December 2012 - Haskell-Cafe

what is the purpose of GHC.Exts.lazy?
by jared simpson 02 Dec '12

02 Dec '12

docs say: The call '(lazy e)' means the same as e, but lazy<http://hackage.haskell.org/packages/archive/base/latest/doc/html/GHC-Exts.h…> has a magical strictness property: it is lazy in its first argument, even though its semantics is strict. why do i want to use magic during strictness analysis?

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Simple GUI Form
by Rune Harder Bak 02 Dec '12

02 Dec '12

Hi I have some input parameters data Input = ... that I need the user to enter in a gui pop-up. (windows people...) The rest of the app is not gui (or perhaps progress could be displayed in a log-window) What is the easiest way to make such a GUI form? It need to compile for both Linux and Windows, so I though WxWidgets was ideal, and I got wx[1] < 0.90 to install (using wxWidgets2.8) on both windows and linux. Now I just need to create the form, but how do you do that? Any clues or links to examples? I have never used wxwidgets on any platform or done any other form of GUI before for that matters. (apart from some Visual Basic ten years ago, and html). I tried looking at wx examples, but I couldn't find this simple use case explained anywhere. I installed wx in the first place because WxGeneric[2] seemed exactly what I needed, but I can't get it to compile using ghc7.4.2 from haskell-platform. Anybody got that working or have some other simple method? Help much appreciated! -Rune [1] http://hackage.haskell.org/package/wx-0.13.2.3 [2] http://hackage.haskell.org/package/WxGeneric

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computation over containers, greatly simplified notation.
by Takayuki Muranushi 01 Dec '12

01 Dec '12

Dear all, I came up with an idea to greatly simplify some kinds of array computations. It should work well with many kinds of arrays. Is this new? https://gist.github.com/4162375 These few days, I've been trying to rewrite a hydrodynamic simulation code that used Data.Vector (~250 lines), to Repa [1] . It seemed promising, but soon I realized that I needed to use Repa.map and Repa.zipWith almost everywhere. I need careful thinking to transform every lines (that used vector's indexing) to Repa's point-free stile. Is there any better ways? Then I realized that I was the author of Paraiso [2], a DSL for stencil computation. One of its feature is to write down array computation just as if it were scalar computation. Basically what I need is ZipList-like Applicative instances for vectors and Repa arrays. Why not they support ZipVector? Because 'pure' of zipList was an infinite list and you can't do infinite vectors. Then I came up with this idea. https://gist.github.com/4162375 the wrapper W does several things: it represents the 'pure,' homogeneous array in a space-efficient manner, and also serves as a newtype-wrapper of Num (and possibly Fractional, Floating...) instances. Questions are: is this technology new? or promising? doomed? It seems to me like a free-Applicative, like the free-Monad theory. Are they related? The function 'backend' helps to mix in the non-zip-like computations. How can we remove the 'undefined' in the 'backend?' Some of Repa computations are Monads. W needs to be a monad transformer to incooperate this. Also I'm grateful to past cafe discussion on existing Zippable implementations [3][4] . [1] hackage.haskell.org/package/repa [2] http://hackage.haskell.org/package/Paraiso [3] http://www.haskell.org/pipermail/haskell-cafe/2009-July/064403.html [4] http://hackage.haskell.org/packages/archive/category-extras/latest/doc/html… -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Are storable-mutable-vectors in two dimensions possible in Haskell?
by Fixie Fixie 01 Dec '12

01 Dec '12

Hello haskellers I am struggling with this package: Data.Vector.Storable.Mutable I am creating vectors like this: MV.new 1000 :: IO (V.MVector (PrimState IO) Int) Now I would like access to this vectors in linear time, like I could have done in C using pointers. The problem is that I do not know which datastructure to keep my vectors in. If I put them in a list, the access is very slow. Even a binary tree is very slow compared to a C-version. I have tried to make an array og vector of storable-mutable vectors, but have not manged to accomplish this - even though there must be a haskell way :-) Any clues? Cheers, Felix

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Why Kleisli composition is not in the Monad signature?
by damodar kulkarni 01 Dec '12

01 Dec '12

Hi, The Monad class makes us define bind (>>=) and unit (return) for our monads. Why the Kleisli composition (>=>) or (<=<) is not made a part of Monad class instead of bind (>>=)? Is there any historical reason behind this? The bind (>>=) is not as elegant as (>=>), at least as I find it. Am I missing something? - Damodar

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Can a GC delay TCP connection formation?
by Jeff Shaw 01 Dec '12

01 Dec '12

Hello, I've run into an issue that makes me think that when the GHC GC runs while a Snap or Warp HTTP server is serving connections, the GC prevents or delays TCP connections from forming. My application requires that TCP connections form within a few tens of milliseconds. I'm wondering if anyone else has run into this issue, and if there are some GC flags that could help. I've tried a few, such as -H and -c, and haven't found anything to help. I'm using GHC 7.4.1. Thanks, Jeff

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