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Wiki contact
by Neil Mitchell 19 Sep '06

19 Sep '06

Hi, Two questions: 1) Is there any way to get ... to be syntax hilighted as dots (or just leaft as is) on the Haskell wiki in <haskell> tags, having it as _._ looks ugly - see http://haskell.org/haskellwiki/Keywords#where - note that the first and last dots are underlined hyperlinks. 2) How do you go about contacting the person responsible for the wiki? Is there a wiki page to drop messages off on? An email address? An email list? - it confused me! Thanks Neil

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Traversing a graph in STM
by Einar Karttunen 19 Sep '06

19 Sep '06

Hello Is there an elegant way of traversing a directed graph in STM? type Node nt et = TVar (NodeT nt et) type Edge et = TVar et data NodeT nt et = NodeT nt [(Node nt et, Edge et)] type MyGraph = Node String Int When implementing a simple depth first search we need a way to mark nodes (= TVars) as visited. In addition multiple concurrent searches should be possible. Is it possible to avoid passing around an explicit Set of visited nodes? And is there a better way of getting TVar identity than StableNames? - Einar Karttunen

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foreach
by Tim Newsham 18 Sep '06

18 Sep '06

I was rewriting some non-haskell code in haskell and came up with this construct: foreach l f = mapM_ f l main = do args <- getArgs foreach args (\arg -> do foreach [1..3] (\n -> do putStrLn ((show n) ++ ") " ++ arg) ) ) which is reminiscent of foreach in other languages. Seems fairly useful and I was wondering how hard it would be to add some syntactic sugar to the "do" construct to make it a little prettier (ie. not require the parenthesis, binding and nested do, as: main = do args <- getArgs foreach args arg foreach [1..3] n putStrLn ((show n) ++ ") " ++ arg) would this type of transformation be possible with template haskell or does this need stronger support from the parser to pull off? Tim Newsham http://www.thenewsh.com/~newsham/

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darcs get-together tuesday evening (2006-09-19 18:00)
by Eric Y. Kow 18 Sep '06

18 Sep '06

Dear darcs users, hackers and observers (*), We're getting together around 18:00 this Tuesday to discuss and/or hack on darcs. This will be after the ICFP sessions for that day, in Portland, Oregon, specifically the Marriott downtown waterfront LL1. David Roundy will be there, Ian Lynagh and me too. Hope to see you there. (*) Sorry for the spam, Haskellers. I figured we might as well try to catch ICFP bystanders. -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

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Re: [Haskell-cafe] Problems interpreting
by Andrea Rossato 18 Sep '06

18 Sep '06

On Mon, Sep 18, 2006 at 12:54:34PM +0200, Albert Crespi wrote: > Thank you very much for your reply! > As I said, it is my first experience with Haskell, I have been programming > in Java and C for some years, and I find this language very different from > them. Anyway I'll try to fix the function with the information that you gave > me. > Thanks again! > You're welcome. By the way, this is what the comments say you are trying to do: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a => a -> [a] -> [a] -> [a] substitute e l1 l2= [c | c <- check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else check_elem xs This is the result: *Main> substitute 1 [1,2,3] [] [2,3] *Main> substitute 1 [1,2,3] [7,8,9] [7,8,9,2,3] *Main> Have fun with Haskell. Ciao Andrea

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Re: Why is type 'b' forced to be type 'm a' and not possibly 'm a -> m a' (Anatoly Zaretsky)
by Vivian McPhail 18 Sep '06

18 Sep '06

> Message: 6 > Date: Fri, 15 Sep 2006 18:36:35 +0300 > From: "Anatoly Zaretsky" <anatoly.zaretsky(a)gmail.com> > Subject: Re: [Haskell-cafe] Why is type 'b' forced to be type 'm a' > and not possibly 'm a -> m a' > To: "Vivian McPhail" <vivian.mcphail(a)paradise.net.nz> > Cc: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: > <a21cb60a0609150836u5695cf41jfbb31530105bec3a(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > On 9/15/06, Vivian McPhail <vivian.mcphail(a)paradise.net.nz> wrote: > > > > class Forkable a where > > fork :: String -> a -> a -> a > > > > ... > > {- > > instance (Monad m, Forkable (m a), Forkable b) => Forkable (m a -> b) where > > fork n a1 a2 a = do > > a' <- a > > fork n (a1 $ return a') (a2 $ return a') > > -} > > > > Let's do manual type checking. > First, fork :: Forkable a => String -> a -> a -> a > So for Forkable (m a -> b) > fork :: String -> (m a -> b) -> (m a -> b) -> m a -> b > Then > fork n a1 a2 a :: b > But you define it as > fork n a1 a2 a = do {...} > So it should be of type Monad t => t a, not just any `b'. > > Instead, you can define > instance (Monad m, Forkable (m b)) => Forkable (m a -> m b) where > ... > Well, I can partially instantiate what I am trying to achieve by enumerating cases. Note that when the first type is a monadic type the computation gets evaluated and then forked, but when the first type is a function it merely gets passed. My problem is that there are a very large number of possible cases. So in the case Forkable (m a -> b), a number of instances of which I can implement (e.g. Forkable (m a -> m a -> m a), Forkable ((m a -> m a) -> m a), and Forkable (m a -> (m a -> m a) -> m a)), I don't see why 'b' should necessarily typecheck to 't t1'. What I would like to be able to do is differentiate between Forkable (m a -> b) and Forkable (<function type> -> b). By the way, the following code typechecks and runs correctly, my problem is that enumerating all possible types requires five factorial (120) different instances, and to a lazy functional programmer who can 'see' the pattern it seems that there must be a nicer way of achieving my end. \begin{code} instance (Monad m, Forkable (m a)) => Forkable (m a -> m a) where fork n a1 a2 a = do a' <- a fork n (a1 $ return a') (a2 $ return a') instance (Monad m, Forkable (m a)) => Forkable (m a -> m a -> m a) where fork n a1 a2 a b = do a' <- a fork n (a1 $ return a') (a2 $ return a') b instance (Monad m, Forkable (m a)) => Forkable ((m a -> m a) -> m a) where fork n a1 a2 a = do fork n (a1 a) (a2 a) instance (Monad m, Forkable (m a)) => Forkable (m a -> (m a -> m a) -> m a) where fork n a1 a2 a b = do a' <- a fork n (a1 $ return a') (a2 $ return a') b \end{code} > Note that to compile it you also need -fallow-undecidable-instances > and -fallow-overlapping-instances. > > -- > Tolik > Thanks for your help so far! > > Cheers, Vivian -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.405 / Virus Database: 268.12.4/449 - Release Date: 15/09/2006

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Problems interpreting
by Carajillu 18 Sep '06

