- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] transformers problem: Could not deduce MonadTrans (StateT s) ??
by Anton Kholomiov 23 Jun '12

23 Jun '12

No, it wants me to define an instance for (StateT s) which is supposed to be defined be the authors of the library. Actually I discovered that I have two libraries called transformers. transformers-0.2.2.0 transformers-0.3.0.0 So when I'm doing import Control.Monad.Trans (0.2.2.0) I get the error And when I'm doing import Control.Monad.Trans.Class (0.3.0.0) It compiles. Anton

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transformers problem: Could not deduce MonadTrans (StateT s) ??
by Anton Kholomiov 23 Jun '12

23 Jun '12

Why this function doesn't compile? phi :: Monad m => StateT s m () phi = lift $ return () I get (ghc-7.4.1) Could not deduce (MonadTrans (StateT s)) arising from a use of `lift' from the context (Monad m) bound by the type signature for phi :: Monad m => StateT s m () at FSMt.hs:28:1-22 Possible fix: add (MonadTrans (StateT s)) to the context of the type signature for phi :: Monad m => StateT s m () or add an instance declaration for (MonadTrans (StateT s)) In the expression: lift In the expression: lift $ return () In an equation for `phi': phi = lift $ return () Failed, modules loaded: none. Anton

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[ANN] GenCheck - a generalized property-based testing framework
by Jacques Carette 23 Jun '12

23 Jun '12

Test.GenCheck is a Haskell library for /generalized proposition-based testing/. It simultaneously generalizes *QuickCheck* and *SmallCheck*. Its main novel features are: * introduces a number of /testing strategies/ and /strategy combinators/ * introduces a variety of test execution methods * guarantees uniform sampling (at each rank) for the random strategy * guarantees both uniqueness and coverage of all structures for the exhaustive strategy * introduces an /extreme/ strategy for testing unbalanced structures * also introduces a /uniform/ strategy which does uniform sampling along an enumeration * allows different strategies to be mixed; for example one can exhaustively test all binary trees up to a certain size, filled with random integers. * complete separation between properties, generators, testing strategies and test execution methods The package is based on a lot of previous research in combinatorics (combinatorial enumeration of structures and the theory of Species), as well as a number of established concepts in testing (from a software engineering perspective). In other words, further to the features already implemented in this first release, the package contains an extensible, general framework for generators, test case generation and management. It can also be very easily generalized to cover many more combinatorial structures unavailable as Haskell types. The package also provides interfaces for different levels of usage. In other words, there is a 'simple' interface for dealing with straightforward testing, a 'medium' interface for those who want to explore different testing strategies, and an 'advanced' interface for access to the full power of GenCheck. See http://hackage.haskell.org/package/gencheck for further details. In the source repository (https://github.com/JacquesCarette/GenCheck) the file tutorial/reverse/TestReverseList.lhs shows the simplest kinds of tests (standard and deep for structures, or base for unstructured types) and reporting (checking, testing and full report) for the classical list reverse function. The files in tutorial/list_zipper show what can be done with the medium level interface (this tutorial is currently incomplete). The brave user can read the source code of the package for the advanced usage -- but we'll write a tutorial for this too, later. User beware: this is gencheck-0.1, there are still a few rough edges. We plan to add a Template Haskell feature to this which should make deriving enumerators automatic for version 0.2. Jacques and Gordon

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Question on proper use of Data.IORef
by Captain Freako 22 Jun '12

22 Jun '12

Hi experts, I fear I don't understand how to properly use *Data.IORef*. I wrote the following code: 1 -- Testing Data.IORef 2 module Main where 3 4 import Data.IORef 5 6 bump :: IORef Int -> IO() 7 bump theRef = do 8 tmp <- readIORef theRef 9 let tmp2 = tmp + 1 10 writeIORef theRef tmp2 11 12 main = do 13 let theValue = 1 14 print theValue 15 theValueRef <- newIORef theValue 16 bump theValueRef 17 return theValue and got this, in ghci: *Main> :load test2.hs [1 of 1] Compiling Main ( test2.hs, interpreted ) Ok, modules loaded: Main. *Main> main 1 *1* I was expecting this: *Main> :load test2.hs [1 of 1] Compiling Main ( test2.hs, interpreted ) Ok, modules loaded: Main. *Main> main 1 *2* Can anyone help me understand what I'm doing wrong? Thanks! -db

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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 22 Jun '12

22 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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Re: [Haskell-cafe] ANNOUNCE : Leksah 0.12.1.2 (fixes some metadata issues)
by Hilco Wijbenga 21 Jun '12

21 Jun '12

On 20 June 2012 18:22, Oliver Batchelor <saulzar(a)gmail.com> wrote: > Check the error given by cairo. > > cabal install cairo-0.12.3.1 Well, it wasn't cairo that was the problem but ... > You probably need to put the .cabal/bin in your PATH, so that the > tools such as c2hs can run. ... indeed, adding .cabal/bin to my PATH allowed the install to finish successfully. Why doesn't cabal abort when a package fails to install? Why doesn't it die if .cabal/bin isn't on the PATH? (Surely, that's easy to test for?)

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relocation R_X86_64_PC32 against undefined symbol
by Henning Thielemann 20 Jun '12

20 Jun '12

Just for the record: I compiled with GHC and got the linker error: /usr/bin/ld: dist/build/.../Module.dyn_o: relocation R_X86_64_PC32 against undefined symbol `..._xyz1_closure' can not be used when making a shared object; recompile with -fPIC Problem was that I forgot to declare Module in the Cabal description.

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[Word8] -> FilePath using ghc's file system encoding
by Joey Hess 20 Jun '12

20 Jun '12

I found myself writing the code below today, and thought I'd write to see if there's a better way, perhaps one that doesn't use unsafePerformIO. A sample use case for this is a program that reads a FilePath from a Handle, and then operates on that file on disk. GHC uses a special encoding for FilePaths that cleanly roundtrips invalid utf8, but for that to work, the FilePath must be read using that encoding too. Neglecting to do that would result in a program that worked for most filenames, but failed on edge case non-utf8 encoded filenames. Another way to deal with this problem is to first hSetEncoding using GHC.IO.Encoding.getFileSystemEncoding, and then read from the Handle using hGetLine, which applies the encoding to the data it reads. However, in this case I needed to use ByteString to read from the Handle, and so needed a way to convert from a ByteString to a FilePath. (Using RawFilePath would be another approach, but reworking my program to use it isn't practical.) Anyway, it seems there should be a pure way to encode a string with GHC's filesystem encoding. ----- import qualified GHC.Foreign as GHC import qualified GHC.IO.Encoding as Encoding import System.IO.Unsafe import Data.Bits.Utils -- from MissingH {- Converts a [Word8] to a FilePath, encoding using the filesystem encoding. - - w82c produces a String, which may contain Chars that are invalid - unicode. From there, this is really a simple matter of applying the - file system encoding, only complicated by GHC's interface to doing so. -} {-# NOINLINE encodeW8 #-} encodeW8 :: [Word8] -> FilePath encodeW8 w8 = unsafePerformIO $ do enc <- Encoding.getFileSystemEncoding GHC.withCString Encoding.char8 (w82s w8) (GHC.peekCString enc) -- see shy jo

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Problem with plugins
by Timo von Holtz 20 Jun '12

