- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Announce: Haskell Platform 2012.2.0.0
by Chris Wong 04 Jun '12

04 Jun '12

On Mon, Jun 4, 2012 at 1:16 PM, Jens Petersen <juhp(a)community.haskell.org> wrote: > Congratulations on the release! > Equally surprising to me is that the number of slashes > also seems to affect the CSS presentation of the website > in Chrome. > > // seems to give the Summer theme, > whereas / gives the Winter one! > > Kind of weird. Anyway I agree it would be better to avoid > the superfluous slashes if possible. Your browser might be caching the file. Try pressing Ctrl-F5 on the winter page to force a reload. > I am going build HP 2012.2 for Fedora 18 soon but > the first proper build may be after ghc-7.4.2 is released. > > Jens > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Announce: Haskell Platform 2012.2.0.0
by Mark Lentczner 04 Jun '12

04 Jun '12

We're pleased to announce the next release of Haskell Platform: a single, standard Haskell distribution for everyone. Download Haskell Platform 2012.2.0.0: http://haskell.org/platform/ The specification, along with installers (including Windows, Macintosh, and Unix installers for a full Haskell environment) are available. Notable version changes with this release: ghc 7.4.1 cabal 1.14 & cabal-install 0.14 mtl 2.1.1 & transformers 0.3.0.0 alex 3.0.1 full package and version list: http://hackage.haskell.org/platform/changelog.html Thanks to: Joachim Breitner, George Colpitts, Duncan Coutts, Jason Dagit, Chris Dornan, Yitzchak Gale, Mikhail Glushenkov, Andres Löh, Gábor Páli, Jens Petersen, Don Stewart, David Terei, Johan Tibell, & Mark Wright for their efforts and help in putting together this release. -- The Platform Infrastructure Team Mark Lentczner, release manager --------------------------------------------------------------------- The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage.

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Announce: haskellifi-trayicon-0.0.1
by Ivan Perez 03 Jun '12

03 Jun '12

Hi everyone, I wrote a small app that suggests default passwords for visible wifi networks (currently only some Telefonica & Jazztel Wifis in Spain, but could be extended). Not a big deal (less than 400 lines of code with the GUI), but some people asked me to release it, so here it is. Disclaimer ======== THIS IS IMPORTANT. This program is not intended for hacker wannabes. This is meant ONLY for network administrators. Use *only* with permission of the wifi admin. Introduction ========= More often than not, network administrators have to deal with clients or other administrators how know very little about wifi security. It is not uncommon to find wifis with default passwords. This program suggests passwords for the wifis it detects, and presents the information in a visually attractive way (tray icon with popup menu) so that your customers/collaborators fully understand HOW EASY IT IS TO BREAK INTO THEIR NETWORKS. NOTE: at the present time, this only works with Telefonica Spain's WPA2 default passwords (and maybe Jazztel's). If the default password of your wifi can be automatically generated from "open" or publicly visible parameters, feel free to write the algorithm in Haskell, send it to me and I'll see if I can add it here. Instructions ========= Code, Installation, readme, and other instructions: https://github.com/ivanperez-keera/haskellifi-trayicon During execution, you should see an icon on your traybar or notification area. Left click will show a menu with the list of wifis it can suggest a password for. Clicking on one will copy the suggested password to the clipboard. Right click on the icon to clear the wifi list or quit the program. Feel free to send me bugs reports and suggestions. Hope it helps somebody. Cheers, Ivan.

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Re-exports of resourcet in conduit
by Myles C. Maxfield 03 Jun '12

03 Jun '12

To: Michael Snoyman CC: haskell-cafe Hello, I'm having a problem working with the conduit library, and was hoping you could help me out. Data.Conduit re-exports ResourceT, MonadResource, and MonadThrow (but not ExceptionT) from Control.Monad.Trans.Resource. I have a conduit which operates on a monad in the MonadThrow class. I am trying to figure out which MonadThrow class this should be (the Data.Conduit.MonadThrow class, or the Montrol.Monad.Trans.Resource.MonadThrow class, since apparently GHC doesn't recognize them as the same, even though one is just a re-export of the other). If a user of this conduit wants to chain this conduit up with something like sourceFile, the underlying monad has to be a member of Data.Conduit.MonadResource and whatever MonadThrow class I chose to use. I would like to be able to use an existing instance to lift the class of the inner monad to the class of the entire monad stack (so I don't have to tell the user of my conduit that they have to define their own instances), and the only rule that I can find that does that is the following from Data.Conduit: Data.Conduit.MonadThrow m => Data.Conduit.MonadThrow (Data.Conduit.ResourceT m) However, GHC doesn't seem to think that Control.Monad.Trans.Resource.ExceptionT is an instance of Data.Conduit.MonadThrow: No instance for (Data.Conduit.MonadThrow (ExceptionT IO)) arising from a use of `.....' Control.Monad.Trans.Resource has a similar instance: Control.Monad.Trans.Resource.MonadThrow m => Control.Monad.Trans.Resource.MonadThrow (Control.Monad.Trans.Resource.ResourceT m) but because sourceFile operates in the Data.Conduit.MonadResource class, and Control.Monad.Trans.Resource.ResourceT isn't a member of that class (it's only a member of Control.Monad.Trans.Resource.MonadResource), that doesn't help: No instance for (Data.Conduit.MonadResource (Control.Monad.Trans.Resource.ResourceT (ExceptionT IO))) arising from a use of `.....' It should be noted that neither module defines anything like the following: MonadResource m => MonadResource (ExceptionT m) It seems like the underlying problem here is that: 1) I am required to use the Control.Monad.Trans.Resource.ExceptionT class, because Data.Conduit doesn't re-export it 2) I am required to use the Data.Conduit.MonadResource class, because sourceFile and others require it 3) There doesn't seem to be an existing instance that bridges between the two. This seems like a fundamental flaw with re-exporting; it can only work if you re-export every single last thing from the original module. This doesn't seem tenable because the orignal module might not be under your control, so its author can add new symbols whenever he/she wants to. I see two solutions to this problem: 1) Re-export Control.Monad.Trans.Resource.ExceptionT in Data.Conduit. This will work until someone adds something to the resourcet package and someone wants to use the new addition and Data.Conduit.ResourceT in the same stack 2) Don't re-export anything in Data.Conduit; make sourceFile and others explicitly depend on types in another module, but this might break compatibility with existing programs if they use fully-qualified symbol names. 3) Make anyone who wants to use a monad stack in MonadThrow and MonadResource define their own instances. This is probably no good because it means that many different modules will implement the same instance in potentially many different ways. I feel like option 2) is probably the best solution here. I'm perfectly happy to issue a pull request for whichever option you think is best, but I don't know which solution you think is best for your project. What do you think? --Myles

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Is Repa suitable for boxed arrays?...
by Stuart Hungerford 03 Jun '12

03 Jun '12

Hi, I have a newb-ish question about Haskell 2D arrays. I need to construct a 2D array of a Haskell data type ("boxed" ?) where each array entry value depends on values "earlier" in the same array (i.e. for entries in previous row/column indexes). Repa (V3.1.4.2) looks very powerful and flexible but it's still not clear to me that it will work with arbitrary values as I haven't been able to get any of the Wiki tutorial array creation examples to work (this is with Haskell platform 2012.2 pre-release for OS/X). I can see I could also construct my own 2D array data structure from Data.Vector instances. Are there any other approaches for general 2D arrays I should be looking at? Thanks, Stu

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Most Important GHC extensions to learn/use?
by Jonathan Geddes 03 Jun '12

