- Haskell-Cafe - Haskell.org

Mitigating state-threading through an application loop
by Michael Serra 21 Dec '11

21 Dec '11

Hello Haskellers, I'm implementing a simple tree-manipulating (for sports tournaments) application prototype, with SDL for graphics and simple user interaction. For reference, I've posted the code on hpaste. <http://hpaste.org/55506> My question is about code organization: everything was simple and elegant until I started writing the program's display/event loop. Every function in this section has to be passed the same parameters - the application window to draw on, the font to display text with, the tree representing the current application state, etc. The font is an especially egregious example of the problem, because it's only used by one function but to get there it must be threaded through all of them (looking at the hpaste, you will see I don't want to call openFont on every invocation of drawTexts; what's needed is to call it once in main and have the resulting value available to drawTxt. So my question: how can I mitigate the ugliness of this state-threading? I understand this is one purpose for monads; am I supposed to implement a monad transformer for this? I think it would be great if I could define a type AppState as a tuple of the various things I need to thread, and specify some kind of automatic as-pattern, so that every function taking this AppState parameter would implicitly have its components bound to certain preset names. I've never seen anyone do this however. What is the right solution?

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ANNOUNCE: partial-lens 0.0.1
by roconnor＠theorem.ca 21 Dec '11

21 Dec '11

Do you miss null references from your old imperative programming days? Wish that the worlds best imperative language had null references? Now your wishes have come true with the new partial-lens package! partial-lens augment edwardk's data-lens package with partial lens. Partial lenses are like regular lenses but have the possibility of not referencing anything. In other words, null references are possible. One notable different with null references from this package is that you can set them without getting a run-time error. Instead setting a null reference is a no-op; however it is possible to determine if setting failed from the return value of the assignment operation. Actually I don't have any applications for partial lenses myself, so if you find this library useful, please let me know. I wrote this mostly because we know what partial lenses are in theory (they are the coalgebras of the (Identity :+: Store b) comonad) but I wanted to see what a real library would look like. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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Test
by Ertugrul Söylemez 21 Dec '11

21 Dec '11

This is a test. Please disregard. -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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OOM when linking, increasingly a problem
by Magnus Therning 21 Dec '11

21 Dec '11

I've started to run out of memory when linking certain packages. The culprits at the moment are crypto-api (Crypto.CPoly) and Agda-executable. A bit of searching revealed that adding ghc-options: -dynamic to the Agda-executable .cabal could allow me to link that package. And it worked. crypto-api is a library package, so I some similar trick isn't really a possibility there :( Is there anything I can do about this? System: Linux (ArchLinux) GHC version: 7.0.3 /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Most software today is very much like an Egyptian pyramid with millions of bricks piled on top of each other, with no structural integrity, but just done by brute force and thousands of slaves. -- Alan Kay

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PADL 2012 Final Call for Participation
by Claudio Russo 20 Dec '11

20 Dec '11

Final Call for Participation ============================ 14th International Symposium on Practical Aspects of Declarative Languages (PADL 2012) http://research.microsoft.com/~crusso/padl12 Philadelphia, Pennsylvania, USA, January 23-24, 2012 Co-located with ACM POPL'12 You are cordially invited to PADL'12. PADL is a forum for researchers and practitioners to present original work emphasizing novel applications and implementation techniques for all forms of declarative concepts, including functional, logic and constraints. PADL'12 is sponsored by Association for Logic Programming with cooperation of ACM SIGPLAN and support by Microsoft Research. Registration ============ To register for PADL'12, please follow the instructions at: https://regmaster3.com/2012conf/POPL12/register.php The early registration deadline is December 24, 2011. The registration fee will cover a copy of the symposium proceedings, refreshments, and an informal dinner. Hotel Information ================= PADL will be co-located with POPL at the Sheraton Society Hill Hotel in Philadelphia. Please visit POPL's web site to make reservations at the special conference rate. Program ======= The symposium will include invited talks by Boon Thau Loo and Don Stewart, and 19 technical papers selected from 41 submissions. The preliminary program is as follows: Invited Talk (9:00-10:00) * Boon Thau Loo Recent Advances in Declarative Networking Break (10:00-10:30) Session 1: Applications (10:30-12:00) * Mayer Goldberg and Guy Wiener A Declarative Approach for Software Modeling * Sergio Antoy and Michael Hanus Contracts and Specifications for Functional Logic Programming * Pedro M. Martins, Julie A. McCann and Susan Eisenbach The Environment as an Argument Lunch (not provided) (12:00-13:30) Session 2: Logic Programming (13:30-15:30) * Yuliya Lierler, Shaden Smith, Mirek Truszczynski and Alex Westlund Weighted-Sequence Problem: ASP vs CASP and Declarative vs Problem-Oriented Solving * Marcello Balduccini and Yuliya Lierler Practical and Methodological Aspects of the Use of Cutting-Edge ASP Tools * Christian Theil Have and Henning Christiansen Efficient tabling of structured data using indexing and program transformation * Dario Campagna, Beata Sarna-Starosta and Tom Schrijvers Optimizing Inequality Joins in Datalog with Approximated Constraint Propagation Break (15:30-16:00) Session 3: Parallelism and Concurrency (16:00-17:30) * Elvira Albert, Puri Arenas and Miguel Gomez-Zamalloa Symbolic Execution of Concurrent Objects in CLP * Pablo Chico De GuzmÃ¡n, Amadeo Casas, Manuel Carro and Manuel Hermenegildo A Segment-Swapping Approach for Executing Trapped Computations * Michael Lesniak Palovca: Describing and Executing Graph Algorithms in Haskell Informal PADL Dinner (Place: TBA) Tuesday, January 24, 2012 Breakfast (8:00-9:00) Invited Talk (9:00-10:00) * Don Stewart Make Things Now! Pragmatic Functional Programming in Haskell Break (10:00-10:30) Session 4: Domain Specific Languages I (10:30-12:00) * Kenny Zhu, Kathleen Fisher and David Walker LearnPADS++: Incremental Inference of Ad Hoc Data Formats * Jeroen Bransen, Arie Middelkoop, Atze Dijkstra and S. Doaitse Swierstra The Kennedy-Warren algorithm revisited: ordering Attribute Grammars * Nicholas Coleman Distributed Policy Specification and Interpretation with Classified Advertisements Lunch (not provided) (12:00-13:30) Session 5: Domain Specific Languages II (13:30-15:30) * Andy Gill and Bowe Neuenschwander Handshaking in Kansas Lava using Patch Logic * Daniel Winograd-Cort, Hai Liu and Paul Hudak Virtualizing Real-World Objects in FRP * Edwin Brady and Kevin Hammond Resource-safe Systems Programming with Embedded Domain Specific Languages * David Broman and Henrik Nilsson Node-Based Connection Semantics for Equation-Based Object-Oriented Modeling Languages Break (15:30-16:00) Session 6: Numerics (16:00-17:00) * Paul Tarau A Declarative Specification of Tree-based Symbolic Arithmetic Computations * Vincent St-Amour, Sam Tobin-Hochstadt, Matthew Flatt and Matthias Felleisen Typing the Numeric Tower

