July 2009 - Haskell - Haskell.org

TABI 0.1: a typeful tagged cross-language calling convention
by Bulat Ziganshin 21 Jul '09

21 Jul '09

Hello, i've uploaded very preliminary version of TABI to http://tabi.googlecode.com/files/tabi-0.1.tar.gz Shortly speaking, while with cdecl calling convention you pass parameters by position, TABI passes parameters by name and annotates each parameter with its type It may be used in following situations: * providing interoperatbility between C, Haskell and any other languages while avoiding complexity of language-specific FFI descriptions. For example, Lua function may receive all parameters as the table * passing complex datastructures between various languages forth and back, providing faster alternative to serialization approach * implementing APIs that may be extended without losing compatibility - new services and parameters may not be supported by all client and server implementations but everything will work together ... read the rest at http://code.google.com/p/tabi/ issue tracker and SVN repository is also there -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

2 1

Anglohaskell - wifi signups
by Philippa Cowderoy 20 Jul '09

20 Jul '09

Wifi signups are Anglohaskell are now on the wiki - please add your details by the 31st of July if you want a wifi account at MS Research for the Friday. Alternatively, reply to this email with your full name, institution, country of residence and email address. The Anglohaskell wiki page can be found at http://www.haskell.org/haskellwiki/AngloHaskell/2009 There's still room for more talks if anyone's got something they fancy giving a go - everything from cutting edge research to "what I did with Haskell on my holiday" is welcome, as is everyone from GHC implentors to newbies frantically writing Hello World on their laptop in the back row. -- flippa(a)flippac.org

1 0

ANNOUNCE: graphviz-2999.1.0.1
by Ivan Lazar Miljenovic 20 Jul '09

20 Jul '09

This is a bug-fix release to fix the various Attribute-related problems in the previous release (2999.0.0.0) spotted mainly by Zsolt Dollenstein. Please disregard version 2999.1.0.0; it had a small bug. Similarly, people should avoid using 2999.0.0.0 unless they don't use any Either-based Attributes. The problem (which took me a while to realise) was that whilst I was using Either to denote that an attribute could take one of two different types of values (e.g. Bool and String) and the parsing for that worked. However, since I was using Show (I know it's bad, it was on my TODO, remember?) to generate the Dot code, then whenever one of these values was converted then either Left or Right would appear, and GraphViz doesn't seem to like that for some reason... I also took the opportunity to define custom types for more Attributes (as they only accept certain String values). Furthermore, GraphViz states that attributes that can accept a boolean value are set to True when the attribute is listed on its own; the library now does this even for those Attributes that use a custom type that combines Bool with something else. One other small change is that the Color Attribute now takes in a list of Colors; if you only want one, use a singleton list (it was either that or a custom datatype that takes either a single Color or a list of Colors...). Note that 2999.1.0.0 accidentally only had singleton Color values for the Color Attribute rather than a list, hence the quick version bump. Nothing else has changed in the library apart from the Attributes. Regarding my request in the previous email about the possible new layout of the DotGraph datatype: How many people actually build DotGraphs by hand or pull them apart by hand? If there were getNodes :: DotGraph -> [Node] and getEdges :: DotGraph -> [Edge] functions available, would you accept an all-in-one data type that just contains a generic list of statements and thus follows upstream more closely? Even leaving aside parsing, this would allow for greater flexibility for creating wild and wacky graphs or for a monadic interface or something for building DotGraphs (which someone has asked me for, but which at the moment I have no idea how to do...). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

sendfile-0.4 released: sockets and freebsd and docs oh my!
by Matthew Elder 19 Jul '09

19 Jul '09

sendfile-0.4: http://hackage.haskell.org/package/sendfile -------------------------------------------------------------------------------- * FreeBSD support contributed by Maxime Henrion (untested!) * sendFile is now unsafeSendFile (continued support for a Handle as the output) * sendFile' is now unsafeSendFile' (continued support for a Handle as the output) * sendFile and sendFile' now take a Socket as the output instead of a Handle in order to be more semantically correct and typesafe (a Handle used for the output which was not created from a Socket will cause runtime errors). Thanks to Johan Tibell for this discussion. * documentation extended / improved Regards, Matthew Elder