03 Jun '12

Haskell Hackers, I'm pretty comfortable with all of Haskell 98 (and 2010, really). But I've always sort of avoided extensions. I realize that this is a bit silly and if I want to continue learning, it probably means delving into the extensions. Which ones are the most important to know from a practical point of view? And which ones from a {Language,Category,Math}-theoretical point of view? (Any other interesting/important points of view I'm missing? :D ) As always, thanks for the feedback. Cheers, --J Arthur

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Need inputs for a Haskell awareness presentation
by C K Kashyap 03 Jun '12

03 Jun '12

Hi folks, I have the opportunity to make a presentation to folks (developers and managers) in my organization about Haskell - and why it's important - and why it's the only way forward. I request you to share your experiences/suggestions for the following - 1. Any thoughts around the outline of the presentation - target audience being seasoned imperative programmers who love and live at the pinnacle of object oriented bliss. 2. Handling questions/comments like these in witty/interesting ways - a) It looks good and mathematical but practically, what can we do with it, all our stuff is in C++ b) Wow, what do you mean you cannot reason about its space complexity? c) Where's my inheritance? d) Debugging looks like a nightmare - we cannot even put a print in the function? e) Static types - in this day and age - come on - productivity in X is so much more - and that's because they got rid of type mess. f) Is there anything "serious/large" written in it? [GHC will not qualify as a good answer I feel] g) Oh FP, as in Lisp, oh, that's AI stuff right ... we don't really do AI. h) Any other questions/comments that you may have heard. 3. Ideas about interesting problems that can be used so that it appeals to people. I mean, while fibonacci etc look good but showing those examples tend to send the signal that it's good for "those kind" of problems. 4. Is talking about or referring to Lambda calculus a good idea - I mean, showing that using its ultra simple constructs one could build up things like if/then etc I'm gonna do my bit to wear the limestone!!! Regards, Kashyap

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Program for displaying graph state in lambda interpreter?
by Benjamin Redelings 02 Jun '12

02 Jun '12

Hi, I have written an interpreter that operates on the lambda calculus augmented with letrec, constructors, case, primitive objects, and builtin operations. I'd like to display the internal state of the intepreter at various points so that I can, um, debug the "programs" that I've written. Currently I've got about 3500 nodes, and I'm dumping a graph that I format using the 'dot' program in graphviz. I might have "let x = 1+2 in x*x" represented as. x/1: <2>+<3> 2: 1.0 3: 2.0 4: <1>*<1> where <1>, <2>, <3>, represent various memory locations. I can display this in graphviz, where each node is a rectangle containing text like "<1>*<1>" and there are arrows from the rectangle to <1>. This works as long as the number of nodes isn't very large. However, it would be nice to know of any programs that are better suited for this. For example, if I could write <>*<> and draw arrows from the placeholders "<>" to the memory location being referenced, that would be easier to read. This is done here http://en.wikibooks.org/wiki/Haskell/Graph_reduction to illustrate that 'square (1+2)' doesn't evaluate 1+2 twice. Any ideas? thanks, -BenRI P.S. If this is too off-topic, please let me know.

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SafeSemapore
by Chris Kuklewicz 02 Jun '12

02 Jun '12

This message is in three parts: a problem, a solution, and a request for help. The problem is that Control.Concurrent.QSem, QSemN, and SampleVar are all still broken. GHC ticket #3160 is still open [1]. These three synchronization mechanisms can be irretrievable broken by a killThread on a blocked waiter. The solution is that I am please to announce that SafeSemaphore has been updated to 0.9.0 on hackage [2] and github. These provides safe solutions to replace QSem and QSemN, actually several such solutions. See the github [2] page (scroll down for README) for a summary of all the modules. The request for help is that I would like to unbreak the Haskell Platform by replacing the guts of QSem, QSemN, and SampleVar with SafeSemaphore. Do you think my replacement is correct or buggy? Can we get #3160 closed? Replacing these will preserve their API but may tinker with corner case undocumented behavior. Should waiters block in FIFO order? Should QSemN starve big or small requests to be fairer? [1] http://hackage.haskell.org/trac/ghc/ticket/3160 (three years old!) [2] http://hackage.haskell.org/package/SafeSemaphore/ [3] https://github.com/ChrisKuklewicz/SafeSemaphore Sincerely, Chris Kuklewicz

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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
by wren ng thornton 01 Jun '12

01 Jun '12

On 6/1/12 6:11 AM, Gábor Lehel wrote: > On Fri, Jun 1, 2012 at 6:29 AM, wren ng thornton<wren(a)freegeek.org> wrote: >> >> TypeFamilies (aka TFs) >> These are really nifty and they're all the rage these days. In >> a formal sense they're equivalent to fundeps, but in practice >> they're weaker than fundeps. > > Is that still true? The reason used to be that we didn't have > superclass equalities, but we do have them now since 7.2. The only > drawbacks I know of relative to FDs are that it's sometimes more > typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow > OverlappingInstances (ick). The superclass equalities was a big thing, but the disparity remains even still. The main problem is that type inference with fundeps remains more powerful than type inference for TFs. There are still patterns of use which infer/check easily with fundeps but which don't have an equivalent TF implementation. I haven't messed with TFs enough to know the details here, but there are other folks on the list who do. In addition there's the issue of overlapping instances, which is currently being discussed elsewhere. -- Live well, ~wren

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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
by wren ng thornton 01 Jun '12

01 Jun '12

On 6/1/12 12:45 AM, Jonathan Geddes wrote: > Thanks, Wren, I really appreciate the detailed response! Though I am > surprised that Template Haskell isn't on your list. From the little I know > of TH it seems like all of the interesting generic/generative stuff is done > with TH. Do the other extensions subsume the need for TH, or is it just not > terribly interesting? TH is plenty interesting, but it's a very different sort of direction to head. The extensions I mentioned are the ones I think everyone assumes a seasoned Haskeller will know. With the exception of TFs and GADTs, those extensions have all been around since the days of GHC 6.6 and Hugs. Thus, most people consider them normal parts of "Haskell" even if they're not in in the Report. Consequently, understanding them is necessary to understand most of the non-H98 code on Hackage. TH, on the other hand, is very much GHC-only and that's unlikely to change. Also, there's a lot of unnecessary(?) grunge there. The basic theory of staged computation which led to TH was laid out in the MetaML/MetaOCaml papers. The version in MetaML is a lot nicer in terms of the theory, so I'd suggest people start there before diving into TH. One of the nice things in MetaML is that there's no limit on the levels of meta-ness. This was removed from TH because it would require having the compiler present in every executable; a sensible limitation, though it complicates the theory. Another major difference is that MetaML only did staging at the term level, whereas TH also allows splices in type signatures, splices which generate type class instances, etc. These extensions make TH *much* more powerful than MetaML, but also make it *much* harder to understand and reason about. IMO the formal theory of these non-term splices hasn't been worked out very well, which is part of the reason why TH is so grungy to work with. I'm a big fan of staged computation, and I'd definitely suggest any sophisticated functional programmer should read the MetaML papers (and the TH papers if so inclined, and the Flask papers to get a different perspective on the theme). But very few packages require understanding TH in order to understand them, and even fewer require understanding TH in order to use them. -- Live well, ~wren

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Requesting Feedback: I Love Haskell, but can't find a place to use it
by Jonathan Geddes 01 Jun '12