20 Jun '12

Hi, I'm currently working on extending the hascat Server. My problem is, that for whatever odd reason it will only work on GHC < 7.0 or alternatively if I execute it with runghc or in ghci. If I compile it with GHC>=7.0 and execute it, then I get this: $ ~/.cabal/bin/hascat config.xml Installing "Root" at / hascat: /home/tvh/.cabal/lib/plugins-1.5.2.1/ghc-7.4.1/HSplugins-1.5.2.1.o: unknown symbol `ghczm7zi4zi1_ErrUtils_zdsinsertzugo3_info' hascat: unloadObj: can't find `/usr/lib/ghc/binary-0.5.1.0/HSbinary-0.5.1.0.o' to unload user error (unloadObj: failed) Installing "Hascat Server Info" at /ServerInfo/ hascat: /home/tvh/.cabal/lib/hascat-system-0.2/ghc-7.4.1/HShascat-system-0.2.o: unknown symbol `base_DataziMaybe_Nothing_closure' hascat: unloadObj: can't find `/usr/lib/ghc/Cabal-1.14.0/HSCabal-1.14.0.o' to unload user error (unloadObj: failed) Installing "Hascat Application Manager" at /Manager/ hascat: /home/tvh/.cabal/lib/hascat-system-0.2/ghc-7.4.1/HShascat-system-0.2.o: unknown symbol `base_DataziMaybe_Nothing_closure' hascat: unloadObj: can't find `/usr/lib/ghc/Cabal-1.14.0/HSCabal-1.14.0.o' to unload user error (unloadObj: failed) Installing "Hascat Application Deployer" at /Deployer/ hascat: /usr/lib/haskell-packages/ghc/lib/zlib-0.5.3.3/ghc-7.4.1/HSzlib-0.5.3.3.o: unknown symbol `base_GHCziForeignPtr_ForeignPtr_con_info' hascat: unloadObj: can't find `/usr/lib/ghc/Cabal-1.14.0/HSCabal-1.14.0.o' to unload user error (unloadObj: failed) Waiting for connections on port 8012 Is there a way to make this work? Greetings Timo

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Re: [Haskell-cafe] Again, version conflicting problem with cabal-install
by Jason Dagit 20 Jun '12

20 Jun '12

In my experience the diamond of death is typically because: * You install package A that uses C-0.1 * Someone uploads C-0.2 to hackage * Later you 'cabal update', this does not rebuild A to use C-0.2, even though it could. * You install package B that uses C, and cabal builds it with C-0.2 * Now you want to build D that uses A and B, but A requires C-0.1 and B requires C-0.2, even though they could be rebuilt to use the same C. Typically, the constraints on A, B, and D would all accept the same version of C. Except that when they were compiled separately and C changed between building A and B, then the compiled versions become fixed on C-0.1 and C-0.2. It's true that it won't solve it in all cases, but in my experience using cabal-dev made the problem go away. I hope that clarifies. Jason On Fri, Feb 3, 2012 at 7:33 AM, Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> wrote: > Well, cabal-dev could not resolve the conflict of "the diamond". > Because the conflict is "depending different version at the SAME > time". > > On Fri, Feb 3, 2012 at 9:20 PM, Jason Dagit <dagitj(a)gmail.com> wrote: >> On Thu, Feb 2, 2012 at 9:54 PM, Magicloud Magiclouds >> <magicloud.magiclouds(a)gmail.com> wrote: >>> Hello, >>> As I recalled, ghc works in staticly link mode. So after one library >>> is compiled, all its build dependencies are useless. Lost, changed, >>> wheresoever, it does not matter. >>> Then why the problem of version conflicting exists? >> >> I'm not sure, but for me this problem has gone away by using cabal-dev >> to build everything. >> >> Jason > > > > -- > 竹密岂妨流水过 > 山高哪阻野云飞 > > And for G+, please use magiclouds#gmail.com.

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Lazy producing a list in the strict ST monad
by Nicu Ionita 20 Jun '12

20 Jun '12

Hi, I'm trying to produce a list in the strict ST monad. The documentation of ST says that the monad is strict in the state, but not in the values. So I expect that, when returning a list, I get back only the Cons (with 2 unevaluated thunks). Now, when I need the first element (head), this will be evaluated (with whatever actions are necessary in the ST universe) and the tail is again a Cons with unevaluated parts. Internally my list is stored in a vector, and the elements are generated phasewise, each phase generating 0 or more elements in the vector, and a fuction splitMove is driving this process (see code below). I would expect that the first phase triggers, generates some moves, then (after these are consumed from the list) the next phase triggers generating the next few moves and so on. But when I trace the phases (Debug.Trace.trace) I get all the trace messages in front of the first move: Moves for fen: rnbqkbnr/pp3ppp/4p3/2pp4/3P4/2NQ4/PPP1PPPP/R1B1KBNR w After move generation... 0 >= 0 : next phase 3 >= 3 : next phase 3 >= 3 : next phase 42 >= 42 : next phase 44 >= 44 : next phase d4c5 g1f3 g1h3 c3b1 ... This seems not to be just an unhappy combination between trace and ST, as also the program without trace is beeing slower than the same implemented with plain lists, which is hard to believe (in many cases the move list is not consumed to the end). I wonder if my expectation is wrong, but I don't find a way to do this. Here is the (incomplete) code: produceList ... = runST $ do ml <- newMList ... listMoves ml -- Transforms a move list to a list of moves - lazy listMoves :: MList s -> ST s [Move] listMoves ml = do sm <- splitMove ml case sm of Just (m, ml') -> do rest <- listMoves ml' return $ m : rest Nothing -> return [] -- Split the first move from the move list and return it together with -- the new move list (without the first move). Return Nothing if there -- is no further move splitMove :: MList s -> ST s (Maybe (Move, MList s)) splitMove ml | mlToMove ml >= mlToGen ml = do mml <- trace trm $ nextPhase ml case mml of Nothing -> return Nothing Just ml' -> splitMove ml' | otherwise = do m <- U.unsafeRead (mlVec ml) (mlToMove ml) case mlCheck ml ml m of Ok -> return $ Just (m, ml1) Skip -> splitMove ml1 Delay -> splitMove ml1 { mlBads = m : mlBads ml } where ml1 = ml { mlToMove = mlToMove ml + 1 } trm = show (mlToMove ml) ++ " >= " ++ show (mlToGen ml) ++ " : next phase"

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Re: [Haskell-cafe] [Haskell-beginners] Function application versus function composition performance
by KC 20 Jun '12

