September 2011 - Haskell-Cafe

GHC/Cabal on AFS
by Tristan Ravitch 06 Sep '11

06 Sep '11

I have the Haskell Platform (and my home directory with my cabal-installed packages) installed on an AFS (a network filesystem) volume and have been noticing a strange issue. Whenever I install a package using cabal-install and it gets to a phase of the build where it needs to load a bunch of packages, the build fails without a useful error. Example: cabal-dev install yesod Resolving dependencies... Configuring yesod-core-0.9.1.1... Preprocessing library yesod-core-0.9.1.1... Preprocessing test suites for yesod-core-0.9.1.1... Building yesod-core-0.9.1.1... [ 1 of 15] Compiling Yesod.Internal.Session ( Yesod/Internal/Session.hs, dist/build/Yesod/Internal/Session.o ) [ 2 of 15] Compiling Paths_yesod_core (dist/build/autogen/Paths_yesod_core.hs, dist/build/Paths_yesod_core.o) [ 3 of 15] Compiling Yesod.Logger (Yesod/Logger.hs,dist/build/Yesod/Logger.o ) [ 4 of 15] Compiling Yesod.Internal.RouteParsing(Yesod/Internal/RouteParsing.hs,dist/build/Yesod/Internal/RouteParsing.o) [ 5 of 15] Compiling Yesod.Internal (Yesod/Internal.hs,dist/build/Yesod/Internal.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package bytestring-0.9.1.10 ... linking ... done. Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package deepseq-1.1.0.2 ... linking ... done. Loading package text-0.11.0.5 ... cabal: Error: some packages failed to install: yesod-0.9.1.1 depends on yesod-core-0.9.1.1 which failed to install. yesod-auth-0.7.1 depends on yesod-core-0.9.1.1 which failed to install. yesod-core-0.9.1.1 failed during the building phase. The exception was: ExitFailure 7 yesod-form-0.3.1 depends on yesod-core-0.9.1.1 which failed to install. yesod-json-0.2.1 depends on yesod-core-0.9.1.1 which failed to install. yesod-persistent-0.2.1 depends on yesod-core-0.9.1.1 which failed to install. If I keep re-running it, it will eventually succeed. It also always makes forward progress (the next attempt will get past text and a few more packages). It seems to be related to the state of the AFS cache; if all of the required packages are in the local AFS cache it usually just works. If the cache has just been flushed (due to other FS operations), this failure pretty much always shows up. Has anyone else experienced anything like this? Alternatively, does anyone have ideas on getting a more useful error message/tracking it down? I didn't see any relevant bugs filed yet, but I wanted to get more information before adding a report.

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Smarter do notation
by Daniel Peebles 05 Sep '11

05 Sep '11

Hi all, I was wondering what people thought of a smarter do notation. Currently, there's an almost trivial desugaring of do notation into (>>=), (>>), and fail (grr!) which seem to naturally imply Monads (although oddly enough, return is never used in the desugaring). The simplicity of the desugaring is nice, but in many cases people write monadic code that could easily have been Applicative. For example, if I write in a do block: x <- action1 y <- action2 z <- action3 return (f x y z) that doesn't require any of the context-sensitivty that Monads give you, and could be processed a lot more efficiently by a clever Applicative instance (a parser, for instance). Furthermore, if return values are ignored, we could use the (<$), (<*), or (*>) operators which could make the whole thing even more efficient in some instances. Of course, the fact that the return method is explicitly mentioned in my example suggests that unless we do some real voodoo, Applicative would have to be a superclass of Monad for this to make sense. But with the new default superclass instances people are talking about in GHC, that doesn't seem too unlikely in the near future. On the implementation side, it seems fairly straightforward to determine whether Applicative is enough for a given do block. Does anyone have any opinions on whether this would be a worthwhile change? The downsides seem to be a more complex desugaring pass (although still something most people could perform in their heads), and some instability with making small changes to the code in a do block. If you make a small change to use a variable before the return, you instantly jump from Applicative to Monad and might break types in your program. I'm not convinced that's necessary a bad thing, though. Any thoughts? Thanks, Dan

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The maximum/minimum asymmetry
by Mario Blažević 05 Sep '11

05 Sep '11

I was recently surprised to discover that the maximum and maximumBy functions always return the *last* maximum, while minimum and minimumBy return the *first* minimum in the list. The following GHCi session demonstrates this: $ ghci GHCi, version 7.2.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :module +Data.List Data.Ord Prelude Data.List Data.Ord> let list = [(1, 'B'), (1, 'A')] Prelude Data.List Data.Ord> maximumBy (comparing fst) list (1,'A') Prelude Data.List Data.Ord> minimumBy (comparing fst) list (1,'B') I would normally consider this kind of gratuitous asymmetry a bug, but seeing that these functions' implementations have been specified in the Haskell 98 Library Report, I guess they are now a permanent feature of the language. Can anybody explain the reason for this behaviour?