01 Jun '12

I love Haskell. It is my absolute favorite language. But I have a very hard time finding places where I can actually use it! I had hoped that compiling Haskell to C with -fvia-C (or would it be just -C?) would allow Haskell to run in new, uncharted territory such as Android (with NDK), IOS, Google's NaCl, etc. But today I learned that GHC's C backend has been deprecated! Is it more difficult than I am imagining to get Haskell to work in these environments? Is it simply a matter of low interest in this kind of work? Or something more fundamental? Am I missing something? I'm hoping that the Haskell->JavaScript efforts will mature enough to make Haskell viable for client-side web apps. (I think the first sign of this will be a self-hosting Haskell->JavaScript compiler.) I use Haskell for Server-Side code with various web frameworks, but over the years more and more of the app logic is moved into client-side JavaScript, leaving the server-side code as little more than a simple validation and security layer over the database and other services. Haskell doesn't have any trouble with this, of course, but it's not exactly a role where it can shine. (Of course this is not true of ALL server-side code, just the kind of apps I have been writing.) So anyway I'd like to request feedback: where can I use Haskell besides simple CLI utilities, dull server code, or project Euler problems? Even if it's just to contribute to getting Haskell in the environments mentioned above, any feedback is welcome! Thanks for reading, --J Arthur

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Re: [Haskell-cafe] Requesting Feedback: I Love Haskell, but can't find a place to use it
by Doug McIlroy 01 Jun '12

01 Jun '12

> > I love Haskell. It is my absolute favorite language. > > But I have a very hard time finding places where I can actually use it! > > have you considered "your head" as such a place that should be easy to find. Excellent advice. Haskell shines unusually brightly on applications that have an algebraic structure. And laziness relieves a plethora of sequencing concerns. I particularly treasure one experience: I asked a guru about the complexity of converting regular expressions to finite-state automata without epsilon transitions (state transitions that don't produce output). The best he knew was O(n^3), which is the cost of removing epsilon transitions from arbitrary automata. Then I wrote about a dozen lines of Haskell to do the job--and running time turned out to be O(n^2). Once I'd written the code, it became clear how to do it in other languages, but I never would have found the theorem without the help of Haskell. (This without even bringing in the heavy artillery of higher-order functions and monads.) Doug McIlroy

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Re: [Haskell-cafe] Requesting Feedback: I Love Haskell, but can't find a place to use it
by Doug McIlroy 01 Jun '12

01 Jun '12

> > I love Haskell. It is my absolute favorite language. > > But I have a very hard time finding places where I can actually use it! > > have you considered "your head" as such a place that should be easy to find. An excellent reason. Haskell shines unusually brightly on applications that have an algebraic structure. Laziness relieves a plethora of sequencing concerns. I particularly treasure one experience: I asked a guru about the complexity of converting regular expressions to finite-state automata without epsilon transitions (state transitions that don't produce output). The best he knew was O(n^3), which is the cost of removing epsilon transitions from arbitrary automata. Then I wrote about a dozen lines of Haskell to do the job--and running time turned out to be O(n^2). Once I'd written the code, it became clear how to do it in other languages, but I never would have found the theorem without the help of Haskell. (This without even bringing in the heavy artillery of higher-order functions and monads.) Doug McIlroy

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Tests by properties: origin?
by Yves Parès 01 Jun '12

01 Jun '12

Out of curiosity, does someone know if QuickCheck was the first test framework working through test by properties associated with random generation or if it drew the idea from something else? Because the idea has be retaken by a lot of frameworks in several languages (see http://en.wikipedia.org/wiki/Quickcheck) but I can't find what was QuickCheck inspiration. (This question pop up when arguing with a Java user about what Haskell brings to the field of development, he pretends it would be astonishing that QuickCheck is the first one using this testing paradigm).

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Confused by ghci output
by Clark Gaebel 01 Jun '12

01 Jun '12

*X> 3^40 `mod` 3 == modexp2 3 40 3 False *X> modexp2 3 40 3 0 *X> 3^40 `mod` 3 0 I'm confused. Last I checked, 0 == 0. Using GHC 7.4.1, and the file x.hs (which has been loaded in ghci) can be found here: http://hpaste.org/69342 I noticed this after prop_sanemodexp was failing. Any help would be appreciated, - Clark

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Re: [Haskell-cafe] How to write Source for TChan working with LC.take?
by Hiromi ISHII 31 May '12

31 May '12

Oops, sorry. The last case's behaviour was not as I expected... A correct log is below: ~~~~ ghci> sourceTChanRaw ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanState ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanYield ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () *blocks* ~~~~ So again, sourceTChanYield blocks here even if it is already supplied with enough values! -- Hiromi ISHII konn.jinro(a)gmail.com

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Finding the average in constant space
by Chris Wong 31 May '12

31 May '12

Hello all I just came up with a way of executing multiple folds in a single pass. In short, we can write code like this: average = foldLeft $ (/) <$> sumF <*> lengthF and it will only traverse the input list once. The code is at: https://gist.github.com/2802644 My question is: has anyone done this already? If not, I might release this on Hackage -- it seems quite useful. Chris

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Haskell Weekly News: Issue 229
by Daniel Santa Cruz 31 May '12