18 Sep '06

Hi, I'm a student and I have to do a program with Haskell. This is the first time I use this languaje, and I'm having problems with the indentation. I want to check if this function is correct, but when I try to make the GHCi interpret it, I get "line 18:parse error (possibly incorrect indentation)" The function is: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a => a -> [a] -> [a] -> [a] substitute e l1 l2= [check_elem c | c <- l1] where check_elem::Eq a => a -> [a] check_elem x = if x == e then return l2 -- Tries to match two lists. If they match, the result consists of the sublist -- bound to the wildcard in the pattern list. (line 18) match :: Eq a => a -> [a] -> [a] -> Maybe [a] match _ _ _ = Nothing {- TO BE WRITTEN -} Thank you for your attention! -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6360687 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Microsoftish Haskell
by Kaveh Shahbazian 17 Sep '06

17 Sep '06

1 - Prologue I am writing C# WEB/Desktop/Embeded applications for 3 years by now and before that, I was writing Visual C++/MFC/COM sort of things. I can say now that everything that come from microsoft is a handcraft of a saleman that has no guaranty and will break before you get home and even try it! For example see generics in C#. These thing are just a code generatore utility that is embeded in Visual Studio 2005 and Microsoft "JUST" has named it "GENERICS". For comparation see generics in Java 5.0. 2 - Question Will Haskell become another pet for Microsoft? If Yes/No, Why? (There are many issues around licensing GHC as you'v seen in this mailing list and I think Haskell already HAS some big problems that prevent others to use it confidently.)

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Serialising types with existential data constructors
by Misha Aizatulin 16 Sep '06

16 Sep '06

hello all, I've been using existentially quantified data constructors like > data Box = forall a. Cxt a => Box a quite successfully for a while. But now I am trying to implement the Load/Save mechanism and getting stuck with that. It's not hard to write a Box into a file, but how do I get it back? Has anyone solved the same problem before? I would be very thankful for any suggestions! If Template Haskell would support finding out all instances of a given class, I could generate a function that would map Names of types to appropriate Box readers. In the file I would write entries like <NameOfTypeInTheBox> : <BoxContents> But sadly, TH doesn't allow that yet. Cheers, Misha

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How to use clipboard in Gtk2Hs
by Yu, Jae-Myoung 16 Sep '06

16 Sep '06

I'm making a text editor with Gtk2Hs, and I want to implement copy & paste with TextView. I read the api document but I can't find how to use the Gtk.Clipboard object. Without context menu basically included in the TextView widget, How can I implement copy & paste?

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Deferred instance declarations (context without type variables)
by Misha Aizatulin 16 Sep '06

16 Sep '06

hello, I have a question about context in type signature. I would like to write a function, say (f :: T -> T) which also relies on an instance of class C being defined for T. The problem is, I don't want this instance defined at the same time f is defined, instead I would like to defer this definition until f is called (in some other module). Naively the code would look like this: ================================= module DefineF where class C a where fC :: a -> a data T = T f :: (C T) => T -> T f T = fC T ================================= module CallF where instance C T where fC = id call = f T ================================= The definition of f in DefineF won't compile because "All of the type variables .. are already in scope". Could you recommend any other way to achieve what I am trying to do? An interesting thing is that I can easily make the code compile with the same meaning by changing the declaration of T to include a dummy type variable: > data T a = T it's just a pity that I have to trick the compiler in such an ugly way. Cheers, Misha

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Re: [Haskell-cafe] Re: ambiguous partially defined type problem
by Maarten 15 Sep '06

15 Sep '06

Dear Brian, Maarten wrote: > Brian Hulley wrote: > > Alternatively, you could wrap the custom part within the node as in: > > data Node = forall cust. ICustom cust => Node cust Common > > getCommon :: Node -> Common > getCommon (Node cust com) = com > Thanks. This really helped. The main thing (I think) that you put the custom part behind an interface. After this I separated the custom and common part of two 'piggy bagged' state transformers, so one can access the functionality separately. The state transformers made into active object by putting them behind a channel in a separate thread and one can invoke actions by writing to the channel. The common functionality provides the connections between the active objects. In this way I would like to create some sort of 'agent' structure, that receive message and process them in their own thread. So far this works quite neat. Wonder if this is they way to go though... Only update (see code below) is a bit ugly (I have no idea why I need fixCastUpdate) and Node itself is probably not necessary, so one level of indirection could be removed. Rest is quite straight forward. Thanks again. Maarten ... (imports) data Node = forall cust. (ICustom cust) => Node cust deriving (Typeable) instance Show Node where -- just for debugging show (Node a) = "Node (" ++ show a ++ ")" class (Show a, Typeable a) => ICustom a where getVal :: forall b cust. (Typeable b, ICustom cust) => a -> (cust -> b) -> Maybe b getVal a f = case cast a of Nothing -> Nothing Just cust -> Just (f cust) -- update :: oif -> (forall a. (ObjectIFace a) => a -> a) -> IO oif update :: a -> (forall b. (ICustom b) => b -> b) -> a update a f = f a instance ICustom Node where getVal (Node n) f = getVal n f update (Node n) f = Node (update n f) type NodeState a = StateT Node (StateT Common IO) a type Connection = Chan (NodeState ()) type Connections = [Connection] instance Show Connection where show o = "Chan (StateT Node (StateT Common IO) ())" -- common part data Common = Common { uid::Integer, connections::Connections } deriving (Show,Typeable) -- custom data data Custom = Custom { val::Integer } deriving (Show,Typeable) instance ICustom Custom where data Custom2 = Custom2 { val2::Integer } deriving (Show,Typeable) instance ICustom Custom2 where -- some function to use common functionality uidM :: NodeState Integer uidM = lift $ gets uid addNodeM :: Connection -> NodeState () addNodeM n = lift $ modify (\s -> addNode s n) where addNode (Common i ns) nn = (Common i (nn:ns)) getNodeM :: Integer -> NodeState Connection getNodeM i = do s <- lift $ get return (getNode s i) where getNode (Common _ ns) i = (ns!!(fromInteger i)) getValM f = do s <- get return (getVal s f) updateM :: forall a b. (ICustom a, ICustom b) => (a -> b) -> NodeState () updateM f = do s <- get let s' = update s (fixCastUpdate f) put s' fixCastUpdate f st = case (cast st) of Nothing -> st Just es -> case cast (f es) of Nothing -> st Just g -> g getStateM = get -- function to create active node functionality action [] = return () action (e:es) = do e s <- get -- just for debugging lift $ lift $ putStrLn $ show s action es newBaseState = do uid <- newUnique return (Common ((toInteger .hashUnique) uid) []) initAction list state = do bs <- newBaseState execStateT (execStateT (action list) state) bs return () send chan action = writeChan chan action sync chan f = do mv <- newEmptyMVar send chan (f' mv) a <- takeMVar mv return a where f' mv = do a <- f lift $ lift $ putMVar mv a newActiveObject action state = do chan <- newChan cs <- getChanContents chan forkIO (action cs state) return chan -- example main = do let n1 = Node (Custom 5) let n2 = Node (Custom2 6) let n3 = Node (Custom2 7) chan <- newActiveObject initAction n1 chan2 <- newActiveObject initAction n3 let l = [chan, chan2] mapM_ (\ch -> send ch (addNodeM chan)) l mapM_ (\ch -> send ch (addNodeM chan2)) l r <- mapM (\ch -> sync ch (getNodeM 0)) l putStrLn $ "r:" ++ show r r2 <- mapM (\ch -> sync ch (uidM)) l putStrLn $ "r2:" ++ show r2 r3 <- mapM (\ch -> sync ch (getValM val)) l putStrLn $ "r3:" ++ show r3 mapM_ (\ch -> send ch (updateM (\s -> s { val2 = 100 }))) l r5 <- mapM (\ch -> sync ch (getStateM)) l putStrLn $ "r5:" ++ show r5 getChar return ()

