- Haskell-Cafe - Haskell.org

[reactive] A pong and integrate
by Limestraël 20 May '10

20 May '10

Hello, I know there is a special mailing list for reactive, but I got no answers when sending to it. Also, maybe my problem is not totally reactive-centered, it concerns integration of an object speed to obtain its position with FRP. As a newbie with reactive, I'm trying to make a simple example, namely a pong. But I'm stuck with the paddle animation. Here is a how I proceed : A paddle: data Paddle = Paddle { paddleCol :: Double, paddleCenter :: Behavior Double } The paddle moves along a column, so only it's Y coordinate changes. paddleCenter is the Y coordinate of its center. This is how I create a paddle: data Direction = PaddleUp | PaddleDown | PaddleStill deriving (Eq, Show) newPaddle :: Behavior Direction -> Double -> Paddle newPaddle direction column = let position = fmap ((initY ^+^)) $ integral *beat* $ fmap ((speed *) . factor) direction factor PaddleUp = -1 factor PaddleDown = 1 factor _ = 0 speed = 50 -- pixels per second initY = 400 -- suppose we have a 800x600 window: paddle is right at the middle *beat = atTimes [0, 0.01..]* in Paddle column position Basically, a paddle is driven by a behavior which tells him to go up, down or to stand still. My problem is with the tuning of the beat, a ticker event which is used for the integration. How do I have to think ? - My speed is in pixels/second, so it has to tick every second? (atTime [0, 1..]) - My app will run at 60fps, so it has to tick 60 times per second? (atTime [0, 1/60..]) The main program (I'm not using any graphics library, just getting keys ('u' for up, 'd' for down) and displaying the current paddle pos) makeKeyboardEvent = do hSetBuffering stdin NoBuffering hSetBuffering stdout NoBuffering (buttonPressedSink, event) <- makeEvent =<< makeClock forkIO . forever $ getChar >>= buttonPressedSink return event main = do keyEvt <- makeKeyboardEvent let dirEvt = fmap key2dir keyEvt paddle = newPaddle (stepper PaddleStill dirEvt) 50 adaptE $ fmap print $ snapshot_ (paddleCenter paddle) $ atTimes [0, 0.01..] key2dir k = maybe PaddleStill snd $ find (($ k) . fst) [((== 'u'), PaddleUp), ((== 'd'), PaddleDown)] When I launch the program, I hit 'u' or 'd' and I see the position increase/decrease. If the beat ticks every second, I see the position being increased by 50 every second, which is normal, but it's a too low refresh rate for an app that is to run at 60fps. But when I set my beat to tick every 60 times per second, the position is well updated, but I clearly see that the display dramatically slows down after a few seconds of execution. Too heavy rate for integrate? That's why I'm asking: how should I use integrate?

6 10

cabal-install
by Serguey Zefirov 20 May '10

20 May '10

Why there is no switch to turn off any use of proxy in cabal-install? Or to supply username/password pair in command line. I have a strange situation: wget works like charm ignoring proxy (I downloaded Cabal and cabal-install to investigate the problem using wget), Firefox works like charm igoring proxy, and only cabal cannot access anything, because it follows proper proxy protocol and our proxy requires username and password. And I don't know of proper way to supply them through http_proxy environment variable. The most annoying part is that it is too much work to download and build cabal using wget and scripts - I once tried it for some other utility and didn't succeed. PS And while I am on it, I'd like to suggest that cabal-install should read common options like credentials from environment or configuration file.

3 5

cabal: problem building ffi shared library and significance of __stginit
by Anthony LODI 19 May '10

19 May '10

Hello, I'm trying to build some haskell code as a .so/.dll so that it can ultimately be used by msvc. I have it working when I compile by hand (listed below) but I can't get the exact same thing built/linked with cabal. On linux everything builds fine, but when I try to link the resulting .so file, I get an error about a missing '__stginit_CInterface' reference. Indeed I couldn't find that name in any of the cabal-generated .dyn_o files. I checked the output of 'cabal build -v' and it seems to be executing about the same thing that I'm executing manually so I'm not sure what could be going wrong. On windows cabal won't even configure since '--enable-shared' seems to imply '-dynamic' (right?), and that's not currently supported. Also, when I remove the line 'hs_add_root(__stginit_CInterface);', and the corresponding forward declaration, the program runs fine! Does ghc no longer need this call or are my toy programs just being lucky sofar? Cheers, - Anthony LODI ========================================= (archlinux32 -- 2.6.32) > hsc2hs CInterface.hsc > ghc --make CInterface > cat dll_init.h #pragma once extern void __stginit_CInterface(void); extern void sp_init(int*, char***); extern void sp_exit(void); > cat dll_init.c #include "HsFFI.h" #include "dll_init.h" void sp_init(int* p_argc, char*** p_argv) { hs_init(p_argc, p_argv); hs_add_root(__stginit_CInterface); } void sp_exit(void) { hs_exit(); } > ghc -c dll_init.c > ghc -o libfoo.so -shared -dynamic CInterface.o CInterface_stub.o dll_init.o > ghc -c ctest.c > ghc -threaded -o ctest -L. -lfoo ctest.o -optl-Wl,-rpath,'$ORIGIN' > ./ctest ========================================= (winxp is similar except without '-threaded' or '-dynamic') ========================================= > cat test.cabal ... library build-depends: base >=4 && <5 exposed-modules: CInterface c-source: dll_init.c ghc_options: -threaded > cabal clean > cabal configure --enable-shared > cabal build

2 2

(no subject)
by R J 19 May '10

19 May '10

This is another proof-layout question, this time from Bird 1.4.7. We're asked to define the functions curry2 and uncurry2 for currying and uncurrying functions with two arguments. Simple enough: curry2 :: ((a, b) -> c) -> (a -> (b -> c))curry2 f x y = f (x, y) uncurry2 :: (a -> (b -> c)) -> ((a, b) -> c)uncurry2 f (x, y) = f x y The following two assertions are obviously true theorems, but how are the formal proofs laid out? 1. curry2 (uncurry2 f) x y = f x y 2. uncurry2 (curry 2 f) (x, y) = f (x, y) _________________________________________________________________ The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. http://www.windowslive.com/campaign/thenewbusy?tile=multiaccount&ocid=PID28…

2 1

Proof format
by R J 19 May '10

19 May '10

Is this how a rigorous Haskeller would lay out the proofs of the following theorems? This is Bird 1.4.6. (i) Theorem: (*) x = (* x) Proof: (*) x = {definition of partial application} \y -> x * y = {commutativity of "*"} \y -> y * x = {definition of "(* x)"} (* x) (ii) Theorem: (+) x = (x +) Proof: (+) x = {definition of partial application} \y -> x + y = {definition of "(x +)"} (x +) (iii) Theorem: (-) x /= (- x) Proof: (-) x = {definition of partial application} \y -> x - y /= {definition of prefix negation, which is not a section} (- x) _________________________________________________________________ Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

2 1

Bug with [Double]
by Dmitry Olshansky 19 May '10

19 May '10

Hello all, It seems that I saw something like this in Cafe recevtly. But I am not sure... In GHC 6.12.1 (Platform 2010 on Windows Vista) I have Prelude> [1,1+2/3..10] [1.0,1.6666666666666665,2.333333333333333,2.9999999999999996,3.666666666666666,4.333333333333332,4.999999999999998,5.666666666666664,6.33333333333333,6.9999999999999964,7.6666666666666625,8.333333333333329,8.999999999999995,9.66666666666666,10.333333333333327] -- It is a bug! Prelude> [1,5/3..10] [1.0,1.6666666666666667,2.3333333333333335,3.0,3.6666666666666665,4.333333333333333,5.0,5.666666666666667,6.333333333333334,7.000000000000001,7.666666666666668,8.333333333333336,9.000000000000004,9.666666666666671] -- correct, but... Prelude> [1,5/3..4] [1.0,1.6666666666666667,2.3333333333333335,3.0,3.6666666666666665,4.333333333333333] -- ... wrong again Prelude> [1,1+2/3..10] :: [Float] [1.0,1.6666667,2.3333335,3.0000002,3.666667,4.333334,5.000001,5.666668,6.333335,7.000002,7.666669,8.333336,9.000003,9.66667] -- correct Prelude> [1,1+2/3..10] :: [Double] [1.0,1.6666666666666665,2.333333333333333,2.9999999999999996,3.666666666666666,4.333333333333332,4.999999999999998,5.666666666666664,6.33333333333333,6.9999999999999964,7.6666666666666625,8.333333333333329,8.999999999999995,9.66666666666666,10.333333333333327] -- wrong Any comments?