31 May '12

Welcome to issue 229 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of May 20 to 26, 2012. Quotes of the Week * Peaker: Harrop taught me that I am a team of experts * applicative: Even if no one else understands me, still, the Glasgow Haskell Compiler understands me * JoeyA: Answers in #haskell are automatically sorted by readability descending. * monochrom: monochromism is a graph morphism that maps graphs to 1-colorable graphs :) * sipa: can we write a TutorialMonad t ? which is used to build and combine monad tutorials? Top Reddit Stories * my Haskell project was 100% funded on Kickstarter Domain: self.haskell, Score: 125, Comments: 22 On Reddit: [1] http://goo.gl/E2wkY Original: [2] http://goo.gl/E2wkY * Building intuition for monads, without mentioning monads (Ertugrul Söylemez) Domain: haskell.org, Score: 56, Comments: 32 On Reddit: [3] http://goo.gl/2leRd Original: [4] http://goo.gl/kMbJR * A gamified tutorial on the sequent calculus Domain: logitext.ezyang.scripts.mit.edu, Score: 51, Comments: 23 On Reddit: [5] http://goo.gl/tD8fZ Original: [6] http://goo.gl/E6zBl * pipes 2.0 - Pipe Finalization Domain: haskellforall.com, Score: 45, Comments: 32 On Reddit: [7] http://goo.gl/boq03 Original: [8] http://goo.gl/AABwV * Haste: Haskell to Javascript Compiler Domain: github.com, Score: 40, Comments: 45 On Reddit: [9] http://goo.gl/sJCcJ Original: [10] http://goo.gl/ZhYbP * Monad Reification in Haskell and the Sunroof Javascript compiler Domain: ittc.ku.edu, Score: 37, Comments: 26 On Reddit: [11] http://goo.gl/LVSVO Original: [12] http://goo.gl/s0PVp * HaskSymb, An Experiment in Haskell Symbolic Algebra. Feedback appreciated! Domain: github.com, Score: 36, Comments: 34 On Reddit: [13] http://goo.gl/d6Xo8 Original: [14] http://goo.gl/TUiJt * Using Ragel, Clang, LLVM and GHC to produce really fast stockmarket data parsers Domain: breaks.for.alienz.org, Score: 35, Comments: 8 On Reddit: [15] http://goo.gl/qEmKb Original: [16] http://goo.gl/ABzsA * Looks like Bryan finished importing Cabal issues to Github — thanks! Domain: github.com, Score: 32, Comments: 6 On Reddit: [17] http://goo.gl/9O8Sa Original: [18] http://goo.gl/bWrLg * What if haskellwiki were run by a Haskell wiki? Domain: haskellwiki.gitit.net, Score: 29, Comments: 14 On Reddit: [19] http://goo.gl/713Yb Original: [20] http://goo.gl/chSPM * New release of Elm available. Looking for feedback on the language so far. Domain: self.haskell, Score: 24, Comments: 21 On Reddit: [21] http://goo.gl/ejS9p Original: [22] http://goo.gl/ejS9p * Generics and Protocol Buffers Domain: breaks.for.alienz.org, Score: 17, Comments: On Reddit: [23] http://goo.gl/z0obj Original: [24] http://goo.gl/R5b3l Top StackOverflow Questions * What's the theoretical basis for existential types? votes: 32, answers: 3 Read on SO: [25] http://goo.gl/ZdAJb * How can a gc find out about object references done from the stack? votes: 20, answers: 4 Read on SO: [26] http://goo.gl/AtXUr * Why is GHC complaining about non-exhaustive patterns? votes: 14, answers: 3 Read on SO: [27] http://goo.gl/9oIsB * Evolving data structure votes: 13, answers: 2 Read on SO: [28] http://goo.gl/gbeLN * What makes a good name for a helper function? votes: 11, answers: 5 Read on SO: [29] http://goo.gl/b6B7o * Haskell “Apply”? [closed] votes: 11, answers: 3 Read on SO: [30] http://goo.gl/EBwsy * Round to nearest integer votes: 10, answers: 1 Read on SO: [31] http://goo.gl/1RaVk * Haskell: lazy versus eager evaluation for insertion sort votes: 6, answers: 1 Read on SO: [32] http://goo.gl/zGrie * Loss of polymorphism after pattern matching votes: 6, answers: 1 Read on SO: [33] http://goo.gl/YmA1y * Church-Rosser Theorem Example in a Functional Programming Language votes: 6, answers: 1 Read on SO: [34] http://goo.gl/FmSMh Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/u2vt5/my_haskell_project_was_100_f… 2. http://www.reddit.com/r/haskell/comments/u2vt5/my_haskell_project_was_100_f… 3. http://www.haskell.org/pipermail/haskell-cafe/2012-May/101338.html 4. http://www.reddit.com/r/haskell/comments/u04vp/building_intuition_for_monad… 5. http://logitext.ezyang.scripts.mit.edu/logitext.fcgi/tutorial 6. http://www.reddit.com/r/haskell/comments/tzp7f/a_gamified_tutorial_on_the_s… 7. http://www.haskellforall.com/2012/05/pipes-20-pipe-finalization.html 8. http://www.reddit.com/r/haskell/comments/txkb0/pipes_20_pipe_finalization/ 9. https://github.com/valderman/haste-compiler#haste 10. http://www.reddit.com/r/haskell/comments/tw997/haste_haskell_to_javascript_… 11. http://www.ittc.ku.edu/csdlblog/?p=88 12. http://www.reddit.com/r/haskell/comments/tvwn7/monad_reification_in_haskell… 13. https://github.com/colah/HaskSymb 14. http://www.reddit.com/r/haskell/comments/u1t8n/hasksymb_an_experiment_in_ha… 15. http://breaks.for.alienz.org/blog/2012/02/09/parsing-market-data-feeds-with… 16. http://www.reddit.com/r/haskell/comments/txfhq/using_ragel_clang_llvm_and_g… 17. https://github.com/haskell/cabal/issues 18. http://www.reddit.com/r/haskell/comments/u2faj/looks_like_bryan_finished_im… 19. http://haskellwiki.gitit.net/ 20. http://www.reddit.com/r/haskell/comments/txn1p/what_if_haskellwiki_were_run… 21. http://www.reddit.com/r/haskell/comments/u69wo/new_release_of_elm_available… 22. http://www.reddit.com/r/haskell/comments/u69wo/new_release_of_elm_available… 23. http://breaks.for.alienz.org/blog/2012/05/20/generics-and-protocol-buffers/ 24. http://www.reddit.com/r/haskell/comments/tw6dq/generics_and_protocol_buffer… 25. http://stackoverflow.com/questions/10753073/whats-the-theoretical-basis-for… 26. http://stackoverflow.com/questions/10710603/how-can-a-gc-find-out-about-obj… 27. http://stackoverflow.com/questions/10699532/why-is-ghc-complaining-about-no… 28. http://stackoverflow.com/questions/10761583/evolving-data-structure 29. http://stackoverflow.com/questions/10740515/what-makes-a-good-name-for-a-he… 30. http://stackoverflow.com/questions/10767407/haskell-apply 31. http://stackoverflow.com/questions/10738569/round-to-nearest-integer 32. http://stackoverflow.com/questions/10681507/haskell-lazy-versus-eager-evalu… 33. http://stackoverflow.com/questions/10728897/loss-of-polymorphism-after-patt… 34. http://stackoverflow.com/questions/10728997/church-rosser-theorem-example-i…

1 0

Unification of PolyTypeable TypeReps ?
by Ismael Figueroa Palet 30 May '12

30 May '12

Hi haskell-cafe In my current project I'm working with type representations created with PolyTypeable, and it turns out that given two type representations I need to find out if they are unifiable. I've looked at polytypeable-utils and see that they have some functions for substituting/applying/free vars, but it doesn't seem like a full solution for what I need. So my question is: is there a library to do this? is there a known way to transform the TypeRep's to make them more amenable to other unification libraries. I noticed that in GHC-api there is an unification library, and that there is also unification-fd. I wanted to ask here first in case someone can point me to the right direction. Thanks! -- Ismael

1 0

What is the difference between runhaskell and compile?
by Magicloud Magiclouds 30 May '12

30 May '12

Hi, I am writing a small program using LDAP hackage. A weird problem occured. When the code was run by runhaskell, things were fine, worked as expected. But when ghc compiled (no any args), and ran, I got this: LDAP error: ldapSimpleBind: LDAPException LdapServerDown(-1): Can't contact LDAP server. There is sure no problem with the server. So I am confused. I thought the two supposed to be the same. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

6 17

A functional programming solution for Mr and Mrs Hollingberry
by Andreas Pauley 30 May '12

30 May '12

Hi all, I'm in the process of learning how to approach problems from a functional perspective, coming from an Object Oriented background (mostly Smalltalk). One of the general concerns/questions raised when talking to people in a similar position is: "How do I design/model a problem when I don't have my trusted classes and objects available?" With this in mind I've created a programming exercise where I imagine an OO programmer would use an object hierarchy with subtype polymorphism as part of the solution. And then I'd like to compare functional implementations of the same problem: https://github.com/apauley/HollingBerries I want to see how elegant a solution I can get in a functional language, given that the problem description is not really elegant at all. It has a few annoying exceptions to the normal rules, typical of what one might get in a real specification from some client. Currently there are 3 implementations: - one in Erlang, my attempt at implementing a functional solution - one in Ruby, my attempt to see how an object hierarchy could be used - one in Clojure, done by one of the people in our FP user group [1] I would love to include some Haskell implementations as well, if any of you are interested. Kind regards, Andreas Pauley 1. http://www.meetup.com/lambda-luminaries/ -- http://pauley.org.za/ http://twitter.com/apauley

