November 2013 - Haskell-Cafe

Canberra interest?
by Tony Morris 18 Nov '13

18 Nov '13

Hello, I am from Brisbane, Australia and it is my mission to teach functional programming to all and especially Australia. I am tasked with this mission by my employer, NICTA, which is funded by the Australia government. With support from NICTA, I usually run (free) courses to assist in this mission and have done so in Sydney, Melbourne and Brisbane. However, I often wonder about any interest in Canberra. I do not find any user interest groups related to Functional Programming or language-specific such as Haskell when I search, so I was hoping to seek out any expressions of interest here. Are any of you from Canberra? If so, are you interested in learning more about functional programming? Would you be interested in attending a 2 or 3 day workshop? Do you have peers who would also be interested? If these people exist, how might I contact them? Thanks for any tips! -- Tony Morris http://tmorris.net/

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Word rigid in "`a' is a rigid type variable..."
by Vlatko Basic 15 Nov '13

15 Nov '13

Hi Cafe, in an example function f :: a -> Bool f a = let b = "x" in a == b compiler complains with `a' is a rigid type variable bound by the type signature for f :: a -> Bool I'm puzzled with the choice of word 'rigid' here. I see these types as - 'b' has "rigid/unchangeable" type (only String), and - 'a' has "soft/variable" type (any type, no constraints). Why is it called rigid? Where does the meaning (in this context) come from? Best regards, vlatko

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Re: [Haskell-cafe] how to call a callback in a haskell dll
by Donn Cave 15 Nov '13

15 Nov '13

Quoth "Kees Bleijenberg" <k.bleijenberg(a)lijbrandt.nl>, > My program creates a Windows dll. The calling program calls a function with > strings as parameters. Works fine. > > I want to add a extra parameter. This extra parameter is a callback: a > function Int -> Int: stdcall. In Haskell I want to call this function. So I > have to convert the pointer to a (formal) Haskell function. > > In the examples for the FFI, Haskell provides the callback. I need an > example or documentation. It's unclear to me whether the Haskell program initially chooses the function and creates a pointer to it, or that happens in C/C++. In the former case, I think Foreign.StablePtr serves that purpose. In the latter case, Haskell will receive the function as something like FunPtr (CInt -> CInt), and you need a "dynamic" foreign function to convert the pointer, like foreign import ccall "dynamic" iiPtrStub :: FunPtr (CInt -> CInt) -> (CInt -> CInt) That's lightly documented in the library docs under Foreign.Ptr. Donn

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Tool for visualisation of graph reduction step by step
by Miguel Ramos 15 Nov '13

15 Nov '13

Hello, It has already been asked for on this list, and I think one does not exist, however: (a) Does anyone know of a tool for the stepwise visualisation of graph reduction? I am aware that it could only be used for minimal examples and that the output would be a bit implementation-specific. My interest is not debugging, but simply producing presentation slides. (b) What tools do people use for plotting static program graphs, such as those in books, papers and slides? I am currently using graphviz (dot) but I am not being able to tame it to make the graphs look good and proper for papers and slides. I also have a minimal interpreter, which can currently output each step to graphviz, but it still does not handle enough of the language to be usable and I don't want to divert the focus of my work. Thank you very much, Miguel Lopes Santos Ramos

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Re: [Haskell-cafe] Tool for visualisation of graph reduction step by step
by Stephen Tetley 15 Nov '13

15 Nov '13

On 14 November 2013 12:03, Miguel Ramos <org.haskell(a)miguel.ramos.name> wrote: > > (b) What tools do people use for plotting static program graphs, such as > those in books, papers and slides? Graphviz can be tamed somewhat with ranking - see the examples in the manual. If you want pretty, hand coded graphs then TikZ is a handsome option. It has the obvious edges with or without arrowheads, but also the "anchor" abstraction which is a very neat way of placing edges.