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Idiomatic usage of the fixpoint library
by Roman Cheplyaka 05 Sep '11

05 Sep '11

Hi, I'm looking for an example of idiomatic usage of the fixpoint library[1]. [1]: http://hackage.haskell.org/package/fixpoint-0.1.1 Here's what I managed to get: {-# LANGUAGE TypeFamilies, FlexibleContexts, UndecidableInstances, FlexibleInstances #-} import Data.Fixpoint newtype Expr = Expr { unExpr :: Pre Expr Expr } instance Functor (Pre Expr) => Fixpoint Expr where data Pre Expr a = Add a a | Const Int project = unExpr inject = Expr instance Functor (Pre Expr) where fmap f (Const x) = Const x fmap f (Add x1 x2) = Add (f x1) (f x2) eval = cata eval' where eval' (Const x) = x eval' (Add x1 x2) = x1 + x2 There are some issues with this code, compared to simply using newtype Fix f = In { out :: f (Fix f) } to build an Expr. 1. Since 'Pre' is a data (not type) family, we cannot simply make use of a functor defined elsewhere. We need to define the functor inside the instance declaration (or at least wrap an existing functor). 2. I wasn't able to derive the Functor instance, getting an error Derived instance `Functor (Pre Expr)' requires illegal partial application of data type family Pre In the data type instance declaration for `Pre' 3. Having to use UndecidableInstances makes me feel a bit uncomfortable. This makes me think that the intended usage is somewhat different. So, could someone give an example? -- Roman I. Cheplyaka :: http://ro-che.info/

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Lens with merge (Semigroup)
by Tony Morris 05 Sep '11

05 Sep '11

A regular Lens can be represented as follows: data CoState a = CoState (a -> b) a newtype Lens a b = Lens (a -> CoState b a) I once read about a lens representation that permits a general "merge" operation. I forget where I read it -- I think it was #haskell IRC. However, as I recall, perhaps incorrectly, the merge operation involves a Semigroup[1] and helps to overcome the fact that Lens is not an Arrow and so does not have a (&&&) operation. I am interested to know what this Lens operation is and the associated merge operation. [1] -- Approximate data SemigroupT f a = SemigroupT (a -> a -> f a) -- Tony Morris http://tmorris.net/

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Call for papers: Programming languages meets program verification
by Nikhil Swamy 04 Sep '11

04 Sep '11

PLPV 2012 The Sixth ACM SIGPLAN Workshop Programming Languages meets Program Verification 24th January, 2012 Philadelphia, USA (Affiliated with POPL 2012) http://research.microsoft.com/en-us/um/people/nswamy/plpv12 -------------------------------------------------------------------------------- Overview The goal of PLPV is to foster and stimulate research at the intersection of programming languages and program verification, by bringing together experts from diverse areas like types, contracts, interactive theorem proving, model checking and program analysis. Work in this area typically attempts to reduce the burden of program verification by taking advantage of particular semantic or structural properties of the programming language. Examples include dependently typed programming languages, which leverage a language's type system to specify and check richer than usual specifications or extended static checking systems which incorporate contracts with either static or dynamic contract checking. We invite submissions on all aspects, both theoretical and practical, of the integration of programming language and program verification technology. To encourage cross-pollination between different communities, we seek a broad scope for PLPV. In particular, submissions may have diverse foundations for verification (Type-based, Hoare-logic-based, Abstract Interpretation-based, etc.), target different kinds of programming languages (functional, imperative, object-oriented, etc.), and apply to diverse kinds of program properties (data structure invariants, security properties, temporal protocols, resource constraints, etc.). -------------------------------------------------------------------------------- Important Dates: Submission: 11th October, 2011 Notification: 8th November, 2011 Final Version: 15th November, 2011 Workshop: 24th January, 2012 -------------------------------------------------------------------------------- Submissions We seek submissions of up to 12 pages related to the above topics---shorter submissions are also welcome. Submissions may describe new work, propose new challenge problems for language-based verification techniques, or present a known idea in an elegant way (i.e., a pearl). Submissions should be prepared with SIGPLAN two-column conference format (http://www.sigplan.org/authorInformation.htm) Submitted papers must adhere to the SIGPLAN republication policy (http://www.sigplan.org/republicationpolicy.htm) Concurrent submissions to other workshops, conferences, journals, or similar forums of publication are not allowed. To submit a paper, access the submission site at http://www.easychair.org/conferences/?conf=plpv2012. -------------------------------------------------------------------------------- Publication Accepted papers will be published by the ACM and will appear in the ACM Digital library. -------------------------------------------------------------------------------- Program Committee Amal Ahmed Indiana University Lennart Augustsson Standard Chartered Koen Claessen Chalmers (co-chair) Martin Giese University of Oslo Daniel Licata Carnegie Mellon University Peter MÃ¼ller ETH Zurich Aleksandar Nanevski IMDEA Matthieu Sozeau INRIA Nikhil Swamy Microsoft Research (co-chair) David Walker Princeton University Dana Xu INRIA -------------------------------------------------------------------------------- Previous PLPVs: http://plpv.org

