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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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