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Why is type 'b' forced to be type 'm a' and not possibly 'm a -> m a'
by Vivian McPhail 15 Sep '06

15 Sep '06

Dear Haskell Cafe, I have a problem I can't get my head around. The code below sets the problem out. What I need to be able to do is commented out. This code works, the only problem is that what I need is that an argument will be evaluated before it is passed, so ((and fries eats) eggs) has a single `eggs` (fries1 eggs2 and3 eats4 egg2) not (fries1 eggs2 and3 eats4 eggs5). The code that doesn't work is commented out at the bottom. I'm not sure the behaviour of ghc is correct, because when it typechecks it tries to unify `b = t t1` but `b` could actually be `t t1 -> t t1`. I want to be able to specify that when the first argument of `b` is of type `m a` that fork should run it and _then_ fork the argument to the first two arguments of 'fork'. The instance for (a -> b) covers the rest of the possibilities. just type "run test[1-4]" to see results. \begin{code} {-# OPTIONS_GHC -fglasgow-exts -fno-monomorphism-restriction #-} module Fork where {--------------------------------------------------------------------------- --} import Prelude hiding (and) import Control.Monad.State {--------------------------------------------------------------------------- --} data NRef = NS0 String | NS1 String NRef | NS2 String NRef NRef deriving(Show) {--------------------------------------------------------------------------- --} data UniqueS = US { nums :: [String] } deriving(Show) type USM a = StateT UniqueS IO a newUniqueS :: UniqueS newUniqueS = US { nums = [ show x | x <- [1..] ] } freshInstance :: String -> USM String freshInstance x = do (f:fs) <- gets nums put $ US { nums = fs } return $ x ++ f {--------------------------------------------------------------------------- --} single x = do x' <- freshInstance x return $ NS0 x' unary x n = do x' <- freshInstance x n' <- n return $ NS1 x' n' binary x n1 n2 = do x' <- freshInstance x n1' <- n1 n2' <- n2 return $ NS2 x' n1' n2' {--------------------------------------------------------------------------- --} foxy = single "foxy" eggs = single "eggs" golden = unary "golden" white = unary "white" fries = binary "fries" eats = binary "eats" {--------------------------------------------------------------------------- --} class Forkable a where fork :: String -> a -> a -> a instance (Forkable a, Forkable b) => Forkable (a -> b) where fork n a1 a2 a = fork n (a1 a) (a2 a) {- instance (Monad m, Forkable (m a), Forkable b) => Forkable (m a -> b) where fork n a1 a2 a = do a' <- a fork n (a1 $ return a') (a2 $ return a') -} {--------------------------------------------------------------------------- --} instance Forkable (USM NRef) where fork n a1 a2 = do a1' <- a1 a2' <- a2 return $ NS2 n a1' a2' {--------------------------------------------------------------------------- --} and = fork "and" test1 = (and foxy eggs) test2 = (and golden white) eggs test3 = (and fries eats) foxy eggs test4 = (eats foxy (and (golden eggs) (white eggs))) run x = runStateT x newUniqueS >>= (putStrLn . show . fst) \end{code -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.405 / Virus Database: 268.12.4/448 - Release Date: 14/09/2006

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parsing long numbers
by Andrea Rossato 15 Sep '06

15 Sep '06

Hello! I'm trying to learn parsing and parser combinations in Haskell, using, as usual, Wadler's Monads in Functional Programming as my text book. Everything works fine except for a small but annoying problem related to "read". I'm sure it must be something easy, some kind of stupid faq. Still I'm not able to find a way out. I created a simple parser and made it an instance of monad and MonadPlus. I then created "iterateP" to combine recursive parsers, and "filterP" to apply some filters. Then I created a function "number", to parse numbers. This function returns a string and works fine. But I wanted to have integers back from parsing. So I created a filter, "digitS", and a new parser for numbers, "number1", that applies recursively digitS and should be returning an Int using "read". The problem is that, when I run it with strings containing a number more then 10 digit long, I get unexpected integers back: *Main> runP number1 "1234567890 and the rest" [(1234567890," and the rest")] *Main> runP number1 "12345678901 and the rest" [(-539222987," and the rest")] Obviously if I use Parsec I can parse that number perfectly. So I tried with another approach: "number2" recursively applies a filter, "digitI", that returns an Int. asNumber is a function that takes a list of single digit integers and returns the corresponding integer. *Main> runP number2 "12345678901 and the rest" [(Just 1234567890," and the rest")] *Main> runP number2 "12345678901 and the rest" [(Just (-539222987)," and the rest")] The very same result. Can you please help me understand why I seem not to be able to get the number I'd like to get? As I said, I think I'm missing something that must be pretty obvious, but still I cannot see it! Thanks for your kind attention. Andrea ps: sorry for such a long message. Moreover, here's the code: module Main where import Control.Monad import Data.Char newtype M a = S {unpack :: String -> [(a,String)]} instance Monad M where return a = S $ \s -> [(a,s)] m >>= f = S $ \s -> [(b,z) | (a,y) <- unpack m s, (b,z) <- unpack (f a) y] instance MonadPlus M where mzero = S $ \x -> [] mplus a b = a `bchoice` b bchoice (S m) (S m1) = S $ \s -> case m s of [] -> m1 s other -> other iterateP m = do { a <- m ; b <- iterateP m ; return (a:b) } `mplus` return [] filterP p = S (\xs -> case xs of [] -> [] (x:xs') -> if p x then [(x,xs')] else []) number = do { a <- filterP isDigit ; b <- number ; return (a:b) } `mplus` return [] digitS = do a <- filterP isDigit return a number1 :: M Int number1 = do a <- iterateP digitS return (read a) -- a different approach maybeAdd a b = do x <- a y <- b return (x + y) asNumber :: [Int] -> Maybe Int asNumber [] = Nothing asNumber (x:[]) = Just x asNumber (x:xs) = Just (x * 10 ^ length xs) `maybeAdd` asNumber xs digitI = do a <- filterP isDigit return $ ord a - ord '0' number2 :: M (Maybe Int) number2 = do a <- iterateP digitI return $ asNumber a runP (S f) = f