7 11

Numerical Analysis
by Pierre-Etienne Meunier 19 May '10

19 May '10

Hello Cafe, Being a complete beginner in the field of numerical analysis, but anyway needing it to solve "real problems", I wrote a few functions recently to solve systems of polynomial equations using the "projected polyhedron" method by Maekawa and Patrikakalis. This requires solving systems of linear equations precisely, thus the simple Gauss method was not enough, and I had to write also an algorithm for the "SVD decomposition". Upon discovering the algol / fortran specifications of these :-( , heavily published in important journals, I thought it would be nice to provide the world with fast reliable implementation of these numerical methods (i.e. not simply bindings to lapack). Moreover, writing numerical things in haskell is much more pleasant than I thought at first. Here are a few random thoughts on this : - The haskell 98 norm does not require enough about IEEE-754 compliance, thus C bindings are still needed to guess for instance the machine epsilons, or manipulating ULPs. Moreover, taking advantage of hardware rounding is not easy, even if the hardware is IEEE-compliant : calling a C function from haskell screws up the speed advantages of hardware rounding, for instance. Maybe the new LLVM backend will make this possible ? - The current Array library is definitely not adapted to production code. It makes debugging tricky, requires a heavy use of Debug.Trace to actually see what happens, and does not seem as fast as one could expect. It seems that each algebra library on hackage redefines part of it, but a unified version would be nice : a discussion within the haskell community seems to be needed... - A numerical analysis library should really take advantage of the parallelism in GHC, especially with the arrival of hardware such as fermi (anyway, I do not know how much haskell is compilable to fermi code). The love for loops and side-effects among this community is hard to understand, but that's more of a cultural problem. Finally, as stated by William Stein, the creator of SAGE, of course it would take thousands of man-years to rewrite these codes in python, but if a language like haskell, and a compiler do 90% of the work, how many man-years are left ? If anyone here has got the time, the team and the will to start such a project, I'd love to contribute ! Cheers, PE

7 13

Re: [Haskell-cafe] Bug with [Double]
by Dmitry Olshansky 19 May '10

19 May '10

But Prelude Data.List> [1,1+2/3..4] :: [Double] [1.0,1.6666666666666665,2.333333333333333,2.9999999999999996,3.666666666666666,4.333333333333332] Prelude Data.List> unfoldr (\n -> let n'=n+2/3 in if n' <= 4 then Just (n',n') else Nothing) 1 :: [Double] [1.6666666666666665,2.333333333333333,2.9999999999999996,3.666666666666666] Prelude Data.List> takeWhile (<=4) $ iterate (+2/3) 1 :: [Double] [1.0,1.6666666666666665,2.333333333333333,2.9999999999999996,3.666666666666666] How 'dodgy' it should be to produce different result? How [a,b..c] works in this case? > > > 2010/5/19 Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com>: >> Dmitry Olshansky <olshanskydr(a)gmail.com> writes: >> >>> Hello all, >>> >>> It seems that I saw something like this in Cafe recevtly. But I am not sure... >>> In GHC 6.12.1 (Platform 2010 on Windows Vista) I have >>> >>> Prelude> [1,1+2/3..10] >>> [1.0,1.6666666666666665,2.333333333333333,2.9999999999999996,3.666666666666666,4.333333333333332,4.999999999999998,5.666666666666664,6.33333333333333,6.9999999999999964,7.6666666666666625,8.333333333333329,8.999999999999995,9.66666666666666,10.333333333333327] >>> >>> -- It is a bug! >> >> No it isn't, because of the dodgy Ord instance for Float and Double values. >> >> -- >> Ivan Lazar Miljenovic >> Ivan.Miljenovic(a)gmail.com >> IvanMiljenovic.wordpress.com >> >

1 0

Type famillies & Lifting IO
by Maciej Piechotka 19 May '10

19 May '10

I started playing with type families. I wanted to achieve, for the beginning, something like: > import qualified Control.Monad.IO.Class as IOC > import Control.Monad.Trans.Class > import Control.Monad.Trans.Cont > import Data.Functor.Identity > class (Monad m, Monad (IO' m)) => MonadIO m where > type IO' m :: * -> * > liftIO :: IO a -> IO' m a > liftM :: m a -> IO' m a It allows to add IO to computation even if computation originally was 'pure'. First step was easy: > instance MonadIO Identity where > type IO' Identity = IO > liftIO = id > liftM = return . runIdentity > > instance MonadIO IO where > type IO' IO = IO > liftIO = id > liftM = id > > instance MonadIO (ST r) where > type IO' (ST r) = IO > liftIO = id > liftM = unsafeSTToIO > > --instance IOC.MonadIO m => MonadIO m where > --type IO' m = m > --liftIO = IOC.liftIO > --liftM = id However I run into problems - this code doesn't want to compile: > instance MonadIO m => MonadIO (ContT r m) where > type IO' (ContT r m) = ContT r (IO' m) > liftIO f = ContT $ \cont -> liftIO f >>= cont > liftM f = ContT $ \cont -> liftM f >>= cont Or this: > instance MonadIO m => MonadIO (ContT r m) where > type IO' (ContT r m) = ContT r (IO' m) > liftIO f = lift . liftIO > liftM f = lift . liftIO In fact there is strange interfering types of ghci: ghci> :t lift . liftIO lift . liftIO :: (m ~ IO' m1, MonadTrans t, Monad m, MonadIO m1) => IO a -> t m a ghci> :t lift . liftIO :: (m ~ IO' m1, MonadTrans t, Monad m, MonadIO m1) => IO a -> t m a <interactive>:1:7: Couldn't match expected type `IO' m' against inferred type `m1' `m1' is a rigid type variable bound by an expression type signature at <interactive>:1:18 NB: `IO'' is a type function, and may not be injective In the second argument of `(.)', namely `liftIO' In the expression: lift . liftIO :: (m ~ (IO' m1), MonadTrans t, Monad m, MonadIO m1) => IO a -> t m a What's the problem? I guess I don't understand something basic about type famillies. Regards

1 0

What do _you_ want to see in FGL?
by Ivan Lazar Miljenovic 19 May '10

19 May '10

Since I've volunteered myself to help maintain/upgrade FGL, what do the people in the community want to see happen with it? Here are some ideas that I have regarding FGL: * I had already started working on a new generic graph class [1] (with initial draft at [2]) to act as a wrapper around FGL and other graph implementations. However, if I'm going to be updating FGL anyway I might as well combine the two and generalise/update FGL instead. Generally speaking, this will involve the following items: - Splitting and expanding the typeclasses involved; most class methods would have default definitions but this will allow people to override the defaults if there's a more efficient way for their datatype. - Having a separate parameter (using associated types?) for the node type rather than just using Int. - Providing more "fundamental" graph operations that may be of use. However, the downside of this is that datatypes such as Data.Graph in containers won't match this anymore due to the lack of node and edge labels. I think this fair price to pay, especially if the mapping aspects are split off into another typeclass. [1]: http://www.haskell.org/pipermail/haskell-cafe/2009-June/063402.html [2]: http://code.haskell.org/~ivanm/Graph.hs * Better fundamental data structures: one of the things that has always annoyed me about FGL is how much it uses tuples; I propose re-defining the Context type to be a record-based data structure. Also, usage of Sets, Maps, etc. where applicable. * Proper instances: make graphs instantiate Eq, Ord, Show and Read (where read . show == id). The Show and Read instances will probably usually be like how Data.Map has them, so helper functions can be defined to help out with this. * Split off the instances: maybe keep one basic graph datatype in FGL, but then have separate packages for others (which means its easier to add new datatypes without needing a new release, etc.). * Split off the various Data.Graph.Inductive.Query functions/algorithms, and merge in the ones I have defined in Graphalyze [3]. [3]: http://hackage.haskell.org/package/Graphalyze * Scrap Data.Graph.Inductive.Graphviz and recommend usage of my graphviz library [4] instead. [4]: http://hackage.haskell.org/package/graphviz * Data.Graph.Inductive.Monad: does anyone actually use this? * Benchmarks and tests. What do other people think? Any other suggestions? Also, once I release the next version of graphviz (hopefully later today), I was going to work on improving SourceGraph [5] by providing command line options, etc. to it; would people prefer I work on FGL instead? [5]: http://hackage.haskell.org/package/SourceGraph -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

