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ICFP 2012 programming contest
by Kevin Hammond 22 Jun '12

22 Jun '12

jPlease consider putting together a team/encourage your students, friends, colleagues to put together a team for the ICFP 2012 programming contest. Entries can be made in ANY LANGUAGE (including one of your own invention) as long as the submitted program can be run by the judges on a standard Linux environment with no network connection (sorry, Vixo!). The competition will run from 1200 UTC on July 13th for 72 hours. There will also be a lightning (24 hour) division for those who can't dedicate a full 72 hours to the contest. This is an open contest. Anybody may participate except for the contest organisers and members of the same group as the the contest chairs. http://icfpcontest2012.wordpress.com/ Did we mention that the contest is sponsored by Facebook and there will be cash prizes (and no entry fee)! What do you have to lose? Book your holiday time and pizza deliveries now! Best Wishes, Kevin -------- Kevin Hammond, Professor of Computer Science, University of St Andrews T: +44-1334 463241 F: +44-1334-463278 W: http://www.cs.st-andrews.ac.uk/~kh In accordance with University policy on electronic mail, this email reflects the opinions of the individual concerned, may contain confidential or copyright information that should not be copied or distributed without permission, may be of a private or personal nature unless explicitly indicated otherwise, and should not under any circumstances be taken as an official statement of University policy or procedure (see http://www.st-and.ac.uk) The University of St Andrews is a charity registered in Scotland : No SC013532

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ANNOUNCE: set-monad
by George Giorgidze 22 Jun '12

22 Jun '12

I would like to announce the first release of the set-monad library. On Hackage: http://hackage.haskell.org/package/set-monad The set-monad library exports the Set abstract data type and set-manipulating functions. These functions behave exactly as their namesakes from the Data.Set module of the containers library. In addition, the set-monad library extends Data.Set by providing Functor, Applicative, Alternative, Monad, and MonadPlus instances for sets. In other words, you can use the set-monad library as a drop-in replacement for the Data.Set module of the containers library and, in addition, you will also get the aforementioned instances which are not available in the containers package. It is not possible to directly implement instances for the aforementioned standard Haskell type classes for the Set data type from the containers library. This is because the key operations map and union, are constrained with Ord as follows. map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b union :: (Ord a) => Set a -> Set a -> Set a The set-monad library provides the type class instances by wrapping the constrained Set type into a data type that has unconstrained constructors corresponding to monadic combinators. The data type constructors that represent monadic combinators are evaluated with a constrained run function. This elevates the need to use the constraints in the instance definitions (this is what prevents a direct definition). The wrapping and unwrapping happens internally in the library and does not affect its interface. For details, see the rather compact definitions of the run function and type class instances. The left identity and associativity monad laws play a crucial role in the definition of the run function. The rest of the code should be self explanatory. The technique is not new. This library was inspired by [1]. To my knowledge, the original, systematic presentation of the idea to represent monadic combinators as data is given in [2]. There is also a Haskell library that provides a generic infrastructure for the aforementioned wrapping and unwrapping [3]. The set-monad library is particularly useful for writing set-oriented code using the do and/or monad comprehension notations. For example, the following definitions now type check. s1 :: Set (Int,Int) s1 = do a <- fromList [1 .. 4] b <- fromList [1 .. 4] return (a,b) -- with -XMonadComprehensions s2 :: Set (Int,Int) s2 = [ (a,b) | (a,b) <- s1, even a, even b ] s3 :: Set Int s3 = fmap (+1) (fromList [1 .. 4]) As noted in [1], the implementation technique can be used for monadic libraries and EDSLs with restricted types (compiled EDSLs often restrict the types that they can handle). Haskell's standard monad type class can be used for restricted monad instances. There is no need to resort to GHC extensions that rebind the standard monadic combinators with the library or EDSL specific ones. [1] CSDL Blog: The home of applied functional programming at KU. Monad Reification in Haskell and the Sunroof Javascript compiler. http://www.ittc.ku.edu/csdlblog/?p=88 [2] Chuan-kai Lin. 2006. Programming monads operationally with Unimo. In Proceedings of the eleventh ACM SIGPLAN International Conference on Functional Programming (ICFP '06). ACM. [3] Heinrich Apfelmus. The operational package. http://hackage.haskell.org/package/operational

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Why does (++ "!") "bla" return "bla!" and not "!bla"?
by Hilco Wijbenga 22 Jun '12

