September 2008 - Haskell-Cafe

StateWriter: a monad-writing exercise
by Anthony LODI 25 Sep '08

25 Sep '08

Hello haskell-cafe, In my application I have a complex state threaded through long computation chains, and I need a way to log all state changes (so that the evolving state can be animated/replayed somewhere else). Initially I tried combining State and Writer monads but this allows for the possibility to change the state, and forget to write a log entry, etc. So I decided to write a separate monad, StateWriter l s, that takes a state-modifying function, l->s->s (l is an ADT for all the allowable state transitions), an initial state s, and only allows s to change by appending 'l' log entries inside the monad. The net result is that I should have read-only access to the current state inside the monad, and all state transitions should be logged (by going through the one function, the log entries serve as witnesses to all state transitions). Anyway, here's my (very rough!) first stab at the problem. This is the first time I've tried writing a monad so any comments/critiques are much appreciated. Also, about the 'StateWriter' idea in general: am I just (poorly?) reimplementing something obvious? Is it unlikely to scale well on real-world problems? Is there some way to combine the existing State and Writer monads to avoid having to do this? If there's nothing seriously wrong here, I was thinking my next step would be to try changing the lists to monoids (like in the Writer monad), and then to try writing a transformer version of the whole thing. Cheers, - Anthony LODI ================================================================================ {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances #-} newtype StateWriter l s a = StateWriter { _runSWriter :: (l -> s -> s) -> [l] -> s -> (a, [l], s) } instance Monad (StateWriter l s) where return a = StateWriter $ \_ ls s -> (a, ls, s) (StateWriter x) >>= f = StateWriter $ \fn ls s -> let (v, ls', s') = x fn ls s in _runSWriter (f v) fn ls' s' class MonadStateWriter m l s | m -> l s where put :: l -> m () get :: m s instance MonadStateWriter (StateWriter l s) l s where put l = StateWriter $ \fn ls s -> ((), ls ++ [l], fn l s) get = StateWriter $ \fn ls s -> (s, ls, s) runSWriter :: StateWriter l s a -> (l -> s -> s) -> s -> (a, [l], s) runSWriter sw fn = _runSWriter sw fn [] -------------------------------------------------------------------------------- data Ops = Inc | Dec deriving (Show) test :: StateWriter Ops Int String test = do put Inc put Inc put Inc val <- get let op = if val == 3 then Dec else Inc put op return "done" stateFn :: Ops -> Int -> Int stateFn Inc s = s + 1 stateFn Dec s = s - 1 runtest = runSWriter test stateFn 0 -- ("done",[Inc,Inc,Inc,Dec],2)

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Re: [Haskell-cafe] Injecting Haskell into C
by Anatoly Yakovenko 25 Sep '08

25 Sep '08

you'll find this example really helpfull ---------- Forwarded message ---------- From: Claude Heiland-Allen <claudiusmaximus(a)goto10.org> Date: 2008/6/5 Subject: Re: [Haskell-cafe] example of FFI FunPtr To: "Galchin, Vasili" <vigalchin(a)gmail.com> Cc: haskell <haskell-cafe(a)haskell.org> Galchin, Vasili wrote: > > Hello, > > I want to model a Haskell function that is a callback from C. I have > only found one example in the unix package's Semaphore.hsc, which apparently > is not used. I want to be able to marshall a Haskell function that is a > first class citizen residing in a Haskell data type and pass to a C function > via FFI. Are there examples of this? Attached is a simple example. The main thing to note is 'foreign import ccall "wrapper"' which gives you a factory for turning Haskell functions into foreign function pointers. More information: http://www.cse.unsw.edu.au/~chak/haskell/ffi/ Claude -- http://claudiusmaximus.goto10.org CallBacker: CallBacker.hs callerback.c callerback.h ghc -O2 -Wall -fffi -o CallBacker CallBacker.hs callerback.c _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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GHC 6.10, OS X, Fink and CPPFLAGS
by Svein Ove Aas 25 Sep '08

25 Sep '08

While trying to test 6.10, I ran into the issue that a number of the libraries it wants are not installed by default, and are most conveniently added via Fink. This includes gmp, binutils, readline, etc. /sw/ is not in the default gcc path, of course, so the usual solution is to add -I/sw/include, -L/sw/lib, etc. to it via the CPPFLAGS and LDFLAGS environment variables, which are supported by most[1] autoconf-based packages. The help text for ghc's configure script does in fact suggest that this is the case, and it has some code meant to read it; broken code, as it happens. As it is, the script fails to propagate those variables to whatever is responsible for compiling the RTS, which fails with a missing gmp.h, bfd.h, and so on. All those files exist live quite happily in /sw/include, and would no doubt appreciate visiting makefiles. So my question is two-fold: Is the help file wrong? Is there some other way to do it? Or is this in fact a bug, and if so, will any of you volunteer to fix it for me? ^_^

