- Haskell-Cafe - Haskell.org

Monad instance for Data.Set, again
by Henning Thielemann 29 Mar '08

29 Mar '08

The blog article http://www.randomhacks.net/articles/2007/03/15/data-set-monad-haskell-macros describes a variant of the Monad class which allows to restrict the type of the monadic result, in order to be able to make Data.Set an instance of Monad (requiring Ord constraint for the monadic result). The same problem arises for container data structures with restricted element types, where the element type restriction depends on the implementation of the container structure (such as UArray). It would be cumbersome to have several class parts, even more, type constraints in type signatures may reveal implementation details. E.g. constraint (Container c x y z) might be needed for a 'zipWith' function, whereas (Container c y x z) is needed if you use 'zipWith' with swapped arguments. Here is another approach that looks tempting, but unfortunately does not work, and I wonder whether this can be made working. module RestrictedMonad where import Data.Set(Set) import qualified Data.Set as Set class AssociatedMonad m a where class RestrictedMonad m where return :: AssociatedMonad m a => a -> m a (>>=) :: (AssociatedMonad m a, AssociatedMonad m b) => m a -> (a -> m b) -> m b instance (Ord a) => AssociatedMonad Set a where instance RestrictedMonad Set where return = Set.singleton x >>= f = Set.unions (map f (Set.toList x)) GHC says: RestrictedMonad.hs:21:13: Could not deduce (Ord b) from the context (RestrictedMonad Set, AssociatedMonad Set a, AssociatedMonad Set b) arising from use of `Data.Set.unions' at RestrictedMonad.hs:21:13-22 Probable fix: add (Ord b) to the class or instance method `RestrictedMonad.>>=' In the definition of `>>=': >>= x f = Data.Set.unions (map f (Data.Set.toList x)) In the definition for method `RestrictedMonad.>>=' In the instance declaration for `RestrictedMonad Set'

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Glome raytracer bug: bad output with -O2 -fasm
by Jim Snow 29 Mar '08

29 Mar '08

I was trying to get Blinn highlights working with my raytracer, and kept getting ugly artifacts. After trying a bunch of things, I finally compiled without -O2, and the artifacts went away. Here's what I mean: http://syn.cs.pdx.edu/~jsnow/glome/Glome.hs-noartifact.png http://syn.cs.pdx.edu/~jsnow/glome/Glome.hs-artifact.png Here's the offending code, run "./make" and "./run" and you should see the artifacts if your setup is the same as mine. (Requires OpenGL.) http://syn.cs.pdx.edu/~jsnow/glome/glome.hs-0.2-bug.tar.gz The artifacts also go away if I use -fvia-C. It doesn't seem to matter whether I use Floats or Doubles in the rendering code. The artifacts also show up with -O1. Have I stumbled across a known compiler bug? Or perhaps an OpenGL bug? (The bug could, of course, be in my code, but then one might expect to get the same erroneous output every time regardless of compiler flags.) To reiterate, I'm using ghc 8.6.2. thanks, -jim

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Sound typeable via type families?
by Dan Doel 28 Mar '08

