February 2007 - Haskell

Haskell-mode 2.2
by Stefan Monnier 09 Feb '07

09 Feb '07

I've just released version 2.2 of the Haskell-mode package for Emacs. It has very few visible changes, mostly some commands to query an underlying interactive hugs/ghci in order to get type/info about specific identifiers, courtesy of Matthew Danish. It does have some non-trivial changes in the indentation code and various other minor details. These have seen very little testing until now, so don't be surprised to find regressions: just complain back to me. As always, the place to find this package is: http://www.iro.umontreal.ca/~monnier/elisp/ -- Stefan

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ANNOUNCE: Data.CompactString 0.1 - my attempt at a Unicode ByteString
by Twan van Laarhoven 09 Feb '07

09 Feb '07

Hello all, I would like to announce my attempt at making a Unicode version of Data.ByteString. The library is named Data.CompactString to avoid conflict with other (Fast)PackedString libraries. The library uses a variable length encoding (1 to 3 bytes) of Chars into Word8s, which are then stored in a ByteString. The structure is very much based on Data.ByteString, most of the implementation is copied from there. Hopefully this means that fusion rules could be copied as well. This is kind of a pre-release, many functions are still missing, and I have not benchmarked yet. I am releasing this in the hopes of getting some feedback on the general idea. Homepage: http://twan.home.fmf.nl/compact-string/ Haddock: http://twan.home.fmf.nl/compact-string/doc/html/ Source: darcs get http://twan.home.fmf.nl/repos/compact-string Twan van Laarhoven

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ANNOUNCE: New Book - Programming in Haskell
by Graham Hutton 09 Feb '07

09 Feb '07

+--------------------------------------------------------------------+ *** NEW BOOK ANNOUNCEMENT *** Programming in Haskell Graham Hutton, University of Nottingham Cambridge University Press, January 2007 Paperback: ISBN-13: 9780521692694 | ISBN-10: 0521692695 Hardback: ISBN-13: 9780521871723 | ISBN-10: 0521871727 DESCRIPTION Haskell is one of the leading languages for teaching functional programming, enabling students to write simpler and cleaner code, and to learn how to structure and reason about programs. This introduction is ideal for beginners: it requires no previous programming experience and all concepts are explained from first principles via carefully chosen examples. Each chapter includes exercises that range from the straightforward to extended projects, plus suggestions for further reading on more advanced topics. The presentation is clear and simple, and benefits from having been refined and class-tested over several years. Features include: freely accessible powerpoint slides for each chapter; solutions to exercises, and examination questions (with solutions) available to instructors; downloadable code that's fully compliant with the latest Haskell release. CONTENTS Preface 1. Introduction 2. First steps 3. Types and classes 4. Defining functions 5. List comprehensions 6. Recursive functions 7. Higher-order functions 8. Functional parsers 9. Interactive programs 10. Declaring types and classes 11. The countdown problem 12. Lazy evaluation 13. Reasoning about programs Appendix A. Standard prelude Appendix B. Symbol table Bibliography Index FURTHER DETAILS: http://www.cs.nott.ac.uk/~gmh/book.html +---------------------------------------------------------------------+ | Dr Graham Hutton Email : gmh(a)cs.nott.ac.uk | | School of Computer Science and IT | | University of Nottingham Web : www.cs.nott.ac.uk/~gmh | | Jubilee Campus, Wollaton Road | | Nottingham NG8 1BB, UK Phone : +44 (0)115 951 4220 | +---------------------------------------------------------------------+ This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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ANN: Open Quark Framework for Java released as Open Source
by Luke Evans 07 Feb '07

07 Feb '07

Further to the seeding of the binary framework and SDK as announced on this list last year, Business Objects is pleased to announce that the Open Quark Framework for Java has now been released as open source under a BSD-style license. The Open Quark Framework for Java includes a language, CAL, similar in many respects to Haskell, which is compiled to efficient byte code on the Java platform. The purpose of Open Quark began, and remains, the ability to conveniently mix logic expressed in a natural functional style with 'regular' Java code. Business Objects' interest in the functional paradigm is that much of the logic to model complex data semantics and to express data transformation is elegantly and reusably represented as pure functions. This contrasts with the corpus of application logic that represents workflow, UI, communications and storage, which is predominantly coded directly in OO languages such as Java but with which the data logic must interoperate. Despite the original motivations for the Quark Framework, CAL is a completely general purpose non-strict functional language and suitable for a broad range of applications. We have made available a suite of documentation, included in the software packages and separately downloadable. These documents should inform those with both a casual interest in the project (and its similarities/differences to Haskell) as well as those who wish to explore the language, SDK and tools in more detail. Open Quark can be downloaded, with or without sources from: http://labs.businessobjects.com/cal Unlike the previous 'seeding' regime, no registration is required to obtain Open Quark. Business Objects remains very active on the project and intends to set up the necessary mechanisms to allow the community to contribute easily to the project. While these arrangements will take a little longer, we are already very happy to receive any/all comments, suggestions and contributions. A discussion forum is already available for such activity (see the main web page). A major priority for the immediate future is to deepen the Eclipse integration in order to improve the convenience for developers using CAL. Luke Evans Chief Scientist, VP Research, Business Objects

