- Haskell-Cafe - Haskell.org

Incorrect behavior in Data.Complex ?
by jared simpson 18 Nov '13

18 Nov '13

Prelude Data.Complex> 0**2 0.0 Prelude Data.Complex> 0**2 :: Complex Double NaN :+ NaN Prelude Data.Complex> exp $ 2 * log 0 :: Complex Double NaN :+ NaN Prelude Data.Complex> log 0 :: Complex Double (-Infinity) :+ 0.0 Prelude Data.Complex> 2 * it :: Complex Double (-Infinity) :+ NaN Prelude Data.Complex> exp it :: Complex Double NaN :+ NaN So Complex uses the default implementation of **. Then when 2*(-inf :+ 0) is evaluated. We do (2 * -inf - 0*0) :+ (2*0 + -inf*0). Which because of -inf*0 sets the imaginary part to NaN. Then exp (-inf :+ NaN) = exp x cos y :+ exp x sin y which becomes 0 * cos NaN :+ 0 * sin NaN. So we end up with NaN :+ NaN. So is there a library I should be using, or is this a bug?

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FP Complete Launches Free Community edition of FP Haskell Center
by Natalia Muska 18 Nov '13

18 Nov '13

https://www.fpcomplete.com/business/blog/fp-complete-launches-free-communit… -- Natalia Muska Marketing Manager FP Complete, Inc. Natalia(a)fpcomplete.com 865-506-6513 skype: natalia.s.muska

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Error cabal update: "Unsucessful HTTP code: 404"
by Vaniax 18 Nov '13

18 Nov '13

Hi, I have a big problem with a fresh installation of the Haskell-Platform 2013.2 on Windows 7. When I try to run `cabal update` I get the following error message: "|Downloading the latest package list from hackage.haskell.org cabal: Failed to download http://hackage.haskell.org/packages/archive/00-index.tar.gz : ErrorMisc "Unsucessful HTTP code: 404" When I try ||`wget http://hackage.haskell.org/packages/archive/00-index.tar.gz`everything works alright. I'm using no proxies or something like that. Maybe someone know whats going wrong? A parallel Windows-XP installation of the platform works fine. Vaniax |

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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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