2 1

evaluate vs seq
by Michael Shulman 15 Sep '06

15 Sep '06

The GHC documentation says that (evaluate a) is not the same as (a `seq` return a). Can someone explain the difference to me, or point me to a place where it is explained? Mike

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Optimization problem
by Rohan Drape 15 Sep '06

15 Sep '06

> > splitStreams [(3,x),(1,y),(3,z),(2,w)] > [(3,[x,z]),(1,[y]),(2,[w])] [snip] > Furthermore it should work on infinite lists. It can't eat the whole > list before producing any output. This doesn't seem to make sense? Only at the end of the list can you know that you've collected all the events for each channel. If you output anything before scanning to the end, you'd not know if there were perhaps more events on that channel? You could accumulate only over a window, which would produce values on an infinite input, though the output list could have multiple packets for each channel. splitterW 3 [(3,x),(1,y),(3,z),(2,w),(3,v)] > [(3,[x,z]),(1,[y]),(2,[w]),(3,[v])] Or accumulate until either there are 'n' messages on a channel or the end of the list is reached? splitterN 3 [(3,x),(1,y),(3,z),(2,w),(3,v),(1,u),(3,t)] > [(3,[x,z,v]),(1,[y,u]),(2,[w]),(3,[t])] Regards, Rohan

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Anonymous types
by Thomas Conway 15 Sep '06

15 Sep '06

Hi all, Is there any deep and meaningful reason why Haskell doesn't have anonymous discriminated union types? I'm thinking of an example like: data Amount = Amount Integer (Mg|G|Kg|T) Now this particular case is perhaps unconvincing - a seperate Units type would be quite sensible, however I'm thinking of translating ASN.1 definitions into Haskell. ASN.1 allows SEQUENCE (record types) and CHOICE (discriminated unions) as a kind of type constructor. Not having to invent names for anonymous SEQUENCE and CHOICE types would be nice. T.

2 1

Bit string
by Thomas Conway 15 Sep '06

15 Sep '06

Hi all, I'm doing some work with ASN.1, and I'm thinking about how to represent the type "BIT STRING". The obvious, and inefficient representation would be > type BitString = [Bool] A reasonable sparse representation might be > type BitString = [Integer] where the integers represent the positions of the non-zero bits, but other implementations (e.g. C & C++ based ones) mostly seem to use dense representations, and there are certain predictability advantages to be had by following existing implementations in this regard. But given that a value of type BIT STRING is allowed to have leading 0 bits, which have no semantic effect, another representation would be > type BitString = [Word64] (I'm a modern type, and believe in 64bit computing ;-), but you could use Word32 if you liked). However, after a few moments' consideration, I realized, that if I was going to use a representation like that, I could probably use > type BitString = Integer which already has [I assume] a carefully optimized implementation. Also, it ends up being significantly more strict than a list representation, which is probably a good thing in most situations. My question for all present is: Have I missed either a problem with using Integer, or have I overlooked a better representation? T.

5 5

ambiguous partially defined type problem
by Maarten 14 Sep '06

14 Sep '06

Dear all, For a project involving I use some partially defined node (in this case a simple record, in my project state transformers) in which the defined part is common to all nodes, and the custom part is different for each node. They have to become anonymous so I can put them in a list of connections from each node to another. For some reason GHC complains of 'ambigous type variable' in the code below. The thing is, part of the code may be undefined, but since I'm (explicitly) not using that part, why would GHC care? Are there other solutions to this problem? Any pointers or comments appreciated. Thanks. Maarten (This code is just some dummy code that contains the essence of the problem. I posted the complete code with piggy bagged state transformers in active objects on haskell(a)haskell.org, but that is rather long and this seems to be the correct mailing list). -- data structure with custom and common part data Node cust = Node cust Common deriving (Show,Typeable) -- anonymous data structure to put use in list data AN = forall ar. (Show ar, Typeable ar) => AN ar instance Show AN where show (AN an) = "AN (" ++ show an ++ ")" -- common part data Common = Common Integer deriving (Show,Typeable) data Custom = Custom Integer deriving (Show,Typeable) data Custom2 = Custom2 Integer deriving (Show,Typeable) -- extract common part, ignoring type of custom part getCommon :: forall gc. (Node gc) -> Common getCommon (Node cust com) = com main = do let a = AN (Node (Custom 5) (Common 10)) let b = case a of (AN a') -> getCommon (case (cast a') of Just a'' -> a'') putStrLn $ "ok:" ++ show b

3 2

program execution and laziness
by Tim Newsham 14 Sep '06

14 Sep '06

I'm having a problem with program execution in win32 which seems to be tied to laziness. The function I'm using is: runCmd cmd outdir dir base ext = do let argv = words cmd (i,o,e,p) <- runInteractiveProcess (head argv) (drop 1 argv) Nothing Nothing hClose i out <- hGetContents o -- print out x <- waitForProcess p hClose o hClose e case x of ExitSuccess -> return 0 (ExitFailure n) -> return n this is hanging indefinitely when I run it in win32. If I add in the print statement (commented out above) it works fine! So I can only imagine that waitForProcess is hanging when I haven't drained the stdout stream. How can I force hGetContents to be strict (or at least to completely process the stream prior to the waitForProcess command)? Tim Newsham http://www.thenewsh.com/~newsham/

4 3

Re: [Haskell-cafe] Slow IO
by Daniel Fischer 14 Sep '06

14 Sep '06

Am Montag, 11. September 2006 17:44 schrieben Sie: > Daniel Fischer wrote: > > > Try Don Stewart's ByteString library > > > > -- and some data that made the programme segfault before now run in a > > couple of seconds. > > But that shouldn't happen! Segfaults aren't performance problems, but > errors. So your program contained a bug (lazyness where it isn't > useful, I suspect), and ByString is hiding it. Maybe I've misused the word segfault. What happened is: The programme consumed more and more memory (according to top), kswapd started to have a higher CPU-percentage than my programme, programme died, system yelling 'Speicherzugriffsfehler', top displays 'kswapd<defunct>'. I believe that means my programme demanded more memory than I have available (only 256MB RAM + 800MB swap). Is that a segfault or what is the correct term? That is probably due to (apart from the stupidity of my IO-code) the large overhead of Haskell lists. So the chunk of the file which easily fits into my RAM in ByteString form is too large as a list of ordinary Strings. However the problem might be too little lazyness, because if I explicitly read the file line by line, memory usage remains low enough -- but ByteString is *much* faster anyway. > > > So even if we just counted newlines, we would have to scan 1,700 million > > (1.7*10^9) chars per second. > > Could any ordinary computer of today do that, using whatever language? > > That rate should be no problem for a 2GHz machine. However, a 200MHz 64 > bit wide bus won't deliver the data fast enough and it is 50x as much as > the best hard disks could deliver in optimal circumstances. I guess, > most of the test cases are a lot smaller than your guesstimate. > I suppose so, too, but not knowing the test data, I assumed a bad case (uniform distribution of line-lengths and test-case-size in the specified range). > > Udo. Bulat: > are you mean arithmetic or geometric average? ;) I meant 'expected value'. If X_i are independent random variables uniformly distributed on [0 .. k], Y is a random variable (independent from the X_i) uniformly distributed on [1 .. n] and Z is the sum of the first Y of the X_i, then the expected value of Z is (n+1)*k/4. So we might call that a weighted arithmetic average, I suppose. Cheers, Daniel -- "In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton

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Optimization problem
by tpledger＠ihug.co.nz 14 Sep '06

14 Sep '06

Magnus Jonsson wrote: [...] > but your example fails on infinite lists [...] > take 2 $ snd $ head $ splitStreams (map (\x -> (0 ,x)) [1..]) Any approach, even sieving, will struggle with infinite lists, won't it? (take 2 . snd . head . splitStreams) [(i, i) | i <- [0..]] Regards, Tom

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Re: [Haskell-cafe] Weak pointers and referential transparency???
by tpledger＠ihug.co.nz 13 Sep '06

13 Sep '06

Brian Hulley wrote: > tpledger(a)ihug.co.nz wrote: [...] > > My reading of the semantics > > (http://haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html…) > > is that you can be sure the proxy *object* is gone. > > My problem is that I don't know what to make of the word > "object" in the context of Haskell ie when can I be sure > that a value is actually being represented as a pointer > to a block of memory and not stored in registers or > optimized out? Or is the compiler clever enough to > preserve the concept of "object" despite such > optimizations? I had been designing my Model/Proxy data > types with the Java notion of "everything is a pointer to > an object" but is this always correct relative to Haskell > as a language or is it just a consequence of the current > GHC implementation? In the context of System.Mem.Weak, but not necessarily GHC, we're concerned solely with garbage collection of heap objects. So yes, that's Java-like. AFAIK. An example of something outside that context is a GHC Int# (unboxed Int). It never inhabits the heap, and isn't allowed to be passed to a function where a polymorphic parameter is expected (such as mkWeak). Regards, Tom

1 0

ffi question
by Maverick 13 Sep '06

13 Sep '06

Hi, I have a question about ffi in Hugs98 (WinHugs-May2006.exe), the Hugs98 documentation says: "Only the ccall, stdcall and dotnet calling conventions are supported. All others are flagged as errors." But I can't get a dotnet import, the ffihugs returns me an error: ffihugs "example.hs" "Prueba.cs" -lm runhugs: Error occurred ERROR "example.hs":6 - Foreign import calling convention "dotnet" not supported Why is this? hugs really don't support the "dotnet" calling convention or I have an error?. Thanks. Alvaro S. --------------------------------- LLama Gratis a cualquier PC del Mundo. Llamadas a fijos y móviles desde 1 céntimo por minuto. http://es.voice.yahoo.com

1 0

[OT] A DSL for state machines
by Stephane Bortzmeyer 13 Sep '06

13 Sep '06

Sorry, this is a bit off-topic but I post here because: * it is about a DSL and many Haskellers are fond of DSLs, * the first implementation is written in Haskell. http://www.cosmogol.fr/ describes a proposal to the IETF to standardize the language used for finite state machines (which are common in IETF standards), much like ABNF is standardized in RFC 4234, to describe grammars. The reference implementation, available at http://www.cosmogol.fr/shadok.html is in Haskell. Bug reports, patches, advices, flames, opinions and criticisms are welcome.

1 0

Why does this program eat RAM?
by John Goerzen 13 Sep '06

13 Sep '06

I have the below program, and I'm trying to run it on an input of about 90MB. It eats RAM like crazy, and I can't figure out why. I do know that the problem is not my custwords function (as you can see, I replaced the call to it with a call to the standard words function on the last line). It seems to be wordfreq, but I don't know why. For information about the problem, see http://changelog.complete.org/posts/535-A-Haskell-solution-to-Lars-Problem.… Here's the code: import System.Environment import Data.List import Data.Char import qualified Data.Map as Map custwords = filter (/= "") . lines . map (conv . toLower) where iswordchar x = isAlphaNum x && isAscii x conv x = if iswordchar x then x else '\n' wordfreq inp = Map.toList $ foldl' updatemap (Map.empty::Map.Map String Int) inp where updatemap nm word = Map.insertWith updatefunc word 1 nm updatefunc _ x = x + 1 freqsort (w1, c1) (w2, c2) = if c1 == c2 then compare w1 w2 else compare c2 c1 showit (word, count) = show count ++ " " ++ word main = do args <- getArgs interact $ unlines . map showit . take (read . head $ args) . sortBy freqsort . wordfreq . words

10 13

Retainer Profiling
by Mark Goldman 13 Sep '06

13 Sep '06

Ok, so I've done a biographical profile and gotten what (I think) are the top offenders in creating objects that never get used. (+RTS -hc -hbvoid) The GHC user's guide suggests that the next step is retainer profiling to see why things are being retained instead of trashed. Is anyone out there able to explain retainer profiling in more detail than the GHC user's guide because I've been guessing on both what I've been seeing and what it all means. Any response would be appreciated. -mdg -- Our problems are mostly behind us, now all we have to do is fight the solutions.

1 0

Weak pointers and referential transparency???
by tpledger＠ihug.co.nz 12 Sep '06

12 Sep '06

Brian Hulley wrote: [...] > Ref.write proxiesRef $! (weakProxy : proxies) (This is nothing to do with your main question, but the strict application looks unnecessary there. Its right hand side is already a constructor cell.) [...] > In other words, if the entry for the proxy > in the table stored in the Model dies, can > I be absolutely 100% sure that this means > the proxy no longer exists in any shape or > form in the running program? My reading of the semantics (http://haskell.org/ghc/docs/latest/html/libraries/base/System-Mem-Weak.html…) is that you can be sure the proxy *object* is gone. As for referential transparency... Fan-in: If you create equivalent proxies in different calls to createProxy, it's possible that they'll end up referring to the same object (e.g. if the compiler or RTS does something fancy, or you use a memoised smart constructor). So then a single live proxy object *could* protect many elements of your Weak Proxy list from scavenging. Fan-out: It would seem perverse to cry "referential transparency!" and spontaneously clone one of your proxy objects. That *could* lead to deRefWeak returning Nothing while an *equivalent* Proxy object is still alive. Might you run your program on an avant-garde distributed RTS? ;-) Regards, Tom