9 48

GADTs and Scrap your Boilerplate
by Oscar Finnsson 19 May '10

19 May '10

Hi, I'm writing a XML (de)serializer using Text.XML.Light and Scrap your Boilerplate (a thttp://github.com/finnsson/Text.XML.Generic) and so far I got working code for "normal" ADTs but I'm stuck at deserializing GADTs. I got the GADT data DataBox where DataBox :: (Show d, Eq d, Data d) => d -> DataBox and I'm trying to get this to compile instance Data DataBox where gfoldl k z (DataBox d) = z DataBox `k` d gunfold k z c = k (z DataBox) -- not OK toConstr (DataBox d) = toConstr d dataTypeOf (DataBox d) = dataTypeOf d but I can't figure out how to implement gunfold for DataBox. The error message is Text/XML/Generic.hs:274:23: Ambiguous type variable `b' in the constraints: `Eq b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Show b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Data b' arising from a use of `k' at Text/XML/Generic.hs:274:18-30 Probable fix: add a type signature that fixes these type variable(s) It's complaining about not being able to figure out the data type of b. I'm also trying to implement dataCast1 and dataCast2 but I think I can live without them (i.e. an incorrect implementation). I guess my questions are: 1. Is it possible to combine GADTs with Scrap your Boilerplate? 2. If so: how do you implement gunfold for a GADT? -- Oscar

6 7

Help with Bird problem 1.4.1
by R J 19 May '10

19 May '10

Newbie trying to get through Bird. Could someone provide a clean solution, with proof (so I can see how these proofs are laid out), to this: Given: f :: Integer -> Integerg :: Integer -> (Integer -> Integer) h :: ...h x y = f (g x y) Questions: a. Fill in the type assignment for "h". b. Which of the following is true: (i) h = f . g (ii) h x = f . (g x) (iii) h x y = f . (g x y) _________________________________________________________________ Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

2 1

Wait forever in main thread
by DPX-Infinity 18 May '10

18 May '10

Hi, I'm writing a program which listens to some D-Bus signals using DBus.Client.onSignal function from dbus-client package. This function runs IO action in separate haskell thread when signal is received. My program does nothing except signal handling, so after setting up signals it has to wait indefinitely. Now I'm using not very clean (I think so) forever $ threadDelay 10000000 . Is there another (I mean, correct) method to do this thing?

6 10

executeFile failing on macosx
by David Powell 18 May '10

18 May '10

"executeFile" is failing for me on Mac OS X 10.5.8, with ghc 6.12.1 when compiling with "-threaded". Compiling without -threaded, or running on linux is fine. When compiled with -threaded, the following snippet produces the error: testProg: /bin/echo: executeFile: failed (Operation not supported) > import System.Posix.Process > main = do > > forkProcess $ executeFile "/bin/echo" False ["Ok"] Nothing Any suggestions for a work around for this would be appreciated. Cheers, -- David Powell

4 5

Windows 7 Permission Denied Problem on Cabal
by Ralph Hodgson 18 May '10

18 May '10

Following a successful install of the Haskell Platform on MAC OSX, I proceed to update my Windows PC. Frustration followed. I am blocked by a "Permission Denied" error when I do a "Setup install": >ghc --make Setup [58 of 58] Compiling Main ( Setup.hs, Setup.o ) Linking Setup.exe ... >Setup configure Configuring Cabal-1.8.0.4... >Setup build Preprocessing library Cabal-1.8.0.4... Building Cabal-1.8.0.4... Registering Cabal-1.8.0.4... >Setup install Setup: permission denied > I changed the permission on folders in the "Haskell Platform" directories to "full control", without any effect. I have searched the web for help but could not find my problem. Can't help but think it is something trivial but what? Help appreciated. Ralph Hodgson, @ralphtq <http://twitter.com/ralphtq>

4 6

GHC Install button grayed out
by David Matuszek 18 May '10

18 May '10

I'm trying to install Haskell Platform 2010.1.0.1 on my Mac, downloaded from http://hackage.haskell.org/platform/ I have: Mac OS X 10.6.3 2 x 2.66 GHz Dual-Core Intel Xeon XCode 3.1.3 Also, I am an admin on this machine. When I try to install GHC-6.12.1-i386.pkg, I agree to the license, the installer skips over Destination Select, and brings me to the Standard Install page, with the Install button grayed out. It remains grayed out for several minutes (and presumably forever). I subsequently tried the 2009 platform, with the same results. Any idea what the problem might be? Thanks, Dave Matuszek PS. I really dislike MacPorts, so please don't suggest that.

2 1

ShowList magic
by Abby Henríquez Tejera 18 May '10

18 May '10

Hi. I'm a Haskell newbie and there's a bit of Haskell code that I don't understand how it works. In the prelude, defining the class Show, the function showList is implemented twice, one for String and another one for other lists: showList cs = showChar '"' . showl cs where showl "" = showChar '"' showl ('"':cs) = showString "\\\"" . showl cs showl (c:cs) = showLitChar c . showl cs and showList [] = showString "[]" showList (x:xs) = showChar '[' . shows x . showl xs where showl [] = showChar ']' showl (x:xs) = showChar ',' . shows x . showl xs The thing is... how does Haskell «know» which to execute? It works even for the blank string: Prelude> show "" "\"\"" Prelude> show [] "[]" Salud, Abby

5 7

ANN: Leksah 0.8
by Jürgen Nicklisch-Franken 18 May '10

18 May '10

This is our official 0.8 release. Anyone still using 0.6 should upgrade. Anyone using an earlier 0.8 release should upgrade too (and delete ~/.leksah-0.8/prefs.lkshp and ~/.leksah-0.8/prefscoll.lkshp). There's lots of new stuff, so please have a look if you can. Let us know which of the things that aren't there you think we should work on next. Some changes in 0.8: * Workspaces (for working on projects with multiple packages) * Better metadata (includes nonexported definitions in workspace packages) * Optional downloading of prebuilt metadata for packages (you can turn this off in the first start dialog) * Better completion (keywords, language extensions, module name, non exported definitions) * Split in a client and server part (Client part doesn't import ghc-api) * Compiles with GHC 6.12 * Many bug fixes and little enhancements * Unstable and very limited Yi mode (vim bindings only for now) Not new but worth mentioning: * <Ctrl> <Enter> evaluates current line or selection in GHCi * <Ctrl> R adds missing imports To building Leksah yourself: * You need GHC 6.10 or 6.12 installed. The preferred way is current Haskell platform. * You need Gtk2Hs including the GtkSourceView2 package. (Currently gtk2hs has no release for 6.12. On Linux we suggest you use the darcs version and install it via cabal install) * cabal install leksah * leksah Making a Gtk application look nice on Windows and OS X is not easy so we recommend using one of these prebuilt binaries: http://leksah.org/packages/leksah-0.8.0.6.exe (Windows) http://leksah.org/packages/leksah-0.8.0.6.dmg (OS X) (Installation of gtk2hs for the development of gtk2hs apps is not done by these installers. Please consult http://leksah.org/download.html for more details about installation! Background information can be found in the Leksah manual: http://leksah.org/leksah_manual.pdf. Known bugs and problems: * If you have installed an earlier 0.8 version please delete your ~/.leksah-0.8/prefs.lkshp and ~/.leksah-0.8/prefscoll.lkshp * The server may occasionally allocate much memory, which has been observed on Windows. You can simply restart leksah (of leksah-server if you have started it separately) * leksah don't run with ghc-6.12.2 (http://http://hackage.haskell.org/trac/ghc/ticket/4038) Have fun Jürgen & Hamish

