- Haskell - Haskell.org

HNOP Status
by Ashley Yakeley 06 Jan '07

06 Jan '07

For code designed not to do anything, HNOP has high hopes. I have recently split the project into two Cabal packages: "nop", a library of no-op services, and "hnop", a program that uses nop to do nothing. Both packages can be found in this repository: darcs get http://semantic.org/hnop/ First and foremost the two packages are intended to be templates for Cabal projects, so I'm interested in making them as canonical and "best practices" as possible. I'm not so interested in showing off clever Haskell programming idioms for doing nothing, unless there's some expository benefit relating to the Cabal or Haddock infrastructure. For instance, in the nop package, "runhaskell Setup test" works. It runs "make" in the "test" directory, which compiles a program and runs it, and then diffs it against reference output. But should I be using QuickCheck or something here? Is there some well-known or Cabal-approved way to run tests on a package? Please send me patches licensed under the simple permissive license in the LICENSE files if you have any improvements. Or I can move this to darcs.haskell.org if it's appropriate. -- Ashley Yakeley Seattle, WA

2 1

ANN: IntelliJIDEA for Haskell
by Tony Morris 04 Jan '07

04 Jan '07

If you prefer IntelliJIDEA for your development, you might be interested in Haskell syntax highlighting. IntellijIDEA for Haskell is released under a BSD licence and can be found here: http://www.workingmouse.com/research/IntelliJIdea%2DHaskell/ -- Tony Morris Workingmouse Research

1 0

FW: WoLLIC'2007 - 2nd CFP
by Simon Marlow 04 Jan '07

04 Jan '07

[ posting on behalf of Ruy de Queiroz <ruy(a)cin.ufpe.br> ] [** sincere apologies for duplicates **] Call for Papers 14th Workshop on Logic, Language, Information and Computation (WoLLIC'2007) Rio de Janeiro, Brazil July 2-5, 2007 WoLLIC is an annual international forum on inter-disciplinary research involving formal logic, computing and programming theory, and natural language and reasoning. Each meeting includes invited talks and tutorials as well as contributed papers. The Fourteenth WoLLIC will be held in Rio de Janeiro, Brazil, from July 2 to July 5, 2007, and sponsored by the Association for Symbolic Logic (ASL), the Interest Group in Pure and Applied Logics (IGPL), the European Association for Logic, Language and Information (FoLLI), the European Association for Theoretical Computer Science (EATCS), the Sociedade Brasileira de Computacao (SBC), and the Sociedade Brasileira de Logica (SBL). PAPER SUBMISSION Contributions are invited on all pertinent subjects, with particular interest in cross-disciplinary topics. Typical but not exclusive areas of interest are: foundations of computing and programming; novel computation models and paradigms; broad notions of proof and belief; formal methods in software and hardware development; logical approach to natural language and reasoning; logics of programs, actions and resources; foundational aspects of information organization, search, flow, sharing, and protection. Proposed contributions should be in English, and consist of a scholarly exposition accessible to the non-specialist, including motivation, background, and comparison with related works. They must not exceed 10 pages (in font 10 or higher), with up to 5 additional pages for references and technical appendices. The paper's main results must not be published or submitted for publication in refereed venues, including journals and other scientific meetings. It is expected that each accepted paper be presented at the meeting by one of its authors. Papers must be submitted electronically at www.cin.ufpe.br/~wollic/wollic2007/instructions.html A title and single-paragraph abstract should be submitted by February 23, and the full paper by March 2 (firm date). Notifications are expected by April 13, and final papers for the proceedings will be due by April 27 (firm date). PROCEEDINGS Proceedings, including both invited and contributed papers, will be published in advance of the meeting. Publication venue: Springer's Lecture Notes in Computer Science. INVITED SPEAKERS: Veronique Cortier (LORIA Nancy) Martin Escardo (Birmingham) Georg Gottlob (Oxford) Achim Jung (Birmingham) Louis Kauffman (U Illinois Chicago) Sam Lomonaco (U Maryland Baltimore) Paulo Oliva (London/QM) John Reif (Duke) Yde Venema (Amsterdam) STUDENT GRANTS ASL sponsorship of WoLLIC'2007 will permit ASL student members to apply for a modest travel grant (deadline: April 1, 2007). See www.aslonline.org/studenttravelawards.html for details. IMPORTANT DATES February 23, 2007: Paper title and abstract deadline March 2, 2007: Full paper deadline (firm) April 12, 2007: Author notification April 26, 2007: Final version deadline (firm) PROGRAM COMMITTEE Samson Abramsky (U Oxford) Michael Benedikt (Bell Labs) Lars Birkedal (ITU Copenhagen) Andreas Blass (U Michigan) Thierry Coquand (Chalmers U, Goteborg) Jan van Eijck (CWI, Amsterdam) Marcelo Finger (U Sao Paulo) Rob Goldblatt (Victoria U, Wellington) Yuri Gurevich (Microsoft Redmond) Hermann Haeusler (PUC Rio) Masami Hagiya (Tokyo U) Joseph Halpern (Cornell U) John Harrison (Intel UK) Wilfrid Hodges (U London/QM) Phokion Kolaitis (IBM Almaden Research Center) Marta Kwiatkowska (U Birmingham) Daniel Leivant (Indiana U) (Chair) Maurizio Lenzerini (U Rome) Jean-Yves Marion (LORIA Nancy) Dale Miller (Polytechnique Paris) John Mitchell (Stanford U) Lawrence Moss (Indiana U) Peter O'Hearn (U London/QM) Prakash Panangaden (McGill, Montreal) Christine Paulin-Mohring (Paris-Sud, Orsay) Alexander Razborov (Steklov, Moscow) Helmut Schwichtenberg (Munich U) Jouko Vaananen (U Helsinki) ORGANISING COMMITTEE Marcelo da Silva Correa (U Fed Fluminense) Renata P. de Freitas (U Fed Fluminense) Ana Teresa Martins (U Fed Ceara') Anjolina de Oliveira (U Fed Pernambuco) Ruy de Queiroz (U Fed Pernambuco, co-chair) Petrucio Viana (U Fed Fluminense, co-chair) WEB PAGE www.cin.ufpe.br/~wollic/wollic2007 ---