2 1

Slow IO
by Daniel Fischer 12 Sep '06

12 Sep '06

Hello all, Now I have an IO-problem, too. SPOJ problem 41 asks basically to determine whether a directed graph allows a path that uses every edge exactly once. The data from which the graphs are to be constructed are contained in a (huge) file, every item (number of test cases, size of test case, edges) on a single line. I've created my own test case file (much smaller, but already 217 MB, 2.2 million lines) to check my programme's performance. Now profiling reveals that over 90% of time and allocation are consumed by reading the file (line by line, reading the whole file as a lazy String and then chopping that to pieces is rather worse). So how do I quickly 1. read an Integer n from the file, 2. read the next n lines and 3. pass the first and last letter of those to the function that builds and checks the graph? When that is achieved, I could see whether my algorithm is good. Thanks for any help, Daniel -- "In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton

4 5

haskell toolchain for slackware
by Andrea Rossato 12 Sep '06

12 Sep '06

Hello! Since there seems to be very little support for haskell in Slackware Linux, I'm putting together a repository with the most useful (for me) hasekll utilities. For the time being there are: 1. ghc-6.4.2 (precompiled binaries packaged for slackware) 2. haddock 3. darsc 4. hscurses 5. HaXml-1.13.2 6. HXT-6.1 with HTTP-20060707 I'm planning to add, very soon, Hugs, hs-plugins and fps-0.7. The repository should be accessible with slapt-get and swaret. I hope dependency checking is working with slapt-get. Here's the repo: http://gorgias.mine.nu/slack/ You can also grab the SlackBuild scripts (I started using Cabal with HaXml, HXT and HTTP) here: http://gorgias.mine.nu/repos/slackBuild/ or darcs get http://gorgias.mine.nu/repos/slackBuild/ I build my stuff in an almost current slack. Ciao Andrea

1 0

Newbied question on IO Monad
by Sara Kenedy 12 Sep '06

12 Sep '06

Hello all, update :: String -> String update sss = ... main = do writeFile "myFile.txt" sss x <- callSystem "myFile.txt" y <- openFile "result.txt" ReadMode zzz <- hGetContents y return zzz I know that function main returns IO String, function update returns String. And my purpose is to get the string zzz from main for the value return of function update. But I do not know which way I can do. Sorry if the question seems ridiculous. Thanks for any help. S.

6 7

RE: [Haskell] Re: H/Direct (HaskellDirect) still maintained?
by Simon Marlow 12 Sep '06

12 Sep '06

On 08 September 2006 17:35, Andreas Marth wrote: > Now that I used a more recent ghc (6.4.2 which has cabal-1.1.4 > installed) I did install comlib with it. > But I am a bit surprised because it did not generate a ihc. So how do > I get a Proxy from an idl-file now? > > If I look at the files in vsHaskell they state "Automatically > generated by HaskellDirect (ihc.exe) ....". > Does anybody know how to generate ihc.exe? comlib is just the COM interface library, it doesn't include ihc (the IDL compiler, which is part of H/Direct). Some of the the Visual Haskell sources were originally generated by ihc, but since then we have modified the sources by hand, and we're no longer using ihc. You should be able to use these files as example code to see how to write your COM interface code. It's a lot of tedious boilerplate, unfortunately. Maybe someone will resurrect or write a replacement for ihc someday. Cheers, Simon

1 0

Heap profiling
by Mark Goldman 12 Sep '06

12 Sep '06

Does anyone know if there is a way around the 20 charachter identifier limitation when heap profiling? I have a number of identifiers that indistinguishably break that limit. -mdg -- Our problems are mostly behind us, now all we have to do is fight the solutions.

2 1

Re[2]: [Haskell] Proposal: unification of style of function/data/type/class definitions
by Bulat Ziganshin 11 Sep '06

11 Sep '06

Hello Taral, Sunday, September 10, 2006, 9:45:33 PM, you wrote: >> data Expr t = If (Expr Bool) (Expr t) (Expr t) >> Expr Int = Lit Int >> Expr Bool | Eq t = Eq (Expr t) (Expr t) > I find this somewhat unreadable due to the implicit "t" parameter not > showing up on the left-hand side... if you mean last line, i tend to agree. i was just a little impulsive and don't mentioned that we define existential here :) it may look as Expr Bool = Eq (Expr t) (Expr t) | Eq t Brian Hulley developed here an idea about implicit "forall" declaration just before class restrictions involving this variable, so it will be equivalent to: Expr Bool = Eq (Expr t) (Expr t) | forall t . Eq t -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

3 4

HaXml and ghci unresolved symbol
by Andrea Rossato 11 Sep '06

11 Sep '06

Hello! probably it's me, but I cannot understand what I'm doing wrong. I'm trying to learn HaXml. I've never used it before and I never did xml processing in Haskell. So I'm a total newbie!! I downloaded and compiled. Everything seems fine. I'm also able to run some examples in the related directory of the source tree. Then I try doing something myself. Very simply, a document with just one element: module Xml where import Text.XML.HaXml xml = "<a>test</a>" doc = xmlParse "test" xml elemName (Document prolog _ (Elem name _ _) _) = name When I try it: *Xml> elemName doc I get a message I've never got before (I'm a newbie!) and I don't know whether the problem is in my code (this would be my first guess), in HaXml or in GHCi. And so I cannot even start understanding out to solve it: During interactive linking, GHCi couldn't find the following symbol: TextziXMLziHaXmlziParse_xmlParse_closure This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. Restart GHCi, specifying the missing library using the -L/path/to/object/dir and -lmissinglibname flags, or simply by naming the relevant files on the GHCi command line. Alternatively, this link failure might indicate a bug in GHCi. If you suspect the latter, please send a bug report to: glasgow-haskell-bugs(a)haskell.org *Xml> xmlParse is defined in Text.Xml.HaXml (xmlParse :: String -> String -> Document) and it should be imported by Text.XML.HaXml. Can you give me please some directions? Thanks for your kind attention.