1 0

Getting a string from url-converted UTF8 input
by Eugene Dzhurinsky 17 May '10

17 May '10

Hello all! Can somebody please explain wha am I doing in wrong way? =============================================================== module UrlEncode where import System import Codec.Binary.UTF8.String as SU import Codec.Binary.Url as U import Data.Maybe main :: IO () main = do args <- getArgs processWithArgs args processWithArgs ("-d":[]) = getLine >>= putStrLn . maybe "" SU.decode . U.decode processWithArgs ("-e":[]) = getLine >>= putStrLn . U.encode . SU.encode processWithArgs _ = putStrLn "Usage: -e (encode) or -d (decode)" =============================================================== With this script if fed with input: =============================================================== 1%29%20%D0%B3%D0%B4%D0%B5%20%D0%BD%D1%8B%D0%BD%D1%87%D0%B5%20%D0%BC%D0%BE%D0%B4%D0%BD%D0%BE%20%D0%B1%D1%80%D0%B0%D1%82%D1%8C%20%D0%BA%D0%BD%D0%B8%D0%B6%D0%BA%D0%B8%20%D0%B2%20%D0%B2%D0%B8%D0%B4%D0%B5%20FB2%3F%0D%0A%0D%0A2%29%20%D0%BA%D0%B0%D0%BA%D0%BE%D0%B9%20%D0%B5%D1%81%D1%82%D1%8C%20%D1%81%D0%BE%D1%84%D1%82%20%D0%BD%D0%B0%20%D0%B6%D0%B5%D0%BB%D0%B5%D0%B7%D0%BA%D1%83%20%D1%82%D0%B8%D0%BF%D0%B0%20%D1%82%D0%B5%D0%BB%D0%B5%D1%84%D0%BE%D0%BD%20%D1%81%20Symbian =============================================================== I am getting the output: =============================================================== 1) 345 =K=G5 <>4=> 1@0BL :=86:8 2 2845 FB2? 2) :0:>9 5ABL A>DB =0 65;57:C B8?0 B5;5D>= A Symbian =============================================================== which is wrong. So what do I miss in encoding the data in UTF? Thank you in advance! -- Eugene Dzhurinsky

3 4

ANN: cil-0.0.1
by Tom Hawkins 17 May '10

17 May '10

CIL [1] is an OCaml library that parses and compiles C down to a simplified subset to ease different forms of static analysis. Frama-C [2] augments CIL with a property specification language (ACSL), which can capture design contracts for C functions. Frama-C's Jessie plugin uses the Why [3] software verification platform, which itself uses several different SMT solvers and proof assistances to verify ACSL function contracts -- a very cool platform, I encourage anyone interested in the verification of C programs to check it out. This Hackage library [4] is an interface to the Frama-C environment. Specifically, it provides equivalent CIL and ACSL data types, allowing CIL and ACSL based analyzers to be written in Haskell. The library installs a Frama-C/OCaml plugin, which dumps the CIL database into Haskell. Internally, both the OCaml and Haskell sides of the interface are generated by a script (GenCIL.hs) that parses the contents of the CIL type definitions (cil_types.mli). Currently it only works for the most basic C programs. A bug in either the generated OCaml plugin or Frama-C causes issues [5]. -Tom [1] http://cil.sourceforge.net/ [2] http://frama-c.com/ [3] http://why.lri.fr/ [4] http://hackage.haskell.org/package/cil [5] http://bts.frama-c.com/view.php?id=481

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What makes Haskell difficult as .NET?
by Daryoush Mehrtash 17 May '10

17 May '10

In this presentation http://norfolk.cs.washington.edu/htbin-post/unrestricted/colloq/details.cgi… the speaker talks about F# on .Net platform. Early on in the talk he says that they did F# because haskell would be "hard to make as a .Net language". Does anyone know what features of Haskell make it difficult as .Net language? Daryoush

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Monadic style with Streams (as in Data.Array.Parallel.Stream)
by Mark Wassell 17 May '10

17 May '10

Hi, This possibly might go against the spirit of what Stream programming is about but I having difficulties converting an imperative algorithm [1] into Haskell and think it would be easier if I was able to write it in a monadic style with operations to read and write from and to the streams. I first tried to approach it by delving into the innards of other Stream functions to devise what I needed. I only got so far and the sticking point was defining the Monad. I then approached it from the Monad side and although what I have is workable, it probably isn't going to perform (for one it uses fromStream and tailS on each read off the front of the stream). So: 1. Is this monadic style within the spirit of what Stream programming is about? 2. Is there anyway to do this more elegantly and without the user of fromStream and tailS, for example. This is the workable solution I have: module StreamMonad where import Data.Array.Parallel.Stream import Data.Monoid import Control.Monad.Writer import Control.Monad.State instance Monoid (Stream a) where mempty = emptyS mappend = (+++) type SM a b c = StateT (Stream a) (Writer (Stream b)) c readS :: SM a b a readS = do s <- get let a = head $ fromStream s put $ tailS s return a writeS :: b -> SM a b () writeS x = tell $ singletonS x t1' :: SM (Int,Int) Int () t1' = mapM_ (\_ -> do (x,y) <- readS writeS x writeS y) [1..2] t1 = fromStream $ snd $ runWriter $ runStateT t1' $ toStream [(1,2),(3,4)] -- At least this works .. t2 = fromStream $ snd $ runWriter $ runStateT t1' $ toStream [(2*x-1,2*x) | x <- [1..] ] Cheers Mark [1] The arithmentic coding and decoding algorithms given in http://mattmahoney.net/dc/dce.html#Section_32

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ANN: Monad.Reader Issue 16
by Brent Yorgey 16 May '10

16 May '10

I am very pleased to announce that Issue 16 of The Monad.Reader is now available [1]. Issue 16 consists of the following three articles: * "Demand More of Your Automata" by Aran Donohue * "Iteratee: Teaching an Old Fold New Tricks" by John W. Lato * "Playing with Priority Queues" by Louis Wasserman Feel free to browse the source files [2]. You can check out the entire repository using darcs: darcs get http://code.haskell.org/~byorgey/TMR/Issue16 If you'd like to write something for Issue 17, please get in touch. The deadline will likely be sometime in September; more details will be forthcoming. -Brent [1] http://themonadreader.files.wordpress.com/2010/05/issue16.pdf [2] http://code.haskell.org/~byorgey/TMR/Issue16

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Speed of Error handling with Continuations vs. Eithers
by Max Cantor 16 May '10