1 0

[ANN] Shellac 0.6, new Vty Shellac backend, Lamba Shell 0.5
by Robert Dockins 04 Jan '07

04 Jan '07

Fellow Haskellers, I am pleased to simultaneously announce the immediate release of the following related packages: Shellac, 0.6 Shellac-readline, 0.3 Shellac-vty, 0.1 Shellac is a framework for building read-eval-print style shells which uses configurable backend plugins. The major new feature of this release is the new Shellac-vty backend package, which uses the new Vty library (http://members.cox.net/stefanor/vty/) to do terminal I/O directly. It currently has basic line editing keybindings, paging, and a command history. The main package and Shellac-readline updates consist of minor API updates. The project page for these packages may be found at: http://www.eecs.tufts.edu/~rdocki01/shellac.html Additionally, the Lambda Shell 0.5 release is now available. The Lambda Shell is a shell environment for defining, evaluating and manipulating expressions of the pure, untyped lambda calculus. It is also a showcase for Shellac and a model for its use. This update contains little new functionality, but is mostly a maintaince update. The Lambda Shell project page may be found at: http://www.eecs.tufts.edu/~rdocki01/lambda.html If you wish to play with the new Vty Shellac backend, you may 'darcs apply' the following patch to the lambda shell darcs sources before building. It will cause the lambda shell to use the Vty backend rather than the readline backend. http://www.eecs.tufts.edu/~rdocki01/lambda-shell-vty.patch Thanks all and happy hacking, Rob Dockins

1 0

ANN: monadLib 3.0.0
by Iavor Diatchki 03 Jan '07

03 Jan '07

Hello, I have placed a new version of 'monadLib' on its web-page: http://www.csee.ogi.edu/~diatchki/monadLib Some of the changes compared to the previous version: * The whole library is in a single module MonadLib.hs (~500 lines) * Simpler and more symmetric API * Removed the (generic) monadic combinators * Removed the search transformer (may add another version of it at some point) * Rewrote some transformers in the "traditional" way (e.g., exceptions and output) * There is an optional module that defines base monads corresponding to each transformer. Comments are very welcome. -Iavor