7 11

ANN: hledger 0.18
by Simon Michael 29 May '12

29 May '12

I'm pleased to announce version 0.18 of hledger! hledger (http://hledger.org) is a haskell library, command-line tool and web application for working with financial data (or anything that can be tracked numerically in an accounting journal). It is inspired by and compatible with John Wiegley's Ledger. hledger reads a plain text file describing transactions and lets you query account balances or transaction details by various criteria. It can also help you record new transactions, or convert CSV data from your bank. Release notes (http://hledger.org/NEWS.html) * web: hledger-web is now based on yesod 1.0 * web: fix js error breaking second use of add form (#72) * web: make `yesod devel` work * the command-line now supports a more powerful query language, consistent with the web UI * hledger now fully supports tags (aka metadata) on both transactions and postings, and querying by tag or tag value * new commands `incomestatement`, `balancesheet`, and `cashflow` provide basic financial statements under certain conditions * format conversion is now done on demand, and the convert command has been dropped. So instead of `hledger convert FILE.csv` just do `hledger -f FILE.csv print` or any other command. You can also pipe any supported format into `hledger -f- CMD` and hledger will try to do the right thing. * support for GHC 6.12 has been dropped; this release has been tested with GHC 7.0.4, 7.2.2, and 7.4.1 * unicode is now handled properly on all supported GHC versions * extensive API, test and internal cleanups Stats: - Release contributors: Simon Michael, xiaoruoruo - 87 days, 155 commits, 6 end-user features and 3 end-user bug fixes since last major release - 216 unit & functional tests (hledger-lib & hledger) - 7836 lines of code (hledger-lib, hledger & hledger-web) Install it: cabal update; cabal install hledger [hledger-web] If you have trouble, see http://hledger.org/MANUAL.html#installing . You can fund a ready-to-run binary for your platform with a donation at http://hledger.org/DOWNLOAD.html . Prosperity and clarity, -Simon

1 0

Estágio com Haskell em SP (Haskell internship at São Paulo, Brazil)
by Felipe Almeida Lessa 28 May '12

28 May '12

(English message follows after the break.) Olá! Estamos procurando um estagiário para trabalhar conosco em São Paulo capital. Nós somos uma startup apaixonada por Haskell com uma boa participação na comunidade e grandes projetos e desafios! Se você está interessado, basta me mandar um e-mail (felipe.lessa(a)gmail.com) com um link para sua conta do GitHub (ou para projetos em que você já tenha trabalhado). Você não precisa ser experiente em Haskell nem ter nenhum código Haskell publicado, mas isso seria um diferencial =). Como a startup está em stealth mode, não posso divulgar o nome da empresa e por isso estou usando meu e-mail pessoal. Até mais! === Hey! We're looking for an intern to work with us in São Paulo, Brazil. We're a startup passionate about using Haskell with great projects and challenges! If you're interested, just send me an e-mail (felipe.lessa(a)gmail.com) with a link to your GitHub account (or to any projects you've worked at). You don't need to be an experienced Haskeller nor have any published Haskell code, but that would be a plus =). As the startup is in stealth mode, I can't disclose the company's name nor use my work e-mail (that's why I'm using my personal one). Cheers, -- Felipe.

1 0

Building pattern and trying monads
by L Corbijn 28 May '12

28 May '12

Hello cafe, I'm working on a project where the main goal of the program is building some complex output (e.g. a Haskell function, module, etc.). In this process there is almost always some partially finished product on which to work. Currently I'm modelling this with a wrapper around StateT containing the partial product but I'm having some doubts about this approach. On one of the projects there is some rule-based transformation which needs to be done. The problem is with the matching and applying the rules. The matches need to retrieve information from the monads in the stack, and the application of a rule changes values in the stack. However when a match fails the stack should be left *untouched* and another rule should be tried. The solution I've in mind depends on the stack being pure. When the monad stack is pure a rule can be applied, returning a maybe value (or having a MaybeT wrapper) and when returning Nothing (failed rule) reverting the stack to it's point before applying the rule. As I'm not quite sure about the design (nor good at software design) I've some questions about this approach. 1. Is there a better approach then using a state monad for building the 'products'? 2. My solution with saving/reverting monad-stacks seems quite a hassle/hack, so is it a good approach or is there something better? Thanks in advance, Lars

4 5

Re: [Haskell-cafe] Building pattern and trying monads
by wren ng thornton 27 May '12

27 May '12

On 5/27/12 8:21 AM, L Corbijn wrote: > 2. My solution with saving/reverting monad-stacks seems quite a > hassle/hack, so is it a good approach or is there something better? One good solution for backtracking is to use logict[1]. I've used it with various state-like monads and it works well (e.g., the IntBindingT monad in unification-fd[2]). [1] http://hackage.haskell.org/package/logict http://okmij.org/ftp/papers/LogicT.pdf [2] http://hackage.haskell.org/package/unification-fd http://hackage.haskell.org/packages/archive/unification-fd/0.7.0/doc/html/C… -- Live well, ~wren

1 0

Record syntax, reopening a can of worms.
by timothyhobbs＠seznam.cz 27 May '12

27 May '12

Hello list, As my first post. I'd like to open a can of worms that I sure has been opened before. That is record syntax. As we all know, there are some type safety problems with our current record syntax. The most obvious is that this compiles without even giving you a warning: >data MyData = > A {a::Int, > b::Int} | > B {c::Int} >foo :: MyData -> Int >foo a@A{} = c a >main :: IO () >main = print $ foo (A 1 2) This compiles, then crashes at run time. Rediculus, why didn't the glorious GHC catch this? This is not some ambigious scenario... One way I found to improve type checking is this: >data MyData = A A' | B B' >data A' = A'{a::Int, > b::Int} >data B' = B'{c::Int} >foo :: MyData -> Int >foo (A a) = c a >main :: IO () >main = print $ foo (A (A' 1 2)) This doesn't compile anymore because a :: A' and c :: B' -> X It seems like the later method is simply better. Better type checking = fewer runtime errors(I actually ran into a crash in a real project because of this lack of proper type checking...). Can we have a nice sugar for doing the later method? Thank you for your time, Timothy

3 5

Problem with packet documentation generated by cabal on windows 7
by Nicu Ionita 27 May '12

27 May '12

Hi cafe, I have a problem with haddock documentation created when installing new packages with cabal on windows. The generated html files have all links in the form j:\Users\...\doc\...\xxx.html, but firefox says, it cannot open that link. Actually all links should be prefixed by "file:///". I wonder if this is only on windows so and if there is a solution to this. Is there a cabal or haddock flag for this? Nicu

5 5

ANN: format-0.1.0.0
by Scott Lawrence 27 May '12

27 May '12

Hey all, I've just uploaded to hackage format-0.1, a small library for handling format strings similar to those used by nginx's log_format directive. The idea is to allow easy parsing of text rendered with such a format string, to make parsing simple, automatically generated text files as easy as possible. So for example, if you want to parse lines consisting of two bits of text, separated by " -- ", then map (scanFormatString "$a -- $b") $ lines text will do so, generating associative arrays with the variable names as keys and their content as values. A `renderFormatString` function, doing the obvious, is also provided. Obligatory blog post with some more details: http://bytbox.net/blog/2012/05/format-haskell.html I hope somebody finds this useful. (This seems the sort of thing which should already have existed, but I couldn't find anything. Did I miss it?) I'm using parsec underneath, both for parsing the format string and the generated parser, and I've payed no attention to optimization. Some simple testing indicates that a reasonably fast computer takes about 1 second to parse 5000 lines with nginx's (and apache's) default format string - pretty slow. If there's demand, I'll be happy to pay more attention to that. -- Scott Lawrence

1 0

How to translate Repa 2 program to efficient Repa 3 code?
by Michael Serra 27 May '12

27 May '12

Hi Haskellers, I've posted this question (and my code) to stack overflow as well ( http://stackoverflow.com/questions/10747079/what-are-the-key-differences-be…) so if anyone here has the answer, I'll post it to that site for the world's reference. Using the Repa 2 API, I have written some simple image convolution tests which run more than fast enough. The trick to getting good performance was to call 'force' after every array transformation. I can't quite figure out the analogous thing to do with Repa 3 - at stackoverflow you can see my Repa 3 code, which runs correctly but very slowly. It is not clear to me how exactly the monadic "computeP" functions in Repa 3 are intended to be used - I have several calls to 'force' in my Repa 2 code, but only 1 call to computeP in the Repa 3 version. I've read the excellent "Numeric Haskell" Repa tutorial by Don S, but it doesn't cover Repa 3. My thanks in advance!

