- Haskell-Cafe - Haskell.org

data types with overlapping component names (in one module)?
by Johannes Waldmann 17 Apr '13

17 Apr '13

What is the current situation: can we have two types with overlapping component names in one module? module M where data T1 = C1 { foo :: Int } data T2 = C2 { foo :: String } It seems not (ghc says: Multiple declarations of 'foo'). This comes close: http://www.haskell.org/ghc/docs/7.6.2/html/users_guide/syntax-extns.html#di… but still requires the definitions to reside in different modules? This is a major pain (it forces me to spread the source over several files), and also a show-stopper when selling Haskell to OO folks, who "naturally" assume that a class also denotes a scope. (And that you could nest them.) Are/were there plans/proposals to address this? - J.W.

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Re: [Haskell-cafe] NaN as Integer value
by wren ng thornton 17 Apr '13

17 Apr '13

On 4/14/13 8:53 PM, Kim-Ee Yeoh wrote: > On Sun, Apr 14, 2013 at 3:28 PM, wren ng thornton <wren(a)freegeek.org> wrote: >> Whereas the problematic >> values due to infinities are overspecified, so no matter which answer you >> pick it's guaranteed to be the wrong answer half the time. >> >> Part of this whole problem comes from the fact that floats *do* decide to >> give a meaning to 1/0 (namely Infinity). > > I'm not sure what you mean about overspecification here, but in > setting 1/0 as +infinity (as opposed to -infinity), there's an easily > overlooked assumption that the limit is obtained "from above" as > opposed to "from below." Setting 1/0 = +inf isn't the problem, or at least not the main one. Of course, there's always the question about which completion of the reals to use--- i.e., whether we have +inf vs -inf, or whether we just have a single point infinity. Rather, the NaN problem comes from trying to resolve things like: inf - inf inf * 0 inf / inf 0 / 0 The overspecification problem I mentioned is that each of these expressions has "too many" solutions. For example, we have the following equations: x * 0 == 0 -- where x is finite inf * y == inf * signum y -- where y /= 0 inf * 0 == ?? x - inf == -inf -- where x is finite inf - y == inf -- where y is finite inf - inf == ?? x / 0 == inf * signum x -- where x /= 0 0 / y == 0 -- where y /= 0 0 / 0 == ?? -- Live well, ~wren

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Derving NFData via Generics from a type that has a vector doesn't work. (was trying to understand out of memory exceptions)
by Anatoly Yakovenko 17 Apr '13

17 Apr '13

-- ok, something in deriving NFData using Generics in a type that has a Vector in it. {-# LANGUAGE DeriveGeneric #-} import Control.DeepSeq import System.IO import GHC.Generics (Generic) import qualified Data.Vector as V import qualified Data.ByteString.Lazy.Char8 as BL 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) -- this runs without blowing up -- main = print $ last $ scanl' (+) (0::Int) [0..] data Simple = Simple (V.Vector Double) deriving (Show, Generic) instance NFData Simple --this blows up main = do let initial = Simple $ V.fromList (take 100 $ repeat 0) sumvs (Simple a) (Simple b) = Simple $ V.zipWith (+) a b print $ last $ scanl' sumvs initial $ repeat $ initial

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version of containers fixed by template-haskell?
by Johannes Waldmann 17 Apr '13

17 Apr '13

Is it still the case that "It just doesn't work to have multiple versions of a wired-in package" (cf. http://hackage.haskell.org/trac/ghc/ticket/5704 )? ghc-7.6.2 comes with containers-0.5.0.0 and template-haskell-2.8.0.0 . It seems I can upgrade to containers-0.5.2.1 and use it with no problems but not alongside template-haskell: ... changes: containers-0.5.0.0 -> 0.5.2.1, Warning: The following packages are likely to be broken by the reinstalls: ... ghc-7.6.2 (Isn't it somewhat bold of cabal-install to offer to break "ghc-7.6.2"? like, this will completely hose the compiler?) That's bad, because I use containers everywhere, and I assume that 0.5.2.1 has some advantages (like, speed) over 0.5.0.0. - J.W.

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Announce: Blip, a bytecode compiler for Python 3 (implemented in Haskell)
by Bernie Pope 17 Apr '13

17 Apr '13

I'm pleased to announce Blip version 0.1.0, a bytecode compiler for Python 3. https://github.com/bjpop/blip Blip compiles Python 3 source files to bytecode. The output bytecode is compatible with the CPython interpreter (the "standard" Python implementation). For example, given a Python 3 source file called foo.py, the command: blip foo.py produces a bytecode file called foo.pyc. The bytecode can be executed by passing it as an argument to a CPython interpreter: python3 foo.pyc This is an early development release of Blip, so it is not ready for serious use, however it supports most of the Python 3 language already. It has been tested on OS X 10.7.5 with GHC 7.4.2. The output has been tested with Python 3.3.0. Currently implemented features: https://github.com/bjpop/blip/wiki/Currently-implemented-features Features not yet implemented (but are on the way): https://github.com/bjpop/blip/wiki/Not-yet-implemented-features Wiki for more information: https://github.com/bjpop/blip/wiki Cheers, Bernie Pope.

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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 16 Apr '13

16 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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