1 0

Haskell Weekly News: January 02, 2007
by dons＠cse.unsw.edu.au 02 Jan '07

02 Jan '07

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20070102 Issue 55 - January 02, 2007 --------------------------------------------------------------------------- Welcome to issue 55 of HWN, a weekly newsletter covering developments in the Haskell community. This week brings a new release of vty and HsColour, and some interesting discussion over the holiday break. Announcements hscolour-1.6. Malcolm Wallace [1]announced HsColour, a popular syntax-highlighter for Haskell code. It can generate ANSI terminal colour codes, HTML, and CSS, and can insert hyperlink anchors for function definitions (useful in conjunction with [2]Haddock). [3]HsColour-1.6 is now available. The major addition is a new LaTeX output mode. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14677 2. http://haskell.org/haddock 3. http://www.cs.york.ac.uk/fp/darcs/hscolour Dimensional: Statically checked physical dimensions. Bj�rn Buckwalter [4]announced version 0.1 of [5]Dimensional, a module for statically checked physical dimensions. The module facilitates calculations with physical quantities while statically preventing e.g. addition of quantities with differing physical dimensions. 4. http://article.gmane.org/gmane.comp.lang.haskell.general/14691 5. http://code.google.com/p/dimensional/ vty 2.0. Stefan O'Rear [6]announced a new major version of [7]vty. Differences from 1.0 include: vty now uses a record type for attributes, instead of bitfields in an Int; vty now supports setting background colors; you can now explicitly specify 'default' colors; vty now supports Unicode characters on output, automatically setting and resetting UTF-8 mode. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/14703 7. http://members.cox.net/stefanor/vty 'Lambda Revolution' tshirts. Paul Johnson [8]announced the creation of a new Haskell tshirt, on the theme of 'The Lambda Revolution'. Tshirts are available from [9]CafePress, and the designs are freely available. 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17916 9. http://www.cafepress.com/l_revolution Discussion Beautiful concurrency. Simon Peyton-Jones [10]mentioned that he's been writing a chapter on concurrency and transactional memory for a new book, 'Beautiful code'. [11]A first draft is available and Simon welcomes constructive suggestions for improvement. The book is aimed at a general audience of programmers, not Haskell geeks, so tries to explain everything necessary. If you are not a Haskell expert, your input would be particularly valuable. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14681 11. http://haskell.org/haskellwiki/Talk:SantaClausProblem Limits to implicit parallelism in functional applications. John DeTreville [12]announced a short paper about how much implicit parallelism there might be in ordinary functional applications. 12. http://article.gmane.org/gmane.comp.lang.haskell.general/14699 Inlining higher order functions. Norman Ramsey [13]asked about fine grained control for inlining in higher order functions. 13. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/11467 Red-black trees as a nested datatype. Jim Apple [14]described how to implement red-black trees as a nested datatype. 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17957 SYB for XML: deserialization and collections. Alexander Jacobson [15]asked about approaches to simplifying boilerplate in HAppS associated with XML serialization and state deserialization. 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17922 Flattening a lisp-style tree. pphetra [16]asked about flattening heterogeneous lists (or trees) in Haskell. 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17976/focus=17976 Jobs Functional programming at Jane Street Capital. Yaron Minsky [17]announced that [18]Jane Street Capital is again looking to hire some top-notch functional programmers. Of particular note is that Jane Street Europe Ltd. now has an office in London, and we are particularly interested in hiring someone for that office with strong systems administration skills in addition to experience with functional programming languages. The ideal candidate has: a commitment to the practical, experience with functional programming languages (such as Haskell). Applicants should also have experience with UNIX and a deep understanding of computers and technology and a strong mathematical background. 