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Final CFP: PADL'12 - Practical Aspects of Declarative Languages 2012
by Claudio Russo 04 Sep '11

04 Sep '11

[Apologies if you receive multiple copies.] Final Call for Papers ===================== 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 Abstract/Paper submission deadline: September 10th/17th, 2011 Conference Description ====================== Declarative languages build on sound theoretical bases to provide attractive frameworks for application development. These languages have been successfully applied to many different real-world situations, ranging from data base management to active networks to software engineering to decision support systems. New developments in theory and implementation have opened up new application areas. At the same time, applications of declarative languages to novel problems raise numerous interesting research issues. Well-known questions include designing for scalability, language extensions for application deployment, and programming environments. Thus, applications drive the progress in the theory and implementation of declarative systems, and benefit from this progress as well. 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, constraints, etc. Topics of interest include, but are not limited to: * Innovative applications of declarative languages * Declarative domain-specific languages and applications * Practical applications of theoretical results * New language developments and their impact on applications * Declarative languages and Software Engineering * Evaluation of implementation techniques on practical applications * Practical experiences and industrial applications * Novel uses of declarative languages in the classroom * Practical extensions such as constraint-based, probabilistic, and reactive languages. PADL'12 welcomes new ideas and approaches pertaining to applications and implementation of declarative languages. In this occasion PADL is co-located, as traditionally, with ACM POPL, which will be held immediately following PADL, January 25-27. The symposium will be held in Philadelphia, Pennsylvania, USA. Important Dates and Submission Guidelines ========================================= Abstract Submission: September 10, 2011 Paper Submission: September 17, 2011 Notification: October 22, 2011 Camera-ready: November 5, 2011 Symposium: January 23-24, 2012 Authors should submit an electronic copy of the full paper in PDF using the Springer LNCS format. The submission will be done through EasyChair conference system. If electronic submission is impossible, please contact the program chairs for information on how to submit hard copies. All submissions must be original work written in English. Submissions must be unpublished and not submitted for publication elsewhere. Work that already appeared in unpublished or informally published workshops proceedings may be submitted. PADL'12 will accept both technical and application papers: * Technical papers must describe original, previously unpublished research results. Technical papers must not exceed 15 pages in Springer LNCS format. * Application papers are a mechanism to present important practical applications of declarative languages that occur in industry or in areas of research other than Computer Science. Application papers will be published in the Springer-Verlag conference proceedings, and will be presented in a separate session. Application papers are expected to describe complex and/or real-world applications that rely on an innovative use of declarative languages. Application descriptions, engineering solutions and real-world experiences (both positive and negative) are solicited. The limit for application papers is 6 pages in Springer LNCS format. Program Committee ================= Marcello Balduccini, Intelligent Systems Department, Kodak Research Labs Edwin Brady, University of St Andrews, Scotland Henning Christiansen, Roskilde University, Denmark Agostino Dovier, University of Udine, Italy Matthew Flatt, University of Utah, USA Gopal Gupta, University of Texas at Dallas, USA John Hughes, Chalmers University of Technology, Sweden; Quviq AB Gabriele Keller, University of New South Wales, Australia Lunjin Lu, Oakland University, USA Marc Pouzet, École normale supérieure, France Ricardo Rocha, University of Porto, Portugal Andreas Rossberg, Google Germany GmbH, Germany Claudio Russo, Microsoft Research Cambridge, UK (co-chair) Kostis Sagonas, Uppsala University, Sweden; NTUA, Greece Satnam Singh, Microsoft Research Cambridge, UK Zoltan Somogyi, University of Melbourne, Australia Eijiro Sumii, Tohoku University, Japan Terrance Swift, Universidade Nova de Lisboa, Portugal; Johns Hopkins University, USA Andrew Tolmach, Portland State University, USA Jan Wielemaker, University of Amsterdam, The Netherlands Roland Yap, National University of Singapore, Republic of Singapore Kwangkeun Yi, Seoul National University, Korea Neng-Fa Zhou, Brooklyn College, City University of New York, USA (co-chair) Contacts ======== For additional information about papers and submissions, please contact the Program Chairs: Claudio Russo Microsoft Research Cambridge,UK Email: crusso <AT> microsoft <DOT> com Neng-Fa Zhou Brooklyn College, The City University of New York, USA Email: zhou <AT> sci <DOT> brooklyn <DOT> cuny <DOT> edu With the Cooperation of ======================= The Association for Logic Programming (ALP) ACM SIGPLAN Microsoft Research, Cambridge ===================================