16 May '10

Based on some discussions in #haskell, it seemed to be a consensus that using a modified continuation monad for Error handling instead of Eithers would be a significant optimization since it would eliminate a lot of conditional branching (everytime >>= is called in the Either monad, there is a conditional. I implemented a ErrCPS monad which does exactly that, but the speed has been disappointing. It runs almost exactly 3x slower than a drop in replacement using the MonadError instance of Either from mtl. mkEMA and midError are basically toy functions but I dont know why Either is so much faster. I've experimented with putting some seq's in the bindErrCPS and even {-# INLINE (>>=) #-} in the Monad instance, but to no avail. I've copy/pasted the code below, any suggestions on optimization, or if this is simply a bad idea would be much appreciated. Strangely, compiling with -O2 seems to have no effect on the speed: -Max > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE Rank2Types #-} > module Main where > > import Control.Applicative > import Control.Monad.Error -- hiding (foldM) > import Control.Monad.Trans > import Control.Monad hiding (foldM) > import System.Random > import Control.Monad.Identity (runIdentity, Identity) > import Control.Monad.Reader.Class > import Data.Time.LocalTime as Time -- for benchmarking > import Data.Time.Calendar (Day) > import Data.Time.LocalTime (getZonedTime) > midError :: MonadError String m => Double -> Double -> m Double > midError a b = if (b < 1) then throwError "check val" > else let r = (a + b) / 2 in r `seq` (return r) > mkEMA l = foldM midError 1 l > newtype ErrCPS e m a = ErrCPS { runErrCPS :: forall r . (e -> m r) -- error handler > -> (a -> m r) -- success handler > -> m r } > > {-# INLINE retErrCPS #-} > retErrCPS :: a -> ErrCPS e m a > retErrCPS x = ErrCPS $ \_ good -> good x > > {-# INLINE bindErrCPS #-} > bindErrCPS :: ErrCPS e m b -> (b -> ErrCPS e m a) -> ErrCPS e m a > bindErrCPS m f = ErrCPS $ \err good -> runErrCPS m err $ \x -> runErrCPS (f x) err good > > instance Monad m => Monad (ErrCPS e m) where > return = retErrCPS > (>>=) = bindErrCPS > main :: IO () > main = do > let n = 500000 > runEither e b g = either b g e > runTest f s = do > sg <- newStdGen > let l = take n $ randomRs (2, 50000) sg > mapM_ (\e -> e `seq` return ()) l > stopwatch $ f (mkEMA l) > (putStr . show) > (putStr . (s ++) . show) > > forever $ do runTest runEither "either: " > runTest runErrCPS "errCPS: " ErrCPS based code seems to run almost exactly 3x slower than the Either based code: errCPS: 37453.226 Action ran in: 30 msec either: 26803.055 Action ran in: 11 msec errCPS: 15840.626 Action ran in: 34 msec either: 32556.881 Action ran in: 10 msec errCPS: 38933.121 Action ran in: 30 msec either: 35370.820 Action ran in: 11 msec ... > > instance (Error e, Monad m) => MonadError e (ErrCPS e m) where > throwError = errCPS > catchError m f = ErrCPS $ \err good -> runErrCPS m (\e -> runErrCPS (f e) err good) good > > > -- * MTL stuff > instance MonadTrans (ErrCPS e ) where lift m = ErrCPS $ \_ good -> m >>= good > instance (MonadIO m) => MonadIO (ErrCPS e m ) where liftIO = lift . liftIO > Random utility stuff > stopwatch :: IO () -> IO () > stopwatch act = do > t1 <- getFastTimeOfDay > act > t2 <- getFastTimeOfDay > putStrLn $ " Action ran in: " ++ show (t2 - t1) ++ " msec" > type FastTimeOfDay = Int > > -- | Return the current trading day. This should respect the > -- fact that the Trading Day ranges from > -- SingTime 6am (UTC -02:00) to SST 5:59 am (UTC -1:59). > getTradingDay :: IO Day > getTradingDay = error "getTradingDay undefined" > > getFastTimeOfDay :: IO FastTimeOfDay > getFastTimeOfDay = getZonedTime >>= > (return . fastFromTimeOfDay . Time.localTimeOfDay . Time.zonedTimeToLocalTime) > > timeOfDayFromFast :: FastTimeOfDay -> Time.TimeOfDay > timeOfDayFromFast fast = Time.TimeOfDay > { Time.todHour = fromIntegral (fast `div` (3600 * 1000)) > , Time.todMin = fromIntegral (fast `div` (60 * 1000)) `mod` 60 > , Time.todSec = fromRational $ (fromIntegral fast) / 1000 > } > > fastFromTimeOfDay :: Time.TimeOfDay -> FastTimeOfDay > fastFromTimeOfDay t = fromIntegral $ > ((Time.todHour t) * 3600000) + > ((Time.todMin t) * 60000) + > (round $ 1000 * Time.todSec t) > > instance (Monad m) => Functor (ErrCPS e m) where > fmap f m = ErrCPS $ \err good -> runErrCPS m err (good . f) > > instance (Monad m) => Applicative (ErrCPS e m) where > pure = return > f <*> a = do f' <- f > a' <- a > return $ f' a' > > errCPS :: forall e m a . e -> ErrCPS e m a > errCPS e = ErrCPS $ \err _ -> err e > >

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What Object Oriented Paradigms Does Haskell Support?
by John Creighton 16 May '10

16 May '10

In response to the discussion What makes Haskell difficult as .NET? http://groups.google.ca/group/haskell-cafe/browse_thread/thread/f61ee38f208… I thought I'd start a discussion on what object oriented features Haskell supports. A good reference is: Haskell's Overlooked Object System http://homepages.cwi.nl/~ralf/OOHaskell/paper.pdf The comparison is often made of Haskell's classes to interface. However, once we define an instance we have something concrete which is closer to a class under the object oriented paradigm. If our instances are paramatric: MyType a = MyType{.......child::a} Then when we can create subclass instances which are sybtypes of our Paramatric type. This gives us inheritance. Overriding might be slightly trickier. We can certainly create new instances of the parent class for a specific subtype. As a minimum we will need to specify each method we created which is not defined in terms of the others (our getters and setters) and perhaps we are allowed to over ride the methods which are derived from the other methods. Anyway, I'll do some re reading of the paper, experiment with some code and see what other options there are.

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ANNOUNCE: hmumps, a MUMPS interpreter in Haskell
by Antoine Latter 15 May '10

15 May '10

Hi folks, As an educational project a few years back I started working on an interpreter for the MUMPS language in Haskell. It's got a REPL and can call into functions defined in external files. It doesn't support a lot of the built-in-functions, nor does it have support for on-disk persistent variabls (globals, in MUMPS speak). But it's on Hackage, in case anyone else is pervers enough to be interested in such a thing. I started it a few years back, so I wouldn't say it has an ideal structure. But it was surprisingly easy to come back to it and extend the parser/runtime in the past few weeks. hackage link: http://hackage.haskell.org/package/hmumps MUMPS wikipedia link: http://en.wikipedia.org/wiki/MUMPS Feedback is appreciated. Antoine

1 0

Help debugging code broken after upgrading debian to GHC 6.12: "invalid argument"
by Brandon Simmons 15 May '10

15 May '10

> > GHC 6.12's runtime handles input and output encoding, instead of > > simply truncating Chars; my guess is it's locale-related. And sure > > enough, I see several non-ASCII characters in mycology.b98 which are > > likely to do the wrong thing if the runtime doesn't know which > > character set to use. > > Yup. Converting mycology.b98 to utf-8 makes it run. > However, why does it run with --quiet on the original file?? That actually makes sense. That mycology Befunge source is actually a combination of Befunge-98 (which can use non-ascii characters. I'd forgotten that) and befunge-93 which uses only printable ascii characters. Without the --quiet flag, we end up inspecting a Bool value returned after taking the `length` of a row and a column, which looks to see if the grid is bigger than 80x25 characters. I guess with the --quiet flag, we ignore that Bool value and so the program never has to take the length of a line, and consequently never sees any of the non-ascii characters (because the source file is written in such a way that a befunge-93 interpreter will naturally stop execution after the '93 section of code. That also explains why simply printing the file string causes the interpreter to fail, even with --quiet. Thanks for the help,

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Help debugging code broken after upgrading debian to GHC 6.12: "invalid argument"
by Brandon Simmons 15 May '10