20 Jun '12

A good functional programming language has a good "code algebra" after parsing to which algebraic transformations can be applied for optimization. For example, reducing the need for generating intermediate data structures. See: fusion. On Tue, Jun 19, 2012 at 3:01 PM, Matt Ford <matt(a)dancingfrog.co.uk> wrote: > Hi All, > > My last question got me thinking (always dangerous): an expression > that doubles a list and takes the 2nd element (index 1), I assume, > goes through the following steps. > > double (double [1,2,3,4]) !! 1 > double ( 1*2 : double [2,3,4]) !! 1 > 1*2*2 : double ( double [2,3,4] ) !! 1 > 1*2*2 : double ( 2*2 : double [3,4] ) !! 1 > 1*2*2 : 2*2*2 : double ( double [3,4] ) !! 1 > 2*2*2 > 8 > > Up until the element requested all the proceeding elements have to > have the expression formed as Haskell process the calculation. > > Now, I want to compare this to using function composition > > ( double . double ) [ 1 ,2, 3, 4 ] !! 1 > > This is the bit I'm unsure of - what does the composition look like. > It is easy to see that it could be simplified to something like: > > ( double . double) (x:xs) = x*4 : (double . double) xs > > This would mean the steps could be written > > (double . double) [ 1,2,3,4 ] !! 1 > (1*4 : (double.double) [2,3,4]) !! 1 > (1*4 : 2*4 : (double.double) [ 3,4 ]) !! 1 > 2*4 > 8 > > Ignoring the start and end steps, it will be half the number of steps > compared to the application version. Significant then, over long > lists. > > So is this true, are composite functions simplified in Haskell in > general so that they may have improved performance over function > application? > > I've seen posts on the internet that say it's a matter of style only: > > http://stackoverflow.com/questions/3030675/haskell-function-composition-and… > . > But my reasoning suggests it could be more than that. > > Perhaps, function application is similarly optimised - maybe by > replacing all functions applications with composition and then > simplifying? Or maybe the simplifying/optimisation step never > happens? As you can see I'm just guessing at things :-) But it's > nice to wonder. > > Many thanks for any pointers, > > Matt. > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > -- -- Regards, KC

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Haskell Implementors' Workshop 2012, Second CFT
by Johan Tibell 19 Jun '12

19 Jun '12

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 Copenhagen, Denmark, September 14th, 2012 The workshop will be held in conjunction with ICFP 2012 http://www.icfpconference.org/icfp2012/ Important dates Proposal Deadline: 10th July 2012 Notification: 27th July 2012 Workshop: 14th September 2012 The Haskell Implementors' Workshop is to be held alongside ICFP 2012 this year in Copenhagen, Denmark. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2012. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop, we hope to study the underlying technology. We want to bring together anyone interested in the nitty-gritty details behind turning plain-text source code into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 minute talks with 10 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to: johan.tibell(a)gmail.com We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Lennart Augustsson (Standard Chartered Bank) * Manuel M T Chakravarty (University of New South Wales) * Gregory Collins - co-chair (Google) * Simon Marlow (Microsoft Research) * David Terei (Stanford University) * Johan Tibell - co-chair (Google)

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Call for Papers: STVR Special Issue on Tests and Proofs
by Achim D. Brucker 19 Jun '12

19 Jun '12

Apologies for duplicates. CALL FOR PAPERS STVR Special Issue on Tests and Proofs http://lifc.univ-fcomte.fr/tap2012/stvr/ The Software Testing, Verification & Reliability (STVR) journal (http://www3.interscience.wiley.com/journal/13635/home) invites authors to submit papers to a Special Issue on Tests and Proofs. Background ========== The increasing use of software and the growing system complexity make focused software testing a challenging task. Recent years have seen an increasing industrial and academic interest in the use of static and dynamic analysis techniques together. Success has been reported combining different test techniques such as model-based testing, structural testing, or concolic testing with static techniques such as program slicing, dependencies analysis, model-checking, abstract interpretation, predicate abstraction, or verification. This special issue serves as a platform for researchers and practitioners to present theory, results, experience and advances in Tests and Proofs (TAP). Topics ====== This special issue focuses on all topics relevant to TAP. In particular, the topics of interest include, but are not limited to: * Program proving with the aid of testing techniques * New challenges 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, SAT and SMT solving * Model-based testing and verification * Automatic bug finding * Debugging of programs combining static and dynamic analysis * Transfer of concepts from testing to proving (e.g., coverage criteria) and from proving to testing * Formal frameworks for test and proof * Tool descriptions, experience reports and evaluation of test and proof * 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 * The processes, techniques, and tools that support test and proof Submission Information ====================== The deadline for submissions is 17th December, 2012. Notification of decisions will be given by April 15th, 2013. All submissions must contain original unpublished work not being considered for publication elsewhere. Original extensions to conference papers - identifing clearly additional contributions - are also encouraged unless prohibited by copyright. Submissions will be refereed according to standard procedures for Software Testing, Verification and Reliability. Please submit your paper electronically using the Software Testing, Verification & Reliability manuscript submission site. Select "Special Issue Paper" and enter "Tests and Proofs" as title. Important Dates: ================ * Paper submission: December 17, 2012 * Notification: April 15, 2013 Guest Editors ============= * Achim D. Brucker, SAP Research, Germany http://www.brucker.ch/ * Wolfgang Grieskamp, Google, U.S.A. http://www.linkedin.com/in/wgrieskamp * Jacques Julliand, University of Franche-Comté, France http://lifc.univ-fcomte.fr/page_personnelle/accueil/8 -- Dr. Achim D. Brucker, SAP Research, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch

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How do people still not understand what FP is about? What are we doing wrong?
by Ben Kolera 18 Jun '12

18 Jun '12

Saw this float by in twitter, and it made me a bit sad. Obviously this is still a large misunderstanding of FP in the larger programming community and it make me wonder what we FP enthusiasts are doing wrong to not get the message out to people. "Programming languages that require random senseless voodoo to get an effect are awesome. Let's make programming hard through poor design." [1] The sad thing about this is that the inverse of this has more truth to it; that "languages that allow people to intersperse side effects anywhere in their computation without thought are flawed by design and allow programmers to do stupid things that hinder the composability, thread safety and ability to reason of/about their code". Has anyone had any experience / success with convincing people that the "senseless voodoo" is actually a boon rather than a bane? Is it even worth trying to convince people so set in their ways? ( Sorry if this is even too off-topic for the cafe. Just needed a place to vent my frustration at this. ) Cheers, Ben [1] https://twitter.com/cwestin63/status/214793627170390018

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: set-monad
by George Giorgidze 18 Jun '12

18 Jun '12

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. The toList and fromList functions are not required to satisfy toList . fromList /= id. This does not hold for Data.Set and it does not hold for the Data.Set.Monad wrapper either. Duplicates are lost when lists are converted into sets. Other instances of fromList and toList also fail to satisfy the property that you desire (e.g., Map). Cheers, George

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Re: [Haskell-cafe] Haskell (GHC 7) on ARM
by Ben Gamari 18 Jun '12