22 Jun '12

Hi all, I'm going through the excellent http://learnyouahaskell.com tutorial. So far it's been pretty easy to follow but now I ran into something that (when I later started reading about maps) do not seem to fully grasp. I think I'm close to understanding why (++ "!") "bla" returns "bla! instead of "!bla" but I seem to be missing the last step. :-) I noticed that ((++) "!") "bla" does indeed return "!bla". So it seems to be related to the infix property of ++? The types of (++) "!", ((++) "!"), and (++ "!") are all the same so that doesn't tell me much. Would someone please nudge me in the right direction? Cheers, Hilco

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: set-monad
by George Giorgidze 22 Jun '12

22 Jun '12

Hi Derek, Thanks for clarifying your point. You are right that (fromList . toList) = id is a desirable and it holds for Data.Set. But your suggestions that this property does not hold for Data.Set.Monad is not correct. Please check out the repo, I have just pushed a quickcheck definition for this property. With a little bit of effort, one can also prove this by hand. Let me also clarify that Data.Set.Monad exports Set as an abstract data type (i.e., the user cannot inspect its internal structure). Also the run function is only used internally and is not exposed to the users. Cheers, George On 19 June 2012 02:21, Derek Elkins <derek.a.elkins(a)gmail.com> wrote: > > On Jun 18, 2012 4:54 PM, "George Giorgidze" <giorgidze(a)gmail.com> wrote: >> >> Hi Derek, >> >> On 16 June 2012 21:53, Derek Elkins <derek.a.elkins(a)gmail.com> wrote: >> > The law that ends up failing is toList . >> > fromList /= id, i.e. fmap g . toList . fromList . fmap h /= fmap g . >> > fmap h >> >> This is not related to functor laws. The property that you desire is >> about toList and fromList. > > Sorry, I typoed. I meant to write fromList . toList though that should've > been clear from context. This is a law that I'm pretty sure does hold for > Data.Set, potentially modulo bottom. It is a quite desirable law but, as > you correctly state, not required. If you add this (non)conversion, you > will get the behavior to which Dan alludes. > > The real upshot is that Prim . run is not id. This is not immediately > obvious, but this is actually the key to why this technique works. A > Data.Set.Monad Set is not a set, as I mentioned in my previous email. > > To drive the point home, you can easily implement fromSet and toSet. In > fact, they're just Prim and run. Thus, you will fail to have fromSet . > toSet = I'd, though you will have toSet . fromSet = I'd, i.e. run . Prim = > id. This shows that Data.Set.Set embeds into but is not isomorphic to > Data.Set.Monad.Set.

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The use of continuation monad in C++
by Bartosz Milewski 22 Jun '12

22 Jun '12

I published a blog for C++ programmers about the advantages of using the continuation monad in dealing with asynchronous API, concurrency, and parallelism. I explained the concepts in Haskell and the translated them into C++. http://fpcomplete.com/asynchronous-api-in-c-and-the-continuation-monad/ -- [:Bartosz:]

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ByteString.getContents fails for files >2GB on OS X
by Shaun Jackman 21 Jun '12

21 Jun '12

Hi, Data.ByteString.Char8.getContents fails for files >2GB on OS X. Is there a fix for this? $ cat getContents.hs main = getContents $ ./getContents <smallFile $ ./getContents <bigFile getContents: <stdin>: hGetBuf: invalid argument (Invalid argument) $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 Mac OS X 10.7.4 64-bit As a workaround, I used ByteString.Lazy instead of the strict ByteString, which worked, but found it was ~4 times slower for my program, so I'd like to get the strict ByteString working with large files. Cheers, Shaun

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Cannot System,Timeout.timeout Network.accept
by Daniel Hlynskyi 21 Jun '12

21 Jun '12

Hello Cafe. I cannot accept socket connection with timeout (Windows, GHC 7.4,1). Consider following code: > import Network > import System.Timeout > main = withSocketsDo $ do > print ":: Starting server ..." > sock <- listenOn (PortNumber 4010) > res <- timeout 1 (accept sock) > print "Timed out" -- Never output! > sClose sock Documentation on http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-Timeout.h… says:"... Standard I/O functions like ... Network.Socket.accept, ... appear to be blocking, but they really don't because the runtime system uses scheduling mechanisms like select(2) to perform asynchronous I/O, so it is possible to interrupt standard socket I/O or file I/O using this combinator." Neither Network.accept, nor Network.Socket.accept can be timed out is example above. Is it bug?

