- Haskell-Cafe - Haskell.org

QuickCheck Questions
by Mark Spezzano 02 Aug '11

02 Aug '11

Hi all, I would appreciate it if someone can point me in the right direction with the following problem. I'm deliberately implementing a naive Queues packages that uses finite lists as the underlying representation. I've already read through Hughes' paper and the article in The Fun of Programming, but I'm still having some difficulties. Specifically: 1. I have a newtype Queue a = Queue [a] and I want to generate Queues of random Integers that are also of random size. How do I do this in QuickCheck? I guess that I need to write a generator and then make my "Queue a" concrete type an instance of Arbitrary? How? 2. If I wanted to specify/constrain the ranges of random Integers generated, how would I do this? 3. If I wanted to specify/constrain the Queue sizes how would I do this? On a separate issue, I also see that QuickCheck 2 has some features not discussed anywhere. For example "Coarbitrary" and "shrink" are a bit of a mystery to me. Any code examples for any of the above questions would help greatly. I've been stuck on this problem for hours. :) Cheers, Mark

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difference between class context and deriving
by Patrick Browne 02 Aug '11

02 Aug '11

What is the difference between using a class context and deriving in data type declaration? Are there certain situations in which one or the other is preferred? data Eq a => Set1 a = NilSet1 | ConsSet1 a (Set1 a) data Set2 a = NilSet2 | ConsSet2 a (Set2 a) deriving Eq (NilSet1) == (NilSet1) -- no instance, error (NilSet2) == (NilSet2) -- True -- seems OK type Type1 = Set1 Integer type Type2 = Set2 Integer -- seems OK data Data1 = Set1 | Set2 -- Seems to have same type :t ConsSet2 1 NilSet2 ConsSet2 1 NilSet2 :: forall t. (Num t) => Set2 t :t ConsSet1 1 NilSet1 ConsSet1 1 NilSet1 :: forall t. (Num t) => Set1 t -- Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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State of play with Haskell-Cocoa (Objective-C) bindings?
by Luke Evans 01 Aug '11

01 Aug '11

I'm planning to start an Objective-C/Cocoa project and would like to write it in Haskell as much as possible. Of course, I can contrive to split the project into app logic (Haskell) and UI (Objective-C) with some sort of minimal interface between them; possibly just plain old C FFI. However, I would like an interface that's as 'automatic' as possible - one that preferably means that I don't have to spend a lot of time maintaining a somewhat heavy-weight special interface layer as I grow my code on both sides of the divide. Preferably the suture line is as far over on the Objective-C side as possible, so that I can write pretty much everything in Haskell. Now, I see a couple of references to some options: - HOC seems very close to what I'm looking for, but the project looks pretty 'dormant'. - I see some mention of an Objective-C FFI, but am uncertain if this is in any current build of GHC. Are there any other options up anyone's sleeves? Anybody working on something interesting I could play with? Until recently, I was concerned about being able to use GC on the Objective-C side, but with Apple's new ARC memory management model, that's no longer an issue. Most Haskell-ObjC bindings that already exist are surely designed for Retain-Release, which I assume is not disrupted by ARC as this is just RR at runtime. -- Luke

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Unbelievable parallel speedup
by John D. Ramsdell 01 Aug '11

01 Aug '11

I've enjoyed reading Simon Marlow's new tutorial on parallel and concurrent programming, and learned some surprisingly basic tricks. I didn't know about the '-s' runtime option for printing statistics. I decided to compute speedups for a program I wrote just as Simon did, after running the program on an unloaded machine with four processors. When I did, I found the speedup on two processors was 2.4, on three it was 3.2, and on four it was 4.4! Am I living in a dream world? I ran the test nine more times, and here is a table of the speedups. 2.35975 3.42595 4.39351 1.57458 2.18623 2.94045 1.83232 2.77858 3.41629 1.58011 2.37084 2.94913 2.36678 3.63694 4.42066 1.58199 2.29053 2.95165 1.57656 2.34844 2.94683 1.58143 2.3242 2.95098 2.36703 3.36802 4.41918 1.58341 2.30123 2.93933 That last line looks pretty reasonable to me, and is what I expected. Let's look at a table of the elapse times. 415.67 176.15 121.33 94.61 277.52 176.25 126.94 94.38 321.37 175.39 115.66 94.07 277.72 175.76 117.14 94.17 415.63 175.61 114.28 94.02 277.75 175.57 121.26 94.10 277.68 176.13 118.24 94.23 277.51 175.48 119.40 94.04 415.58 175.57 123.39 94.04 277.62 175.33 120.64 94.45 Notice that the elapse times for two and four processors is pretty consistent, and the one for three processors is a little inconsistent, but the times for the single processor case are all over the map. Can anyone explain all this variance? I have enclosed the raw output from the runs and the script that was run ten times to produce the output. John

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A language that runs on the JVM or .NET has the advantage of Oracle & Microsoft making those layers more parallelizable.
by KC 01 Aug '11

01 Aug '11

Are there plans a foot (or under fingers) to make a version of Haskell that runs on the JVM? -- -- Regards, KC