17. http://article.gmane.org/gmane.comp.lang.haskell.general/14683 18. http://www.janestcapital.com/tech.html Blog noise [19]Haskell news from the blogosphere. * [20]Secret Santas in Haskell III: Lather, Rinse, Repeat 1 * [21]More Haskell in Java 7 or 8? * [22]Generalized vs. dynamic interfaces * [23]Type classes in Java * [24]Static vs dynamic typing: do what thou wilt * [25]Hoogle 4 progress * [26]Hoogle Progress, HsMan features * [27]The York Haskell Compiler: 1000 patches! * [28]Evaluating cellular automata is co-monadic * [29]Tying Knots Generically * [30]Rails -> Ruby -> Haskell * [31]A Sermon on Programming Languages * [32]Thoughtful revision control * [33]readFile' * [34]Code unraveller * [35]The Haskell Meta Tutorial * [36]Monads * [37]My Haskell Experience * [38]Communities * [39]Darcs: The source code management system of the future? * [40]On strong type systems * [41]On Programming Languages and Productivity 19. http://planet.haskell.org/ 20. http://mult.ifario.us/articles/2006/12/22/secret-santas-in-haskell-iii-coll… 21. http://blogs.msdn.com/ralflammel/archive/2006/12/21/more-haskell-in-java-7-… 22. http://blogs.msdn.com/ralflammel/archive/2006/12/23/generalized-vs-dynamic-… 23. http://syntaxfree.wordpress.com/2006/12/23/et-tu-brute/ 24. http://syntaxfree.wordpress.com/2006/12/24/static-vs-dynamic-typing-do-what… 25. http://neilmitchell.blogspot.com/2006/12/hoogle-4-progress.html 26. http://neilmitchell.blogspot.com/2006/12/hoogle-progress-hsman-features.html 27. http://yhc06.blogspot.com/2006/12/1000-patches.html 28. http://sigfpe.blogspot.com/2006/12/evaluating-cellular-automata-is.html 29. http://sigfpe.blogspot.com/2006/12/tying-knots-generically.html 30. http://rubynations.wordpress.com/2006/12/28/evolutionrails-ruby-haskell/ 31. http://www.dailyspeculations.com/wordpress/?p=101 32. http://koweycode.blogspot.com/2006/12/distributed-chiming-in.html 33. http://koweynlg.blogspot.com/2006/12/meh.html 34. http://koweycode.blogspot.com/2006/12/unraveller.html 35. http://koweycode.blogspot.com/2006/12/haskell-metatutorial.html 36. http://hierodule.livejournal.com/69052.html 37. http://kawagner.blogspot.com/2006/12/my-haskell-experience.html 38. http://gravityboy.livejournal.com/31930.html 39. http://www.iovene.com/content/view/93/1/ 40. http://blog.tmorris.net/strong-type-systems/ 41. http://www.neilmix.com/2007/01/01/on-programming-languages-and-productivity/ Quotes of the Week * chessguy: [in regards to #haskell] man, it's amazing the difference between what happens when someone asks for help here, and what happens when they ask for help in another language channel * cjeris: It's amazing what some languages do to make thinking impossible, seemingly justified by the assumption that no one thinks anyway, so it's more important to make non-thinking programming as easy as possible. * edwinb: I've just walked past a poster advertising a gig by a band called 'The Awkward Squad'. I assume this means they provide output, play concurrently, and people take exception to them. * kfish: Apparently @pl also doubles as the command for producing an unintelligible flip-stream * Binkley: [Monads as clothes] using unsafePerformIO is kind of like going naked in public, might be safe in some contexts, but you really don't want to know what happens if you do it in a really bad one * quazimodo: I know why you guys are so ready to deal with me and put up with noob questions ... you program so fast compared to other language users that you have time to mess around? * iulus: IO, IO, it's off to bind we go ... * dons: all your imperative are belong to us * Logan Capaldo: I like constructing things with type errors. It lets me read the error and try and figure out what I'm really trying to do * glguy: In true Haskell form, after I realized what I was actually doing... all my functions melted down to about 2 lines each * jcreigh: I've found learning Haskell makes me feel vastly inferior to Haskell coders. ('Oh,', they say, 'That's just a fold over the hyper-monad fluxbox list. Here's the one-line replacement for your entire program.') * Saizan: [New Year's Fun] Yesterday I was so drunk I was trying to typecheck the people at the party... 'What's your monad!?' Code Watch Wed Dec 27 17:03:48 PST 2006. Manuel M T Chakravarty. [42]Parse and desugar equational constraints. With -findexed-types, equational constraints can appear in contexts wherever class predicates are allowed. The two argument types need to be boxed and rank 0. 42. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18551 About the Haskell Weekly News Each week, 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. Headlines are available as [48]PDF. The Haskell Weekly News is also [49]available in Spanish translation. To help create new editions of this newsletter, please see the [50]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [51]http://www.cse.unsw.edu.au/~dons/code/hwn 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://www.cse.unsw.edu.au/~dons/code/hwn/archives/20070102.pdf 49. http://haskell.org/haskellwiki/HWN/es 50. http://haskell.org/haskellwiki/HWN 51. http://www.cse.unsw.edu.au/~dons/code/hwn