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Real CMS Application in Haskell
by Haisheng Wu 04 Sep '11

04 Sep '11

Hello guys, I googled for a real open source CMS application in Haskell but have no luck. So I'm wondering if Haskell is used very little in Web development area. Thanks. -Simon

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Enumeratee that generates data
by tsuraan 04 Sep '11

04 Sep '11

I'm trying to write a program whose network behaviour is analogous to a web proxy. A client connects to my server and gives me some data, my server connects to an upstream server and gives data to it, my server gets data from upstream, and gives data to the client. I'd like to write this using the enumerator library, since it seems like it should be possible to do that cleanly. Now, the guts of an echo server are pretty easy: runIteratee $ enumSocket cs sock $$ iterSocket sock That's just plain pretty. You can even put interesting data tranformations (Enumeratees) in there to make it behave a bit differently. What I'd like to do is have an Enumeratee that connects to the upstream server, forwards the request, and then uses one of the Data.Enumerator.List functions to generate a response that is the response from the upstream server. It doesn't look like this is possible though, since Enumeratees are pretty much one output per input. Is my basic model broken here? It's occurred to me that what I actually want to do is tie the enumSocket clientSock to an iterSocket upstreamSock, do the reverse, and then do two runIteratee calls (one in a forkIO, I guess?). That seems uglier, I'm not sure if there would be complications in making sure both Iteratees stop running correctly. Is that the right way to do, or am I just missing some capability that Enumeratees have?

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ANNOUNCE: Hs2lib-0.5.5
by Phyx 04 Sep '11

04 Sep '11

What is it? ======== A preprocessor and library which allow you to create dynamic libs from arbitrary annotated Haskell programs with one click. It also allows you to use the generated lib in C, C++ and C# just by including the generated header files. At a minimum it can be considered the inverse of c2hs. Where to get it? ============ You can get it from Hackage (http://hackage.haskell.org/package/Hs2lib) or by using cabal (cabal install Hs2lib). Documentation, Mailing List, Source, etc ======================= Go to http://mistuke.wordpress.com/category/hs2lib/ for information. Or for a tutorial http://www.scribd.com/doc/63918055/Hs2lib-Tutorial What's New? ========= - Currently Supported: * Autogerates free functions for any StablePtr used. * Added support for memory leak tracing with the --debug flag http://mistuke.wordpress.com/2011/08/09/tracing-lexer-memory-leaks/ * Fixed a bug in the library that caused an error in marshalling * No longer frees strings, in order to prevent heap corruptions on calls from c# * Fixed an issue with lists and type synonyms * Fixed an alignment issue with stdcall * Renamed the hs2c# pragma to hs2cs * Fixed an error in parsing pragmas * Fixed a majot marshalling bug having to do with lists of pointer types * Started on an implementation of an automatic test generator to test exported functions. * Re-arranged the namespaces generated in C#. Now Functions and types are put in different namespaces. - Not Currently supported: * Infix constructors * Control over the autogenerated free functions for StablePtr * Exporting of type/functions with the same name in different modules * Exporting polymorphic functions Details ===== A more detailed documentation will follow in the next few weeks. Why is the first version 0.4.8? -------------- This project has been in development for a very long time. Nearly two years on and off. It was developed mainly to facilitate the creation of Visual Haskell 2010. This is just the first public release of this tool.

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