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ANNOUNCE: HROOT-0.5.0.1 - haskell binding to ROOT
by Ian-Woo Kim 01 Aug '11

01 Aug '11

Hi, everyone, I am pleased to announce a haskell binding to the ROOT analysis package, HROOT: http://hackage.haskell.org/package/HROOT >From wikipedia: ROOT (http://root.cern.ch) is an object-oriented program and library developed by CERN. It was originally designed for particle physics data analysis and contains several features specific to this field, but it is also used in other applications such as astronomy and data mining. ROOT includes * histogramming and graphing to view and analyze distributions and functions, * curve fitting (regression analysis) and minimization of functionals, * statistics tools used for data analysis, * matrix algebra, * four-vector computations, as used in high energy physics, * standard mathematical functions, * multivariate data analysis, e.g. using neural networks, * image manipulation, used, for instance, to analyze astronomical pictures, * access to distributed data (in the context of the Grid), * distributed computing, to parallelize data analyses, * persistence and serialization of objects, which can cope with changes in class definitions of persistent data, access to databases, * 3D visualizations (geometry) * creating files in various graphics formats, like PostScript, PNG, SVG, * interfacing Python and Ruby code in both directions, * interfacing Monte Carlo event generators. Current implementation of HROOT is very limited to histogramming and graphing, but complete binding will be planned to be provided soon. This will help haskell community for doing numerical analysis. To install HROOT, you have to install ROOT prior to HROOT. It's fairly easy to install. The homepage of HROOT is now on http://ianwookim.org/HROOT Enjoy this new addition. I'd love to hear feedback. Part of this work was done in my "HacPhi 2011" activity. Thank you for HacPhi organizers and all the participants. Ian-Woo

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Re: [Haskell-cafe] XCode Dependency for HP on Mac
by Tom Murphy 31 Jul '11

31 Jul '11

On 7/30/11, Mark Lentczner <mark(a)glyphic.com> wrote: > Hiho - I'm the maintainer of the Mac installer for HP. I thought I'd > chime in a bit: > An expert :) > On Mac OS X, "developer tools" is essentially synonymous with "Xcode". > That is, to get the set of standard utilities needed for development > on compiled executables (notably the "binutils"), you install Xcode. > True, it also includes the IDE called Xcode, but the vast bulk [...] Remember everyone: install just the system tools and dev tools (only haskell required parts), and your install is 1 or 2 GB instead of 11GB. > As several have pointed out, you can download Xcode for free. It turns out this is true. It's very hard to find the dl link to an XCode that doesn't require Lion, but it actually is there. > (I haven't bought a new Air... > yet... but perhaps someone can check to see if the Xcode installer is > one the SSD volume already?) > It doesn't, and it's not on the USB restore disk either. > It is conceivably possible to build and distribute some of those > tools, but not the whole bundle. But the difficulty of getting such a > build just right, and all the pieces in the right place, seems absurd > to attempt to recreate when Apple has done it, and gives it away for > free. Apple's versions of bintools also includes many extensions extra > options for the OS X environment (like supporting multi-arch binaries) > Finally, there is also licensing questions regarding the parts > supplied by the OS vendor (headers, stub libs, debug libs, etc....) > So in response to the question "is there a way to install the HP without XCode?" the answer is pretty much no. Ok. It's a shame, but thanks for the insight! Thanks, Tom

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Re: [Haskell-cafe] [Haskell-beginners] No instance for (Show a)
by Mark Spezzano 31 Jul '11

31 Jul '11

Hi, You might need a class constraint. instance (Show a) => Show (Stack a) where This basically lets Haskell know that if your "a" type is Showable then "Stack a" is also Showable. Let me know if this works. Cheers, Mark Spezzano On 31/07/2011, at 6:49 PM, Ovidiu Deac wrote: > For some reason ghc complains about not being able to call show on an > Integer (?!?!?) > > Please enlighten me! > > ovidiu > > This is the hspec: > it "shows one element" > ( show (push 1 EmptyStack) ≡ "Stack(1)") > > ...this is the code: > > data Stack a = > EmptyStack | > StackEntry a (Stack a) > deriving(Eq) > > > instance Show (Stack a) where > show s = > "Stack(" ⊕ (showImpl s) ⊕ ")" > where > showImpl EmptyStack = "" > showImpl (StackEntry x _) = show x > > ...and this is the error: > > src/Stack.hs:12:22: > No instance for (Show a) > arising from a use of `showImpl' > In the first argument of `(++)', namely `(showImpl s)' > In the second argument of `(++)', namely `(showImpl s) ++ ")"' > In the expression: "Stack(" ++ (showImpl s) ++ ")" > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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Properties for Foldable
by Conal Elliott 30 Jul '11

30 Jul '11

Is there a collection of laws associated with the Foldable class? Or for Traversable? - Conal

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Function returning the type of its argument.
by Grigory Sarnitskiy 29 Jul '11

29 Jul '11

It is not possible in Haskell, moreover it seems to be too alien to Haskell. Nevertheless, is there an extension which would allow to write a function that returns the type of its argument?

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