc, with a very small and easy to understand surface api. Hopefully it'll help with the thing I've seen in other languages (cough ruby cough) with every test framework reinventing this idea, and not all frameworks having all the matchers I want to use. Let me know if you have any feedback/thoughts Tom

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When - O'Reilly book on Parallel and Concurrent Haskell
by amslonewolf＠gmail.com 16 Apr '13

16 Apr '13

Are there updates on when Simon Marlow's O'Reilly book on Parallel and Concurrent Haskell is going to be released?

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Monad fold
by Christopher Howard 16 Apr '13

16 Apr '13

So, I'm doing something like this foldl (>>=) someA list :: Monad m => m a where list :: Monad m => [a -> m a], someA :: Monad m => m a Is there a more concise way to write this? I don't think foldM is what I want -- or is it? -- frigidcode.com

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primitive operations in StateT
by Clark Gaebel 16 Apr '13

16 Apr '13

Hi list! I want to use MVectors in a StateT monad transformer. How do I do that? StateT isn't a member of 'PrimMonad', and I have no idea how to make it one. Regards, - Clark

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