5 7

Nice cygwin application to be used with ghci
by Maurício 11 Sep '06

11 Sep '06

Hi, I've just uploaded a package ('rlwrap') to cygwin that I like to use with ghci. You can use it like this: rlwrap ghcii.sh and then you will use ghci as if it were readline aware (i.e., you can press up arrow to get last typed lines etc.). 'rlwrap' is very stable and I never had unexpected results while using it. Since the issue of ghci integration with terminals has been raised here sometimes, I thought some guys here would be interested (actually, I found rlwrap looking for a better way to use ghci). Best, Maurício

2 1

Re: [Haskell] Re: Proposal: unification of style of function/data/type/class definitions
by Neil Mitchell 10 Sep '06

10 Sep '06

Hi > I don't see the problem. You can read all of Bulat's > examples as "'thing being declared' 'relationship' 'value' > given that 'context'", so this one is ""sequence" has type > "[m a] -> m [a]" given that "m" is a "Monad"". So viewed > that way, they're all consistent with each other. How about name, context, value as the three elements: class <name> | <context> where <value> data <name> | <context> = <value> type <name> | <context> = <value> <name> | <context> :: <value> For class and data, you can argue that it isn't really a <value>, its <inner> or something. But type definately has a <value>. And if you want data and type consistent, you need to put them like that. And then, if you want functions consistent with types, you have to do the above. Or at least thats what my brain interprets as logical, and on the plus side it changes data/type/class which I personally don't like, but doesn't change function declarations, which I am not aware of anyone having issue with. Thanks Neil

2 1

Re: [Haskell-cafe] HaXml and ghci unresolved symbol
by Andrea Rossato 10 Sep '06

10 Sep '06

Il Sat, Sep 09, 2006 at 05:20:55PM -0400, Mark T.B. Carroll ebbe a scrivere: > FWIW I have the same problem - I can't use HaXml with ghci. > So it's not just you. (-: Indeed! So I found my first bug in ghc... And now I'll dig into ghc bug reports to see if someone is working on the problem... This is going to be hard, but it's the way you start learning something....;-) Thanks for your kind attention. Below the full error messages with the code producing them. Andrea The code below works fine with Hugs: Main> :load xml.hs Main> main prova Main> If I try to compile it with ghc I get this: [11:16:06][andrea@laptop:~/devel/haskell/xml]$ ghc --make xml.hs -package HaXml -package hxml -o prova Chasing modules from: xml.hs Compiling Main ( xml.hs, xml.o ) Linking ... /usr/local/lib/hxml-0.2/lib/libhxml.a(HaXmlAdapter.o)(.text+0x12c1): In function `spZc_dflt': : undefined reference to `TextziXMLziHaXmlziPretty_zdwelement_infó /usr/local/lib/hxml-0.2/lib/libhxml.a(HaXmlAdapter.o)(.text+0x1435): In function `spZy_dflt': : undefined reference to `TextziXMLziHaXmlziPretty_zdwelement_infó /usr/local/lib/hxml-0.2/lib/libhxml.a(HaXmlAdapter.o)(.rodata+0xa4): undefined reference to `TextziXMLziHaXmlziPretty_zdwelement_closure` /usr/local/lib/hxml-0.2/lib/libhxml.a(HaXmlAdapter.o)(.rodata+0xc4): undefined reference to `TextziXMLziHaXmlziPretty_zdwelement_closure` collect2: ld returned 1 exit status Loading HaXml and hxml in ghci will produce: [11:15:39][andrea@laptop:~/devel/haskell/xml]$ ghci -package HaXml -package hxml ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Loading package Cabal-1.1.4 ... linking ... done. Loading package haskell98-1.0 ... linking ... done. Loading package haskell-src-1.0 ... linking ... done. Loading package HaXml-1.13.1 ... linking ... done. Loading package hxml-0.2 ... linking ... ghc-6.4.2: /usr/local/lib/hxml-0.2/lib/hxml.o: unknown symbol `TextziXMLziHaXmlziTypes_AttValue_con_info` ghc-6.4.2: unable to load package `hxml-0.2' This is the code: module Main where import Text.XML.HaXml import HaXmlAdapter -- from hxml xml = "<a>prova</a>" doc = xmlParse "tmp" xml getContent (Document prolog _ (Elem name _ content) _) = content cont = getContent doc showDoc :: [Content] -> IO () showDoc [] = return () showDoc (x:xs) = do putStrLn $ showContent x showDoc xs main = showDoc cont

3 4

nondet function
by Ashley Yakeley 10 Sep '06

10 Sep '06

Is it possible to write nondet? nondet :: a -> a -> a nondet _|_ _|_ = _|_ nondet _|_ q = q nondet p _|_ = p nondet p q = p or q nondet evaluates its arguments in parallel, and returns the first one of them to evaluate. It's thus a bit different from the "par" of GPH. This isn't referentially transparent, of course, but maybe it could be written in the IO monad: nondet p q = unsafePerformIO (getnondet p q) getnondet :: a -> a -> IO a -- Ashley Yakeley, Seattle WA

4 3

Re[2]: [Haskell] Proposal: unification of style of function/data/type/class definitions
by Bulat Ziganshin 10 Sep '06

10 Sep '06

Hello Neil, Sunday, September 10, 2006, 2:21:50 PM, you wrote: >> class Monad m | Functor m, Monoid m where ... > It also makes grep'ing easier. yes, i also had problems finding class declarations in base package >> sequence :: [m a] -> m [a] | Monad m > I don't like this. In the other two instances you are moving the most > important information (the name of the thing) to the front. In this > the name is at the front, but the instances move to the end, which > isn't really where they should be. And following the function | rule > function | predicates = body > I would have said that logically, you want: > sequence | Monad m :: [m a] -> m [a] > (of course, this might present a problem for parsing...) > Note this also makes sense compared to your: > data Data | Classes = Alternatives you have omitted "patterns, or parameters shape" part: function shapes_of_parameters | guards_for_parameters = result so, in my proposal: data T a b | Eq a, Monad b = .... and function signature obey the same rule - first write shape of function type, then narrow type variables used to some classes: addEncoding :: Encoding m -> h -> m (WithEncoding m h) | Monad m, ByteStream m h Of course, that's more important is readability. it's discussible whether we need to know that m is monad and h is stream before reading shape of function type btw, the following is anyway most readable: sequence :: [Monad a] -> Monad [a] ;) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

1 0

Re: [Haskell-cafe] HaXml and ghci unresolved symbol
by Andrea Rossato 10 Sep '06

10 Sep '06

Il Sat, Sep 09, 2006 at 04:34:04AM +0100, Marco André F. de Almeida ebbe a scrivere: > I know this will not solve your problem, but just so that you > know that with Hugs, you code works without any problems. > If you don't want/need to compile the program, I guess one interpreter > (Hugs) is as good as the other (GHCi) ;-) > > Marco Hi Marco! For the time being, that is to say, for learning xml transformation in haskell Hugs is just fine. But I'd like to start working on a project of a command line tool with hscurses that requires xml processing. I'm going to do that mostly as a learning exercise and I'm still in the preliminary phase, looking for libraries and tools. I'd like to start writing real code for the xml part, though. Thanks, Andrea

1 0

NaN, Infinity literals
by Tamas K Papp 09 Sep '06

09 Sep '06

Hi, Is there a way to use NaN and Infinity as literals, or at least to test if a value is NaN or Infinity? I tried *Main> let nan=0/0 *Main> nan NaN *Main> nan==0/0 False so "storing" the value does not work... Thanks, Tamas