1 0

Haskell Weekly News: Issue 126 - July 18, 2009
by Brent Yorgey 18 Jul '09

18 Jul '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090718 Issue 126 - July 18, 2009 --------------------------------------------------------------------------- Welcome to issue 126 of HWN, a newsletter covering developments in the [1]Haskell community. [2]Hac phi is next weekend! With almost 30 people already registered, it looks like we're going to have a fantastic time hacking in Philadelphia. It's still not too late to [3]register! Announcements GHC 6.10.4. Ian Lynagh [4]announced a new patchlevel release of GHC, 6.10.4. This version has very few changes over 6.10.3, but fixes some bugs that could be critical for a few users. See the [5]release notes for details. shelltestrunner 0.6 released. Simon Michael [6]announced the first release of [7]shelltestrunner, a small tool for testing any command-line program by running it through "shell" tests defined with a simple file format. generator 0.5.1. Yair Chuchem [8]announced the release of the [9]generator package, which implements an alternative list monad transformer, a list class, and related functions. GLURaw 1.0.0.0. Sven Panne [10]announced a new [11]GLURaw package, containing full support for all GLU functionality and similar in spirit to the OpenGLRaw package: it is a 1:1 mapping of the C interface, no libraries or headers are needed at build time, and the GLU API entries are resolved dynamically at runtime. OpenGLRaw 1.0.1.0. Sven Panne [12]announced a new version of the [13]OpenGLRaw package, which adds support for a number of OpenGL extensions. ObjectName 1.0.0.0. Sven Panne [14]announced a (tiny) new package, [15]ObjectName, which contains a class corresponding to the general notion of explicitly handled identifiers for API objects, e.g. a texture object name in OpenGL or a buffer object name in OpenAL. StateVar 1.0.0.0. Sven Panne [16]announced the [17]StateVar package, which further modularizes the OpenGL/OpenAL packages. It implements state variables, which are references in the IO monad, like IORefs or parts of the OpenGL state. data-ordlist-0.0.1 and NumberSieves-0.0. Leon Smith [18]announced the release of two new packages: [19]Data.OrdList offers a convenient way for efficiently dealing with lists that you happen to know are ordered, and includes operations such as union, merge, exclusive union, intersection, and difference. [20]NumberSieves includes the Sieve of O'Neill, from "The Geniune Sieve of Eratosthenes" by Melissa O'Neill, which offers an incremental primality sieve based on priority queues. Also included are two array-based generalizations of the Sieve of Eratosthenes: one for factoring a large quantity of small numbers, and another for calculating the phi function for a large quantity of small numbers. graphviz-2999.0.0.0. Ivan Lazar Miljenovic [21]announced a new release of the [22]graphviz package for Haskell, which provides bindings to the [23]GraphViz suite of tools. The biggest and most important change in this release is that all 152 attributes utilised/supported by GraphViz are now specified and supported. uncommon IMO problem - toilet management. Henning Thielemann [24]announced a [25]Haskell package for managing toilet use at the International Mathematical Olympiad. darcs 2.3 beta 4. Petr Rockai [26]announced another darcs 2.3 beta release, which features better Windows support. If you're on Windows, you should be able to install it with 'cabal install darcs-beta' -- give it a try! Google Summer of Code Progress updates from participants in the 2008 [27]Google Summer of Code. space profiling. Gergely Patai has been working on a [28]heap profile manager. fast darcs. Petr Rockai put out another [29]another darcs 2.3 beta release, and made a [30]bunch of other progress including getting darcs up and running on win32, working on hashed-storage, and optimizing 'darcs show contents'. Discussion is closing a class this easy? Conor McBride [31]asked for feedback on some code intended to effectively create a closed type class. laziness blowup exercise. Thomas Hartman [32]challenged readers to squash a memory leak. Blog noise [33]Haskell news from the [34]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Gergely Patai: [35]Introducing the heap profile manager. * FP Lunch: [36]Folding Statistics. * Petr Rockai: [37]soc progress 8. * Greg Bacon: [38]Monadic takeWhile. * Petr Rockai: [39]darcs 2.3 beta 4. * David Amos: [40]Counting symmetries using transversals. * Magnus Therning: [41]XML prettifier in Haskell. * Petr Rockai: [42]soc progress 7. Quotes of the Week * Berengal: For me, understanding the basics/reasoning behind haskell's type system was just a minute meditating on the phrase "what's the square root of hello?" * bitwize: The oleg is to functional studliness as the farad is to capacitance: a hopelessly large base unit. * maartenm: euclidate: to promote a conjecture to an axiom just for the sake of simplicity * RobertGreaye: Some suggest the original English remained in Britain when the North American colonies were founded; others claim it was brought to the Americas by the British settlers, leaving a pale imitation back in Britain. The truth is much stranger: the original English was actually smuggled out of Britain to the West Indies in a wardrobe belonging to General Sir Ralph Abercromby, where it ended up on the island of Trinidad after Sir Ralph took possession of that territory in the name of the British Crown. About the Haskell Weekly News New editions are posted to [43]the Haskell mailing list as well as to [44]the Haskell Sequence and [45]Planet Haskell. [46]RSS is also available, and headlines appear on [47]haskell.org. To help create new editions of this newsletter, please see the information on [48]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [49]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://haskell.org/haskellwiki/Hac_%CF%86 3. http://haskell.org/haskellwiki/Hac_%CF%86/Register 4. http://www.haskell.org//pipermail/haskell/2009-July/021508.html 5. http://haskell.org/ghc/docs/6.10.4/html/users_guide/release-6-10-4.html 6. http://article.gmane.org/gmane.comp.lang.haskell.general/17367 7. http://hackage.haskell.org/package/shelltestrunner 8. http://article.gmane.org/gmane.comp.lang.haskell.general/17366 9. http://hackage.haskell.org/package/generator 10. http://article.gmane.org/gmane.comp.lang.haskell.general/17364 11. http://hackage.haskell.org/package/GLURaw 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17363 13. http://hackage.haskell.org/package/OpenGLRaw 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17362 15. http://hackage.haskell.org/package/ObjectName 16. http://article.gmane.org/gmane.comp.lang.haskell.general/17361 17. http://hackage.haskell.org/package/StateVar 18. http://article.gmane.org/gmane.comp.lang.haskell.general/17355 19. http://hackage.haskell.org/package/data-ordlist 20. http://hackage.haskell.org/package/NumberSieves 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61494 22. http://hackage.haskell.org/package/graphviz 23. http://www.graphviz.org/ 24. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61448 25. http://hackage.haskell.org/package/toilet-0.0.1 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/61302 27. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 28. http://just-bottom.blogspot.com/2009/07/introducing-heap-profile-manager.ht… 29. http://web.mornfall.net/blog/darcs_2.3_beta_4.html 30. http://web.mornfall.net/blog/soc_progress_8.html 31. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/61456 32. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/61283 33. http://planet.haskell.org/ 34. http://haskell.org/haskellwiki/Blog_articles 35. http://just-bottom.blogspot.com/2009/07/introducing-heap-profile-manager.ht… 36. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=232 37. http://web.mornfall.net/blog/soc_progress_8.html 38. http://feedproxy.google.com/~r/gbacon/~3/Q-Y-KapB8iI/monadic-takewhile.html 39. http://web.mornfall.net/blog/darcs_2.3_beta_4.html 40. http://haskellformaths.blogspot.com/2009/07/counting-symmetries-using-trans… 41. http://therning.org/magnus/archives/695 42. http://web.mornfall.net/blog/soc_progress_7.html 43. http://www.haskell.org/mailman/listinfo/haskell 44. http://sequence.complete.org/ 45. http://planet.haskell.org/ 46. http://sequence.complete.org/node/feed 47. http://haskell.org/ 48. http://haskell.org/haskellwiki/HWN 49. http://code.haskell.org/~byorgey/code/hwn/