1 1

gona to join in haskell-cafe
by Dante.py 26 May '12

26 May '12

Hello,friends! I come from China, wishing to enjoy haskell with you. -- 张晔 Dante.py 中山大学09级数学与应用数学专业个人主页：http://dantepy.yslsg.org/

2 1

Correspondence between libraries and modules
by Alvaro Gutierrez 26 May '12

26 May '12

Hi there, I've only dabbled in Haskell, so please excuse my ignorance: why isn't there a 1-to-1 mapping between libraries and modules? As I understand it, a library can provide any number of unrelated modules, and conversely, a single module could be provided by more than one library. I can see how this affords library authors more flexibility, but at a cost: there is no longer a single, unified view of the library universe. (The alternative would be for every module to be its own, hermetic library.) So I'm very interested in the rationale behind that aspect of the library system. Thanks! Alvaro

8 20

Template Haskell antiquotation in user-defined quasiquoters
by Sam Lindley 26 May '12

26 May '12

Template Haskell supports antiquotation for built-in quasiquotes, e.g.: [| \x -> x + $([|3 * 4|]) |] However, as far as I can tell, there is no way of supporting antiquotation in user-defined quasiquoters, because the only way to specify a new quasiquoter is through a quoteExp function of type String -> Q Exp. Of course, it is perfectly possible to write a parser for some fragment of Haskell inside your quoteExp function, but that seems crazy given that Template Haskell or rather GHC already implements a parser for the whole language. I know about Language.Haskell.Exts.Parser in haskell-src-exts, which provides parseExp :: String -> ParseResult Exp, but that Exp is a different type to the one provided by Template Haskell.<http://hackage.haskell.org/packages/archive/haskell-src-exts/1.9.0/doc/html…> I'm also aware of Dominic Orchard's syntax-trees package, which supports converting between the two representations using a cunning hack that pretty-prints the haskell-src-exts representation to a string and uses Template Haskell to parse it back. Is there a saner way of simulating antiquotation in user-defined quasiquoters? Sam -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

4 3

webcam library on github
by . 26 May '12

26 May '12

Hello Cafe, since I haven't found anything like that, I wrote a small library [1] to read from a webcam in Haskell, using V4L on Linux. It uses the v4l2 package and repa for images. Is anyone interested in contributing to that, or giving some hints on how it could be made faster (I am not sure if I use repa correctly for best performance at all times), or just in emitting any other constructive comments? If yes, please give me a shout. Cheers, Christian PS I would put the whole thing on hackage, but my cabal currently doesn't want to do an sdist. [1] https://github.com/cgo/hsimage

4 4

Typed TemplateHaskell?
by Ilya Portnov 25 May '12

25 May '12

Hello Haskell world. One well-known missfeature of TemplateHaskell is it's «untypedness». I mean, that if you have say an expression (Exp or Q Exp), you do not know it's type (does it represent String or Int or whatever). And GHC does not know too. So, one could easily construct bad-typed expressions using TH, and GHC will complain only on generated code, not on TH code. Such a missfeature was a reasoned decision, because adding some typing into TH will add much complexity to compiler and to language. As far as can I see, using features of last GHC one could write typed TH library relatively easily, and saving backwards compatibility. For example, now we have Q monad and Exp type in "template-haskell" package. Let's imagine some new package, say "typed-template-haskell", with new TQ monad and new polymorphic type Exp :: * -> *. Using last GHC's features, one will easily write something like "expr :: Exp String", which will mean that "expr" represents a string expression. And we will need a new function, say runTQ :: TQ a -> Q a (or some more complicated type), which will turn TypedTemplateHaskell's constructs into plain TH. One question will be on quotations and antiquotations: they are implemented to work untyped TH constructs, how to implement compatibility with new typed TH? We could add some support of them to GHC... Or, maybe define a phantom type (say, Any), and some conversion functions, say untyped :: Language.Haskell.TH.Syntax.Exp -> Language.Haskell.TH.Typed.Syntax.Exp Any. Main idea is: seems new GHC enables us to implement "typed TH" without modifying GHC itself, only by writing a library. Is it so? Any further thoughts? With best regards, Ilya Portnov.

4 3

"Generic figures and their glueings" book ..... please help
by Vasili I. Galchin 24 May '12

24 May '12

Hello, I think the above book by Reyes, Reyes and Zolfaghari is quite intriguing. At the same time I can't get my mind around the notions of "generic figures" and "change of figures"! The first seems to be objects in the base category and the latter morphisms in the base category. Whoever has read this book and understands it please give me some pointers. Sorry the vague call for help. Kind regards, Vasili

1 0

Re: [Haskell-cafe] Can Haskell outperform C++?
by Roman Werpachowski 24 May '12

24 May '12

> From: "Richard O'Keefe" <ok(a)cs.otago.ac.nz> > Subject: Re: [Haskell-cafe] Can Haskell outperform C++? > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: <5F6605A2-DFE0-4AEA-9987-3B07DEF34C42(a)cs.otago.ac.nz> > Content-Type: text/plain; charset=us-ascii > > On 17/05/2012, at 2:04 PM, Gregg Lebovitz wrote: > >> Richard, >> >> Thank you. This is an example of what I had in mind when I talked about changing the playing field. Do you have a slide deck for this lecture that you would be willing to share with me? I am very interested in learning more. > > > No slide deck required. The task is "generating alternating permutations". > > Method 1: generate permutations using a backtracking search; > when a permutation is generated, check if it is alternating. > > Method 2: use the same backtracking search, but only allow extensions > that preserve the property of being an alternating permutation. Gregg, what makes Method 2 so much harder than Method 1 to implement in C or C++? Regards, RW

10 23

Re: [Haskell-cafe] Converting wiki pages into pdf
by do_not_reply_to_this_address＠t-online.de 24 May '12

24 May '12

I invested an enormous amount of time into this problem. Accordingly I got a very well working solution. http://de.wikibooks.org/wiki/Benutzer:Dirk_Huenniger/wb2pdf http://en.wikibooks.org/wiki/File:Haskell.pdf I am happy If you find it useful. Yours Dirk Hünniger > Thu, 08 Sep 2011 05:36:44 -0700 > Hello all > I am trying to write a Haskell program which download html pages from > wikipedia including images and convert them into pdf . I wrote a > small script > > import Network.HTTP > import Data.Maybe > import Data.List > > main = do > x<- getLine > htmlpage<- getResponseBody =<< simpleHTTP ( getRequest x ) -- > open url > --print.words $ htmlpage > let ind_1 = fromJust . ( \n -> findIndex ( n `isPrefixOf`) . > tails $ htmlpage ) $ "" > ind_2 = fromJust . ( \n -> findIndex ( n `isPrefixOf`) . > tails $ htmlpage ) $ "" > tmphtml = drop ind_1 $ take ind_2 htmlpage > writeFile "down.html" tmphtml > > and its working fine except some symbols are not rendering as it > should be. Could some one please suggest me how to accomplish this > task. > > Thank you > Mukesh Tiwari >

1 0

Formalisation for types of monads
by Yves Parès 24 May '12

24 May '12

When explaining monads to beginners (having an imperative background), I found myself to say that there is *roughly* three "groups" of monads (because they're always worried about their cost, i.e. their incidental complexity): - Function-oriented monads (e.g. State, Reader, Cont) - Reductible data-oriented monads (e.g. Maybe, Identity, Writer) - Irreductible data-oriented monads (e.g. List, free monads) (which, as I understood, have a (>>=) operation that has a quadratic complexity if not used properly) Are there others "groups" I'm missing and is there terms to formalize them?

