- Haskell-Cafe - Haskell.org

Fighting the monad stack, MonadIO
by Adam Smyczek 11 Apr '08

11 Apr '08

For a small webapi binding I try to implement a session like monad by building a stack including BrowserAction from Network.Browser module as following: newtype RBAction a = RBAction { exec :: ErrorT String (StateT RBState BrowserAction) a } deriving (Functor, Monad, MonadState RBState) I would like the RBAction to implement MonadIO as well, but fight with the liftIO function for hours now, without success. Any idea how the implementation of liftIO could look like? Thanks for help, Adam

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ghc
by Cetin Sert 11 Apr '08

11 Apr '08

Hi, Is GHC required to be installed on the target OS I compile Haskell binaries for in order for these binaries to run? I need a quick answer on that! By the way there is no computer in the 4 or so networks I have online access to on which ghc is not installed, which might be a sign people like haskell here in Heidelberg/Mannheim, Germany. Best Regards, Cetin Sert

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Pattern match failure
by Jackm139 10 Apr '08

10 Apr '08

I'm trying to write a function to recognize a context free grammar, but I keep getting pattern match failure errors. This is what I have: data Grammar c = Brule c c c | Rule c c gez = [(Brule 'S' 'p' 'D'),(Brule 'D' 't' 'E'),(Rule 'E' 'j')] recog :: String -> String -> [Grammar Char] -> Bool recog a b list = case list of [Brule x y z] -> if a == [x] then recog [z] b list else recog a b list [Rule x y] -> True how can I solve this pattern matching error? -- View this message in context: http://www.nabble.com/Pattern-match-failure-tp16600643p16600643.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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instance Monad m => Functor m
by Hans Aberg 10 Apr '08

10 Apr '08

Using 'hugs -98', I noticed it accepts: instance Monad m => Functor m where fmap f x = x >>= return.f Has this been considered (say) as a part of the upcoming Haskell Prime? Hans Aberg

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Haskell library install question
by Galchin, Vasili 10 Apr '08

10 Apr '08

Hello, I have configured, built and installed a library. When I "runhaskell Setup.hs install", I noticed the message Registering unix-2.2.0.0... In what sense is it being registered? Can I query this registry information? Thanks, Vasili

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PhD or PostDoc opening in Dresden, Germany
by Janis Voigtlaender 10 Apr '08

10 Apr '08

There is a research position on offer in the project described at: http://wwwtcs.inf.tu-dresden.de/~voigt/project/ The official job advertisement (in German) can be found at: http://www.verw.tu-dresden.de/StellAus/einzelstelle.asp?id=783 The most important details are: - full-time faculty position - payment according to German public sector scale E 13 TV-L Ost (gross monthly salary expected to start around 2680 Euro) - no teaching duties - initial appointment for up to 30 months - no knowledge of German required (but English is) Please contact me directly with any further questions. Please also forward to potentially interested students. The closing date for applications is 15th May 2008. Best wishes, Janis Voigtlaender -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Lock-Free Data Structures using STMs in Haskell
by Pete Kazmier 10 Apr '08

10 Apr '08

I recently read the STM paper on lock-free data structures [1] which I found very informative in my quest to learn how to use STM. However, there are a few things I do not fully understand and was hoping someone might be able to explain further. In the STM version of the ArrayBlockingQueue, the following type is defined: data ArrayBlockingQueueSTM e = ArrayBlockingQueueSTM { shead :: TVar Int, stail :: TVar Int, sused :: TVar Int, slen :: Int, sa :: Array Int (TVar e) } It's unclear to me why the Array's elements must be wrapped in TVars. Why aren't the TVars on shead, stail, and sused sufficient? Here is the only function that reads from the queue: readHeadElementSTM :: ArrayBlockingQueueSTM e -> Bool -> Bool -> STM (Maybe e) readHeadElementSTM abq remove block = do u <- readTVar (sused abq) if u == 0 then if block then retry else return Nothing else do h <- readTVar (ihead abq) let tv = sa abq ! h -- Why are the array elements wrapped in TVars? e <- readTVar tv if remove then do let len = slen abq let newh = h `mod` len writeTVar (shead abq) $! newh writeTVar (sused abq) $! (u-1) else return () return (Just e) It is not immediately obvious to me why the elements need to be wrapped in TVars. Could someone help elaborate? The other question is in regards to section 2 where STM is introduced. The authors define the following: decT :: TVar Int -> IO () decT v = atomically (do x <- readTVar v if x == 0 then retry else return () writeTVar v (x-1)) And then go on to show how easy it is to compose STM types with this function: decPair v1 v1 :: TVar Int -> TVar Int -> IO () decPair v1 v2 = atomically (decT v1 `orElse` decT v2) Will this actually compile? I was under the impression that 'orElse' could only combine STM types, not IO () types. Thank you, Pete [1] Anthony Discolo, Tim Harris, Simon Marlow, Simon Peyton Jones, and Satnam Singh. Lock-free data structures using STMs in Haskell. In Eighth International Symposium on Functional and Logic Programming (FLOPS.06), April 2006.