2 1

Quotes on thinking
by Rishiyur Nikhil 02 Jan '07

02 Jan '07

> Quotes of the Week: (from Haskell Digest, Vol 41, Issue 1) > > * cjeris: It's amazing what some languages do to make thinking > impossible, seemingly justified by the assumption that no one > thinks anyway, so it's more important to make non-thinking > programming as easy as possible. You may be interested in the following quotes: Alfred North Whitehead (1861-1947): "It is a profoundly erroneous truism, repeated by all copybooks and by eminent people when they are making speeches, that we should cultivate the habit of thinking of what we are doing. The precise opposite is the case. Civilization advances by extending the number of important operations which we can perform without thinking about them." [Example: compare long division with Roman numerals and Arabic numerals] Operations of thought are like cavalry charges in a battle--they are strictly limited in number, they require fresh horses, and must only be made at decisive moments." Joe Stoy: "If you want to reduce ... design time, you have to stop thinking about something you used to have to think about." Nikhil

1 0

PLAN-X 2007: Last Call for Participation
by Torsten Grust 02 Jan '07

02 Jan '07

Last Call for Participation P L A N - X 2 0 0 7 Programming Language Technologies for XML An ACM SIGPLAN Workshop collocated with POPL 2007 Nice, France -- January 20, 2007 www.plan-x-2007.org Please join us for PLAN-X 2007, the fifth workshop in the PLAN-X series, dedicated to the interaction and integration of programming language technology and the world of XML. The XML data model and its associated languages add interesting twists to programming language practice as well as theory. Just like its four predecessors, the PLAN-X 2007 workshop turns the spotlight on how programming language technology can embrace and explain streaming XML transformations, types for XPath and XML updates, web service contracts, tree patterns in XQuery, LINQ and XML Schema, and more. PLAN-X 2007 will feature eight talks, four system demonstrations, extensive opportunity for discussion, and a keynote address by Christoph Koch (U Saarland, Germany). PLAN-X 2007 will be held in the Plaza Hotel (Nice, France) all-day on Saturday, January 20, 2007, just after and collocated with POPL 2007, the ACM SIGPLAN - SIGACT Symposium on Principles of Programming Languages (January 17-19, 2007). -- PROGRAM (Saturday, January 20, 2007) 09:00--10:00 Welcome Invited Talk by Christoph Koch (U Saarland, Germany) XPath Leashed 10:00--10:30 Coffee break 10:30--12:00 Session 1: Research Papers Streaming XML Transformations Using Term Rewriting (Alain Frisch, Keisuke Nakano) How to Recognise Different Kinds of Tree Patterns From Quite a Long Way Away (Jan Hidders, Philippe Michiels, Jerome Simeon, Roel Vercammen) Lux: A Lightweight, Statically Typed XML Update Language (James Cheney) 12:00--01:30 Workshop lunch (provided) 01:30--03:10 Session 2: Research Papers and Demo Presentations A Theory of Contracts for Web Services (Giuseppe Castagna, Nils Gesbert, Luca Padovani) XML Transformation Language Based on Monadic Second Order Logic (Kazuhiro Inaba, Haruo Hosoya) Demo: MTran: An XML Transformation Language Bases on Monadic Second Order Logic (Kazuhiro Inaba, Haruo Hosoya) Demo: XCentric: A Logic-Programming Language for XML Processing (Jorge Coelho, Mário Florido) Demo: LINQ to XSD (Ralf Lämmel) Demo: GeLaBa: A Framework to Define Classes of XML Documents and to Automatically Derive Specialized Infrastructures (Benoit Pin, Georges André Silber) 03:10--04:00 Interactive Demos and Coffee break 04:00--05:30 Session 3: Research Papers XPath Typing Using a Modal Logic with Converse for Finite Trees (Pierre Geneves, Nabil Layaida, Alan Schmitt) Deciding Equivalence of Top-Down XML Transformations in Polynomial Time (Sebastian Maneth, Helmut Seidl) A Logic Your Typechecker Can Count On: Unordered Tree Types in Practice (Nate Foster, Benjamin C. Pierce, Alan Schmitt) -- REGISTRATION PLAN-X 2007 is held in cooperation with POPL 2007. You can register for the workshop via the POPL 2007 registration process (online or offline). Please visit http://www.regmaster.com/conf/popl2007.html Registration rates are shown below. Note that you can upgrade an existing POPL 2007 registration to include PLAN-X 2007. Workshop-only registration is possible as well. ACM or SIGPLAN Member $89 Non-Member $99 Student $89 Your registration includes a copy of the PLAN-X 2007 informal proceedings, coffee breaks, and lunch. -- PLAN-X 2007 Workshop Chairs - General Chair - Program Chair Torsten Grust Giorgio Ghelli TU München U Pisa Munich, Germany Pisa, Italy grust(a)in.tum.de ghelli(a)di.unipi.it -- PLAN-X 2007 Program Committee - Michael Benedikt (Lucent, USA) - Daniela Florescu (Oracle, USA) - Alain Frisch (INRIA Rocquencourt, France) - Giorgio Ghelli, Chair (U Pisa, Italy) - Haruo Hosoya (U Tokyo, Japan) - Anders Møller (U Aarhus, Denmark) - Mukund Raghavachari (IBM T.J. Watson Research Center, USA) - Alan Schmitt (INRIA Rhône-Alpes, France) - Sophie Tison (U Lille, France) - Philip Wadler (U Edinburgh, UK)