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Re: [Haskell-cafe] The Riddle of the Buddhist Monk
by Patrick Browne 20 Dec '11

20 Dec '11

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The Riddle of the Buddhist Monk
by Patrick Browne 20 Dec '11

20 Dec '11

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CfP: Extended Submission Deadline TAP 2012 (International Conference on Test and Proofs)
by Achim D. Brucker 20 Dec '11

20 Dec '11

(Apologies for duplicates) ************************************************************ *** TAP 2012 offers extended submission deadlines due to *** *** numerous requests received: *** *** Abstract submission: January 11, 2012 *** *** Paper submission: January 18, 2012 *** ************************************************************ *** TAP 2012 solicits both full papers and (industrial) *** *** experience/tool papers *** *** in combining proofs and (security) testing. *** ************************************************************ CALL FOR PAPERS Full and short Research Paper, Industrial Experience Papers, Tool Papers Abstract submission: Jan 11, 2012, Paper submission Jan 18, 2012 6th INTERNATIONAL CONFERENCE ON TEST AND PROOFS (TAP 2012) http://lifc.univ-fcomte.fr/tap2012/ May 31 - June 1, 2012, Prague, Czech Republic Part of the TOOLS Federated Conferences 2012 http://tools2012.fit.cvut.cz/ The TAP conference is devoted to the convergence of proofs and tests, to the application of techniques from both sides and their combination for the advancement of software quality. Test and Proof seem to be contradictory techniques: if you have proved your program to be correct, it is fruitless to comb it for bugs; and if you are testing it, that is surely a sign that you have given up on any hope to prove its correctness. Accordingly, proofs and tests have, since the onset of software engineering research, been pursued by distinct communities. However, the development of both approaches lead to the discovery of common issues and to the realization that each may need the other. The emergence of model checking has been one of the first signs that contradiction may yield to complementarity. Further evidence give test data generation techniques from models or programs which boil down to constraint resolution techniques for relatively large formula; the advent of powerful SMT solvers have therefore powered new testing techniques. Finally, since formal, proof-based verification is costly, testing invariants and background theories can be helpful to detect errors early and to improve cost effectivity. Summing up, in the past few years an increasing number of research efforts have encountered the need for combining proofs and tests, dropping earlier dogmatic views of incompatibility and taking instead the best of what each of these software engineering domains has to offer. The TAP conference aims to bring together researchers and practitioners working in the converging fields of testing and proving, and will offer a generous allocation of papers, panels and informal discussions. Topics of interest cover theory definitions, tool constructions and experimentations and include (other topics related to TAP are welcome): - Transfer of concepts from testing to proving (e.g., coverage criteria) and from proving to testing - Program proving with the aid of testing techniques - New problematics in automated reasoning emerging from specificities of test generation - Verification and testing techniques combining proofs and tests - Generation of test data, oracles, or preambles by deductive techniques such as: theorem proving, model checking, symbolic execution, constraint logic programming - Model-based testing and verification - Generation of specifications by deduction - Automatic bug finding - Debugging of programs combining static and dynamic analysis - Formal frameworks - Tool descriptions and experience reports - Case studies combining tests and proofs - Applying combination of test and proof techniques to new application domains such as validating security procotols or vulnerability detection of programs Important Dates: ================ Abstract submission (extended): January 11, 2012 Paper submission (extended): January 18, 2012 Notification: March 2, 2012 Camera ready version: March 19, 2012 TAP conference: May 31 - June 1, 2012 TOOLS conferences (TOOLS, ICMT, SC, TAP): May 29 - June 01, 2012 Conference Chairs: Bertrand Meyer Program Chairs: Achim D. Brucker and Jacques Julliand Program Committee (to be extended): =================================== Nazareno Aguirre, Bernhard K. Aichernig, Paul Ammann, Dirk Beyer, Nikolaj Bjorner, Robert Clarisó, Marco Comini, Catherine Dubois, Gordon Fraser, Angelo Gargantini, Alain Giorgetti, Patrice Godefroid, Martin Gogolla, Arnaud Gotlieb, Reiner Hähnle, Bart Jacobs, Thierry Jéron, Gregory Kapfhammer, Nikolai Kosmatov, Victor Kuliamin, Karl Meinke, Jeff Offutt, Holger Schlingloff, T.H. Tse, Margus Veanes, Luca Viganò, Burkhart Wolff, Fatiha Zaidi Submission: =========== Please submit your papers via http://lifc.univ-fcomte.fr/tap2012/. TAP 2012 will accept two types of papers: - Research papers: full papers with at most 16 pages in LNCS format (pdf), which have to be original, unpublished and not submitted elsewhere. - Short contributions: work in progress, (industrial) experience reports or tool demonstrations, position statements; an extended abstract with at most 6 pages in LNCS format (pdf) is expected. Accepted papers will be published in the Springer LNCS series and will be available at the conference. The contents of previous TAP proceedings is available at: http://www.informatik.uni-trier.de/~ley/db/conf/tap/ -- Dr. Achim D. Brucker, SAP Research, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, Fax: +49-6227 78-50502, http://www.brucker.ch

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Splitting off many/some from Alternative
by Gregory Crosswhite 20 Dec '11

20 Dec '11

Hey everyone, I am sure that it is too late to do more than idly speculate about this, but could we split the some/many methods out from Alternative? They simply don't make sense except in a subset of possible Alternatives --- in most cases they just result in an infinite loop. That is to say, it is not just that they are extraneous functionality, but most of the time they are *impossible* functionality to even implement! In a way, it is a lie to be including them in Alternative since people making use of the class might incorrectly (but quite reasonably!) assume that any type that is an instance of Alternative *has* a well-defined some and many method, when this is actually the exception rather than the rule. It is only recently that I have been able to grok what some and many are even about (I think), and they seem to only make sense in cases where executing the Alternative action results in a portion of some input being consumed or not consumed. "some v" means "consume at least one v and return the list of items consumed or fail", and "many v" means "consume zero or more v and return the list of items consumed or the empty list of none are consume". It thus makes sense for there to be some subclass of Alternative called something like "Consumptive" that contains these methods. The majority of "Alternative" instances would no longer have these methods, and again that would actually be an improvement since in such cases some/many were unworkable and did nothing but cause infinite loops anyway. Normally it would be unthinkable to even consider such a change to the base libraries, but I suspect that there are not more than a couple of packages out there that make active use of the some/many instances of Alternative; it is really only parser packages that need some/many, and users most likely use the versions included with the packages themselves rather than the Alternative version. Could we verify that this is the case, and if so split them away? I thought I heard a trick whereby people were able to grep all the source on Hackage, but I can't remember what it was. :-) Just a thought. :-) Thanks, Greg