15 May '10

>On May 14, 2010, at 20:24 , Brandon Simmons wrote: >> The other baffling thing is this: if the debugging line 426 is >> uncommented, then even running: >> >> $> runghc Befunge.hs --quiet mycology.b98 >> >> ...will fail. But all we're doing is a call to `putStr`! Why would >> that trigger an error?! Maybe there was a bug in my code that was > > > GHC 6.12's runtime handles input and output encoding, instead of > simply truncating Chars; my guess is it's locale-related. And sure > enough, I see several non-ASCII characters in mycology.b98 which are > likely to do the wrong thing if the runtime doesn't know which > character set to use. Thanks, that makes a lot of sense. Is there something I can read to enlighten myself as to how I would go about getting my program to be liberal about non-ascii characters in a file? Also, I noticed yesterday that $> cabal list --installed shows the 'old-locale' package: * old-locale Synopsis: locale library Latest version available: 1.0.0.2 Latest version installed: 1.0.0.2 License: BSD3 I'm not sure if that could be part of the issue, or whether that is normal. Thanks again, Brandon Simmons http://coder.bsimmons.name/ > > -- > brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com > system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu > electrical and computer engineering, carnegie mellon university KF8NH

2 1

Help debugging code broken after upgrading debian to GHC 6.12: "invalid argument"
by Brandon Simmons 15 May '10

15 May '10

I developed my Befunge interpreter that I posted here: http://coder.bsimmons.name/blog/2010/01/a-befunge-93-interpreter/ ...under GHC 6.8 Debian packages, and after revisiting the code, my interpreter fails on a particular file in a way that is truly baffling me. I don't even know where to begin. I am certain this code was working before, because the Befunge source it fails on is in my darcs test suite: The error I get is an InvalidArgument IOError. I'm not even sure what that means. Here are the imports I'm using in the module in case that makes the problem with my upgrade obvious: import System.Environment import Data.Char import Control.Arrow (first,second,(&&&),(|||)) import Data.Array.Unboxed import Control.Monad.State.Strict import System.Random import System.IO import System.IO.Error If anyone wants to look at my code and help me with this, they can download the three attached files and run the following: $> runghc Befunge.hs aturley.bf $> runghc Befunge.hs --quiet aturley.bf ...which should both print a pretty picture. Now try the file giving me problems: $> runghc Befunge.hs --quiet mycology.b98 ... which should give correct output (a bunch of line of "GOOD: blah...."). And finally: $> runghc Befunge.hs mycology.b98 ...which will (for me) simply print: "error processing options: invalid argument" The problem seems to have to do with line 427 and the call to `buildGrid` and the Bool value `wasBig`: while debugging I can print the array returned on that line, but trying to print the Bool simply fails with the error. The other baffling thing is this: if the debugging line 426 is uncommented, then even running: $> runghc Befunge.hs --quiet mycology.b98 ...will fail. But all we're doing is a call to `putStr`! Why would that trigger an error?! Maybe there was a bug in my code that was there before, but wasn't showing its head because something was being lazier than it is now? If anyone wants to take the time to help me with this, I would be most grateful. SIncerely, Brandon Simmons http://coder.bsimmons.name/

3 2

MultiParamTypeClasses, FunctionalDependencies and FlexibleInstances using GHCi
by Julian Fleischer 14 May '10

14 May '10

Hello, i'm playin' around with GHCs Haskell and some extensions. I'm already aware of that functional dependencies are "very very tricky", but there is something I don't understand about there implementation in GHC. I've constructed my own TypeClass "Num" providing a signature for (+), having multiple params a, b and c. I'm than declaring a (flexible) Instance for Prelude.Num, simply using (Prelude.+) for the definition of my (+) - and it does not work as I expect it to. First, this is the code: > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, TypeSynonymInstances, FlexibleInstances #-} > import qualified Prelude > > class Num a b c | a b -> c where > (+) :: a -> b -> c > > instance (Prelude.Num x) => Num x x x where > (+) = (Prelude.+) now if I load it into GHCi and type "3 + 4" i get a whole bunch of error-messages. I do understand that > (3::Prelude.Int) + (4::Prelude.Int) works, since I've explicitly declared 3 and 4 to be Prelude.Int and there is a functional dependency stating that (+) :: a b determines the results type c, by the Instance declaration cleary c will be the same as a and b. Now, if I type > 3 + 4 it does not work, and i really don't understand why. If i ask GHCi for 3's type ($ :t 3) it will answer "3 :: (Prelude.Num t) => t". But, if 3 and 4 are Prelude.Nums and there is an instanfe Num x x x for x of Prelude.Num - than why can't GHC deduce from the definitions that 3 and 4, both Prelude.Nums, can be used with (+) since there is an instance for Prelude.Num and my class Num - and the result will of course be something of Prelude.Num? best regards, Julian

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Modular type inference
by Simon Peyton-Jones 14 May '10

14 May '10

Friends Many of you will know that I've been muttering about re-engineering GHC's type inference engine for some time now. Dimitrios, Tom, Martin and I have just completed an epic paper describing the Glorious New Framework that forms the substance of the above mutterings: http://haskell.org/haskellwiki/Simonpj/Talk:OutsideIn We'd love comments and feedback. Dimitrios and I plan to roll up our sleeves and implement it in June. Simon Modular type inference with local assumptions: OutsideIn(X) Abstract. Advanced type system features, such as GADTs, type classes, and type families have have proven to be invaluable language extensions for ensuring data invariants and program correctness among others. Unfortunately, they pose a tough problem for type inference, because they introduce local type assumptions. In this article we present a novel constraint-based type inference approach for local type assumptions. Our system, called OutsideIn(X), is parameterised over the particular underlying constraint domain X, in the same way as HM(X). This stratification allows us to use a common metatheory and inference algorithm. Going beyond the general framework, we also give a particular constraint solver for X = type classes + GADTs + type families, a non-trivial challenge in its own right.

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Re: [Haskell-cafe] Why is TChan GHC specific?
by Edward Amsden 14 May '10

14 May '10

On Wed, May 12, 2010 at 3:29 PM, Peter Robinson <thaldyron(a)gmail.com> wrote: > As far as I know, TChan needs the 'retry' combinator which requires GHC's RTS. > Same is true for TMVar, I think. (sorry for the doubling peter, I forgot reply-all) OK. I'm new to this and probably didn't know where to look, but I didn't know that 'retry' was GHC specific.

3 3

tweak vacuum-* output
by Ozgur Akgun 14 May '10

14 May '10

Hi all, I am using vacuum-opengl and vacuum-ubigraph to visualise and analyse some of my data structures. They are quite helpful most of the time, however sometimes I feel the need to tweak the generated output -- such as removing the auto-generted identifiers from constrcutor names, pack some things together, or similar. Is there a way to configure their output? And for the vacuum-ubigraph option, I like it's output generally, however while creating the expression tree, is doesn't respect my structures. If there is a flag or so to fix this issue, I'd appreciate it. The Problem is like the following: data Expr = Sum Expr Expr | Mult Expr Expr | Single Int e = Sum (Single 2) (Mult (Single 3) (Single 4)) And it orients the tree in such a way that Mult looks like the root node, instead of Sum, as I would expect. Thanks, Ozgur Akgun

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Concurrent Rings in Erlang and Haskell: A Haskell answer to Joe Armstrong's Erlang Challenge
by Aran Donohue 14 May '10

14 May '10

I wrote this a couple of weeks ago and forgot about it. I thought it might be of interest to some on this list: http://workshop.arandonohue.com/ring/

2 1

Ideas of a polymorphic Tree in Haskell
by Edgar Z. Alvarenga 14 May '10

14 May '10

Hi, I created a small Genetic Algorithm program, replicating this work ("Statistical mechanics of program systems" - JP Neirotti, N. Caticha, Journal of Physics A Math and Gen) made in Lisp. When a restricted the problem just for one type, the Haskell program worked perfectly with much less lines and a lot faster than the Lisp program. The problem was when I tried to generalize to polymorphic types. I'm using this datatype to represent a program: data Tree a = Bin (String, (a -> a -> a)) (Tree a) (Tree a) | Un (String, (a -> a)) (Tree a) | V And can convert a Tree to a function with: treeToFunc :: Tree a -> a -> a treeToFunc (Bin f l r) = (snd f) <$> treeToFunc l <*> treeToFunc r treeToFunc (Un f u) = (snd f).(treeToFunc u) treeToFunc V = id I already create another datatype to represent a polymorphic program (using GADT or existentials), but doesn't see a way to convert this kind of tree to a function. Anyone has any idea? Thanks, Edgar