4 6

question about linking with sqlite3
by Canhua 24 May '12

24 May '12

Hi, I made a cabal package that use HDBC-sqlite3 on Windows. When cabal install it, I got an error from ld.exe: cannot find -lsqlite3. How can I tell ld.exe where sqlite3.dll is plcaed? Thanks.

1 1

Parallel cooperative multithreading?
by Benjamin Ylvisaker 24 May '12

24 May '12

Has anyone ever worked on implementing something like this in Haskell? http://www.cs.hmc.edu/~stone/papers/ocm-unpublished.pdf The outline of the idea: - Concurrent programming is really hard with the popular frameworks today. - For most purposes parallel programming is hard, in some part because it requires concurrent programming. Of course there are attempts to do non-concurrent parallel programming, but I hope it's not too controversial to say that such frameworks are still on the fringe. - Cooperative concurrency is way easier than preemptive concurrency because between invocations of pause/yield/wait, sequential reasoning works. - Historically, cooperative concurrency only worked on a single processors, because running threads in parallel would break the atomicity of sequential blocks (between invocations of p/y/w). - Researchers have been poring tons of effort into efficiently running blocks of code atomically. - Hey, we can do parallel cooperative multithreading! The paper discusses implementations in Lua, C++ and C, but I think Haskell could be an awesome substrate for such a framework. Has anyone thought about this? Ben

4 3

Re: [Haskell-cafe] [Haskell-beginners] Most C++ compilers will not optimize x^2.0 as x*x but instead will do an expensive ...
by Tom Murphy 24 May '12

24 May '12

Just to play devil's advocate, if you look back at the list, KC has written a lot of helpful and informative messages in the past. Tom On 5/23/12, Bardur Arantsson <spam(a)scientician.net> wrote: > On 05/24/2012 04:31 AM, Brandon Allbery wrote: >> On Wed, May 23, 2012 at 10:19 PM, Bardur Arantsson >> <spam(a)scientician.net>wrote: >> >>> This has come up before -- this KC person probably has a broken mail >>> client which doesn't set appropriate References headers. >>> >> >> That, however, ignores the rest of it; the lack of references in this >> case >> forms a pattern with the other things I noted, in that a conversation is >> apparently being held in the form of single observations emitted at the >> point of observation instead of being collected and presented *as* a >> conversation. >> > > Right. I was actually just about to respond to (only) KC in person, but > perhaps unwisely, decided to "hijack" your response to add a little > explanation for everyone. > > You are of course right that not quoting context and just randomly > spewing out small bits of text is not really suitable for a mailing list. > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

1 0

Most C++ compilers will not optimize x^2.0 as x*x but instead will do an expensive ...
by KC 24 May '12

24 May '12

exponentiation and logarithm. So, I believe this C++ versus Haskell versus (your language of choice) is a Penn & Teller misdirection. Whereas, another level of indirection solves everything. -- -- Regards, KC

3 3

Haskell Weekly News: Issue 228
by Daniel Santa Cruz 24 May '12

24 May '12

Welcome to issue 228 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of May 13 to 19, 2012. Announcements Stefan Wehr issued a call for presentations for the Commercial Users of Functional Programmers (CUFP 2012) conference to be held in Copenhagen, Denmark during September 13 to 15, 2012. [1] http://goo.gl/Rl1T9 Quotes of the Week * hpc measures understanding in angstroms per (second * liter) ^ (1/15) Top Reddit Stories * ANNOUNCE: forthcoming O'Reilly book on Parallel and Concurrent Haskell Domain: haskell.org, Score: 101, Comments: 13 On Reddit: [2] http://goo.gl/UY9L6 Original: [3] http://goo.gl/oqLo6 * GHC 7.6 will have support for deferred type errors Domain: hackage.haskell.org, Score: 55, Comments: 4 On Reddit: [4] http://goo.gl/sPknX Original: [5] http://goo.gl/ieM9s * What happens when you mix three research programming languages together Domain: blog.ezyang.com, Score: 41, Comments: 7 On Reddit: [6] http://goo.gl/ujmF1 Original: [7] http://goo.gl/jME65 * Reactive-Banana 0.6: FRP in Haskell, supports both WxHaskell and generation to Javascript! Domain: apfelmus.nfshost.com, Score: 39, Comments: 3 On Reddit: [8] http://goo.gl/XrFxl Original: [9] http://goo.gl/Uanl6 * Keter: It's Alive! (Web app deployment system for Haskell) Domain: yesodweb.com, Score: 35, Comments: 6 On Reddit: [10] http://goo.gl/RyuLI Original: [11] http://goo.gl/EVcDT * Reasoning about Stream Processing with Effects Domain: personal.cis.strath.ac.uk, Score: 26, Comments: 26 On Reddit: [12] http://goo.gl/TI8bj Original: [13] http://goo.gl/n2rS0 * Patches for the new GHC-iPhone (v4) builds but needs a little more debugging Domain: hip-to-be-square.com, Score: 26, Comments: 27 On Reddit: [14] http://goo.gl/7QvqK Original: [15] http://goo.gl/zLCzr * Commutative Edits and Exceptions -- Categorically Dual(ish)? Domain: bosker.wordpress.com, Score: 24, Comments: 2 On Reddit: [16] http://goo.gl/1quI5 Original: [17] http://goo.gl/ET2ym * Parallel Haskell Digest 10: SPJ on Cloud Haskell, concurrent channels, meta-par, Accelerate, and StackOverflow'ing like crazy Domain: well-typed.com, Score: 21, Comments: 3 On Reddit: [18] http://goo.gl/z1UiV Original: [19] http://goo.gl/iPiAx * Interactive Programming with LambdaCalc Domain: dynamicaspects.org, Score: 20, Comments: 6 On Reddit: [20] http://goo.gl/IeEis Original: [21] http://goo.gl/n3Bqo * BitParser - RFC on a new parsing library for parsing data at the bit level. Similar in functionality to Erlang's bit syntax Domain: github.com, Score: 18, Comments: 8 On Reddit: [22] http://goo.gl/a7Qmq Original: [23] http://goo.gl/SL1u4 Top StackOverflow Questions * Haskell: Between a list and a tuple votes: 10, answers: 3 Read on SO: [24] http://goo.gl/6eiPg * Why is it so uncommon to use type signatures in where clauses? votes: 9, answers: 4 Read on SO: [25] http://goo.gl/ih3cK * Is there any good Haskell indentation script for Vim? votes: 9, answers: 2 Read on SO: [26] http://goo.gl/t1D3Y * Haskell “exceptions” votes: 8, answers: 2 Read on SO: [27] http://goo.gl/5WfTT * Is it possible to program and check invariants in haskell? votes: 8, answers: 3 Read on SO: [28] http://goo.gl/5wNE6 * The “handle” function and Real World Haskell votes: 7, answers: 2 Read on SO: [29] http://goo.gl/K8WK0 * Haskell: Flatten binary tree votes: 7, answers: 2 Read on SO: [30] http://goo.gl/eSokk * How can I uninstall a version of a Cabal package? votes: 6, answers: 1 Read on SO: [31] http://goo.gl/Gl1Mi * Fault tolerant JSON parsing votes: 6, answers: 1 Read on SO: [32] http://goo.gl/NBsVH * Can a Haskell or Haskell OS thread waiting on Network.Socket.accept not be killed on Windows? votes: 6, answers: 1 Read on SO: [33] http://goo.gl/8NHKB * efficient functional data structure for finite bijections votes: 6, answers: 2 Read on SO: [34] http://goo.gl/bXA1Ke Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19157 2. http://www.haskell.org/pipermail/haskell/2012-May/023328.html 3. http://www.reddit.com/r/haskell/comments/trf72/announce_forthcoming_oreilly… 4. http://hackage.haskell.org/trac/ghc/wiki/Status/May12 5. http://www.reddit.com/r/haskell/comments/tm1mc/ghc_76_will_have_support_for… 6. http://blog.ezyang.com/2012/05/what-happens-when-you-mix-three-research-pro… 7. http://www.reddit.com/r/haskell/comments/tptuo/what_happens_when_you_mix_th… 8. http://apfelmus.nfshost.com/blog/2012/05/15-frp-banana-0-6.html 9. http://www.reddit.com/r/haskell/comments/toghk/reactivebanana_06_frp_in_has… 10. http://www.yesodweb.com/blog/2012/05/keter-its-alive 11. http://www.reddit.com/r/haskell/comments/tsyv5/keter_its_alive_web_app_depl… 12. https://personal.cis.strath.ac.uk/~raa/posts/2012-01-06-streams.html 13. http://www.reddit.com/r/haskell/comments/tm7ur/reasoning_about_stream_proce… 14. http://hip-to-be-square.com/~blackh/ghc-iphone-new/v4/ 15. http://www.reddit.com/r/haskell/comments/tnnqc/patches_for_the_new_ghciphon… 16. http://bosker.wordpress.com/2012/05/10/on-editing-text/ 17. http://www.reddit.com/r/haskell/comments/ts3sz/commutative_edits_and_except… 18. http://www.well-typed.com/blog/66 19. http://www.reddit.com/r/haskell/comments/tta8y/parallel_haskell_digest_10_s… 20. http://dynamicaspects.org/blog/2012/05/15/back-to-the-future/ 21. http://www.reddit.com/r/haskell/comments/tokmh/interactive_programming_with… 22. https://github.com/axman6/BitParser/tree/bs-state 23. http://www.reddit.com/r/haskell/comments/tmhca/bitparser_rfc_on_a_new_parsi… 24. http://stackoverflow.com/questions/10626943/haskell-between-a-list-and-a-tu… 25. http://stackoverflow.com/questions/10609936/why-is-it-so-uncommon-to-use-ty… 26. http://stackoverflow.com/questions/10663888/is-there-any-good-haskell-inden… 27. http://stackoverflow.com/questions/10611382/haskell-exceptions 28. http://stackoverflow.com/questions/10656402/is-it-possible-to-program-and-c… 29. http://stackoverflow.com/questions/10578572/the-handle-function-and-real-wo… 30. http://stackoverflow.com/questions/10592920/haskell-flatten-binary-tree 31. http://stackoverflow.com/questions/10576868/how-can-i-uninstall-a-version-o… 32. http://stackoverflow.com/questions/10604808/fault-tolerant-json-parsing 33. http://stackoverflow.com/questions/10608602/can-a-haskell-or-haskell-os-thr… 34. http://stackoverflow.com/questions/10669217/efficient-functional-data-struc…