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Sharing on equality
by Johan Brinch 19 Dec '11

19 Dec '11

Hey all, Can GHC eliminate one of two equal ByteStrings, when they are compared and turns out to be equal? Say i have a map, ByteString -> Int. I now do a lookup on a ByteString and if it exists, I insert this ByteString into a list. Is it possible to avoid using more memory, than used by the keys in the map + the list structure? I.e. is it possible to eliminate the redundant ByteStrings somehow? I guess, this could be done by having lookup return the key as well, and then insert this key into the list, however, that's a bit ugly and somewhat anti-intuitive. Here's an example program, to illustrate the idea: https://gist.github.com/1472418 -- Johan Brinch

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Idiomatic Aeson
by Rune Harder Bak 19 Dec '11

19 Dec '11

Hi I'm trying to parse a nested structure using Aeson (from Facebook) It looks something like this: { "education": [ { "school": { "name": "A" }, }, { "concentration": [ { "name": "B" } } ]} And besides this there is a lot of fields I don't need. (And a lot of other fields I do need) What is the idiomatic aeson way of doing this? I'm using Aeson because it's fast, and it seams to have all the relevant combinators, just can't get them to play nicely together. I was thinking something like getInfo (Object m) = (,) <$> m .: "school" .: "name" <*> mapM (.: "name") (m .: "concentracion") but of cause the types don't add up. the verbose style like getInfo (Object m) = do school <- m .: "school" schoolName <- case school of Object m' -> m' .: "name" l -> typeMismatch "schoolName" l concentration <- m .: "concentration" concentrationName <- case concentration of Array l -> forM (V.toList l) $ \m' -> case m' of Object m' -> m' .: "name" l -> typeMismatch "concentrationName" l ) o -> typeMismatch "parseList" o return (schoolName,concentrationName) quickly explodes. So I wrote my own functions of the following sort parseName name _ (Object m) = m .: name parseName _ err l = typeMismatch err l parseList p (Array l) = mapM p $ V.toList l parseList _ o = typeMismatch "parseList" o so I can do something like getInfo (Object m) = (,) <$> parseName "school "school" m >>= parseName "name" "schoolName" <*> parseName "concentration" "concentration" m >>= parseList (parseName "name" "concentrationName") And then variants for for .:?, but then it gets messy with >>= and maybe. Getting rid of the m in the above I moved to arrows, and.. it seems I'm not understanding the standard combinators. So.. what is the idiomatic way of doing this in Aeson? It shouldn't be necessary to move to template-haskell/generics to do this! Thanks!

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compare lengths of lists lazily
by Henning Thielemann 19 Dec '11

19 Dec '11

Shortest and most obfuscating solution I found is: import Data.Ord (comparing) import Control.Applicative ((<$)) compareLength :: [a] -> [a] -> Ordering compareLength = comparing (()<$) :-)

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PEPM'12 - second call for participation
by Simon Thompson 19 Dec '11

19 Dec '11

ACM SIGPLAN 2012 Workshop on Partial Evaluation and Program Manipulation http://www.program-transformation.org/PEPM12 January 23-24, 2012. Philadelphia, PA, USA (co-located with POPL'12) Second Call For Participation Online registration is open at https://regmaster3.com/2012conf/POPL12/register.php Early registration deadline is December 24, 2011 Program is now available http://www.program-transformation.org/PEPM12/Program The PEPM Symposium/Workshop series brings together researchers and practitioners working in the broad area of program transformation, which spans from refactoring, partial evaluation, supercompilation, fusion and other metaprogramming to model-driven development, program analyses including termination, inductive programming, program generation and applications of machine learning and probabilistic search. PEPM focuses on techniques, supporting theory, tools, and applications of the analysis and manipulation of programs. In addition to the presentations of regular research papers, the PEPM program includes tool demonstrations and `short paper' presentations of exciting if not fully polished research. PEPM has established a Best Paper award. The winner will be announced at the workshop. INVITED TALKS Compiling Math to High Performance Code Markus Pueschel (ETH Zuerich, Switzerland) http://www.inf.ethz.ch/~markusp/index.html Specification and verification of meta-programs Martin Berger (University of Sussex, UK) http://www.informatics.sussex.ac.uk/users/mfb21/ VENUE The conference is co-located with POPL and will be held at the Sheraton Society Hill Hotel in Philadelphia's historic district. For hotel rate details and booking please see the POPL webpage: http://www.cse.psu.edu/popl/12/ Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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[Alternative] summary of my understanding so far
by Gregory Crosswhite 19 Dec '11

19 Dec '11

Hey everyone! First of all, it sounds like we all agree that the documentation for Alternative needs to be improved; that alone would clear a lot of the confusion up. I think that a fairly convincing case has also been made that removing many/some from the typeclass doesn't help too much since they are generically defined in terms of the other methods. Put another way, arguing that removing many/some makes Alternative more safe would be like arguing that removing "forever" from the definition of Monad (assuming it were currently a method rather than a function) made Monad more safe. (On the other hand, it might be nice if many/some were not featured so prominently above other functions/combinators in the module.) As a corollary to the above paragraph, if the many/some methods *were* moved to a subclass --- call it, "Parser" --- then essentially this subclass would be redundant. Nonetheless, such a subclass could still be useful because it supplies more information to the user about how the type behaves. That is, while any user of an instance of Alternative can always theoretically use something like many/some, in practice a user might want to add the Parser constraint to their type just to get an extra guarantee that many/some not only exist but are well-behaved. Although many/some cause infinite loops for the current instance of Maybe and [], forever also causes loops for (return (undefined)) for any Monad. Thus, even in the likely event that we decide to keep many/some in Alternative, it still makes sense to have Alternative instances for Maybe and [], despite the fact that they don't play well with many/some for non-empty values. In fact, if anything the existence of the Maybe and [] instances provides a strong reason *to* have the many/some methods inside Alternative, precisely because it gives us a customization point that allows us to make many and some provide well-defined answers for all values of these types. To quote Ross Paterson's proposals: instance Alternative Maybe where ... some Nothing = Nothing some (Just x) = Just (repeat x) many Nothing = Just [] many (Just x) = Just (repeat x) instance Alternative [] where ... some [] = [] some (x:xs) = repeat (repeat x) many [] = [[]] many (x:xs) = repeat (repeat x) The only price that we pay for these instances is that, while some and many are still solutions of • some v = (:) <$> v <*> many v • many v = some v <|> pure [] they no longer the *least* solutions of these equations. In my opinion this is a relatively small price to pay since they nonetheless *are* solutions to these questions, and they have the nice property that they converge sensibly. In fact, in a sense they are the least solutions to the equations that out of all the solutions that converge, though I don't know enough about the theory involved to use the proper technical terminology to express what I really mean, or even if what I just wrote was true. :-) Anyway, as the above discussion illustrates, the existence of pure types that are instances of Alternative actually *adds* to the case of keeping some and maybe in Alternative. So in conclusion: 1) Documentation really needs to be improved 2) some/many cannot be physically separated from Alternative, but there *might* be an advantage to creating a subclass for them anyway purely for the sake of conveying more information about a type to users 3) Maybe and [] are sensible instances of Alternative, even if many/some often enters an infinite loop. 4) It is possible to provide special instance of many/some that satisfy the equations of many/some, with the slight disadvantage that these solutions are no longer the "least" solutions. Based on all of this, at this moment in time it seems to me that the most sensible way forward is to fix the documentation, tweak the definition of Alternative to no longer require the least solutions of the equations, and then to adopt the new instances for Maybe and []. Thoughts? Cheers, Greg