18 Jun '12

Joshua Poehls <joshua(a)poehls.me> writes: > Hello Ben, > Hello, Sorry for the latency. I'm currently on vacation in Germany so I haven't had terribly consistent Internet access. I've Cc'd haskell-cafe@ as I've been meaning to document my experiences anyways and your email seems like a good excuse to do this. > I just got a Raspberry Pi and I'm interested in running Haskell on it. So > far I'm running Arch Linux ARM and I noticed there is is a GHC package > available, but it is version 6.12.3. > > Doing research online I saw that you did some work on adding GHCi support > for ARM in GHC 7.4. I was wondering if you had anything to share related to > running GHC/GHCI 7.4 on ARM, specifically Arch Linux ARM. > Indeed my ARM linker patch was merged into GHC 7.4.2. This means that ARM is now a fully supported target platform for GHC, complete with GHCi support (which is pretty much necessary for many ubuiquitous packages). My ARM development environment consists of a PandaBoard (and before this a BeagleBoard) running Linaro's Ubuntu distribution. For this reason, I'm afraid I won't be of terribly much help in the case of Arch. That being said, GHC has a wonderful build system which has never given me trouble (except in cases of my own stupidity). Just installing the required development packages and following the build instructions on Wiki[1] is generally quite sufficient. I've also have some notes of my own[2] where I've collected my own experiences. As you likely know, GHC doesn't have a native ARM code generator of its own, instead relying on LLVM for low-level code generation. For this reason, you'll first need to have a working LLVM build. I have found that the ARM backend in the 3.0 release is a bit buggy and therefore generally pull directly from git (HEAD on my development box is currently sitting at 7750ff1e3cbb which seems to be stable). That being said, I haven't done much work in this space recently and it's quite likely that the 3.1 release is better in this respect. Compiling LLVM is one case where I have found the limited memory of my BeagleBoard (which is I believe is similar to the Raspberry Pi) to be quite troublesome. There are cases during linking where over a gigabyte of memory is necessary. At this stage the machine can sit for hours thrashing against its poor SD card making little progress. For this reason, I would strongly recommend that you cross-compile LLVM. The process is pretty straightforward and I have collected some notes on the matter[3]. The size of GHC means that in principle one would prefer to cross-compile. Unfortunately, support[4] for this is very new (and perhaps not even working/complete[5]?) so we have to make due with what we have. The build process itself is quite simple. You will want to be very careful to ensure that the ghc/libraries/ trees are sitting on the same branch as your ghc/ tree (with the ghc/sync-all script). Inconsistency here can lead to very ambiguous build failures. Otherwise, the build is nearly foolproof (insofar as this word can describe any build process). The build takes the better part of a day on my dual-core Pandaboard. I can't recall what the maximum memory consumption was but I think it might fit in 512MB. Lastly, if I recall correctly, you will find that you'll need to first build GHC 7.0 bootstrapped from 6.12 before you can build 7.4 (which requires a fairly new stage 0 GHC). Let the list know if you encounter any issues. I'll try to dust off my own development environment once I get back to the states next week to ensure that everything still works. I've been meaning to setup the PandaBoard as a build slave as Karel's has been failing for some time now (perhaps you could look into this, Karel?). Moreover, perhaps I can find a way to produce redistributable binaries to help others get started. Cheers, - Ben [1] http://hackage.haskell.org/trac/ghc/wiki/Building [2] http://bgamari.github.com/posts/ghc-llvm-arm.html [3] http://bgamari.github.com/posts/cross-compiling_llvm.html [4] http://hackage.haskell.org/trac/ghc/wiki/CrossCompilation [5] http://www.haskell.org/pipermail/cvs-ghc/2012-February/070791.html

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Single board computers on HaskellWiki
by Henk-Jan van Tuyl 18 Jun '12

18 Jun '12

L.S., As more and more people start to develop software in Haskell for single board computers, I created the HaskellWiki pages Arduino[0], ARM[1], Raspberry Pi[2], BeagleBoard[3] and PandaBoard[4]. I hope experts on the matter will give these pages more body (for example, describe what is necessary to get Haskell programs running on these platforms) and add links to relevant web pages. The PandaBoard page needs an image, I couldn't find one with public domain license. If you don't know how to edit HaskellWiki pages, read the contribute page[5] first. To get your appetite wet, if you are not involved in this yet, take a look at the Arduino Weather Clock[6]; this is really a fun project. Regards, Henk-Jan van Tuyl [0] http://www.haskell.org/haskellwiki/Arduino [1] http://www.haskell.org/haskellwiki/ARM [2] http://www.haskell.org/haskellwiki/Raspberry_Pi [3] http://www.haskell.org/haskellwiki/BeagleBoard [4] http://www.haskell.org/haskellwiki/PandaBoard [5] http://www.haskell.org/haskellwiki/HaskellWiki:Contributing [6] http://www.geeky-gadgets.com/arduino-weather-clock-13-01-2010/ -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Receiving UDP messages on Windows 7
by Edward Amsden 18 Jun '12

18 Jun '12

Hi all, I have the following program, which I'm running using runghc 7.4.1 with HP2012.2 on Windows 7: == {-# LANGUAGE OverloadedStrings #-} module Main where import Network.Socket hiding (send, sendTo, recv, recvFrom) import Network.Socket.ByteString import qualified Data.Text as T import qualified Data.Text.Encoding as T import qualified Data.Text.IO as T port :: String port = show (5555 :: Int) main :: IO () main = withSocketsDo $ do addrInf:_ <- fmap (filter ((== AF_INET) . addrFamily)) $ getAddrInfo (Just (defaultHints {addrFlags = [AI_PASSIVE]})) Nothing (Just port) putStrLn "Address info: " print addrInf sock <- socket (addrFamily addrInf) Datagram defaultProtocol putStrLn "Socket created" bindSocket sock (addrAddress addrInf) putStrLn "Socket bound" let procMessages = do (msg, addr) <- recvFrom sock 1024 let addrTxt = T.pack $ show addr msgTxt = T.decodeUtf8 msg outputTxt = T.concat [addrTxt, " says ", msgTxt] T.putStrLn outputTxt procMessages procMessages == I'm trying to receive incoming UDP packets on port 5555. Unfortunately, when I run the program it does not receive packets. It prints the address info, and the messages that the socket has been created and bound. When I run Wireshark I can see that there are indeed incoming UDP packets arriving on port 5555 (from another computer on the local network running a proprietary program). The other bit of information that may be useful is that the machine has 2 network interfaces. However, when I replace the Nothing parameter of getAddrInfo with (Just "192.168.1.3") which is the address of the correct NIC, the behavior is as before. Is there something I'm missing?

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Noticed this change about infix decls in GHC 7.4.2
by Christopher Done 18 Jun '12

18 Jun '12

So I have some module, in a work project that I'm portnig from GHC 6.12.3 to GHC 7.4.2, module Data.Monoid.Operator where import Data.Monoid (++) :: Monoid a => a -> a -> a (++) = mappend infixr 5 ++ This compiles happily on GHC 6.12.3, but on 7.4.2 says: src/Data/Monoid/Operator.hs:9:10: Ambiguous occurrence `++' It could refer to either `Data.Monoid.Operator.++', defined at src/Data/Monoid/Operator.hs:8:1 or `Prelude.++', imported from `Prelude' at src/Data/Monoid/Operator.hs:3:8-27 (and originally defined in `GHC.Base') It seems that it used to assign higher priority to the declared thing in the current module over the imported one. Is this intentional? I'd suspect not, given that if I comment the binding out: -- (++) :: Monoid a => a -> a -> a -- (++) = mappend I get: src/Data/Monoid/Operator.hs:9:10: The fixity signature for `++' lacks an accompanying binding (The fixity signature must be given where `++' is declared) Which seems to contradict the previous error message. Bug? Ciao! P.S. Yes, I know I can fix this by not importing Prelude. P.P.S Yes, I know there's a <> or <+> defined now, but I prefer (++), thanks.