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problems building hpodder
by Karl Hasselström 09 Apr '08

09 Apr '08

I'm trying to build hpodder 1.1.2 with ghc 6.8.2. I successfully downloaded, built, and installed (the latest versions of) all its dependencies, but when building hpodder itself, I get FeedParser.hs:146:26: `Content' is not applied to enough type arguments Expected kind `??', but `Content' has kind `* -> *' In the type `Content -> Either String String' In the type `String -> Content -> Either String String' In the type signature for `strof_either': strof_either :: String -> Content -> Either String String I don't know enough Haskell to even determine what kind of problem this is -- wrong version of some dependency? Wrong version of ghc? Help appreciated. :-) ( FWIW, the hackage auto-builder thingie seems to have the exact same problem I have: http://hackage.haskell.org/packages/archive/hpodder/1.1.2/logs/failure/ghc-… ) -- Karl Hasselström, kha(a)treskal.com www.treskal.com/kalle

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What's the difference?
by PR Stanley 09 Apr '08

09 Apr '08

Hi What is the difference between data T0 f a = MkT0 a instance Eq (T0 f a) where ... and data T0 f a = MkT0 a instance Eq a => Eq (T0 f a) where ... I've only seen the "=>" operator used for declaring extended classes but never with class instances. By the way, what is the correct terms for the "=>" and the "->"? Cheers Paul

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RE: [Haskell-cafe] type families and type signatures
by Sittampalam, Ganesh 09 Apr '08

09 Apr '08

OK, thanks. I think I'm finally understanding :-) Cheers, Ganesh -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Martin Sulzmann Sent: 09 April 2008 07:21 To: Ganesh Sittampalam Cc: Manuel M T Chakravarty; haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] type families and type signatures Manuel said earlier that the source of the problem here is foo's ambiguous type signature (I'm switching back to the original, simplified example). Type checking with ambiguous type signatures is hard because the type checker has to guess types and this guessing step may lead to too many (ambiguous) choices. But this doesn't mean that this worst case scenario always happens. Consider your example again type family Id a type instance Id Int = Int foo :: Id a -> Id a foo = id foo' :: Id a -> Id a foo' = foo The type checking problem for foo' boils down to verifying the formula forall a. exists b. Id a ~ Id b Of course for any a we can pick b=a to make the type equation statement hold. Fairly easy here but the point is that the GHC type checker doesn't do any guessing at all. The only option you have (at the moment, there's still lots of room for improving GHC's type checking process) is to provide some hints, for example mimicking System F style type application by introducing a type proxy argument in combination with lexically scoped type variables. foo :: a -> Id a -> Id a foo _ = id foo' :: Id a -> Id a foo' = foo (undefined :: a) Martin Ganesh Sittampalam wrote: > On Wed, 9 Apr 2008, Manuel M T Chakravarty wrote: > >> Sittampalam, Ganesh: > >>> No, I meant can't it derive that equality when matching (Id a) >>> against (Id b)? As you say, it can't derive (a ~ b) at that point, >>> but (Id a ~ Id b) is known, surely? >> >> No, it is not know. Why do you think it is? > > Well, if the types of foo and foo' were forall a . a -> a and forall b > . b -> b, I would expect the type-checker to unify a and b in the > argument position and then discover that this equality made the result > position unify too. So why can't the same happen but with Id a and Id > b instead? > >> The problem is really with foo and its signature, not with any use of >> foo. The function foo is (due to its type) unusable. Can't you >> change foo? > > Here's a cut-down version of my real code. The type family Apply is > very important because it allows me to write class instances for > things that might be its first parameter, like Id and Comp SqlExpr > Maybe, without paying the syntactic overhead of forcing client code to > use Id/unId and Comp/unComp. It also squishes nested Maybes which is > important to my application (since SQL doesn't have them). > > castNum is the simplest example of a general problem - the whole point > is to allow clients to write code that is overloaded over the first > parameter to Apply using primitives like castNum. I'm not really sure > how I could get away from the ambiguity problem, given that desire. > > Cheers, > > Ganesh > > {-# LANGUAGE TypeFamilies, GADTs, UndecidableInstances, > NoMonomorphismRestriction #-} > > newtype Id a = Id { unId :: a } > newtype Comp f g x = Comp { unComp :: f (g x) } > > type family Apply (f :: * -> *) a > > type instance Apply Id a = a > type instance Apply (Comp f g) a = Apply f (Apply g a) type instance > Apply SqlExpr a = SqlExpr a type instance Apply Maybe Int = Maybe Int > type instance Apply Maybe Double = Maybe Double type instance Apply > Maybe (Maybe a) = Apply Maybe a > > class DoubleToInt s where > castNum :: Apply s Double -> Apply s Int > > instance DoubleToInt Id where > castNum = round > > instance DoubleToInt SqlExpr where > castNum = SECastNum > > data SqlExpr a where > SECastNum :: SqlExpr Double -> SqlExpr Int > > castNum' :: (DoubleToInt s) => Apply s Double -> Apply s Int castNum' > = castNum > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

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