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Re: [Haskell-cafe] [Haskell] JustHub 'Sherkin' Release
by Chris Dornan 21 Jun '12

21 Jun '12

[This discussion was started on the Haskell mailing list in response to an announcement I made which you can read here http://justhaskell.org/2012/06/13/announce-justhub-sherkin/ At issue is whether the JustHub Haskell distribution for Enterprise Linux and the hub hackage for sandboxing development projects and integrating multiple GHC tool chains is redundant because all of the functionality is covered by the Nix Haskell distribution, allowing as they do multiple Nix Haskell releases to be deployed simultaneously.] (cd = Chris Dornan, al = Andres Löh) cd: > The key feature is the way it integrates the multiple tool chains into cd: > a single virtual tool chain with a sandboxing mechanism provided by cd: > the hub hackage. cd: > I think the Nix distribution might be a natural fit for 'hub' cd: > integration -- I would be happy to work with the Nix people on that cd: > (though it can't be right now). al: If this adds anything in terms of functionality that Nix doesn't already provide, al: then I would be interested to know. From having a quick look at your al: announcements, it's not quite clear though if it does. Hi Adres, >From the original announcement (on the above link): The JustHub distribution is based on the Hub system for sandbox development that allows each work tree to work in its own sandboxed environment with a specific tool chain and a private user-package database. All of the standard Haskell tools inter-operate cleanly and transparently with the sandboxes and operate in the normal way outside of them. Sandboxed environments (hubs) can be shared between work trees as well as being (re)named, annotated, replicated, swapped, archived, locked and removed. Proper package deletion with the option of garbage collecting orphaned code and documentation is also supported. Where is this functionality provided by Nix? These are the 'key features' that I emphasized in the clarification. I have loaded GHC-7.4.1 platform and the GHC-7.0.4 into Nix and it installs all of the ghc drivers into a single bin directory in the user's profile. I am guessing that running `ghc` will generally get you the latest compiler you have installed (7.4.1 in my case); specific releases can be invoked with ghc-7.0.4, etc. This hardly covers all of the above functionality! I think the Nix distribution is excellent and I strongly agree with its functional philosophy -- which I think is the right way to distribute Haskell. Quite related to this (in my mind anyway) are the user-level facilities for managing the package databases that each work tree uses -- the problems that cabal-dev was created to solve. What I have done is to create a system that manages the environment each source work tree uses. If you are in a 2012.2.0.0-based project work tree then the ghc-driver will detect that and invoke the right tools. The 2012.2 platform uses cabal-instal-0.14 and that is what you will get when you invoke cabal in such a work tree. However in work trees based on earlier version of the compiler (e.g., GHC-7.2.2), cabal-install-0.10.2 will be used because cabal-install-0.14.0 doesn't interoperate very well with cabal-0.10 and earlier (see https://github.com/haskell/cabal/issues/932) Also in such a work tree you will get all of the tools that were shipped with the GHC-7.2.2 and all through issuing the usual command 'ghc', 'ghci', 'ghc-pkg', etc). Without some system to help the user invoke the right tools in the right context, having to invoke each version of the compiler explicitly can get awkward to use quite quickly. Think about installing a package into a 7.0.4-based work tree where 'ghc' runs version 7.4.1. Cabal will reach for 7.4.1 unless told otherwise. The only practical way to do this is to build a PATH where the right tools get run on the default commands ('ghc', 'ghc-pkg', etc), or equivalently use intelligent drivers that make sure the right tools get invoked (the method JustHub uses). It is also sometimes helpful to be able to start from the minimal collection of packages that you get with compiler (sans platform packages) and build your collection from there. The JustHub system allows you to do this, even on installations that are working double time as Haskell Platform instantiations. Oftentimes less is more in working out the right package combination! Production Haskell development requires this kind of control over the environment, we have all been doing this for years and it isn't technically difficult. But frankly it sure is tedious, especially when you have worked with something better. And it means that everyone has to hand-build their own developments -- that is wasteful. GHC+cabal do provide an awesome build system but the package management mechanisms aren't as easy to use as they could be. There is no proper mechanism for erasing packages for instance (arguably because it is meaningless -- reinforcing my point) and the mechanisms for managing package databases are inconsistent across the tools (differing between 'cabal' and 'ghc'/ghc-pkg') and not very friendly (requiring the setting up of environment variables and the use of special flags). I have also found it highly useful to be able to name, replicate, archive and share environments between different work trees All of this together (including the missing bits) can make for quite a steep learning curve for those gearing up for writing production Haskell code, and it can get in the way to a surprising degree even for those that have been doing it for a while (or at least that has been my experience). You may dismiss that as pointless or irrelevant but it _is_ a large part of what I am trying get right and it did feature prominently in the announcement. I can see elements of this present in the Nix distribution and cabal-dev and similar tools. But our current standard tools don't make it easy for a developers to unpack a production work tree, ensure the relevant tool chain is installed and just work with it in the precise configuration it needs; each developer has to solve that for themselves. I think the Nix functional approach to distributions is definitely going in the right direction in giving user better control over which packages are installed but, unless I have missed something, I don't think it is so definitive that there isn't room for other developments. Finally a couple of gripes with Nix as a replacement for JustHub. JustHub has a special emphasis on Enterprise Linux (RHEL/CentOS/SL) developers. Because the EL family of O/Ses have such long lifetimes for each O/S release they pose special problems. The GCC tools shipped with RHEL5 are just too long in the tooth for use with recent versions of GHC, which is why JustHub ships with GCC-4.6.1 and binutils-2.21. As we have seen the Nix Linux package doesn't work with CentOS 5 and I have tried to get the Nix distribution loaded onto an up-to-date CentOS 6 system. As far as I could tell that required building the package manager from source but the build failed with an opaque libtool-related message. Because the RPMs are available for Fedora that was much easier to use. Also I could not determine which releases of GHC you are supporting. It looks as the distribution is focused on the Haskell Platform, in which case only those releases of GHC will be available, when they get scheduled for inclusion in the platform. This is a more selective collection than I am aiming to provide with JustHub, but perhaps more importantly, it makes it hard for anyone trying to get hold of an older release of the compiler to find it and load it into Nix. Chris