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PEPM12: Second Call For Participation
by oleg＠okmij.org 19 Dec '11

19 Dec '11

ACM SIGPLAN 2012 Workshop on Partial Evaluation and Program Manipulation http://www.program-transformation.org/PEPM12 January 23-24, 2012. Philadelphia, PA, USA (co-located with POPL'12) Second Call For Participation Program is now available http://www.program-transformation.org/PEPM12/Program Online registration is open at https://regmaster3.com/2012conf/POPL12/register.php Early registration deadline is December 24, 2011 The PEPM Symposium/Workshop series brings together researchers and practitioners working in the broad area of program transformation, which spans from refactoring, partial evaluation, supercompilation, fusion and other metaprogramming to model-driven development, program analyses including termination, inductive programming, program generation and applications of machine learning and probabilistic search. PEPM focuses on techniques, supporting theory, tools, and applications of the analysis and manipulation of programs. In addition to the presentations of regular research papers, the PEPM program includes tool demonstrations and `short paper' presentations of exciting if not fully polished research. PEPM has established a Best Paper award. The winner will be announced at the workshop. INVITED TALKS Compiling Math to High Performance Code Markus Pueschel (ETH Zuerich, Switzerland) http://www.inf.ethz.ch/~markusp/index.html Specification and verification of meta-programs Martin Berger (University of Sussex, UK) http://www.informatics.sussex.ac.uk/users/mfb21/ VENUE The conference is co-located with POPL and will be held at the Sheraton Society Hill Hotel in Philadelphia's historic district. For hotel rate details and booking please see the POPL webpage: http://www.cse.psu.edu/popl/12/

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Type checker for haskell-src-exts (was: Typechecking Using GHC API)
by Sean Leather 19 Dec '11

19 Dec '11

On Thu, Dec 15, 2011 at 11:07, Niklas Broberg wrote: > Envisioned: The function you ask for can definitely be written for > haskell-src-exts, which I know you are currently using. I just need to > complete my type checker for haskell-src-exts first. Which is not a small > task, but one that has been started. That's good to know! I presume it's something like Haskell98 to start with? I'd be even more impressed (and possibly also concerned for your health) if you were going to tackle all of the extensions! I've been trying to find a student to write a haskell-src-exts type checker for me. It should use a particular kind of mechanism though, using constraints similar to [1]. Then, I want to adapt that to do transformations. What approach are you using? Maybe I can somehow steal your work... ;) Regards, Sean [1] http://www.staff.science.uu.nl/~heere112/phdthesis/

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Adding state to a library
by Kevin Jardine 18 Dec '11

18 Dec '11

I have a library of functions that all take cfg parameter (and usually others) and return results in the IO monad. It is sometimes useful to drop the config parameter by using a state-like monad.. I have found that I can wrap all my functions like so: withLibrary cfg f = f cfg stateF a b c d =

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ANNOUNCE: Anansi 0.4.2 (literate programming pre-processor)
by John Millikin 18 Dec '11

18 Dec '11

Anansi is a preprocessor for literate programs, in the model of NoWeb or nuweb. Literate programming allows both computer code and documentation to be generated from a single unified source. Home page: https://john-millikin.com/software/anansi/ Hackage: http://hackage.haskell.org/package/anansi-0.4.2 ----- This release has a couple cool new features, suggested by Dirk Laurie. Markdown loom =============== Markdown, a lightweight markup language similar to ReStructuredText, is used often on web forums. Use [[ :loom anansi.markdown ]] in your documents to enable. User-defined line pragmas ======================== Users can now add, modify, and disable line pragmas in tangled output based on file extension. This makes it much easier to combine code and data in a single source file. By default, line pragmas are enabled for the C, C++, C#, Haskell, Perl, and Go languages. Customizing the pragma format is easy. Use the ${line}, ${path}, and ${quoted-path} substitutions in an :option. This example code will insert comments into tangled Python code: ------------------------------------------- :# Insert comments into any tangled output file with a name ending in ".py" :option anansi.line-pragma-py=# Generated from ${path} line ${line} ------------------------------------------- To disable line pragmas for a particular file type, just set its pragma format to an empty string: ------------------------------------------- :option anansi.line-pragma-pl= -------------------------------------------

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Announce: The Haskell Platform 2011.4
by Don Stewart 18 Dec '11

18 Dec '11

We're pleased to announce the release of the Haskell Platform: a single, standard Haskell distribution for everyone. Download the Haskell Platform 2011.4.0.0: http://haskell.org/platform/ The specification, along with installers (including Windows, Apple and Unix installers for a full Haskell environment) are available. The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. What you get is specified here: http://hackage.haskell.org/platform/contents.html Thanks! -- The Platform Infrastructure Team P.S. Special thanks to Mark Lentczner for his excellent work on this release.

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ANNOUNCE: hxournal-0.5.0.0 - A pen notetaking program written in haskell
by Ian-Woo Kim 18 Dec '11

18 Dec '11

Hi, everyone, I am very pleased to announce a pen notetaking program: hxournal, which is written entirely in haskell using gtk2hs. I uploaded the package on hackage. This program accompanies with two library packages, xournal-parser and xournal-render for parsing and rendering xournal format file. http://hackage.haskell.org/package/hxournal http://hackage.haskell.org/package/xournal-parser http://hackage.haskell.org/package/xournal-render Installing hxournal should be very simple: > cabal update > cabal install hxournal hxournal can be currently regarded as a clone of xournal, which is a notetaking program developed in C. (See http://xournal.sourceforge.net) As xournal, hxournal can take wacom tablet X11 input in subpixel unit so that it can result in very smooth notetaking experience. Currently, basic pen operations and eraser operations, file open/save operations, rectangular selection, cut/copy/paste operations have been implemented. So the application is semi-usable. The file format is the same as xournal but gunzipped. So to view/edit xoj files generated from xournal, just gunzip the xoj files and read them in hxournal. Gunzipped xoj files generated from hxournal are readable in xournal program. One NEW special feature of hxournal compared with xournal: This program can make a split view (horizontal and vertical and arbitrary combination of them) of the same document similarly to emacs buffers and windows. Please try Horizontal/Vertical Split in View menu of the program. This will be convenient when notetaking a long document. The git repository is located at https://www.github.com/wavewave/hxournal The program web page and development web/wiki pages will be announced soon (it will be linked from package webpage on hackage anyway) and the detailed manual will be presented there. Thank you for your interest. Enjoy haskell notetaking! Ian-Woo Kim