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how to call a callback in a haskell dll
by Kees Bleijenberg 15 Nov '13

15 Nov '13

My program creates a Windows dll. The calling program calls a function with strings as parameters. Works fine. I want to add a extra parameter. This extra parameter is a callback: a function Int -> Int: stdcall. In Haskell I want to call this function. So I have to convert the pointer to a (formal) Haskell function. In the examples for the FFI, Haskell provides the callback. I need an example or documentation. Kees

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ANNOUNCE: Haskell Pdf Presenter (hpdfp) 0.2.2
by Michael D. Adams 15 Nov '13

15 Nov '13

ANNOUNCE: Haskell Pdf Presenter (hpdfp) 0.2.2 I'm pleased to announce the first public release of the Haskell Pdf Presenter (hpdfp). Project home page: http://michaeldadams.org/projects/haskell-pdf-presenter/ Project on Hackage: http://hackage.haskell.org/package/haskell-pdf-presenter/ Project repository: http://michaeldadams.org/repos-pub/hg/haskell-pdf-presenter/ What it is ========== The Haskell Pdf Presenter (hpdfp) is a tool for presenting PDF-based slide shows. For example, as a presenter, I like to see the next slide before it is shown to the audience as well as how much allotted time remains. I was never quite satisfied with the existing tools for this, so I wrote my own and hpdfp is the result. Though I originally wrote it for my own use, the tool has matured over time, and I now wish to share it with others. Aside from being a useful tool, I think hpdfp is a good example of elegance and economy in program design. Despite being only a thousand lines long, this program is full of features that may not be obvious at first glance so please be sure to look at the available help. Status ====== I've been using this tool in my own talks for over a year now, and it is fairly stable and feature complete. I've only tested it on Ubuntu so I don't know if it ports to other platforms. I'd love to hear any field reports about how it runs on other platforms. Future Directions ================= I have a long TODO list of ideas I may implement in the future, but most development has been driven by the practical considerations that I discover as I use the tool. If you have a use case for a particular feature, please let me know. ================ Feature requests, bug reports, suggestions or feedback as well as contributions to the documentation or implementation are most welcome. Michael D. Adams

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Aeson js question
by hc＠j4.pe 14 Nov '13

14 Nov '13

Hello everyone, I'd use aeson to parse JSON. I did much of the translation JSon in Haskell data but I'm stuck on one point. Suppose I have a JSon object that is in this form: {\"Ports\":{\"22/tcp\":[{\"HostIp\":\"0.0.0.0\",\"HostPort\":\"49154\"}],\"80/tcp\":[{\"HostIp\":\"0.0.0.0\",\"HostPort\":\"49155\"}]} I do not know how to "bind" the key name and value knowing that the key changes name "randomly". Example with port 21 and port 8888: {\"Ports\":{\"21/tcp\":[{\"HostIp\":\"0.0.0.0\",\"HostPort\":\"49154\"}],\"8888/tcp\":[{\"HostIp\":\"0.0.0.0\",\"HostPort\":\"49155\"}]}} All code : http://lpaste.net/95607 Thank you for your help.

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ANNOUNCE: Haskell Communities and Activities Report (25th ed., November 2013)
by Janis Voigtlaender 14 Nov '13

14 Nov '13

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (25th edition, November 2013) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2013/report.pdf http://haskell.org/communities/11-2013/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of April 2014: target deadline for contributions to the May 2014 edition of the HC&A Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Mihai Maruseac and Janis Voigtlaender <hcar at haskell.org>

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off-topic question: why invent new Haskell-like compile-to-js functional languages when we have awesome js_of_ocaml ?
by Ömer Sinan Ağacan 13 Nov '13

13 Nov '13

Hi all, Sorry for off-topic question, I'm asking this question here because as Haskell community we invent lots of new Haskell-like(sometimes a subset of Haskell like Fay, sometimes different languages with Haskell syntax and Haskell-like features like Elm and recent language with row polymorphic records -- I don't remember it's name) languages. What I'm wondering is that js_of_ocaml project looks like solving all problems we have. It's supports 100% of OCaml(it translates OCaml bytecode), and OCaml is arguably better than all languages we invented(awesome module system, records, strict by default, all OCaml libraries(except FFI ones) work, like functional data structures, parser generators etc.) (I'm assuming we won't have JS backend for GHC anytime soon) So in short, I don't understand why use Haskell like compile-to-js languages instead of js_of_ocaml (other than learning purposes -- JS may be a nice high-level target language to practice writing compilers etc.) Please note that I'm not using any compile-to-js languages in my work, so I may not be able to make a fair comparison, but to me all other compile-to-js langauges look worse after seeing js_of_ocaml. Of course, if I had a JS backend for GHC(which would mean compiling Haskell to JS with all extensions) it would be best solution, but for now I think js_of_ocaml is superior to all other solutions. I'm wondering your opinions about this. Thanks, --- Ömer Sinan Ağacan http://osa1.net

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