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The Layout Rule
by Michael D. Adams 21 Jun '12

21 Jun '12

I am looking for background material on how GHC and other Haskell compilers implement the layout rule. Are there any papers, documentation, commentary, etc. that discus the actual implementation of this rule (even if only a paragraph or two)? I've already looked at the parsing code in GHC and UHC. Do any other Haskell compilers have interesting approaches for implementing the layout rule? I am writing a paper about a new formalism for indentation sensitive languages and I want to ensure I've covered the appropriate background material on existing implementations of the layout rule. Michael D. Adams mdmkolbe(a)gmail.com

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Haskell Weekly News: Issue 232
by Daniel Santa Cruz 21 Jun '12

21 Jun '12

Welcome to issue 232 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 10 to 16, 2012. Quotes of the Week * irene-knapp: ewtoombs: the universe is already an interactive quantum physics simulator * Philipp: "While in Python every problem can be solved with a big enough Hashtable, in Haskell, every problem can be solved with an additional type parameter." * kulin: man, i am loving this existential quantification, i feel like now i will use it incorrectly to solve every problem for sure * mekeor: xmonad is awesome (ignore the ambiguity, please) Top Reddit Stories * Waldo - the Haskell powered codebase behind xkcd’s Umwelt comic Domain: self.haskell, Score: 75, Comments: 16 On Reddit: [1] http://goo.gl/M2fGv Original: [2] http://goo.gl/M2fGv * “ARM is now a fully supported target platform for GHC, complete with GHCi support” Domain: haskell.org, Score: 60, Comments: 17 On Reddit: [3] http://goo.gl/O0XK3 Original: [4] http://goo.gl/0Fajp * 3D Mandelbrot in OpenGL from imperative to functional style. Domain: yannesposito.com, Score: 36, Comments: 8 On Reddit: [5] http://goo.gl/5MKK1 Original: [6] http://goo.gl/nzpvl * ANN: clckwrks - a Haskell blogging and CMS framework Domain: groups.google.com, Score: 35, Comments: 21 On Reddit: [7] http://goo.gl/ROrHt Original: [8] http://goo.gl/WneBw * web server C10k benchmark: snap vs. erlang, go, node.js, java Domain: github.com, Score: 35, Comments: 5 On Reddit: [9] http://goo.gl/r1Q1j Original: [10] http://goo.gl/eFlTm * GADTs Domain: haskellforall.com, Score: 33, Comments: 74 On Reddit: [11] http://goo.gl/B41Ef Original: [12] http://goo.gl/rcL6I * Announcing Elm 0.2: Haskell integration (Yesod, HAppStack, and Snap), better error messages, larger examples, and more. Domain: self.haskell, Score: 30, Comments: 8 On Reddit: [13] http://goo.gl/ENBPH Original: [14] http://goo.gl/ENBPH * command line options using applicative without Template Haskell Domain: github.com, Score: 28, Comments: 13 On Reddit: [15] http://goo.gl/zqKGw Original: [16] http://goo.gl/eVSAb * Haskell and Yesod - Book on writing web applications in Haskell Domain: shop.oreilly.com, Score: 28, Comments: 5 On Reddit: [17] http://goo.gl/0RdvE Original: [18] http://goo.gl/9qCEu Top StackOverflow Questions * Tying the Knot with a State monad votes: 33, answers: 3 Read on SO: [19] http://goo.gl/cMcRf * Lazy evaluation of terms in an infinite list in Haskell votes: 32, answers: 4 Read on SO: [20] http://goo.gl/IyRJ1 * What is going on with the types in this ghci session? votes: 17, answers: 1 Read on SO: [21] http://goo.gl/3RVnN * the seq function and strictness votes: 14, answers: 2 Read on SO: [22] http://goo.gl/f8u9G * Is it possible to “hide” language extensions? votes: 11, answers: 2 Read on SO: [23] http://goo.gl/q4JQx * Rebinding do notation for indexed monads votes: 11, answers: 1 Read on SO: [24] http://goo.gl/Z4lnx * Haskell: “how much” of a type should functions receive? and avoiding complete “reconstruction” votes: 9, answers: 1 Read on SO: [25] http://goo.gl/2FFHu * Intelligent purely functional sets votes: 8, answers: 1 Read on SO: [26] http://goo.gl/I7Tzb * Is there a better way to express the absolute error function in point-free notation? votes: 8, answers: 2 Read on SO: [27] http://goo.gl/FFFek * GADTs vs. MultiParamTypeClasses votes: 7, answers: 2 Read on SO: [28] http://goo.gl/3DYKI Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/uved7/waldo_the_haskell_powered_co… 2. http://www.reddit.com/r/haskell/comments/uved7/waldo_the_haskell_powered_co… 3. http://www.haskell.org/pipermail/haskell-cafe/2012-June/101704.html 4. http://www.reddit.com/r/haskell/comments/uv07c/arm_is_now_a_fully_supported… 5. http://yannesposito.com/Scratch/en/blog/Haskell-OpenGL-Mandelbrot/ 6. http://www.reddit.com/r/haskell/comments/v4pzk/3d_mandelbrot_in_opengl_from… 7. https://groups.google.com/forum/#!msg/clckwrks/58AwDSRRGoI/prrMP6fhtqMJ 8. http://www.reddit.com/r/haskell/comments/uyr0y/ann_clckwrks_a_haskell_blogg… 9. https://github.com/ericmoritz/wsdemo/blob/results-v1/results.md 10. http://www.reddit.com/r/haskell/comments/v595l/web_server_c10k_benchmark_sn… 11. http://www.haskellforall.com/2012/06/gadts.html 12. http://www.reddit.com/r/haskell/comments/v5ewa/gadts/ 13. http://www.reddit.com/r/haskell/comments/uugne/announcing_elm_02_haskell_in… 14. http://www.reddit.com/r/haskell/comments/uugne/announcing_elm_02_haskell_in… 15. https://github.com/pcapriotti/optparse-applicative 16. http://www.reddit.com/r/haskell/comments/uw7eq/command_line_options_using_a… 17. http://shop.oreilly.com/product/0636920023142.do 18. http://www.reddit.com/r/haskell/comments/v64f5/haskell_and_yesod_book_on_wr… 19. http://stackoverflow.com/questions/11060565/tying-the-knot-with-a-state-mon… 20. http://stackoverflow.com/questions/10972429/lazy-evaluation-of-terms-in-an-… 21. http://stackoverflow.com/questions/11003535/what-is-going-on-with-the-types… 22. http://stackoverflow.com/questions/11046590/the-seq-function-and-strictness 23. http://stackoverflow.com/questions/11035676/is-it-possible-to-hide-language… 24. http://stackoverflow.com/questions/11042685/rebinding-do-notation-for-index… 25. http://stackoverflow.com/questions/10967832/haskell-how-much-of-a-type-shou… 26. http://stackoverflow.com/questions/10971237/intelligent-purely-functional-s… 27. http://stackoverflow.com/questions/11006772/is-there-a-better-way-to-expres… 28. http://stackoverflow.com/questions/10994508/gadts-vs-multiparamtypeclasses

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