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Wheres this going wrong
by Neil Rutland 23 Mar '06

23 Mar '06

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strange ghc program behaviour
by Koen.Roelandt＠mineco.fgov.be 22 Mar '06

22 Mar '06

Hi, I'm a Haskell newbie using HaXml for the conversion of xml files into html files. I'm using GHC 6.4 to compile the program. When I run the program, it will not convert the whole file: the document tree is incomplete and will stop when the limit of e.g. 8k has been reached. E.g. An xml file of 12.3k will result in a file of 8k and will stop at 8k An xml file of 15.7k will result in a file of 16k An xml file of 36k will result in a file of 24k In an output to the screen (and not to a file), the whole document tree is being generated. Am I missing something obvious here (memory issues)? On compilation, ghc issues warnings about the class FiniteMap and Module IOExts being deprecated. Is this the cause? Any ideas? Cheers, K.

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Returning a list element?
by Neil Rutland 22 Mar '06

22 Mar '06

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Re: Associated types and data constructors (was: the MPTC Dilemma (please solve))
by Manuel M T Chakravarty 22 Mar '06

22 Mar '06

David Menendez: > This is something I've been wondering about for a while. Can you do that > sort of thing with associated types? > > As another example, consider this type class for composible continuation > monads: > > class Monad m => MonadCC p sk m | m -> p sk where > newPrompt :: m (p a) > pushPrompt :: p a -> m a -> m a > withSubCont :: p b -> (sk a b -> m b) -> m a > pushSubCont :: sk a b -> m a -> m b > > You can use instances of this class to create backtracking monads, along > these lines: > > data Tree m a = HZero | HOne a | HChoice a (m (Tree m a)) > newtype SR p m a = SR (forall ans. ReaderT (p (Tree m ans)) m a) > > instance MonadCC p sk m => MonadPlus (SR p m) > > With associated types, the MonadCC class becomes: > > class Monad m => MonadCC m where > type Prompt m a > type SubCont m a b > > newPrompt :: m (Prompt m a) > pushPrompt :: Prompt m a -> m a -> m a > withSubCont :: Prompt m b -> (SubCont m a b -> m b) -> m a > pushSubCont :: SubCont m a b -> m a -> m b > > Since |Prompt m| is determined by |m|, can we eliminate the prompt > parameter from SR? > > newtype SR' m a > = SR' (forall ans. ReaderT (Prompt m (Tree m ans)) m a) > > instance MonadCC m => MonadPlus (SR' m) Yes, that's right. > That would presumably lead to SR' having the type > > SR' :: (MonadCC m) => ReaderT (Prompt m (Tree m ans)) m a -> SR' m a > > That doesn't seem like it should be a problem, since it's impossible to > create a value of type |ReaderT (Prompt m (Tree m ans)) m a| unless m is > an instance of MonadCC, but it also puts a context on a newtype data > constructor, which isn't currently allowed. If you go by the rules in the associated types paper, you need to put that context there. However, whether that context is really needed is the topic of an ongoing discussion. Without that sort of context, you can write programs where type errors are raised a bit later than if you have the context, but type safety doesn't seem to be affected by those contexts. The fact that newtype cannot have a context is an argument in favour of not requiring the context. As your example shows, associated types (in comparison to FDs) not only eliminate type arguments from classes, but also from data type declarations. IMHO this improves the readability of programs. Manuel

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Arrows, wxFruit
by Gerbrand van Dieijen 21 Mar '06