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I've just heard of a neat security measure that when you compile the code it generates different object code...
by KC 17 Dec '11

17 Dec '11

... with the same functionality. Thus, your program would be a moving target to hackers. Would this be challenging with ghc? -- -- Regards, KC

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Typechecking Using GHC API
by Sh NJP 17 Dec '11

17 Dec '11

Hi, I do some pre-processing on a normal Haskell code ( -F ). The pre-processor needs to know the type of each expression. What are the possibilities to do so? Can I use GHC API to employ GHC type checker? If yes, any good tutorial? Is it too naive to think of a function, f :: String -> AnnotatedAST , that takes Haskell code and returns its corresponding abstract syntax tree annotated with types? Thanks, /shayan

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gtk2hs: adding widget during runtime
by Gary Klindt 17 Dec '11

17 Dec '11

Hello Haskellers, currently I work with the gtk2hs library to create a graphical user interface. Everything works fine, but there is one problem: How can I insert widgets into boxes during runtime? I tried something like: main = do initGUI window <- windowNew box <- vBoxNew True 0 l1 <- labelNew $ Just "label eins" bc <- buttonNewWithLabel "insert!" onClicked bc $ (\b -> do lh <- labelNew $ Just "neues label!" boxPackStart b lh PackNatural 0 widgetQueueDraw lh widgetQueueDraw b ) box boxPackStart box l1 PackNatural 0 boxPackStart box bc PackNatural 0 containerAdd window box widgetShowAll window onDestroy window mainQuit mainGUI That compiles fine, but the GUI never shows a "neues label!". What I want is a widget W, which takes a list L, and produces sub widgets for each element. There are also some Entry widgets and a button B. By clicking on that button B, I want to add an element to the list L, and add a sub widget to the widget W. Thank you for reading, Gary

2 2

What happens if you get hit by a bus?
by Michael Litchard 17 Dec '11

17 Dec '11

I'm learning what it means to be a professional Haskell programmer, and contemplating taking on side jobs. The path of least resistance seems to be web applications, as that is what I do at work. I've been investigating what some web developers have to say about their trade. One article addresses the question above. His answer was that he uses RoR which has a large community and he is therefore easily replaceable. My question, for freelancers in general, and web developers in particular is this: How do you address this question? I imagine potential clients would need to be assuaged of their fears that hiring me would lead to a lock-in situation at best, and no one to maintain a code base at worst. Lock-in won't be part of my business model, also sooner or later we part ways with the client. When the client wonders, "What happens then?", what is a good answer?

11 13

Dutch national FP day, January 6, Utrecht
by S D Swierstra 16 Dec '11

16 Dec '11

On http://www.cs.uu.nl/wiki/bin/view/FPDag2012/WebHome you will find the program and registration information about the next Dutch National Functional Programming day, on January 6, Utrecht University. Please forward this mail to anyone interested. Any questions can be directed to me. Hoping to meet you all, Doaitse Swierstra

2 1

Proposal to incorporate Haskell.org
by Don Stewart 16 Dec '11

16 Dec '11

Hello everyone. The haskell.org committee[1], in the interest of the long-term stability of the open source Haskell community infrastructure, has decided to incorporate haskell.org as a legal entity. This email outlines our recommendation, and seeks input from the community on this decision. The committee's proposal is that haskell.org incorporate as an entity under the Software Freedom Conservancy umbrella organization (the same group that Darcs joined recently): http://sfconservancy.org/ If we proceed with this move, haskell.org will be a legal entity, and registered as a non-profit, allowing us to more directly accept (US tax-deductible) donations, and to invest in assets that benefit the Haskell open source community. We welcome your feedback on the proposal attached below. -- Don Stewart (on behalf of the Haskell.org committee) ------------------------------------------------------------------------ = A proposal for the incorporation of Haskell.org = In recent years, haskell.org has started to receive assets, e.g. money from Google Summer Of Code, donations for Hackathons, and a Sparc machine for use in GHC development. We have also started spending this money: in particular, on hosting haskell.org itself. There is also interest in running fundraising drives for specific things such as Hackathon sponsorship and hosting fees. However, haskell.org doesn't currently exist as a legal entity, meaning that these assets have had to be held on our behalf by other entities, such as Galois and various universities. This leads to tricky situations, with no-one being sure who should decide how the haskell.org assets can be used. To solve these problems, we propose that haskell.org applies to become a member project of the Software Freedom Conservancy (SFC) <http://conservancy.softwarefreedom.org/>. The SFC is a non-profit organization that provides free financial and administrative services to open source projects. Additionally, it has 501(c)(3) status, meaning donations from the US are tax-deductible. The SFC would hold haskell.org's money and other assets, and would be able to accept donations on behalf of haskell.org. The haskell.org committee, as described here [2], will make decisions on spending assets and other decisions related to governing the non-profit. Before proceeding, we are inviting input from the community in the form of specific objections or queries regarding the plan. We've tried to answer some of the most likely questions: Q: Does this mean that my Haskell project must now be covered by a copyleft licence such as GPL? A: No, but Haskell projects using haskell.org resource should use an Open Source licence <http://www.opensource.org/licenses/alphabetical>. Q: Will it still be possible to use community.h.o to host non-open-source material, such as academic papers? A: An overall minority of such content, as is the current situation, is not a problem. Q: Will it still be possible to have job ads on the haskell.org mailing lists and website? A: Yes. Q: Will this affect our ability to host the Haskell Symposium <http://www.haskell.org/haskell-symposium/> and Industrial Haskell Group<http://industry.haskell.org/> webpages within haskell.org? A: No. Q: What will be the relationship between haskell.org and other organizations such as the Haskell Symposium and Industrial Haskell Group? A: Those organisations will continue to exist as separate entities. Q: If an umbrella non-profit organisation "The Haskell Foundation" was created, would haskell.org be able to join it? A: Yes. It's likely that in such a scenario, the Haskell Foundation would become the owner of the haskell.org domain name, with the cost divided between the members. The entity that is part of the SFC would be renamed "community.haskell.org" in order to avoid confusion. [1]: http://www.haskell.org/haskellwiki/Haskell.org_committee [2]: http://www.haskell.org/haskellwiki/Haskell.org_committee#Operation