28 Mar '08

Hello, Recently, someone (I forgot who, exactly) mentioned in #haskell this article: http://okmij.org/ftp/Haskell/types.html#unsound-typeable where Oleg demonstrates that Typeable makes the type system unsound, and one can get back unsafeCoerce (which is used in the implementation, I suspect). However, this evening, Ryan Ingram showed how to construct a safe Typeable using GADTs. The problem with that, of course, is that GADTs aren't open, so one can only have a predefined number of Typeables. However, it occurred to me that data families don't have that problem, so I took a shot and came up with the following: {-# LANGUAGE TypeFamilies, ScopedTypeVariables, GADTs , MultiParamTypeClasses, FlexibleInstances, OverlappingInstances #-} class Typeable t where data TypeRep t :: * typeRep :: TypeRep t instance Typeable Int where data TypeRep Int = TInt typeRep = TInt instance Typeable Integer where data TypeRep Integer = TInteger typeRep = TInteger instance Typeable () where data TypeRep () = TUnit typeRep = TUnit data TEq a b where Refl :: TEq a a class (Typeable a, Typeable b) => TEquality a b where teq :: TypeRep a -> TypeRep b -> Maybe (TEq a b) instance (Typeable a) => TEquality a a where teq _ _ = Just Refl instance (Typeable a, Typeable b) => TEquality a b where teq _ _ = Nothing cast :: forall a b. (TEquality a b) => a -> Maybe b cast a = do Refl <- teq ta tb return a where ta :: TypeRep a ta = typeRep tb :: TypeRep b tb = typeRep Which, somewhat to my surprise, actually works. And, of course, if one tries to pull off Oleg's trick: newtype Oleg a = Oleg { unOleg :: a } instance Typeable (Oleg a) where TypeRep (Oleg a) = ... typeRep = ?? Well, it won't work, because TypeRep () ~/~ TypeRep (Oleg a). However, obviously, this depends on overlapping instances (if there's some other way, I'd be happy to know; if type inequality contexts are available, I wasn't able to find them), and I've heard that type families don't play well with overlap. Does that not apply to data families? Will this construction still work in 6.10 and beyond? Also, this doesn't seem to be a suitable basis for Dynamic. Trying to extend the GADT solution presented resulted in errors unless incoherent instances were turned on (clearly not a good sign), and even then, it didn't actually work. Is it possible to do better, and come away with something that will actually work for Dynamic, and be sound? Is there some other trick waiting to pull unsafeCoerce out of this setup (seems doubtful, as it isn't used in the code, and the type families folks have no doubt done plenty of work to ensure soundness)? Comments appreciated. -- Dan

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What Does This Message Mean? (Re: [Haskell-cafe] compilation succeeds -- execution fails)
by Jason Dusek 28 Mar '08

28 Mar '08

The message unknown symbol `___stginit_cedictzm0zi1zi1_DataziCharziCEDICTziMatter_' says that it can't find the initializer for` Data.Char.CEDICT.Matter` in `cedict-0.1.1` (this is 'z-encoding', if I remember correctly). So, the odd thing is, that is not the part with the C FFI stuff in it -- that's in `Data.Char.CEDICT.Lists`. Is there a usual thing that is wrong when the initializer can not be found? -- _jsn

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[gsoc] mingw64 ghc port
by Bulat Ziganshin 28 Mar '08

28 Mar '08

Hello haskell-cafe, it's probably a bit too late, but i recalled that there is one project that will be very useful - it's porting ghc to mingw64 platform, allowing it to generate 64-bit windows platforms. may be someone will find it interesting -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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[GSoC] Parallel Benchmarking and Profiling
by Etienne Laurin 28 Mar '08

28 Mar '08

Hello, I am putting together a student proposal to participate in Google's Summer of Code with one of the following project ideas. Parallel programming benchmarking and benchmark suite - http://hackage.haskell.org/trac/summer-of-code/ticket/1544 Are there open source projects and real world applications that rely on GHC's parrallel programming primitives and libraries? I have found many references to LOLITA, but it seems to be old and not available online. The idea page suggests porting existing benchmark suites such as PARSEC, but PARSEC is 5G of C code. Most of it seems to come from existing applications already written in C. It might also be interesting to automate the discovery of optimal strategies through empirical data, and to modify the thresholds dynamically. Parallel profiling tools for GHC - http://hackage.haskell.org/trac/summer-of-code/ticket/1559 Simon Marlow wrote on the idea page that Gransim was ported to a more up-to-date GHC. The documentation available on the web seems to be for GHC 0.29 but it describes many options for logging and visualising the activity of threads and processors over time. Getting GHC to display that information on the frontpanel would make a nice project. Do you have any comments or suggestions? Thank you, Etienne Laurin