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[Proposal] Data type declarations are implicitly moduled
by Chris Moline 07 Feb '07

07 Feb '07

Hi, I'm not sure if something like this has been proposed already but I thought I'd bring it up. The idea is to allow multiple data declarations to share constructors by having them be implicitly declared inside a module. So, for example, data Dir = Left | Right deriving Show data Arrived = Arrived | Left deriving Show main = do print Dir.Left print Arrived.Left will work just fine. Nothing is required by this proposal that isn't already done (with the exception of adding syntactic support to the compiler of course). The above can be accomplished today by simply putting Dir and Arrived into their own modules and then importing them. There is a question of do we provide some means of abbreviating these implicit module names. If this is considered desirable, one way of accomplishing this is to add an as-clause to the data declaration syntax, which will function similary to where-clauses and deriving syntax. Ex, data Dir = Left | Right as D deriving Show This will effectively be the same as declaring Dir in its own module and then importing it using as syntax. __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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HSXML version 1.13. Overloading polyvariadic functions
by oleg＠pobox.com 06 Feb '07

06 Feb '07

This message announces the version 1.13 of HSXML. HSXML is a library for writing and transforming typed semi-structured data in Haskell -- in S-expression syntax, with the extensible set of `tags', and statically enforced content model restrictions. A particular application is writing web pages in Haskell. We obtain HTML, XHTML or other output formats by running the Haskell web page in an appropriate rendering monad. The benefit of representing XML-like documents as a typed data structure/Haskell code is static rejection of bad documents -- not only those with undeclared tags but also those where elements appear in wrong contexts. The web page http://pobox.com/~oleg/ftp/Scheme/xml.html#typed-SXML gives further motivation and description. It points to an example of authoring web pages in HSXML and a complex example of context-sensitive HSXLT transformations: producing structurally distinct HTML and XML/RSS from the same master file. The complete source code with several examples is available at http://pobox.com/~oleg/ftp/Haskell/HSXML.tar.gz The new version of HSXML contains more examples, many of which have been very kindly suggested by shelarcy. One example in particular, quote.hs, demonstrates overloading of a polyvariadic function. That may seem impossible: a polyvariadic function typically has the type forall t. C t => t. How overload on t? How to overload on arguments or the result of the function if we don't know even the number of arguments let alone their types. It's all in the future, when the function is applied. As it turns out, if we make preparations in the present, we can overload the future.

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Dynamic database updated by list using existential types
by Rene de Visser 05 Feb '07