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Re: Functions of type foo :: f a -> g a
by Gordon J. Uszkay 14 May '10

14 May '10

Thank you for all these answers - I learned a lot (especially that the responses come in REALLY quickly, so the digest option is not a good choice). I think I need to rephrase the problem, taking into account what I have learned. class (Functor f, Functor g) => Foo f g where foo :: f a -> g a as per John Meacham's suggestion to isolate the transform. Really, f and g are 'containers' which I want to reshape (and there can be many different re-shapings of g for each f, and different f's can map to the same g, so no fundeps relation holds in general). I am trying to use classes as an interface description, with this transformation being part of that interface, and also to allow me to experiment with different constraints (e.g. Monad f, Applicative f, etc). Edward Kmett points out (thank you for that detailed reply!) that there is no canonical foo -- which is a good point, and is indeed the right model -- "natural transformation" was likely a red herring. What I am trying to do is take a container of data f, and partition it into a structure g as a 'chunking' of that data. The f container is a potentially infinite stream of data obtained from a generator, and I want to be able to control how much data is extracted, so an eager 'fmap' won't be sufficient (an eager process will be applied to the 'chunks' defined by g). I am going to purse the hylomorphism model, but am interested if anyone else has any similar situations, and if using classes to manage the interface is the right strategy. Thanks! Gordon J. Uszkay uszkaygj(a)mcmaster.ca

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debugging a hanging program: where to start?
by Aran Donohue 14 May '10

14 May '10

Hi Cafe, I have a program that I can reliably cause to hang. It's concurrent using STM, so I think it could be a deadlock or related issue. I also do some IO, so I think it could be blocking in a system call. It only hangs when compiled with -threaded. I tried building with -prof, and running with -hc -xt to get a clue where in which function it is stopping, but the resulting profile didn't help much. I've started sprinkling print statements, but I thought I'd trawl for better tips and techniques. Any suggestions on approaches? Thanks, Aran

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Re: Frama-C
by Tom Hawkins 14 May '10

14 May '10

On Sun, May 9, 2010 at 12:54 AM, Lee Pike <leepike(a)gmail.com> wrote: > Tom, > >> Have you used any of these tools? They're pretty cool. I'd be >> interested to hear you opinion. >> >> http://frama-c.com/ > > Not yet. We were considering using them for a C security-analysis, but > rolled-our-own stuff in Haskell. Do you guys support ACSL or something similar? One major short comings of Frama-C is its inability to prove a formula false. As such, the tool will not return any counter examples to help with debugging. > > I'll ask around Galois if there's been any experience with it. > > Did anyone respond to your post about a Haskell interface? Negative. I'm looking into writing a bit of OCaml to dump out the Cil+ACSL stuff, which I can then read with Haskell. One thing I really like about Frama-C is its use of multiple solvers. Some work better than others, and it's very easy to see this in the gWhy GUI, as well as overall proof coverage. It would be nice to have a Haskell library where we could stream in a bunch of proof obligations and it would farm it out to different solvers, possibly running on different machines. It could also keep track of which obligations have proved true or false, so it doesn't wast time running the obligation on other solvers -- gWhy doesn't currently do this. You kind of alluded to these benefits when you recommended using SMT-LIB instead of Yices for AFV, but I didn't appreciated it until now. -Tom

2 3

Data creation pattern?
by Eugene Dzhurinsky 13 May '10

13 May '10

Hello, all! I need to create objects like this data Object = MyObject { param1, param2, param3 :: String } from the input file param_1_param1=value11 param_2_param1=value21 param_2_param2=value22 param_1_param3=value13 param_2_param3=value23 param_1_param2=value12 so general pattern of recognizing parameter name and value is param_{id}_{property name}={property value} so I need to create function > parseDataFile :: [String] -> [Object] For now I can think on splitting the task into 2 functions > groupSameObjectParameters :: [String] -> [[String]] which will group the lines with same id into a list, and then apply function > createObject :: [String] -> Object so overall solution will be > parseDataFile :: [String] -> [Object] > parseDataFile = map createObject . groupSameObjectParameters however I have no neat idea about how to create instance of MyObject - I need to supply all of parameters to the constructor at once. Also I don't like the idea of rearranging list first and then create objects from another list, because the list can be relatively large. In imperative programming with mutable objects I would create an array, then for each line get the id and try to find if there is the object in the array at 'id' index. If no - create one. Then set appropriate property from value. Is it possible to do something similar in Haskell? Thank you in advance! -- Eugene N Dzhurinsky

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ANNOUNCE: yaml, data-object-yaml and data-object-json 0.3.0
by Michael Snoyman 13 May '10

13 May '10

Hi all, We're happy to announce the newest release of the three above packages. Notable changes include: * Anton Ageev wrote support for anchors and aliases into both the yaml and data-object-yaml package. * Anton also wrote support in data-object-yaml for merge keys. * The yaml package is now based on the iteratee package, replacing the previous cursor interface. This should hopefully make it easier to use, and perhaps more performant. * data-object-json API was modified to match (as much as possible) the data-object-yaml API. * In addition, there was a version bump for quite a number of the underlying libraries. Overall, data-object-yaml has not changed in any significant way for the user, and the data-object-json API changes should make it much more palatable. As always, feedback is welcome! Michael

1 0

Major Emacs mode for reading GHC Core files
by Johan Tibell 13 May '10

13 May '10

Hi all, I wrote a minimal major mode for reading (and editing) GHC Core files. It provides syntax highlighting and removal of commonly ignored annotations, similar to what's offered by ghc-core. * Usage Dump the simplifier output in a file with a .hcr suffix: ghc -c -ddump-simpl -O2 Test.hs > /tmp/Test.hcr Open the .hcr file in Emacs. Use `ghc-core-clean-buffer` to remove commonly ignored annotations and namespace prefixes from the buffer. * Prerequisites You need to have haskell-mode installed. * Installation Download the file from http://johantibell.com/files/ghc-core.el and put it in your ~/.emacs.d directory. Add (load-file "~/.emacs.d/ghc-core.el") to ~/.emacs.d/init.el Cheers, Johan

1 0

ANN: precis-0.3.1 - Cabal package diff tool
by Stephen Tetley 13 May '10

13 May '10

Hello all I've upload precis to Hackage - a diff tool for Cabal packages. It compares two cabal files looking at changes to the exposed and internal modules lists, then diffs the exposed modules. It only works on extant .hs files - i.e. it won't work on .hsc files. Also, I haven't tested if it works on .lhs files - parsing is left to haskell-src-exts. When it diffs new and old revisions of an exposed module it should report: - changes to the export list - changes to instance declarations - changes to exported data declarations - changes to the exported type signatures of exported functions The report it produces is not very pretty. A declaration is marked as *conflict* if its textual representation has changed - I compare the string output from pretty printing the new and old versions rather than look deeply into the AST. Though low-tech, this seems to work well in practice. I expect there are some bugs... Thanks to Malcolm Wallace for CppHs, and Niklas Broberg for haskell-src-exts. Best wishes Stephen

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corner case in Text.JSON 0.4.3
by Martin Hilbig 13 May '10