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Re: [Haskell-cafe] Web server libraries
by Sterling Clover 28 Mar '08

28 Mar '08

Yipe. It's just been pointed out to me that the hvac repo was missing a key file. I just committed it and tried a fresh pull and build, and it seems to work properly now. Apologies to all who couldn't get it working. Regards, Sterl. On Fri, 28 Mar 2008, Sterling Clover wrote: > While hvac, which I announced here recently, is not yet ready for > primetime, so to speak, you may want to take a look at it -- with a > few tweaks it should match your specs. (darcs get http:// > community.haskell.org/~sclv/hvac/<http://community.haskell.org/%7Esclv/hvac/> ) > [...]

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MonadMemory class
by Ariel J. Birnbaum 28 Mar '08

28 Mar '08

Two questions came to mind while thinking of memory references in Haskell: 1. Is there a "standard" equivalent of the following: class (Monad m) => MonadMemory m r | m -> r where new :: a -> m (r a) read :: r a -> m a write :: r a -> a -> m () What kind of axioms should an instance of this class satisfy? 2. How would a "pure" instance of this class look like (obvious unsafePerformIO-based solutions aside)? Is it even possible in pure Haskell? Thanks much! -- Ariel J. Birnbaum

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Recursion problem in infinite list model
by Hans Aberg 28 Mar '08

28 Mar '08

When experimenting with list index sets (i.e. lists more general than provided by Haskell), I arrived at the problem that in the example code below for example h(list 6) does not (in Hugs) write out the beginning of the list lazily. It does work for list 6 first (list 6) rest (list 6) 10 -+ (list 3) and so on. So it seems to be a problem with h, also suggested by heap profiling, perhaps similar to that of "foldl". So is it possible to fix it? The function h x is derived by unwinding the monadic construct corresponding to (for lists) do {i <- [x..]; return i} So there are not many possibilities of change. Below, (-+) corresponds, for lists, to ":", "first" to "head", and "rest" to "tail". Hans Aberg -------- data List a = List(Integer->a) instance Show a => Show (List a) where show (List f) = "[" ++ show (f(0)) ++ concat ["," ++ show (f(toInteger i))| i<-[1..]] ++ "]" list :: Integer -> List Integer list x = List(\z -> x+z) (-+) :: a -> List a -> List a x -+ (List y) = List(f) where f 0 = x f k = y(k-1) first :: List a -> a first (List f) = f 0 rest :: List a -> List a rest (List y) = List(f) where f k = y(k+1) h :: List a -> List a h x = (-+) (first x) (h (rest x)) --------

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Running classical logic
by Jim Apple 28 Mar '08

28 Mar '08

We can't safely extract a t out of IO t, but we can run an IO t, interact with the environment, and then see the t. Griffin ( http://citeseer.ist.psu.edu/griffin90formulaeastypes.html ) explained that we can do the same thing with (t -> r) -> r by interacting with the context. That is, he defined a function c :: ((t -> r) -> r) -> t buy interaction with the reduction context of calls to c. I don't expect such a function to be definable in Haskell without some magic, but even with magic, I'm not sure how to write such a function. On #haskell a couple of days ago, we came up with data Empty deriving Typeable toEmpty :: a -> Empty toEmpty = unsafeCoerce fromEmpty :: Empty -> a fromEmpty = unsafeCoerce type Bot = forall x . x griffinC :: Cont Bot a -> IO a griffinC (Cont f) = do key <- newUnique catch (evaluate $ f $ \v -> throwDyn (hashUnique key,toEmpty v)) (\x -> case x of DynException d -> case fromDynamic d of Just (u,y) -> if u == hashUnique key then return $ fromEmpty y else throwIO x Nothing -> throwIO x _ -> throwIO x) The Unique is used to keep the later griffinC calls from being cast to the wrong types. The more I look at this, the less I understand it. Any clues? Jim P.S. Thanks to #haskell, and especially roconnor for the fine help

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