10 16

DB vs read/show for persisting large data
by C K Kashyap 16 Dec '11

16 Dec '11

Hi, It has been on my todo list for some time now. I'd like to write a GTD tool that has dependency tracking support. Haskell seems like a good choice for this. I was wondering if there has been any past attempts with this? One thing that has been bothering me has been this - the persistence of data. Should I use sqlite(or someother DB) or should I use Haskell's read/show functions to read from and write to a file? I am slightly not inclined towards NOT using DB because I want to implement all the business logic in Haskell. I want to avoid having to generate SQL. It'll be great if I could get some feedback on the "read/show" approach - is this even a viable option? Regards, Kashyap

8 11

Parallel Matrix Multiplication
by mukesh tiwari 16 Dec '11

16 Dec '11

Hello all I am trying to learn parallel Haskell and I have gone through couple of resources ( Real world Haskell and http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/AFP08… ). I understood par and pseq and I wrote matrix multiplication using these two function but it does not look. Although I did not get all these details but none of the sparks converted which is really important for parallel programming. Could some one please tell me how to improve this. Also kindly recommend me some literature for parallel programming in Haskell. Regards Mukesh Tiwari import Data.List import Control.Parallel parHelp :: ( Num a ) => [ a ] -> [ a ] -> a parHelp [] [] = 0 parHelp ( x : xs ) ( y : ys ) = ret where ret = par a ( pseq a ( a + parHelp xs ys ) ) where a = x * y helpMult :: ( Num a ) => [ a ] -> [ [ a ] ] -> [ a ] helpMult _ [] = [] helpMult x ( y : ys ) = ret where ret = par a ( pseq a ( a : helpMult x ys ) ) where a = parHelp x y mult :: ( Num a ) => [ [ a ] ] -> [ [ a ] ] -> [ [ a ] ] mult [] _ = [] mult ( x : xs ) ys = ret where ret = par a ( pseq a ( a : mult xs ys ) ) where a = helpMult x ys main = print $ mult [[1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4] ] ( transpose [[1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4 ] , [ 1 .. 4] ]) [user@haskell Programming]$ ghc -O2 -threaded -rtsopts Matpar.hs [1 of 1] Compiling Main ( Matpar.hs, Matpar.o ) Linking Matpar ... [user@haskell Programming]$ ./Matpar +RTS -N2 -s ./Matpar +RTS -N2 -s [[10,20,30,40],[10,20,30,40],[10,20,30,40],[10,20,30,40]] 85,480 bytes allocated in the heap 5,216 bytes copied during GC 47,328 bytes maximum residency (1 sample(s)) 22,304 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 0 collections, 0 parallel, 0.00s, 0.00s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed Parallel GC work balance: -nan (0 / 0, ideal 2) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 1 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 2 (bound) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 3 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) * SPARKS: 84 (0 converted, 0 pruned)* INIT time 0.00s ( 0.00s elapsed) MUT time 0.00s ( 0.00s elapsed) GC time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.00s ( 0.00s elapsed) %GC time 33.3% (13.5% elapsed) Alloc rate 42,761,380 bytes per MUT second Productivity 33.3% of total user, 45.0% of total elapsed gc_alloc_block_sync: 0 whitehole_spin: 0 gen[0].sync_large_objects: 0 gen[1].sync_large_objects: 0 [user@haskell Programming]$

2 2

smt solver bindings
by Dimitrios Vytiniotis 16 Dec '11

16 Dec '11

I've a quick question: Are there Haskell wrappers for the Z3 C API around? Thanks! d-

4 3

[Alternative] some/many narrative
by Gregory Crosswhite 16 Dec '11

16 Dec '11

Okay, so how about the following as a user narrative for some and many? many v = repeatedly execute the action v and save each obtained result until v fails; at that point, *succeed* and return a list with all of the results that had been collected some v = like many v, but if v does not succeed even once then *fail* instead of return the empty list Note: If v *never* fails, then at best many/some will return infinite lists, and at worst they will repeat your action forever. Cheers, Greg P.S.: And yes, I know that many of you will complain that these definitions are not mathematically precise or what not, but it is important to be able to build up narratives at a sufficiently high level that they explain the essentials of what is going on for the benefit of users who are unfamiliar with the precise mathematical language used to precisely define their semantics.

5 7

How to get a file path to the program invoked?
by dokondr 16 Dec '11

16 Dec '11

Hi, When my program starts it needs to know a complete path to the directory from which it was invoked. In terms of standard shell (sh) I need the Haskell function that will do equivalent to: #!/bin/sh path=$(dirname $0) How to get this path in Haskell? getProgName :: IO String defined System.Environment only returns a file name of the program without its full path. Thanks!

12 21

indentation blues
by Martin DeMello 15 Dec '11

15 Dec '11

The vim autoindent for haskell is really bad :( Is there a better indent.hs file floating around somewhere? Alternatively, is the emacs haskell mode better enough that it's worth my time learning my way around emacs and evil? martin

8 11

ANNOUNCE: keera-network-sms-diamondcard
by Ivan Perez 14 Dec '11

14 Dec '11

My fellow haskellers, I wrote a very simple library to send SMSs using the DiamondCard VoIP provider. At the present time, it uses the HTTP interface to contact the provider (there's a SOAP interface, but I haven't been able to make it play along with shoap). The current API is quite straightforward: sendSMS :: String -> String -> String -> String -> [ String ] -> IO SMSDeliveryResult sendSMS accountID pinCode message fromNumber toNumbers I don't like submitting alpha-stage software to hackage, so it's only in github for now: https://github.com/ivanperez-keera/keera-network-sms-diamondcard In a few days I plan to release a small GTK frontend that I've been using for a few months. If you give either of them a try, I'd be glad to know where it got you. Cheers, Ivan Perez

1 0

A Mascot
by heathmatlock 14 Dec '11

14 Dec '11

I liked Go's mascot, and I figure it couldn't hurt to have our own. I spent the past hour making this: http://i.imgur.com/Mib6Q.png What do you think? -- Heath Matlock +1 256 274 4225

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Off The Beaten Track: Call for Participation
by David Walker 14 Dec '11

14 Dec '11

Off The Beaten Track: Underrepresented Problems for Programming Language Researchers http://www.cs.princeton.edu/~dpw/obt/ A Workshop Co-located with POPL 2012 Philadelphia, USA January 28, 2012 Come and join us for OBT -- we have a great program filled with a diverse set of problems and ideas for programming language researchers. And at the end of the day, we will have an open mic session to discuss directions for the PL community. Our program is now up here: http://www.cs.princeton.edu/~dpw/obt/obt_program.html And you can register here: https://regmaster3.com/2012conf/POPL12/register.php A broader explanation of the workshop goals: Programming language researchers have the principles, tools, algorithms and abstractions to solve all kinds of problems, in all areas of computer science. However, identifying and evaluating new problems, particularly those that lie outside the typical core PL problems we all know and love, can be a significant challenge. Hence, the goal of this workshop is to identify and discuss problems that do not often show up in our top conferences, but where programming language researchers can make a substantial impact. The hope is that by holding such a forum and associating it directly with a top conference like POPL, we can slowly start to increase the diversity of problems that are studied by PL researchers and that by doing so we will increase the impact that our community has on the world. While many workshops associated with POPL have become more like mini-conferences themselves, this is not the goal for Off the Beaten Track. The workshop will be informal and structured to encourage discussion. It will also be centered around problems and problem areas as opposed to fully-formed solutions.