21 Mar '06

Hello, I am working on a thesis project which involves for a large part constructing a gui. In short, I want to create a combinator library that gives (a subset of) functionality, reporting libraries as Crystal reports offer. I read about Yampa and Fruit and wxFruit and both seem a very elegant way to construct a gui.Unfortunately, Fruit uses a rather low-level library which makes it of less use. wxFruit looks very nice, but the library does not seem to be maintained anymore. First, is wxFruit still maintained, or are there any plans to further develop it. Or, can you recommend other libraries/frameworks to construct a gui using combinators, that use higher level libraries such as wxHaskell? (An other possibility would be to use wxHaskell only and construct the combinators entirely myself. This might be the most flexible approach.) Second, I can't get wxFruit to compile on my machine. When I compile I use the following command-line: ghci -fglasgow-exts -package data -package wx -i/users/gjhdieij/dowload/afrp-0.4/src WXFruit.hs The packages than almost loads, but I still get the following errors (GHC 6.4, wxHaskell 0.9.4) . Does anyone know a sollution? /users/gjhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:169:46: Ambiguous occurrence `>>^' It could refer to either `AFRPUtilities.>>^', defined at /users/gjhdieij/dow load/afrp-0.4/src/AFRPUtilities.hs:106:2 or `Control.Arrow.>>^', imported from AFRP at /users/g jhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:90:0-10 /users/gjhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:178:48: Ambiguous occurrence `>>^' It could refer to either `AFRPUtilities.>>^', defined at /users/gjhdieij/dow load/afrp-0.4/src/AFRPUtilities.hs:106:2 or `Control.Arrow.>>^', imported from AFRP at /users/g jhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:90:0-10 (etc.)

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First Question
by Neil Rutland 21 Mar '06

21 Mar '06

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Looking for an efficient tree in STM
by Einar Karttunen 21 Mar '06

21 Mar '06

Hello Does anyone have an efficient tree implemented in STM that supports concurrent updates in an efficient fashion? This seems suprisingly hard to implement - a normal binary tree with links as TVar is very slow and does not scale very well. - Einar Karttunen

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hs-plugins/ haskelldb/ ELF zlib incompatibility.
by Marc Weber 21 Mar '06

21 Mar '06

I get this error: calhost db=store uid=marc pwd=" DB/Direct: Daan Leijen (c) 1999, HWT (c) 2003-2004, Bjorn Bringert (c) 2005 Connecting to database... DBDirect: user error (loadShared: couldn't load `/usr/lib/libz.so' because /usr/lib/libz.so: invalid ELF header Any idea how to fix it ? (What to watch for?) latest cvs ghc, latest darcs hs-plugins (dons told me to ask the haskelldb how they managed to run it..) I've tried compiling with -lz option. Same result. I'm also posting here because I don't know how frequently the haskelldb mailinglist is read yet.. It doesn't seem to be used by that much people yet. Marc

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RE: [Haskell-cafe] Type classes
by Geest, G. van den 20 Mar '06

20 Mar '06

I suppose you want to define compareEntries like this: > compareEntries (Entry x) (Entry y) = compareEntries x y An option is to just implement it the following way (Haskell98!): > class DatabaseEntry e where > entryLabel :: e -> String > formatEntry :: e -> String > compareEntries :: e -> e -> Ordering > > data Entry a = Entry a > > instance DatabaseEntry a => DatabaseEntry (Entry a) where > entryLabel (Entry e) = entryLabel e > formatEntry (Entry e) = formatEntry e > compareEntries (Entry x) (Entry y) = compareEntries x y Gerrit -----Original Message----- From: haskell-cafe-bounces(a)haskell.org on behalf of Max Vasin Sent: Mon 3/20/2006 3:46 PM To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Type classes Hi! I'm currently experimenting with a bibliography generation tool for LaTeX. It will (if it will be finished) use BibTeX databases but bibliography styles will be written in Haskell. I want styles to be able to transform database entries into some style specific data type, so I define > class DatabaseEntry e where > entryLabel :: e -> String > formatEntry :: e -> String > compareEntries :: e -> e -> Ordering Then I define > data Entry = forall a. (DatabaseEntry a) => Entry a > instance DatabaseEntry Entry where > entryLabel (Entry e) = entryLabel e > formatEntry (Entry e) = formatEntry e How can I define compareEntries for this instance? -- WBR, Max Vasin. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Type classes
by Max Vasin 20 Mar '06

20 Mar '06

Hi! I'm currently experimenting with a bibliography generation tool for LaTeX. It will (if it will be finished) use BibTeX databases but bibliography styles will be written in Haskell. I want styles to be able to transform database entries into some style specific data type, so I define > class DatabaseEntry e where > entryLabel :: e -> String > formatEntry :: e -> String > compareEntries :: e -> e -> Ordering Then I define > data Entry = forall a. (DatabaseEntry a) => Entry a > instance DatabaseEntry Entry where > entryLabel (Entry e) = entryLabel e > formatEntry (Entry e) = formatEntry e How can I define compareEntries for this instance? -- WBR, Max Vasin.

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