6 5

Weak pointers and referential transparency???
by Brian Hulley 09 Sep '06

09 Sep '06

Hi, I have the following data structures: import System.Mem.Weak data Proxy = ... data Model = Model { _proxiesRef :: !(Ref.T [Weak Proxy]), ...} (Ref.T is just a lifted IORef) I was writing code like: createProxy :: MonadIO m => Model -> m Proxy createProxy Model{_proxiesRef = proxiesRef} = do proxies <- Ref.read proxiesRef let proxy = Proxy{ ... } weakProxy <- liftIO $ mkWeakPtr proxy Nothing Ref.write proxiesRef $! (weakProxy : proxies) return proxy where the intention is that whenever the returned proxy is no longer used, the table will no longer hold onto it (the table gets scavenged whenever I ask for the list of values in it) so there will be no space leak as proxies are created and discarded. However when I looked at the above code more closely, it struck me that Haskell gives no guarantees that the returned proxy is actually the same object that is referenced by the table, because of referential transparency. So my question is: will the above code work as expected and is this because we are allowed to assume non-referential transparency in situations like the above ie are we allowed to assume that all occurrences of the identifier "proxy" are always bound to a pointer to the same physical object and that all other uses of this value in the program are not going to cause it to be represented by multiple physical objects or otherwise split apart into registers etc? In other words, if the entry for the proxy in the table stored in the Model dies, can I be absolutely 100% sure that this means the proxy no longer exists in any shape or form in the running program? Thanks, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

1 0

How to round off frational number?
by Sara Kenedy 09 Sep '06

09 Sep '06

Hello all, I try to find some functions in Haskell library to deal with numeric such that the result in the following format (but I did not find) For example, >1/3 0.33 >1/6 0.17 >8/3 2.67 >9/3 3.00 It seems a so "baby" question, but I really did not find the answer after trying on that for some hours. Thanks for any help.

3 3

sections of noncommutative operators
by Michael Shulman 09 Sep '06

09 Sep '06

A propos of sections of subtraction, and thence to sections of other noncommutative operators, as a Haskell newbie I was surprised to discover (the hard way!) that (< 0) and ((<) 0) mean different things. I had typed (< 0) when I meant to type ((<) 0). No compiler errors, of course, and I had a devil of a time finding that bug. My initial reaction was that (< 0) should be an error and you should have to write ((>) 0); now I realize that the section notation is more fundamental, since (<) itself is actually a "double section". And of course I should have written (> 0) anyway; it's probably my lisp background that tripped me up. But is there any way this could be made less confusing? Do any of the online tutorials warn against this sort of mistake, and I just missed it? Mike

4 5

Derived Read instance for types with infix constructors (ghc 6.4.1)
by Misha Aizatulin 09 Sep '06

09 Sep '06

hi, the Haskell Report 10.4 says that "The result of show is readable by read if all component types are readable" however if I define a type like data T = A | T `And` T deriving (Read, Show) then *Main> show $ A `And` A "A And A" *Main> (read "A And A") :: T *** Exception: Prelude.read: no parse *Main> In fact, I wasn't able to guess, what I should type so that the value (A `And` A) gets parsed. I have ghc 6.4.1. Looking into the code of the derived instance I see that it expects Text.Read.Lex.lex to return (Symbol "And") for the constructor. If I understand the code for lex correctly, then it parses things as Symbol if they consist only of "!@#$%&*+./<=>?\\^|:-~" How then do I read values of type T defined above? Thanks in advance for any directions. Cheers, Misha

7 12

[Newbie] Cannot derive Eq and Show. Why?
by Peter Arrenbrecht 09 Sep '06

09 Sep '06

Hi all I am stumped again. The following code generates the error "ERROR file:.\Cube.hs:12 - An instance of IArray UArray a is required to derive Eq (Cube a b)" in Hugs. But I did specify the IArray UArray k constraint. So what is wrong? module Cube( Cube(..) ) where import Data.Array.Unboxed data (Ord k, IArray UArray k, IArray UArray v) => Cube k v = Empty | Values (UArray Int k) (UArray Int v) | Subs (UArray Int k) (Array Int (Cube k v)) deriving (Show, Eq) Thanks for any help -peo -- View this message in context: http://www.nabble.com/-Newbie--Cannot-derive-Eq-and-Show.-Why--tf2239265.ht… Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

2 1

Reading integers
by Bertram Felgenhauer 08 Sep '06

08 Sep '06

Hi, prompted by a discussion about http://www.spoj.pl/problems/MUL/ on #haskell I implemented some faster routines for reading integers, and managed to produce a program that passes this problem. Currently I have a single module that provides reading operations for Integers and Ints. I'm not quite sure what to do with it. Ideally, read should use faster code for parsing numbers. This is hard to do though, because read for numbers is specified in terms of lex, which handles arbitrary tokens. One nasty side effect of this, besides bad performance, is that for example read ('"':reapeat 'a') :: Int diverges, even though there's obviously no possible parse. Incorporating faster integer reading code into read without breaking backward compatibility is not trivial. The next obvious place to put the functions is the Numeric module. This sounds like a good idea until one realizes that readSigned (which is the obvious thing to use for signed integers) again uses lex, with all the complications above. A third option is to put it all in a separate module. This would probably be hard to find, so few people would use it. I'm pondering breaking compatibility with Haskell 98 to some extent and implementing read instances for Int and Integer that don't diverge on weird inputs, but handle all valid numbers that Haskell 98 supports. This still requires some work to support octal and hexadecimal numbers, whitespace and parentheses. What do you think? The code (quite unpolished) is available with darcs get http://int-e.home.tlink.de/haskell/readinteger (browsing directly doesn't work) It also includes a simple benchmark for comparing reads, Numeric.readDec and my readInteger functions. Results for my computer are included as well. regards, Bertram

5 9

Monad laws
by Deokhwan Kim 08 Sep '06

08 Sep '06

What is the practical meaning of monad laws? (M, return, >>=) is not qualified as a category-theoretical monad, if the following laws are not satisfied: 1. (return x) >>= f == f x 2. m >>= return == m 3. (m >>= f) >>= g == m >> (\x -> f x >>= g) But what practical problems can unsatisfying them cause? In other words, I wonder if declaring a instance of the Monad class but not checking it for monad laws may cause any problems, except for not being qualified as a theoretical monad? Cheers, -- Deokhwan Kim

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Does this function already exist in the libraries?
by Jason Dagit 08 Sep '06

08 Sep '06

Hello, Sometimes I find that I have two lists I want to combine, but I only want to combine them for as much as I have elements in both lists to combine and then the rest I just want to pass through unmodified. I've now found the following function useful in three different programs I've written: zipWith' :: (a -> a -> a) -> [a] -> [a] -> [a] zipWith' _ [] ys = ys zipWith' _ xs [] = xs zipWith' f (x:xs) (y:ys) = f x y : zipWith' f xs ys Does it already exist or maybe there is another natural solution to use in its place? Thanks, Jason

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