1 0

acid state
by Anatoly Yakovenko 13 Dec '11

13 Dec '11

So I am trying to understand how acid state works. The HelloWorld example has a type Message = String data Database = Database [Message] $(deriveSafeCopy 0 'base ''Database) -- Transactions are defined to run in either the 'Update' monad -- or the 'Query' monad. addMessage :: Message -> Update Database () addMessage msg = do Database messages <- get put $ Database (msg:messages) It seems to me that since the Dababase is a list of messages every update would require acid-state to rewrite the list into the file, so each update would get slower as the list gets bigger, but what I am seeing is that updates are constant time regardless of the size of the list. So how does it work?

2 2

AFRP is not (necessarily) imperative
by Ertugrul Söylemez 13 Dec '11

13 Dec '11

Hello fellows, after a few discussions on IRC and via private mail I feel obligated to point out that arrows and in particular AFRP do not force you to use an imperative style in any way. You can use a style very similar to SHE's idiom brackets. I will demonstrate this using the Netwire library. The following code has a very imperative feel to it and also looks quite ugly: myWire = proc _ -> do fps <- avgFpsInt 1000 100 -< () t <- time -< () let x = 3 + t y <- integral 0 -< t returnA -< printf "%8.2f %8.2f %8.2f" fps x y Let's improve this code. The magic lies in identifying behaviors from classic FRP. The arrow variables from the above code can be seen as the behaviors, but that's not very useful for getting rid of the imperative style. A better way to look at it is that every wire that ignores its input (i.e. has a fully polymorphic input type) is a behavior, so let's find them. First of all it is impossible to write a proper Num instance for wires. The underlying problem is the same as for writing a Num instance for functions. However, the Wire type forms a vector space, and the next release of Netwire will include the corresponding instances (see the vector-space package by Conal Elliot). With them we can write: x = constant 3 ^+^ time The x wire is our first behavior. Passing x to a generic wire is simply regular arrow composition, giving you behaviors as functions of other behaviors, hence: y = integral 0 <<< x Also fps is just a simple behavior: fps = avgFpsInt 1000 100 To get to the final output there are multiple ways. Perhaps the nicest way is to exploit the Applicative instance, giving you: myWire = liftA3 (printf "%8.2f %8.2f %8.2f") fps x y where fps = avgFpsInt 1000 100 x = constant 3 ^+^ time y = integral 0 <<< x Looks much more declarative, no? For more complicated compositions or generic wires use banana brackets or the combinator syntax in arrow notation. Examples: myWire = proc x -> (someWire -< x) ^+^ (otherWire -< 15) myWire = proc x -> (| f (someWire -< x) (otherWire -< 15) |) This closely resembles idiom bracket notation, but allows you to be explicit about inputs, takes care of creating side channels and gives you the full power of arrows, including ArrowChoice and ArrowLoop. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

3 2

Chuan-Kai Lin's Unimo framework
by Jean-Luc Delatre 13 Dec '11

13 Dec '11

Does anybody has an hands on experience of using Chuan-Kai Lin's Unimo framework? https://sites.google.com/site/chklin/research It looks interesting but it seems it amounts to add an extra layer of interpretation (the monads encoding as a data type) and the paper lacks examples of actual uses of the framework. jld

2 2

Linear types using kinds?
by Gregory Crosswhite 13 Dec '11

13 Dec '11

Hey everyone! So, I was on LtU reading about F* http://lambda-the-ultimate.org/node/4318 and an idea just occurred to me which *must* have occurred to someone smarter than me, so I would love your feedback on the following question: Could Haskell be extended to support linear/affine types (in order to have, for example, more efficient resource management) by extending the kind system to include such types? Or, even if the answer were yes, does the use of monads and higher-order types (like those used in "runST" function for the ST monad) make linear/affine types completely redundant in Haskell. Thanks a lot! Greg

1 0

indentation blues
by Ben Kolera 13 Dec '11

13 Dec '11

I am fairly new to haskell, but I really like the emacs haskell mode. It is a bit strict but it generally does what I want it to. Unfortunately I can't really compare to the haskell vim mode since I only did Scala and Perl back when I was a heavy vim user. The one useful thing that I can add is that there are some really good packages out there for modal vi keybindings in emacs. If you truly like the vim keybindings better then you can still use them in emacs. I hope that helps a little bit. On Tue, Dec 13, 2011 at 10:50 AM, Martin DeMello <martindemello(a)gmail.com> wrote: > The vim autoindent for haskell is really bad :( Is there a better > indent.hs file floating around somewhere? Alternatively, is the emacs > haskell mode better enough that it's worth my time learning my way > around emacs and evil? > > martin > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Module name space question
by Christoph Breitkopf 12 Dec '11

12 Dec '11

Hi, I recently asked about what interfaces to implement for a new data type. Following the rule that the last 10% of work take the second 90% of time, some other questions have come up. If anyone wants to look at the code in question: http://www.chr-breitkopf.de/comp/IntervalMap Some time ago, I was looking for a data structure to search in sets of possibly overlapping intervals, and found only Data.SegmentTree, which did not fit my needs (nice term for "I did not understand the type signatures"). So I started to roll my own IntervalMap implementation. But yesterday I used Hayoo instead of Hoogle for the first time, and found Data.IntervalMap.FingerTree. This has an Interface that I _do_ understand, and, while slightly different from my aims (depite the name, it seems more like a multi-map and does not offer lookup of specific keys), it raises some tasks/questions: Tasks: Benchmark against my implementation, and maybe scrap own if it performs worse. And questions: 1. Why doesn't Hoogle find this? 2. Module name space pollution I would have been so bold to name my module Data.IntervalMap. Now, since even Ross Patterson uses a submodule name instead, what is the accepted practice? If I decide to release my class after all, what should be the name? Data.IntervalMap.RedBlackTree ? Or even Data.IntervalMap.MyDomaindNameOrSomething. An I even have a module Data.IntervalMap.Interval, so if Ross had decided to put his Interval type into a separate module, even FingerTree vs RedBlackTree would not have prevented a name clash. 3. Are there more wothwile operations on this data structure? Data.IntervalMap.FingerTree has 'dominators', which I missed. A function to extract the interval with the largest endpoint might be useful, too. Any more ideas? TIA, Chris