05 Feb '07

The following code took me several hours to work out. I post it here, so perhaps other can avoid the difficulties of working out how to use existential types. Improvements welcome. If anyone thinks it is worth it I could add it to the wiki under existential types (I didn't find the examples there very helpful). Hopefully my newreader won't destroy the layout. Rene. {-# OPTIONS -fglasgow-exts #-} module Record where -- Demo of an extensible updatable database using Data.Dynamic and Data.Map -- which can be updated by a list of updates which are implemented -- using existential types. import Control.Concurrent.MVar import qualified Data.Map as M import Data.Dynamic type Tables = M.Map Int Dynamic -- the dynamic database -- look up a table in the database based on it's type class Typeable a => GetType a where getValue :: Tables -> MVar a getValue tables = case M.lookup index tables of Just d -> case fromDynamic d of Just e -> e _ -> error $ "Table with index " ++ show index ++ " not in database." where index = tabIndex (undefined::a) tabIndex :: a -> Int -- Manually create indexes for each table. This could be probably done automatically -- using Oleg's type to integer code. instance GetType IntColl where tabIndex _ = 1 instance GetType CatString where tabIndex _ = 2 -- Define the update function for the table class (GetType m) => Table m where apply :: (m ->m) -> Tables -> IO () apply f tables = modifyMVar_ table (\v -> return $! f v) where table :: MVar m table = getValue tables -- Existential type to allow updates to various tables data SRec = forall m. Table m => SRec (m -> m) update_db tables (SRec f) = apply f tables -- helper function -- Define how the update is done to each table class Mod_def r m | r -> m where apply_record :: r -> m -> m -- Define a type for each table data IntColl = IntColl Int deriving (Typeable, Show) data CatString = CatString String deriving (Typeable, Show) instance Mod_def Int IntColl where apply_record i (IntColl j) = IntColl (i + j) instance Mod_def String CatString where apply_record i (CatString j) = CatString (i ++ j) -- Register the tables instance Table IntColl instance Table CatString -- Create an update record updater :: forall r m. (Table m, Mod_def r m) => r -> SRec updater rec = SRec ((\db -> apply_record rec db) :: (m -> m)) -- Some simple tests to get the typing right newDB :: (GetType a) => a -> IO Tables newDB x = do var <- newMVar x return $! M.singleton (tabIndex x) $ toDyn var test2 :: SRec -> IO () test2 f = newDB (IntColl 3) >>= (\ db -> update_db db f) test3 = test2 (updater (1::Int)) newTable :: (Typeable (MVar a), GetType a) => a -> IO (Int, Dynamic) newTable x = do mv <- newMVar x return (tabIndex x, toDyn mv) test4 = do initial_db <- sequence [newTable $ IntColl 5, newTable $ CatString " again"] let db = M.fromList initial_db mapM (update_db db) [updater (1::Int), updater "hello"] content <- readMVar (getValue db :: MVar IntColl) print content

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Haskell Xlib bindings
by Dimitry Golubovsky 05 Feb '07

05 Feb '07

Hi, This message does not answer the original question, but may be useful if anyone really wants to work on the X protocol interface to Haskell. Just adding from my past experience. I had a (experimental, unfinished, and most likely it never will be finished) project to bind directly to the X11 protocol rather than to Xlib, and thus to have full access to all information exchanged between the client and server sides (the original question was about Xlib hiding some info). The protocol transport layer binding was done using FFIPKG, and the working directory is here: http://www.golubovsky.org/repos/hsxwm/ All those modules with cryptic names are results of hsffig parsing C data structures' definitions from X.h and Xproto.h Some example usage is in the HSX11 subdirectory. Hskell files in the main directory were my attempts to model the evilwm window manager as an example, but because of lack of time I never got any far on that project. In HSX11 there is a definition of the Transceivable class which does binary serialization/deserialization of protocol packets, and also a module XAuthority which implements the magic cookie protocol. Also some generic code for few X11 requests and responses. Finally, I was able to display a window with some cyrillic characters in it (to show that Unicode works). The largest problem was to obtain automatic serialization code for all those packet structures which might be most likely done with Template Haskell, but some functionality was missing in it back then. -- Dimitry Golubovsky Anywhere on the Web

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Haskell Xlib bindings
by Rob Hoelz 04 Feb '07

04 Feb '07

Hello everyone, This question probably belongs in GUI, but I tried posting there a week ago and have yet to get a response. My question is this: Why do the Haskell Xlib bindings have no way to extract the property event from an event pointer? For example, the following functions are available: get_KeyEvent, get_ButtonEvent, get_MotionEvent, etc. but there's no get_PropertyEvent. Why was this left out? It seems pretty critical to the functionality of an Xlib program to someone who's been programming in C with Xlib for quite some time. Thanks, Robert Hoelz

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ANNOUNCE: Haskell XML Toolbox Version 7.1
by Uwe Schmidt 02 Feb '07

02 Feb '07

Haskell XML Toolbox 7.1 I would like to announce a new version of the Haskell XML Toolbox. To be honest, there is not very much new. The main change is the step from cvs to darcs. The documentation has source links into the darcs repository. the darcs repo is at http://darcs.fh-wedel.de/hxt HXT 7.1 works with ghc-6.4 and ghc-6.6 A tutorial is availiable in the Haskell wiki: http://www.haskell.org/haskellwiki/HXT More information and download: http://www.fh-wedel.de/~si/HXmlToolbox/index.html Please email comments, bugs, etc. to hxmltoolbox(a)fh-wedel.de Uwe -- University of Applied Sciences, Wedel, Germany http://www.fh-wedel.de/~si/index.html

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