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lightweight alternative to mtl?
by Anton Kholomiov 18 Jun '12

18 Jun '12

What are you using instead of mtl? I need simple type for State. The more classy it gets the harder error messages are to understand. I've installed new package. Silently it installed new mtl. And here I'm staring into three lines of code for half an hour trying to understand where I misused the types. Compiler tells me long story about functional dependencies. After downgrade it takes me a second to find the mistake. Ohh type mismatch, forgot to pass an argument. It's class for strict and lazy states. Maybe it's better to take approach of containers (the same interface and different modules)? Anton

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Re: [Haskell-cafe] Safe Haskell at the export symbol granularity?
by Ryan Newton 18 Jun '12

18 Jun '12

Thanks David. I'm glad to see it was discussed in the wiki. (Btw, my 2 cents is that I like the comment pragmas more than new keywords.) The issue that I think doesn't make it into the wiki is of splitting, not modules, but* type-classes*. That's where I think it becomes a more serious issue. Do you think a symbol-level Safe Haskell would be able to distinguish one method of a type class as unsafe, while the others are safe? -Ryan P.S. In my two examples -- There's only one "Acc" type and Accelerate's "fold" can pretty easily be moved into an .Unsafe module, though it breaks the one-giant-module-for-the-whole-programming-model thing it has going now. In the Par example on the other hand type classes are used to abstract over different implementations, so that's where we run into the safe/unsafe factoring problem.

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Data.Binary hanging when used with socket - feature?
by Paul Brenman 17 Jun '12

17 Jun '12

I'm using the 0.5.1.0 binary package (i.e. Data.Binary/Data.Binary.Get) to encode/decode a lazy bytestring. Unfortunately, decode/get are hanging, possibly due to the underlying chunking logic in Data.Binary.Get. The lazy bytestring is being populated from a socket (via "socketTohandle sock ReadWriteMode . hGetContents"). Periodically, encoded data structures are received from the socket. When the encoded data structure happens to end exactly on a chunk boundary, and in my specific case the final encoded item is an Int(64), the Data.Binary.Get.getBytes function wants to force the next chunk to be read into the lazy bytestring before returning the decoded item (see comment in file at line 333 that reads "forces the next chunk before this one is returned"). Unfortunately, since the socket is not yet ready to send anything the call to decode/get hangs. The assumption seems to be that the lazy bytestring isn't populated from a source that can potentially block. Is this reading correct? If so, are there any workarounds? Appreciate any advice - production code.

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event handler
by Corentin Dupont 17 Jun '12

17 Jun '12

Hi folks, I'm trying to make a simple event driven engine. It simply consists of two functions: - "addEvent", where you pass the event name with a callback, - "triggerEvent" where you pass the event name with the data. the data shall be passed to the callback of the corresponding event. I have trouble making it correctly typed. Here is my try: * type Player = Int --dummy types for the example type Rule = Int data EventEnum = NewPlayer | NewRule deriving Eq data Data = P Player | R Rule data Handler = H (Data -> IO ()) addEvent :: EventEnum -> Handler -> [(EventEnum, Handler)] -> [(EventEnum, Handler)] addEvent e h es = (e,h):es triggerEvent :: EventEnum -> Data -> [(EventEnum, Handler)] -> IO () triggerEvent e d es = do let r = lookup e es case r of Nothing -> return () Just (H h) -> h d* The trouble is that I want the user to be only able to add an event that is compatible with its handler: For example the event NewPlayer should have a handler of type Player -> IO (). The data passed when triggering this event should be only of type Player. How can I do that? It sound like dependant typing... Thanks! Corentin

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Haskell is rapidly approaching the top 20
by Henk-Jan van Tuyl 17 Jun '12

17 Jun '12

L.S., From the TIOBE web site[0]: -----✂---- TIOBE Programming Community Index for June 2012 June Headline: Haskell is rapidly approaching the top 20 Last month we asked ourselves the question what language could become the next big new programming language. We suggested several candidates such as Scala, Erlang and Clojure. Clearly, the new thing was expected to come from the functional programming field. A functional language not explicitly mentioned was Haskell. And this month it was Haskell that jumped from #35 to #25. Looking at the TIOBE trend graph of Haskell (starting in 2003) it shows a constant rise, with peaks in 2006, 2010 and now in 2012. This certainly sounds promising. -----✂---- Regards, Henk-Jan van Tuyl [0] http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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ANN: GHC-7.4.2-Eden - Parallel Haskell on multicore and cluster systems
by Eden - Functional parallel Programming (Mischa Dieterle) 17 Jun '12

17 Jun '12

We are happy to announce a new release of Eden, a parallel extension of Haskell. The release comprises: * The GHC-7.4.2-Eden compiler: GHC-7.4.2 extended with the Eden parallel runtime system. * The Eden modules: a library defining the Eden language constructs. * The Eden skeleton library: a comprehensive library with common task and data parallel patterns. * The Eden trace viewer EdenTV: an visualisation and analysis tool for eventlogs of parallel program runs. All resources are available via: http://www.mathematik.uni-marburg.de/~eden Eden extends Haskell with a small set of syntactic constructs for explicit process specification and creation. While providing enough control to implement parallel algorithms efficiently, it frees the programmer from the tedious task of managing low-level details by introducing automatic process handling, synchronisation and communication (via head-strict lazy lists). Eden is tailored for distributed systems but works equally well on multicore architectures. Processes work within disjoint address spaces and do not share any data. This simplifies Eden's implementation as there is no need for global garbage collection. A tutorial is available from http://www.mathematik.uni-marburg.de/~eden/paper/edenCEFP.pdf The compiler release includes binary packages for Linux and Windows, and a source distribution, available from http://www.mathematik.uni-marburg.de/~eden/?content=down_eden_7_4&navi=down Other features: - Extensions for GHC's event logging support. - Built-in serialisation of Haskell data structures (orthogonal to evaluation), see "Orthogonal Serialisation for Haskell", available from http://www.diku.dk/hjemmesider/ansatte/berthold/jbpubl.html - Optimised local communication. - Stand-alone version for multicore, using middleware MPI or PVM for clusters. Eden libraries and tools are available separately from hackage: - Eden modules: http://hackage.haskell.org/package/edenmodules (already included in the Eden compiler) - Eden skeleton library: http://hackage.haskell.org/package/edenskel - EdenTV: http://hackage.haskell.org/package/edentv (uses gtk2hs and glade) When built using a standard GHC, the packages will produce a threaded simulation of Eden.