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Where is OpenGL loadMatrix ?
by minh thu 25 Sep '08

25 Sep '08

Hi, I can't find the loadMatrix function in http://hackage.haskell.org/packages/archive/OpenGL/latest/doc/html/Graphics… Should I use loadIdentity then multMatrix instead ? Thank you, Thu

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Google Android
by Galchin, Vasili 25 Sep '08

25 Sep '08

Hello, Do there currently (or in the works) exist FFI bindings for Google's Android API? Kind regards, Vasili

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Am I doing it right?
by Creighton Hogg 25 Sep '08

25 Sep '08

Hi Haskellers, So as another step in my education, I wanted to ask if the following code at least feels 'morally' correct as a find/replace function, replacing each occurrence of the sub-list before with after in the list. unfoldr felt like the most natural higher order function to capture the recursion, but I imagine there are still slicker ways to have done this because I am creating an intermediate list-of-lists. However, doing it this way seems at least moderately speedy given that I was able to read in, find/replace, and write out a 120 MB file in 30 seconds on a fairly dinky linux box. replace :: (Eq a) => [a] -> [a] -> [a] -> [a] replace before after = concat . unfoldr aux where aux [] = Nothing aux str | take l str == before = Just (after,drop l str) | otherwise = Just (take 1 str,drop 1 str) l = length before Thanks, Creighton

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Haskell board game
by Rafael C. de Almeida 25 Sep '08

25 Sep '08

Hello, I'm interested in doing a simple board game on haskell. For that I want to be able to draw stuff like the possible player movements and I want to be able to display very simple animations. I want to know what graphical interface library you suggest to me. I have almost no prior experience with graphical interfaces of any kind, so I rather start with something easy and straightforward. I have no need for great performance or anything like that. I'd like if it runs on windows with much trouble, that is, it'll be easy to package it for windows without requiring the user to install anything besides my game. My first thought was to use GTK's gtkTable, but I'm unsure how easy it is to make it work on windows. Beside that, I'm not sure it would be the easiest API for me to use. []'s Rafael

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need help making sense of the type ...
by Daryoush Mehrtash 25 Sep '08

25 Sep '08

I am having hard time making sense of the types in the following example from the Applicative Programming paper: http://www.cs.nott.ac.uk/~ctm/IdiomLite.pdf ap :: Monad m ⇒ m (a → b ) → m a → m b ap mf mx = do f ← mf x ← mx return (f x ) Using this function we could rewrite sequence as: sequence :: [ IO a ] → IO [ a ] sequence [ ] = return [ ] sequence (c : cs ) =* return (:) 'ap' c *'ap' sequence cs I am specifically confused over the type of "m" in: return (:) 'ap' c "c" is obviously an instance of IO a monad. "return (:)" on the other hand (at least as I would expect it) is an instance of " ->" monad. a) are the above statements correct? b) if so, does it make sense for the "ap" function to have two different instances of the "m"? thanks for you help daryoush

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Injecting Haskell into C
by Roman Cheplyaka 24 Sep '08

24 Sep '08

I have a C function whose behaviour is customized by user-supplied function (think of libc qsort). Typically these user-supplied functions are written in C, but I'd like to use FFI to write them in Haskell. Precisely, I'd like to write high-order function which will generate these functions (e.g. mkCompare :: ... -> (Ptr a -> Ptr a -> Int) for feeding the result to qsort). As I understand, there are two ways to do that. Either Haskell code is called from C, or C code is called for Haskell. So my questions are: 1. Are they both possible? 2. If yes, which is better performance-wise? (C function is performance-critical). If generated function is called many times, how big an overhead is going to be? -- Roman I. Cheplyaka :: http://ro-che.info/ kzm: My program contains a bug. How ungrateful, after all I've done for it.

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Re: [Haskell] ICFP programming contest results
by David Roundy 24 Sep '08

24 Sep '08

On Wed, Sep 24, 2008 at 1:06 AM, Malcolm Wallace <malcolm.wallace(a)cs.york.ac.uk> wrote: > The ICFP programming contest results presentation: > http://video.google.com/videoplay?docid=-4697764813432201693 > > Feel free to pass on this link to any other appropriate forum. Yikes. Haskell did pretty terribly! Anyone have an idea why, given our success in previous contests? David (speaking as someone who was traveling that weekend and didn't have time to compete)

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