1 0

ANNOUNCE: graphviz-2999.0.0.0
by Ivan Lazar Miljenovic 18 Jul '09

18 Jul '09

I am pleased to announce a new release of the graphviz package for Haskell, which provides bindings to the GraphViz [1] suite of tools. [1] http://www.graphviz.org/ Probably the biggest and most important change in this release is that AFAICT, all 152 attributes utilised/supported by GraphViz [2] are now specified and supported by this library. However, I'm not fully sure how well the parsing of these attributes will turn out; if you notice a bug/problem then please let me know. [2] http://graphviz.org/doc/info/attrs.html I've specified this as being the first in the 2999.0 series of releases. I will switch to 3000.0 when the generic graph class has been released and graphviz switched to using it rather than just FGL. One other future change that I'm considering is to improve the parsing ability of the Dot language. At the moment, graphviz assumes the following layout is followed: * Graph attributes * Nodes with their attributes (clusters are supported only for creation, not parsing). * Edges with their attributes. To match the behaviour of upstream, this will need to be changed into just a list of statements, where a statement is one of five things [3]: * A Node * An Edge * An attribute (either for the graph overall, nodes or for edges) * "ID '=' ID" (not quite sure what this is; some kind of assignment) * A subgraph (clusters are a specific type of subraph) As the way of defining an attribute for a specific grouping of nodes/edges/subgraphs is to have them all listed after the attribute definition (whereas those beforehand do not have this attribute), the imperative nature of the Dot language does not allow us to split these statements up as we currently do. [3] http://graphviz.org/doc/info/lang.html As such, I'm asking which of the following two choices people would prefer: 1. Follow upstream so that it can fully parse a Dot graph 2. Keep it as it is, so that it is possible to consider all edges, etc. easily. Other major changes to this release: ==================================== * Fixed a bug where the Show instance and read function for DotEdge had the from/to nodes the wrong way round. This was not immediately noticed since the Graph -> DotGraph functions created them the wrong way round, so for those users who only used these this was not apparent. Spotted by Neil Brown. * Greatly improved Attribute usage: almost all attributes are now covered with allowed values. * Extend DotGraph to include whether a graph is strict or not and if it has an ID. Also move the directedGraph field. * Make "Dot" refer to the actual dot command and DotArrow refer to the ArrowType (rather than DotCmd and Dot as before). * Make the Data.GraphViz.ParserCombinators module available to end users again, but not re-exported by Data.GraphViz; it has a warning message up the top not to be used. It is there purely for documentative purposes. * Use extensible-exceptions so that base < 4 is once again supported. * Follow the PVP rather than using dates for versions: http://www.haskell.org/haskellwiki/Package_versioning_policy Note that this means that any library/application using more than a trivial sub-set of graphviz will most likely need to be updated. However, now that the PVP is being followed it should be easier to tell in future when updates will be required. Other items I'm wanting to do in future releases: ================================================= * Allow user to choose whether or not the graph is meant to be directed or undirected. * Improve parsing to fully (or at least follow more closely) support Dot. * Improve clustering/subgraph support. * Use a PrettyPrinter rather than Show to generate Dot output. * Improve Output support. * Find and fix the handle closing bug with graphvizWithHandle. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