1 0

FunctionalOWL: FP Usergroup in Germany, area Bielefeld
by raichoo 16 Jun '12

16 Jun '12

Announcing FunctionalOWL in Bielefeld! Next Wednesday will be the first meetup of the FunctionalOWL at Hackerspace Bielefeld. The meetup is aimed at people who are interested in functional programming. FunctionalOWL wants to provide a wide range of interesting topics and opportunities to share and acquire knowledge. Become a part of it. When: Wed 20 June @ 19:00 Where: Sudbrackstraße 42, Bielefeld Followup meetups are planned to happen every 3rd Wednesday at the same location. We have various beamers and whiteboards. Internet and caffeinated lemonade are also available. For more information and contact, check out our website. Feel free to drop a message on our pubML or in our Chatroom. www.hackerspace-bielefeld.de Kind regards, raichoo, Hackerspace Bielefeld

1 0

Performance with do notation, mwc-random and unboxed vector
by Dmitry Dzhus 16 Jun '12

16 Jun '12

Hello everyone. I wonder why using do notation with `<-` can ruin the performance. In essence the problem is that, for some action `f :: m Double`, running the code (in my case, `standard` from mwc-random). f for million times is fast but the code do v <- f return v is slower about a hundred times. Consider this simple source where we generate an unboxed vector with million pseudo-random numbers: ---- 8< ----- import qualified Data.Vector.Unboxed as VU import System.Random.MWC import System.Random.MWC.Distributions (standard) count = 1000000 main = do g <- create e' <- VU.replicateM count $ standard g return () ---- >8 ----- Being compiled with -O2, this runs for 0.052 s on my machine. Changing the replicateM line to use do notation brings the runtime down to 11.257 s! See below: ---- 8< ----- import qualified Data.Vector.Unboxed as VU import System.Random.MWC import System.Random.MWC.Distributions (standard) count = 1000000 main = do g <- create e' <- VU.replicateM count $ do v <- standard g return v return () ---- >8 ----- I don't quite understand why this happens. I'm using GHC 7.4.1 on Linux x86_64 system. Compiling *both* versions with profiling enabled changes runtime to 5.673 sec, which is exactly half the runtime of slow version without profiling, and this is awkward (double calculations occuring in do block?). Does anybody have an idea if this is a problem with my do, or with mwc-random, or with vector (my notation disallowing efficient unboxing?).

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Re: [Haskell-cafe] ANN: Portackage - a hackage portal
by Andrew Seniuk 15 Jun '12

15 Jun '12

Hi ___, Thank you for your suggestion to provide JSON, which seconds Derek's earlier suggestion. I have anonymised your name since you wrote to me but didn't CC the list. :) As much as I wish the web interface was up to snuff, it is really too slow. While I continue to try to resolve that, there is something like a JSON feed available now. I'm new to these things and may be doing it wrong. Any advice would be appreciated, either in accelerating the web interface or in providing the JSON. I have no idea whether it parses or not. JSON Schema: http://www.fremissant.net/portackage/packages.schema.json.txt Hopefully compressed JSON data: http://www.fremissant.net/portackage/packages.php Raw JSON data in case the compressed doesn't work: http://www.fremissant.net/portackage/packages.json The hackage data updated as of today (June 14). For now this is only the packages which expose modules (i.e. not including binary packages). No particular reason, I just ran out of time this afternoon and thought I'd post what I've got. The fields present in the JSON are those available on the HTML table version. There are several dozen other fields which come up more or less infrequently in the Hackage packages, and they could all be included. The complete list of modules is included (rather than the summary versions used in the HTML table version). A next step would be for the server to incrementally update the JSON data based on the Hackage RSS feed. Kind Regards, Andrew On Wed, Jun 13, 2012 at 7:58 AM, ___ wrote: > Hi Andrew > > Portackage is a great site for browsing packages, but it's really > really slow for filtering per column. What about offering an > additional non-javascript version of the pages -- searching with plain > text in a browser works quite well. > > On 4/30/12, Andrew Seniuk <rasfar(a)gmail.com> wrote: >> Oops, the three links in summary are: >> >> module tree: http://fremissant.net/portackage/modules.php >> library packages: http://fremissant.net/portackage/portackage.php >> other packages: http://fremissant.net/portackage/binary.php >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > -- > ___

1 2

Installing pandoc / json with ghc 6.12.1
by Rico Moorman 15 Jun '12

15 Jun '12

Hello, I recently tried to install the latest version of pandoc on my debian stable box (with ghc 6.12.1) using virthualenv as sandboxing mechanism. But instead of a normal installation, compilation of the json package (more specifically the Text.JSON.Parsec module) hangs and causes ghc to consume around 2GB of RAM (out of 3GB) and the system to swap and hang. For reference ... I just created a plain virthualenv (for sandboxing) and after that ran (tmp) foo@bar:/tmp$ cabal install json I also tried if this has something to do with version 0.5 (the latest version) only, to no avail. Are there other users of pandoc/json or related packages who encountered/solved this issue? Am I doing something wrong here? Best regards, Rico

2 1

class instances for kinds with finite ty constrs, does this make sense?
by ex falso 15 Jun '12

15 Jun '12

Hi, i've been using DataKinds for a while and there seems to be a recurring annoyance that could be possibly eliminated with a language extension. It goes something like this: data Tuple (l :: [*]) where Unit :: Tuple '[] Comma :: a -> Tuple as -> Tuple (a ': as) data Proxy k = Proxy class TupleLength (l :: [*]) where tupleLength :: Proxy l -> Integer instance TupleLength '[] where tupleLength _ = 0 instance (TupleLength as) => TupleLength '(a : as) where tupleLength _ = 1 + tupleLength (Proxy :: Proxy as) so far so good. but now if we want to use TupleLength: printLength :: (TupleLength l) => Tuple l -> IO () printLength t = print $ tupleLength t we always have to put the class restriction (TupleLength l) there, even though all possible type constructors of [*] have a TupleLength instance defined! If we had a way to signal that all ty constrs are catered for then the compiler could check this, and deduce that l::[*] is always an instance. Well that's the hypothesis. Of course this would only make sense for kinds with finite no. of ty constructors, namely DataKinds-lifted types. Does this make sense? ex

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Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
by Arlen Cuss 15 Jun '12

15 Jun '12

(resending to café, turns out I wasn't subbed from this address.) Hi Magicloud, This is correct; because you've hidden the type-variables away by universally quantifying them, there's no more level of specificity you can get back *out* of them than just "some kind of Map" (Job = M.Map k b, where k ≠ k0, b ≠ b0). If you have a Job type which can store *any* kind of Map (forall k a. Job (Map k a)), then that means you could have a Job with a Map Int Bool, and a Job with a Map String (Float -> Float), and they'd both have the same type "Job". You can't do anything with the values within, because you're being too permissive about what a Job is. You may want "data Job k a = Job (Map k a)", *or* if you do actually use one kind of Map only, then why not "data Job = Job (Map Int String)" (substituting your real types for Int and String). In this case, you could also consider using newtype ("newtype Job = Job { getJob :: Map Int String }") to provide the guarantee that you're getting a Job (and not any Map Int String) without performance loss. Let me know if I've been more confusing than helpful; Arlen On Thursday, 14 June 2012 at 1:16 PM, Magicloud Magiclouds wrote: > Hi there, > Thanks for the reply. To be clear, all I want is to "avoid having to > type type variables all over the place". What should I do? My original > code with RankNTypes and ImpredicativeTypes does not work.... > > The "type Job = forall k a. M.Map k a" works now. But function uses > it does not. Compiler complains about "Couldn't match expected type > `Job' with actual type `M.Map k0 b0'". > > On Wed, Jun 13, 2012 at 9:15 PM, Daniel Peebles <pumpkingod(a)gmail.com (mailto:pumpkingod@gmail.com)> wrote: > That doesn't require existential quantification, but it'll need Rank-2 typesif you ever do anything with Job. Unfortunately, a universally quantifiedJob like what you wrote (or what Magicloud seems to want) is only inhabitedby the empty Map. > > > An existentially quantified Job, as you might get with > > > data Job = forall k a. Job (Map k a) > > > does let you wrap up any Map containing anything in it, but unfortunatelythe only thing you can do with that map afterwards is ask for "structural"properties about it, like whether it's empty or how many elements it has init. You could ask to enumerate the elements in it, but you wouldn't be ableto touch any of them because you wouldn't know what their types were. > > > So I'm not really sure how to interpret the question. Was the goal to have aheterogeneous Map, maybe? Or just to avoid having to type type variables allover the place? Both of those are possible but require a bit moresophistication with types. > > > -Dan > > > > > On Wed, Jun 13, 2012 at 7:32 AM, Ismael Figueroa Palet<ifigueroap(a)gmail.com (mailto:ifigueroap@gmail.com)> wrote: > > > > Do you want to hide the specific types of the job? Presumably to thendefine a type JobList = [Job] ?You can do that with the ExistentialQuantification extension. > > > > type Job = forall k a. Map k atype JobList = [Job] > > > > ??Note you can't unpack the types k a once you have hidden them. But thetypechecker can use it to ensure some static property.Also you could use unsafeCoerce to do some casts, but *only if you are*sure* that things will go OK*. > > > > > > > 2012/6/13 Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com (mailto:magicloud.magiclouds@gmail.com)> > > > > > Hi,I've forgotten this.This is OK:type Job k a = Map k aAnd this is OK:{-# LANGUAGE RankNTypes #-} -- or LiberalTypeSynonyms?type Job = forall a. forall k. Map k a > > > > > Then how to write it like this?type Job = Map k a--竹密岂妨流水过山高哪阻野云飞 > > > > > And for G+, please use magiclouds#gmail.com (http://gmail.com) > > > > > _______________________________________________Haskell-Cafe mailing listHaskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > > > > > > > > > > > > > > > > --Ismael > > > > > > > _______________________________________________Haskell-Cafe mailing listHaskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > -- > 竹密岂妨流水过 > 山高哪阻野云飞 > > And for G+, please use magiclouds#gmail.com (http://gmail.com) > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Current uses of Haskell in industry?
by Chris Smith 14 Jun '12