1 0

ghci readline support on FreeBSD: simple patch
by Brandon S. Allbery KF8NH 31 Dec '06

31 Dec '06

The problem people have been having with ghci on FreeBSD turns out to be a bootstrapping issue: if the bootstrap compiler was not built with readline support, or it doesn't include ghc-pkg (the FreeBSD bootstrap ghc has been stripped down to bare essentials, so guess what's not there?), then ghc won't be built with readline support unless you build the stage2 and stage3 compilers. Since ports only builds stage1, with the crippled bootstrap ghc, the result is no readline in ghc itself. (But the readline package is built, so Haskell programs which themselves use readline work fine.) The attached patch is a simplistic, hacky workaround which stubs out the offending configure test. Since ports ghc declares readline as a dependency, this is safe (or at least safe enough). Drop it into / usr/ports/lang/ghc/files and (re)build ghc (either "cd /usr/ports/ lang/ghc; make install clean -DFORCE_PKG_REGISTER", or "portupgrade - f ghc", or deinstall and reinstall from scratch). You will probably want to do something to keep it from being removed or to copy it back when you update the ports tree with portsnap or cvsup. I'm not going to bother reporting this to the port maintainer because it'll just get stuck in the same queue that's holding the ghc-6.6 upgrade. :) -- brandon s. allbery [linux,solaris,freebsd,perl] allbery(a)kf8nh.com system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

1 1

Converting a 'streaming' monad into a list
by Ryan Ingram 31 Dec '06

31 Dec '06

Hi everyone... it's my newbie post! I am trying to create a monad which allows computations to output data to a stream. (Probably such a thing already exists, but it's a good problem for my current skill level in Haskell) For example: streamDemo = do output 1 output 2 output 5 makelist streamDemo -- [1,2,5] I modelled my implementation around the state monad, but with a different execution model: class (Monad m) => MonadStream w m | m -> w where output :: w -> m () run :: m a -> s -> (s -> w -> s) -> s -- basically foldl on the stream values makelist m = reverse $ run m [] (flip (:)) -- s is the type of the object to stream, r is the return type type StreamFunc s r = forall b. b -> (b -> s -> b) -> (r,b) newtype Stream s r = Stream { run' :: StreamFunc s r } instance Monad (Stream s) where return r = Stream (\s _ -> (r,s)) Stream m >>= k = Stream (\s f -> let (r,s') = (m s f) in run' (k r) s' f) instance (MonadStream w) (Stream w) where output w = Stream (\s f -> ((),f s w)) run m st f = snd $ run' m st f What I don't like is how makelist comes out. It feels wrong to need to use reverse, and that also means that infinite streams completely fail to work. But I think it's impossible to fix with the "foldl"-style "run". Is there a better implementation of "makelist" possible with my current definition of "run"? If not, what type should "run" have so that it can work correctly? As an example, I want to fix the implementation to make the following code work: fibs :: Stream Integer () fibs = fibs' 0 1 where fibs' x y = output y >> fibs' y (x+y) fiblist :: [Integer] fiblist = makelist fibs take 5 fiblist -- [1,1,2,3,5], but currently goes into an infinite loop Thanks, -- ryan

6 8