PEPM'10 - Call for Papers (Deadline: 6 Oct 09) - Invited Speakers announced
by Janis Voigtlaender 17 Jul '09

17 Jul '09

=============================================================== CALL FOR PAPERS ACM SIGPLAN 2010 Workshop on Partial Evaluation and Program Manipulation (PEPM'10) Madrid, January 18-19, 2010 (Affiliated with POPL'10) http://www.program-transformation.org/PEPM10 =============================================================== INVITED SPEAKERS: * Lennart Augustsson (Standard Chartered Bank, UK) * Jeremy Siek (University of Colorado at Boulder, USA) IMPORTANT DATES: * Paper submission: Tue, October 6, 2009, 23:59, Apia time * Author notification: Thu, October 29, 2009 * Camera-ready papers: Mon, November 9, 2009 To facilitate smooth organization of the review process, authors are asked to submit a short abstract by October 1, 2009. SUBMISSION CATEGORIES: * Regular research papers (max. 10 pages in ACM Proceedings style) * Tool demonstration papers (max. 4 pages plus max. 6 pages appendix) TRAVEL SUPPORT: Students and other attendants in need can apply for a SIGPLAN PAC grant to help cover expenses. For details, see http://www.sigplan.org/PAC.htm. SCOPE: The PEPM Symposium/Workshop series aims to bring together researchers and practitioners working in the areas of program manipulation, partial evaluation, and program generation. PEPM focuses on techniques, theories, tools, and applications of analysis and manipulation of programs. The 2010 PEPM workshop will be based on a broad interpretation of semantics-based program manipulation in a continued effort to expand the scope of PEPM significantly beyond the traditionally covered areas of partial evaluation and specialization and include practical applications of program transformations such as refactoring tools, and practical implementation techniques such as rule-based transformation systems. In addition, it covers manipulation and transformations of program and system representations such as structural and semantic models that occur in the context of model-driven development. In order to reach out to practitioners, there is a separate category of tool demonstration papers. Topics of interest for PEPM'10 include, but are not limited to: * Program and model manipulation techniques such as transformations driven by rules, patterns, or analyses, partial evaluation, specialization, program inversion, program composition, slicing, symbolic execution, refactoring, aspect weaving, decompilation, and obfuscation. * Program analysis techniques that are used to drive program/model manipulation such as abstract interpretation, static analysis, binding-time analysis, dynamic analysis, constraint solving, type systems, automated testing and test case generation. * Analysis and transformation for programs/models with advanced features such as objects, generics, ownership types, aspects, reflection, XML type systems, component frameworks, and middleware. * Techniques that treat programs/models as data objects including meta-programming, generative programming, deep embedded domain-specific languages, program synthesis by sketching and inductive programming, staged computation, and model-driven program generation and transformation. * Application of the above techniques including experimental studies, engineering needed for scalability, and benchmarking. Examples of application domains include legacy program understanding and transformation, DSL implementations, visual languages and end-user programming, scientific computing, middleware frameworks and infrastructure needed for distributed and web-based applications, resource-limited computation, and security. We especially encourage papers that break new ground including descriptions of how program/model manipulation tools can be integrated into realistic software development processes, descriptions of robust tools capable of effectively handling realistic applications, and new areas of application such as rapidly evolving systems, distributed and web-based programming including middleware manipulation, model-driven development, and on-the-fly program adaptation driven by run-time or statistical analysis. PROCEEDINGS: There will be formal proceedings published by ACM Press. In addition to printed proceedings, accepted papers will be included in the ACM Digital Library. Selected papers may later on be invited for a journal special issue dedicated to PEPM'10. SUBMISSION GUIDELINES: Papers should be submitted electronically via the workshop web site. Regular research papers must not exceed 10 pages in ACM Proceedings style. Tool demonstration papers must not exceed 4 pages in ACM Proceedings style, and authors will be expected to present a live demonstration of the described tool at the workshop (tool papers should include an additional appendix of up to 6 extra pages giving the outline, screenshots, examples, etc. to indicate the content of the proposed live demo at the workshop). Authors using Latex to prepare their submissions should use the new improved SIGPLAN proceedings style (sigplanconf.cls). PROGRAM CO-CHAIRS: * John Gallagher (Roskilde University, Denmark, and IMDEA Software, Spain) * Janis Voigtländer (Technische Universität Dresden, Germany) PROGRAM COMMITTEE MEMBERS: * Wei-Ngan Chin (National University of Singapore, Singapore) * Michael Codish (Ben-Gurion University of the Negev, Israel) * Jim Cordy (Queen's University, Canada) * Nate Foster (University of Pennsylvania, USA) * Haifeng Guo (University of Nebraska at Omaha, USA) * Patricia Johann (University of Strathclyde, UK) * Oleg Kiselyov (FNMOC, USA) * Günter Kniesel (University of Bonn, Germany) * Viktor Kuncak (Ecole Polytechnique Fédérale de Lausanne, Switzerland) * Yanhong Annie Liu (State University of New York at Stony Brook, USA) * Andres Löh (Utrecht University, Netherlands) * Jan Midtgaard (Roskilde University, Denmark) * David Monniaux (National Center for Scientific Research (CNRS) and VERIMAG laboratory, France) * Akimasa Morihata (University of Tokyo, Japan) * Alberto Pettorossi (Università di Roma Tor Vergata, Italy) * João Saraiva (Universidade do Minho, Portugal) * Ganesh Sittampalam (Credit Suisse, UK) * Fausto Spoto (Università di Verona, Italy) * Harald Søndergaard (University of Melbourne, Australia) * Walid Taha (Rice University, USA)