14 Jun '12

It turns out I'm filling in for a cancelled speaker at a local open source user group, and doing a two-part talk, first on Haskell and then Snap. For the Haskell part, I'd like a list of current places the language is used in industry. I recall a few from Reddit stories and messages here and other sources, but I wonder if anyone is keeping a list. -- Chris

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[Announce] AusHac 2012 - The Aussie Haskell Hackathon
by Alex Mason 14 Jun '12

14 Jun '12

We know many of you have been holding your breaths for this (hopefully none of you have passed out though!) for quite a while... but we’re pleased to announce that we’re finally getting around to announcing AusHac 2012! This year, we’re moving AusHac to Sydney proper: Atlassian has kindly offered the use of their offices next to Darling Harbour, so there’s more places to stay and eat than in previous years! It’s right across the harbour from Google’s headquarters where FP-Syd is hosted, and we’re starting on the Friday after next month’s FP-Syd. When: Friday 20th - Sunday 22nd of July Where: Atlassian 173/185 Sussex Street Sydney NSW 2000, Australia (02) 9262 1443 Map: http://goo.gl/maps/JoNg If you're looking for places to stay, check out AirBnB (http://www.airbnb.com) the prices are very reasonable (around $100/night within walking distance of Atlassian if you’re lucky). This year we’d like to see a few more talks on interesting things people are working on (Yes, the stuff you are working on IS interesting, and we want to hear about it!). If you’d like to give a talk, please let us know in the sign up sheet. Speaking of which, here it is: http://tinyurl.com/AusHac2012SignUp Please let us know if you’re considering coming along, so we can organise things like adequate power etc. We’d love for you to come along for the whole time or just a few hours, we’re happy for people to drop in when they want, on any day. The more people we have come along, the more awesome it will be. We hope to see you there! Cheers, Alex Mason and Ivan Miljenovic

1 0

What extension do I need to write "type Job = Map k a"?
by Magicloud Magiclouds 14 Jun '12

14 Jun '12

Hi, I've forgotten this. This is OK: type Job k a = Map k a And this is OK: {-# LANGUAGE RankNTypes #-} -- or LiberalTypeSynonyms? type Job = forall a. forall k. Map k a Then how to write it like this? type Job = Map k a -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Haskell Weekly News: Issue 231
by Daniel Santa Cruz 14 Jun '12