6 5

Cabal: Disjunctive Build-Depends
by Matthew Farkas-Dyck 12 Dec '11

12 Dec '11

Hello all. I have written some code that can be compiled with either of two libraries, with no modification. How can I tell Cabal? I tried "||" but it failed to parse. I could find which package is available in the build script, and then call defaultMainNoRead with the appropriate GenericPackageDescription, but I'm not sure how to find this system-agnostically. Thanks for any help. Cheers, Matthew Farkas-Dyck

2 2

HaXml 1.13 -> 1.22 upgrade
by Michael Orlitzky 12 Dec '11

12 Dec '11

I'm trying to migrate one my programs from the old HaXml API to the new. Please, someone save me. I'm currently stuck with this, which works in 1.13. All of the filters work on Content, so I make some from the root element with the "root_elem = CElem root" line. -- |Takes an XML String as an argument, and returns the -- status that was parsed from it. Should only be used -- on XML string where a <status> is a top-level element. parse_status :: String -> [Status] parse_status xml_data = catMaybes maybe_status where (Document _ _ root _) = xmlParse xml_file_name xml_data root_elem = CElem root status_element = (single_status root_elem) maybe_status = map status_from_content status_element In the new API, xmlParse returns the root element with type (Element i) rather than just Element. And the Content constructor I have to use is (CElem (Element i) i), but I have no way to pass the correct 'i' to it. I just want to parse a few elements from an XML file.

3 4

Re: [Haskell-cafe] Module name space question
by John Lato 12 Dec '11

12 Dec '11

> From: Christoph Breitkopf <chbreitkopf(a)googlemail.com> > > Hi, > > I recently asked about what interfaces to implement for a new data type. > Following the rule that the last 10% of work take the second 90% of time, > some other questions have come up. > > If anyone wants to look at the code in question: > http://www.chr-breitkopf.de/comp/IntervalMap > > Some time ago, I was looking for a data structure to search in sets of > possibly > overlapping intervals, and found only Data.SegmentTree, which did not fit > my needs > (nice term for "I did not understand the type signatures"). I can't answer any of your specific questions, although I've published a related structure in the splaytree package (http://hackage.haskell.org/package/splaytree) The interface is minimal, although it suits my needs and I'd be happy to extend it if someone else found it worthwhile. The difference between my data structure (which I've called a RangeSet) and a standard IntervalSet is that overlapping ranges are combined, and if part of a range is deleted, the affected range is modified and possibly split e.g. > let set1 = singleton $ range 0 5 > set2 = insert (range 2 5) set1 after this, set2 has one node representing the range 0-7. Staying on topic, my package puts everything under `Data.SplayTree.xxx`, which is different from the fingertree package organization. AFAICT the selection of one convention over the other is fairly arbitrary, although more packages seem to use the "one second-level hierarchy" structure (e.g. bytestring, text, containers). John L.

1 0

No instance for (Read POSIXTime)
by dokondr 12 Dec '11

12 Dec '11

Hi, I need to read / write epoch seconds from / to plain text files. When I try to read POSIXTime that I use in my own data type: data TimedClassRecT = TCR {timeStamp :: POSIXTime, classCosMap :: Map.Map String Float} deriving (Eq, Read, Show) I get the following error: No instance for (Read POSIXTime) arising from the 'deriving' clause of a data type declaration Possible fix: add an instance declaration for (Read POSIXTime) or use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Read TimedClassRecT) What should I do to provide Read instance for POSIXTime? I would rather not implement it myself. Thanks!

2 3

Tracing Prelude.read exceptions
by dokondr 12 Dec '11

12 Dec '11

Hi, I got quite used to a sequence providing simple data persistence : 1) Store my data to a file: writeFile fileName (show someData) 2) Some time later read this data back: line <- readFile fileName let someData = read line :: SomeDataType Having this done hundreds of times I now got stuck with step 2) trying to read moderately complex structure back. I get read exception in run-time: fromList *** Exception: Prelude.read: no parse I have checked and rechecked my types, data files, etc. - and still no idea. So my question: Is there any way to trace Prelude.read exceptions to see exactly on what data element read fails in run-time? Thanks!

3 2

Recommended class instances for container type
by Christoph Breitkopf 12 Dec '11

12 Dec '11

Hello, I'm in the process of implementing a container data type, and wonder what class instances are generally considered necessary. E.g. is it ok to start out with a Show that's adequate for debugging, or is it a 'must' to include instances of everything possible (Eq, Ord if possible, Read, Show, Functor, ...). And what about the more experimental things? Say, DeepSeq, Typeable, Data? I'd like to keep this simple at start, and I've admittedly not followed recent developments in Haskell-land (recent meaning the last 10 years or so. I _do_ know about hierachical modules ;-) ). OTOH, if not having such instances makes it impossible to do things the modern way, I'd probably take the time to implement (and maybe understand) them. Thanks, Chris

6 8

How did iteratees get their names?
by Douglas McClean 12 Dec '11

12 Dec '11

I love iteratees as a paradigm for IO, but I am having trouble developing a relationship with the names. Could someone explain their origin? It seems like if iteratees consume things, enumerators produce them, andenumeratees do both that names like Sink, Source, and Transformer or Consumer, Producer, and Transformer might be more relatable choices? Is there some reason apart from convention why these names wouldn't fit the concepts well? -- J. Douglas McClean

5 6

Why doesn't this work? (palindrome :: IO)
by Alexej Segeda 12 Dec '11

12 Dec '11

Hi! A couple of months ago, I wrote an exam in an introductory Haskell course and failed, all because of an assignment that I was convinced would work, but for some reason, it didn't. The assignment was to write a function that would take a line, then determine whether it's a palindrome or not. My code follows: palindrome :: IO () palindrome = do putStr "Type in a word" s <- getLine case s of (s == reverse s) -> putStrLn (s ++ " is a palindrome") otherwise -> putStrLn (s ++ " is not a palindrome") The interesting thing is, that if I change the "case ... of" statement to an "if ... then ... else" statement, this magically starts to work. Since I no longer am enrolled (I have to take the course next year), I can't ask a teacher, but my curiosity still bugs me. Why doesn't this work? And why does it work with a "if ... then ...else" statement?

6 10

Generating Code
by L Corbijn 11 Dec '11

11 Dec '11

Hello, In an attempt to reduce the amount of boring repetitive work to update the OpenGLRaw package I've created a generator to do it partially for me. It currently uses haskell-src-exts for representing the haskell source of the modules. Though haskell-src-exts does an excellent job for representing haskell source, it seems to be more aimed at parsing haskell source and doing something with it, rather than generating it. For cpp macros, say for the use of different calling conventions used, can't be used directly, nor is there a really good way to use comments (at least so it seems to me). So I'm interested if there are other libraries that are more suitable to the task of generating haskell code for library use, and thus generate 'human readable' exported code (so no TH). I'm also interested in how other projects generate code for their packages. Greetings, Lars Corbijn

10 15