1 0

ANNOUNCE: GHC version 6.10.4
by Ian Lynagh 16 Jul '09

16 Jul '09

============================================================== The (Interactive) Glasgow Haskell Compiler -- version 6.10.4 ============================================================== The GHC Team is pleased to announce a new patchlevel release of GHC. This release contains a number of bugfixes relative to 6.10.3, so we recommend upgrading. Release notes are here: http://haskell.org/ghc/docs/6.10.4/html/users_guide/release-6-10-4.html How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: http://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language; the current language version is Haskell 98, agreed in December 1998 and revised December 2002. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces (C, whatever). GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page http://www.haskell.org/ghc/ GHC developers' home page http://hackage.haskell.org/trac/ghc/ Haskell home page http://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: http://hackage.haskell.org/trac/ghc/wiki/Contributors Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: http://hackage.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site run by Trac: http://hackage.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at http://www.haskell.org/mailman/listinfo/glasgow-haskell-users http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see http://www.haskell.org/mailman/listinfo/ Some GHC developers hang out on #haskell on IRC, too: http://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: http://www.haskell.org/ghc/reportabug

1 0

IFL 2009: Third Call for Papers
by IFL 2009 16 Jul '09

16 Jul '09

1 0

Re: ANNOUNCE: generator 0.5.1
by Yair Chuchem 16 Jul '09

16 Jul '09

Sebastian Fisher wrote: > The nested monadic tails of your lists seem more similar to the > nested monadic data described in the ICFP'09 paper with Oleg > Kiselyov and Chung-chieh Shan [1]. The ideas described in that paper > are on Hackage and your ListT seems similar to the List type in > Data.Monadic.List [2] although our version not only uses monadic > tails but also monadic heads. See [3] for a tutorial. A small correction: the tail of ListT is not monadic, but "unpacking" it (getting either a Nil or a Cons) is. If just the tail was monadic one couldn't create a list which might be empty depending on a monadic result, For example: linesFromUser :: ListT IO String linesFromUser = takeWhile (not . null) . joinM $ repeat getLine linesFromUser is a list of lines from the user until an empty line, not including the empty line. the first line entered by the user determines if the list is empty or not. cheers, Yair

1 0