14 Jun '12

Welcome to issue 231 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 03 to 09, 2012. Announcements Paolo Capriotti announced the patchlevel release of GHC 7.4.2. "This release contains a number of bugfixes relative to 7.4.1, so we recommend upgrading." [1] http://goo.gl/X3Z85 Quotes of the Week * roconnor: there's a right way, and a wrong way, and then there is the ekmett way. Top Reddit Stories * Haskell Platform 2012.2.0.0 released Domain: hackage.haskell.org, Score: 104, Comments: 33 On Reddit: [2] http://goo.gl/ZNa9h Original: [3] http://goo.gl/JjJg2 * Why free monads matter Domain: haskellforall.com, Score: 61, Comments: 47 On Reddit: [4] http://goo.gl/hfKlc Original: [5] http://goo.gl/JPpsp * 3 Flavors of MVar: Lock, Var, Barrier Domain: neilmitchell.blogspot.com, Score: 43, Comments: 20 On Reddit: [6] http://goo.gl/3eEkO Original: [7] http://goo.gl/ek7Qq * Introducing the HERMIT Equational Reasoning Framework Domain: ittc.ku.edu, Score: 36, Comments: 20 On Reddit: [8] http://goo.gl/UC0sF Original: [9] http://goo.gl/ifS7f * Complicating conduit? Domain: yesodweb.com, Score: 35, Comments: 45 On Reddit: [10] http://goo.gl/W4rfE Original: [11] http://goo.gl/rU0vK * Beautiful Code, Compelling Evidence: OpenGL, Cairo tutorial [PDF, 33 pages] Domain: renci.org, Score: 33, Comments: 2 On Reddit: [12] http://goo.gl/qKY8w Original: [13] http://goo.gl/r5SVj * Forklift - a pattern for performing monadic actions in a worker thread Domain: apfelmus.nfshost.com, Score: 30, Comments: 7 On Reddit: [14] http://goo.gl/B6brV Original: [15] http://goo.gl/mWVAv * Dependently typed programming with singletons (Eisenberg, Weirich)[PDF] Domain: cis.upenn.edu, Score: 29, Comments: 3 On Reddit: [16] http://goo.gl/y3fYv Original: [17] http://goo.gl/RqM5m * Continuation-based relative-time FRP Domain: paolocapriotti.com, Score: 22, Comments: 6 On Reddit: [18] http://goo.gl/CFSK7 Original: [19] http://goo.gl/cP0cJ * Explaining the Prompt monad with a clearer representation Domain: joeysmandatory.blogspot.com, Score: 21, Comments: 42 On Reddit: [20] http://goo.gl/ZZM93 Original: [21] http://goo.gl/Im7SF * Lazily Evaluate length of a list Domain: self.haskell, Score: 21, Comments: 29 On Reddit: [22] http://goo.gl/cKUOt Original: [23] http://goo.gl/cKUOt Top StackOverflow Questions * Killing a thread when MVar is garbage collected votes: 22, answers: 4 Read on SO: [24] http://goo.gl/cvoqw * How does one write efficient Dynamic Programming algorithms in Haskell? votes: 16, answers: 3 Read on SO: [25] http://goo.gl/mkMTp * How to apply a polymorphic function to a Dynamic value votes: 11, answers: 1 Read on SO: [26] http://goo.gl/3jIVr * Haskell Typeclass for Tuples votes: 11, answers: 3 Read on SO: [27] http://goo.gl/pvDah * Monadic expressions in conditionals - GHC compiles, cabal refuses votes: 10, answers: 2 Read on SO: [28] http://goo.gl/QXMXh * GHC Optimization: Collatz conjecture votes: 10, answers: 2 Read on SO: [29] http://goo.gl/AfiEC * Haskell: Lists vs Streams votes: 10, answers: 3 Read on SO: [30] http://goo.gl/qn1dE * Is my program Turing-complete? votes: 9, answers: 2 Read on SO: [31] http://goo.gl/PLf5q * How to find the optimal processing order? votes: 9, answers: 1 Read on SO: [32] http://goo.gl/P3XZ2 * How to do fast data deserialization in Haskell votes: 9, answers: 1 Read on SO: [33] http://goo.gl/Synm2 * What's so bad about OverlappingInstances? votes: 9, answers: 1 Read on SO: [34] http://goo.gl/M17Rp Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.glasgow.user/22181 2. http://hackage.haskell.org/platform/?2012.2.0.0 3. http://www.reddit.com/r/haskell/comments/uipvz/haskell_platform_2012200_rel… 4. http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.ht… 5. http://www.reddit.com/r/haskell/comments/utxq2/why_free_monads_matter/ 6. http://neilmitchell.blogspot.com/2012/06/flavours-of-mvar_04.html 7. http://www.reddit.com/r/haskell/comments/uk9wu/3_flavors_of_mvar_lock_var_b… 8. http://www.ittc.ku.edu/csdlblog/?p=105 9. http://www.reddit.com/r/haskell/comments/ujdet/introducing_the_hermit_equat… 10. http://www.yesodweb.com/blog/2012/06/complicating-conduit 11. http://www.reddit.com/r/haskell/comments/urc75/complicating_conduit/ 12. http://www.renci.org/wp-content/pub/tutorials/BeautifulCode.pdf 13. http://www.reddit.com/r/haskell/comments/uqc33/beautiful_code_compelling_ev… 14. http://apfelmus.nfshost.com/blog/2012/06/07-forklift.html 15. http://www.reddit.com/r/haskell/comments/uq44h/forklift_a_pattern_for_perfo… 16. http://www.cis.upenn.edu/~eir/papers/2012/singletons/paper.pdf 17. http://www.reddit.com/r/haskell/comments/uic9g/dependently_typed_programmin… 18. http://paolocapriotti.com/blog/2012/06/04/continuation-based-relative-time-… 19. http://www.reddit.com/r/haskell/comments/uklsq/continuationbased_relativeti… 20. http://joeysmandatory.blogspot.com/2012/06/explaining-prompt-monad-with-sim… 21. http://www.reddit.com/r/haskell/comments/unpaz/explaining_the_prompt_monad_… 22. http://www.reddit.com/r/haskell/comments/uomz4/lazily_evaluate_length_of_a_… 23. http://www.reddit.com/r/haskell/comments/uomz4/lazily_evaluate_length_of_a_… 24. http://stackoverflow.com/questions/10871303/killing-a-thread-when-mvar-is-g… 25. http://stackoverflow.com/questions/10906053/how-does-one-write-efficient-dy… 26. http://stackoverflow.com/questions/10889682/how-to-apply-a-polymorphic-func… 27. http://stackoverflow.com/questions/10899804/haskell-typeclass-for-tuples 28. http://stackoverflow.com/questions/10867300/monadic-expressions-in-conditio… 29. http://stackoverflow.com/questions/10876110/ghc-optimization-collatz-conjec… 30. http://stackoverflow.com/questions/10942450/haskell-lists-vs-streams 31. http://stackoverflow.com/questions/10878846/is-my-program-turing-complete 32. http://stackoverflow.com/questions/10879838/how-to-find-the-optimal-process… 33. http://stackoverflow.com/questions/10902405/how-to-do-fast-data-deserializa… 34. http://stackoverflow.com/questions/10942136/whats-so-bad-about-overlappingi…

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Perth Functional Programmers meetup group launched
by Todd Owen 13 Jun '12

13 Jun '12

We are pleased to announce that a functional programming user group has recently been formed in Perth, Australia. Being a small community, we aim to keep the group as inclusive as possible, and welcome new members from all levels of experience and language backgrounds. (That said, Haskellers currently account for about half our membership, followed by F#, and then a heap of other languages). As this is a global mailing list, I would also like to invite our colleagues from around Australia, and indeed anywhere in the world, to get in touch if you are ever passing through Perth. We're always happy to share a beer and talk about programming! For more information, visit: http://www.meetup.com/PerthFP/

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Warning when attempting to install cabal-dev on the latest HP
by Edward Amsden 13 Jun '12

13 Jun '12

http://hpaste.org/69885 The warning in that paste occurs when I attempt $ cabal update $ cabal install cabal-dev This is curious because it wants to install a previous version of mtl. I can't figure out why. There doesn't seem to be any dependency between cabal-dev and mtl. According to hackage, the dependencies for cabal-dev are: cabal-dev -> (base, Cabal) Cabal -> (base, filepath) filepath -> (base) base -> () The dependencies for mtl are (just to show no conflicts) mtl -> (base, transformers) transformers -> (base) base -> () And there's not a versioning conflict with base. Why is cabal insisting on backdating mtl then?

1 1

RawFilePath vs System.FilePath
by Kazu Yamamoto 13 Jun '12

13 Jun '12

Hello, After releaseing new Haskell Platform, many people can now use RawFilePath (e.g. ByteString) for System.*. However, there is no System.FilePath.ByteString which manipulates RawFilePath. How do you guys manipulate RawFilePath as file path? Is there a plan to implement System.FilePath.ByteString? # I know the system-filepath package. But I would like to know # answers to the questions above. Regards, --Kazu

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[Meetup] Haskell web development - Munich - this Friday
by Vincent Ambo 13 Jun '12

13 Jun '12

Hej, would anyone be interested in a short dated meetup in Munich this Friday? (June 15th) Michael Snoyman of Yesod will be giving a talk at the Big Tech Day and a meeting afterwards would be possible on Friday afternoon. I'd like if this meeting were somewhat focused on web development in Haskell (Yesod/WAI, Happstack, Snap, clckwrks, hakyll, Elm, .... (please don't be angry if I forgot your specific project, our web development tools are growing at an out of control speed!)), complaining about how annoying Javascript's type 'system' is and generally having a bit of fun (definitely nothing formal or with a strict schedule). We've also not decided on a location yet, so if you have any ideas for that please bring them up here :-) Cheers, Vincent

1 0

Derived Read for records
by Conal Elliott 13 Jun '12

13 Jun '12

Someone at work just asked me about the inflexibility of the derived Read instances for records -- specifically that they require fields to be given in the same order as in the type definition and that fields cannot be omitted. I hadn't been aware of these restrictions. A few questions: * Are there known work-arounds? * Is it particularly difficult to synthesize more flexible Read instances? * Do people just not mind the restrictions? Thanks, -- Conal

1 0