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Haskell Meeting Munich
by Heinrich Hördegen 23 May '12

23 May '12

Dear all, our local hackathon in Munich on the 12th of May was a real success. Thanks to all those who joined! Look at the site for a picture: http://www.haskell-munich.de/dates Our next regular meeting is on Monday, the 28th of May at Cafe Puck at 19h30. This is a holiday in Germany. We hope that many of you will come! If you plan to do so, please click the button on the above mentioned site. Until then, I wish all of you a nice time, Heinrich -- hoerdegen(a)funktional.info www.funktional.info

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HPC question
by David Turner 23 May '12

23 May '12

Dear Haskell Cafe, I've been playing around with the -fhpc option to GHC and have come across some things being highlighted yellow in the HTML markup for which I can't work out the right way to 'cover' them. A cut down program that shows what I'm talking about is at http://hpaste.org/68888 Looking at the output of hpc show, I see: 47 0 Main 15:6-15:9 ExpBox False 48 2 Main 15:6-15:9 ExpBox False 49 0 Main 15:6-15:9 ExpBox False 50 0 Main 15:6-15:9 ExpBox False 51 1 Main 22:13-22:14 TopLevelBox ["=="] 52 0 Main 22:13-22:14 TopLevelBox ["/="] 53 1 Main 22:17-22:20 TopLevelBox ["showsPrec"] 54 0 Main 22:17-22:20 TopLevelBox ["showList"] I.e. each of those bits whose highlighting I don't understand is made of some overlapping boxes, one of which is used. Now, if I fiddle in the .tix file by hand to fake a usage of (/=) by changing the 52nd entry from 0 to 1, the Eq instance isn't highlighted any more in the HTML output. More strangely, if I then remove the usage of (==) by changing the 51st entry from 1 to 0, the Eq instance still isn't highlighted. A similar effect happens with the Show instance. It seems to be highlighting based only on the last entry in the .tix file, where there are two or more identically-placed boxes. Is this right? I'd have expected that if I use (==) then the deriving (Eq) clause should be considered 'used'. Secondly, I can't work out what the four boxes in position 15:6-15:9 are supposed to be. If I use -ddump-simpl I can see many calls to 'tick' but there's no mention of numbers 47, 49 or 50. Perhaps they've been simplified away? I'm afraid I don't know what else to try dumping to get at the instrumented code before the simplifier's had a go at it. I'm using Haskell Platform 2011.2.0.1 (GHC 7.0.3) on Windows if that's relevant. Thanks in advance, David

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Re: [Haskell-cafe] Please critique my code (a simple lexer)
by Lorenzo Bolla 23 May '12

23 May '12

> On Tue, May 22, 2012 at 4:13 PM, John Simon <zildjohn01(a)gmail.com> wrote: > >> data Lexer = Lexer String >> >> makeLexer :: String -> Lexer >> makeLexer fn = Lexer fn > > `makeLexer` is redundant. You can simply use `Lexer`. L.

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cool tools
by Chris Dornan 23 May '12

23 May '12

I have been playing around with the latest cabal-install (0.14.0) and it is working really nicely. Having unpacked a cabal bundle you can now type 'cabal install' inside the root and it will work everything out as if you had asked to install directly from the repo -- very nice. I have also noticed that GHC is suggesting alternatives when it encounters missing identifiers. This gives a strong sense of helpfulness that I think accurately reflects the long and sustained (decades-long) effort that has gone into making the GHC diagnostics as useful as possible. The tools are so good because the developers have been paying attention to the gripes. But we should sometimes say thank you too... it is much appreciated. Chris

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Can Haskell outperform C++?
by Janek S. 23 May '12

23 May '12

Hi, a couple of times I've encountered a statement that Haskell programs can have performance comparable to programs in C/C++. I've even read that thanks to functional nature of Haskell, compiler can reason and make guarantess about the code and use that knowledge to automatically parallelize the program without any explicit parallelizing commands in the code. I haven't seen any sort of evidence that would support such claims. Can anyone provide a code in Haskell that performs better in terms of execution speed than a well-written C/C++ program? Both Haskell and C programs should implement the same algorithm (merge sort in Haskell outperforming bubble sort in C doesn't count), though I guess that using Haskell-specific idioms and optimizations is of course allowed. Jan

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Found quote: "One of the ineradicable illusions about C++ is that the compiler should be able to see through ...
by KC 23 May '12

23 May '12

all the abstractions and obfuscations an “advanced” C++ program contains. First and foremost, C++ should be seen as a language that enables complexity management. The features one has grown fond of in this concept, like operator overloading, object orientation, automatic construction/destruction, etc., are however mostly unsuitable for efficient low-level code." >From "Introduction to High Performance Computing for Scientists and Engineers" CRC Press, 2011. So if one avoids such C++ complexity management features and a Haskell compiler can see through most of the complexity management features of Haskell; they should be comparable. :) Casey -- -- Regards, KC

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