13 May '10

hi, since i got no answer from the maintainer, maybe someone else can take care of it, or at least point out, what i did wrong. so, i recently stumbled upon some error while using Text.JSON 0.4.3 [1]: Text/JSON/String.hs:(127,4)-(137,49): Non-exhaustive patterns in case indeed ghc warned: [5 of 7] Compiling Text.JSON.String ( Text/JSON/String.hs, dist/build/Text/JSON/String.o ) Text/JSON/String.hs:127:4: Warning: Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: [] from looking at the code i couldn't see how this would ever happen, but you can reproduce it be running the files from [2]: $ ./test < problem Ok (JSArray [JSString (JSONString {fromJSString = "this"}),JSString (JSONString {fromJSString = "is"}),JSString (JSONString {fromJSString = "some"}),JSString (JSONString {fromJSString = "json"}),JSObject (JSONObject {fromJSObject = [("that",JSString (JSONString {fromJSString = "works"}))]})]) test: Text/JSON/String.hs:(127,4)-(137,49): Non-exhaustive patterns in case the patch i put there fixes it (at least for me) to return an Error instead of dying: $ ./test < problem Ok (JSArray [JSString (JSONString {fromJSString = "this"}),JSString (JSONString {fromJSString = "is"}),JSString (JSONString {fromJSString = "some"}),JSString (JSONString {fromJSString = "json"}),JSObject (JSONObject {fromJSObject = [("that",JSString (JSONString {fromJSString = "works"}))]})]) Error "Unexpected end of String: does" Error "Malformed JSON: invalid token in this context not\"]" test: <stdin>: hGetLine: end of file have fun martin hilbig [1]: http://hackage.haskell.org/package/json [2]: http://friendpaste.com/3IvnChRMoczf0mIKpOtrYE

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Why is TChan GHC specific?
by Edward Amsden 12 May '10

12 May '10

I'm currently just getting into playing around with concurrency in haskell, primarily because I find STM intriguing. In looking through the docs I noticed that the transactional channels are GHC specific. http://hackage.haskell.org/packages/archive/stm/2.1.1.2/doc/html/Control-Co… I dug around the wiki and Google but I couldn't find the reason for this.

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ghc api renamed source
by Phyx 12 May '10

12 May '10

Hi all, I was wondering if it's possible to get to the renamed source before actually typechecking, I currently have parsed <- parse modName checked <- typecheckModule parsed let renamed = tm_renamed_source checked value = tm_typechecked_source checked but if typechecking failes, I get no information at all (an error), while I should be able to still get the renamed source. Any suggestions? Cheers, Phyx

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hSetEncoding on socket handles
by David Powell 12 May '10

12 May '10

Greetings, I am having trouble sending unicode characters as utf8 over a socket handle. Despite setting the encoding on the socket handle to utf8, it still seems to use some other encoding when writing to the socket. It works correctly when writing to stdout, but not to a socket handle. I am using ghc 6.12.1 and network-2.2.1.7. I can get it to work using System.IO.UTF8, but I was under the impression this was no longer necessary? I also don't seem to understand the interaction between hSetEncoding and hSetBinaryMode because if I set the binary mode to 'False' and the encoding to utf8 on the socket, then when writing to the socket the string seems to be truncated at the first non-ascii codepoint. Here is a test snippet, which can be used with netcat as a listening server (ie. nc -l 1234). > import System.IO > import Network > main = do > let a="λ" > s <- connectTo "127.0.0.1" (PortNumber 1234) > hSetEncoding s utf8 > hSetEncoding stdout utf8 > hPutStrLn s a > putStrLn a > hClose s Thanks, David

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Tsuru Capital is hiring
by Simon Cranshaw 12 May '10

12 May '10

We are looking to hire a programmer that has experience programming in a functional language, and is familiar with Haskell. There is a wide range of tools and applications that we want to build over time, including graphical interfaces and profiling tools for our trading system, parsers for new market feeds, and algorithms for trading strategies. If you have experience in any one of these areas it's a definite bonus. You will be working with a small team of developers, and from time to time will be working along side Traders to assist in tool development. Annual compensation will be in the region of USD150k with potential for growth. The team is based in Tokyo but there are projects that could be done remotely so feel free to apply for this type of work also. http://tsurucapital.com/en/jobs.html#programmer

1 0

comprehending monad execution
by rick＠rickmurphy.org 12 May '10

12 May '10

All: I've been analyzing monads for a while now and have a question on execution. I suspect I've missed something basic. In the snippet below the ghc debugger shows that in just one single step Discriminate has been substituted for g. Even with :trace turned on in the debugger there is no evidence of an execution step to carry out the substitution. This leads me to assume a compile time operation identifies Discriminate as a substitute for g. Could someone correct my assumption or explain how this works? Note: this snippet was extracted from [1]. > {-# LANGUAGE EmptyDataDecls, Rank2Types #-} > module LEM where > import Control.Monad.Identity > > data F -- no constructors > > data Discriminate a b = L a | R b > instance Monad (Discriminate e) where > return = R > R x >>= f = f x > L x >>= f = L x > > h :: (forall m. Monad m => ((a -> m F) -> m F)) -> a > h g = case g (\x -> L x) of > R x -> error "Not reachable" > L x -> x > > -- A proof term for a -> NOT NOT a > lift :: forall a m. Monad m => a -> ((a -> m F) -> m F) > lift x = \k -> k x > > t = h $ lift True > ghci trace ... C:\haskell\lem>ghci lem.lhs GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling LEM ( lem.lhs, interpreted ) Ok, modules loaded: LEM. *LEM> :b h Breakpoint 0 activated at lem.lhs:(14,2)-(16,17) *LEM> :trace t Loading package mtl-1.1.0.1 ... linking ... done. Stopped at lem.lhs:(14,2)-(16,17) _result :: a = _ [lem.lhs:(14,2)-(16,17)] *LEM> :step Stopped at lem.lhs:(14,8)-(16,17) _result :: a = _ g :: (a -> Discriminate a F) -> Discriminate a F = _ [lem.lhs:(14,8)-(16,17)] *LEM> :hist -1 : h (lem.lhs:(14,2)-(16,17)) -2 : t (lem.lhs:22:6-18) <end of history> [lem.lhs:(14,8)-(16,17)] *LEM> My previous analysis includes running single step execution on Wadler's I monad here [2], where I am able to observe execution of the I monad. [essence.lhs:32:33-37] *Essence> :hist -1 : unitI (essence.lhs:7:34) -2 : unitI (essence.lhs:7:2-34) -3 : interp (essence.lhs:32:26-38) -4 : interp (essence.lhs:(31,2)-(39,44)) -5 : test (essence.lhs:54:41-51) -6 : showval (essence.lhs:(26,2)-(28,45)) -7 : showI (essence.lhs:9:34-42) -8 : showI (essence.lhs:9:2-42) -9 : test (essence.lhs:54:34-52) -10 : test (essence.lhs:54:2-52) <end of history> [1] http://okmij.org/ftp/Computation/lem.html [2] http://homepages.inf.ed.ac.uk/wadler/papers/essence/essence.ps Thanks much in advance for your help. Rick

1 0

Functions of type foo :: f a -> g a
by Gordon J. Uszkay 11 May '10

11 May '10

I would like to build a class that includes a method to move data from one arbitrary functor to another, a natural transformation. The structures might be more than just functors, but I can start with that. I ran into some practical issues with resolving the type variables for my multiparameter type class, which I can resolve with functional dependencies. I can also not isolate the natural transformation from my overall operation, muddling it with the element transformation. I was wondering if anyone had any words of advice, example or warning about this kind of function or method in general? Class (Functor f, Functor g) => Foo f g a where foo :: f a -> g a bar :: (a->b) -> g a -> g b Thanks, Gordon J. Uszkay, McMaster University uszkaygj(a)mcmaster.ca

4 3

How efficient is read?
by Tom Hawkins 11 May '10

11 May '10

I have a lot of structured data in a program written in a different language, which I would like to read in and analyze with Haskell. And I'm free to format this data in any shape or form from the other language. Could I define a Haskell type for this data that derives the default Read, then simply print out Haskell code from the program and 'read' it in? Would this be horribly inefficient? It would save me some time of writing a parser. -Tom

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