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  • Closure vs Anonymous function (difference?)

    - by Maxim Gershkovich
    Hi, I have been unable to find a definition that clearly explains the differences between a closure and an anonymous function. Most references I have seen clearly specify that they are distinct "things" yet I can't seem to get my head around why. Could someone please simplify it for me? What are the specific differences between these two language features? Which one is more appropriate in what scenarios?

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  • When .VHD get updated?

    - by Recce2jp
    Hello, I have a VM with a .vhd file however, when I connet to the VM (by RDP) and make some changes (create a directory and add a small file with notepad) the .VHD file stays unchanged. The 'Date modified' of the .VHD stays at the starttime of the VM. 1/How can we force an update of het VM while the VM is running? 2/Where aree the temporary changes kept? Hypervisor: Microsoft Hyper-V server 2008R2 (free download version) regards, Maxim

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  • Des fonctions d'Ubuntu 13.04 seront développées à huis clos, vers une fermeture de l'OS ?

    Des fonctions d'Ubuntu 13.04 développées à huis clos Canonical adopte une nouvelle méthode de développement pour Raring Ringtail, vers une fermeture de l'OS ? À peine Canonical a publié Ubuntu 12.10, que la firme livre déjà quelques informations sur son successeur. Comme il est de coutume, l'OS est d'abord passé par la traditionnelle étape de définition de son nom de code. Ubuntu 13.04 sera baptisé « Raring Ringtail », d'après un billet de blog de Mark Shuttleworth, fondateur de Canonical. Cette mouture sera essentiellement centrée sur la mobilité. Le coeur du système d'exploitation sera nettoyé au maxim...

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  • First TDD, Simple 2-tier C# Project - what do I unit test?

    - by Joel
    This is probably a stupid question but my googling isn't finding a satisfactory answer. I'm starting a small project in C#, with just a business layer and a data access layer - strangely, the UI will come later, and I have very little (read:no) concept / control over what it will look like. I would like to try TDD for this project. I'm using Visual Studio 2008 (soon to be 2010), I have ReSharper 5, and nUnit. Again, I want to do Test-Driven Development, but not necessarily the entire XP system. My question is - when and where do I write the first unit test? Do I only test logic before I write it, or do I test everything? It seems counter-productive to test things that have no reason to fail (auto-properties, empty constructors)...but it seems like the "No new code without a failing test" maxim requires this. Links or references are fine (but preferably to online resources, not books - I would like to get started ASAP). Thanks in advance for any guidance!

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  • Image Drawing on UIView

    - by user1180261
    I'm trying to create an application where I can draw a lot of pictures at a specific point (determined for each image) on one view. I have a coordinates where I need draw a picture, width and height of it For example: I have 2 billion jpeg's images. for each images I have a specific origin point and size. In 1 second I need draw on view 20-50 images in specific point. I have already tryid solve that in the next way: UIGraphicsBeginImageContextWithOptions(self.previewScreen.bounds.size, YES, 0); [self.previewScreen.image drawAtPoint:CGPointMake(0, 0)]; [image drawAtPoint:CGPointMake(nRect.left, nRect.top)]; UIImage *imagew = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); [self.previewScreen setImage:imagew]; but in this solution I have a very big latency with displaying images and big CPU usage WBR Maxim Tartachnik

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  • how to make "tweet" button active in Twitter Anywhere TweetBox

    - by user322293
    I added a Twitter Anywhere TweetBox to my blog maxim.tumblr.com like this: var idvar = "tweetbox"; twttr.anywhere(function (T) { T("#tweetbox").tweetBox({ label: "Tweet me:", height: 100, width: 210, defaultContent: "@maxgrinev Hello!", onTweet: function (tweet, htmlTweet) { document.getElementById(idvar).setAttribute("style", "display: none;"); } }); }); Everythink works fine except the tweet button is not active by default. You have to edit the box to make it active (look here: http://maxgrinev.tumblr.com/). How can I make it active by default?

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  • Data Mining Resources

    - by Dejan Sarka
    There are many different types of analyses, each one with its own pros and cons. Relational reports have a predefined structure, and end users cannot change it. They are simple to use for end users. Reports can use real-time data and snapshots of data to show the state of a report at specific points in time. One of the drawbacks is that report authoring is limited to IT pros and advanced users. Any kind of dynamic restructuring is very limited. If real-time data is used for a report, the report has a negative impact on the performance of the source system. Processing of the reports might be slow because the data comes from relational database management systems, which are not optimized for reporting only. If you create a semantic model of your data, your end users can create ad-hoc report structures. However, the development is more complex because a developer is needed to create these semantic models. For OLAP, you typically use specialized database management systems. You get lightning speed of analyses. End users can use rich and thin clients to interactively change the structure of the report. Typically, they do it graphically. However, the development of an OLAP system is many times quite complex. It involves the preparation and maintenance of an enterprise data warehouse and OLAP cubes. In order to exploit the possibility of real-time restructuring of reports, the users must be both active and educated. The data is usually stale, as it is loaded into data warehouses and OLAP cubes with a scheduled process. With data mining, a structure is not selected in advance; it searches for the structure. As a result, data mining can give you the most valuable results because you can discover patterns you did not expect. A data mining model structure is limited only by the attributes that you use to train the model. One of the drawbacks is that a lot of knowledge is needed for a successful data mining project. End users have to understand the results. Subject matter experts and IT professionals need to understand business problem thoroughly. The development might be sometimes even more complex than the development of OLAP cubes. Each type of analysis has its own place in an enterprise system. SQL Server has tools for all kinds of analyses. However, data mining is the most advanced way of analyzing the data; this is the “I” in BI. In order to get the most out of it, you need to learn quite a lot. In this blog post, I am gathering together resources for learning, including forthcoming events. Books Multiple authors: SQL Server MVP Deep Dives – I wrote an introductory data mining chapter there. Erik Veerman, Teo Lachev and Dejan Sarka: MCTS Self-Paced Training Kit (Exam 70-448): Microsoft SQL Server 2008 - Business Intelligence Development and Maintenance – you can find a good overview of a complete BI solution, including data mining, in this book. Jamie MacLennan, ZhaoHui Tang, and Bogdan Crivat: Data Mining with Microsoft SQL Server 2008 – can’t miss this book if you want to mine your data with SQL Server tools. Michael Berry, Gordon Linoff: Mastering Data Mining: The Art and Science of Customer Relationship Management – data mining from both, business and technical perspective. Dorian Pyle: Data Preparation for Data Mining – an in-depth book about data preparation. Thomas and Ronald Wonnacott: Introductory Statistics – if you thought that you could get away without statistics, then you are not serious about data mining. Jiawei Han and Micheline Kamber: Data Mining Concepts and Techniques – in-depth explanation of the most popular data mining algorithms. Michael Berry and Gordon Linoff: Data Mining Techniques – another book that explains data mining algorithms, more fro a business perspective. Paolo Guidici: Applied Data Mining – very mathematical book, only if you enjoy statistics and mathematics in general. Forthcoming presentations I am presenting two data mining related sessions during the PASS Summit in Charlotte, NC: Wednesday, October 16th, 2013 - Fraud Detection: Notes from the Field – I am showing how to use data mining for a specific business problem. The presentation is based on real-life projects. Friday, October 18th: Excel 2013 Advanced Analytics – I am focusing on Excel Data Mining Add-ins, and how to use them together with Power Pivot and other add-ins. This is the most you can get out of Excel. Sinergija 2013, Belgrade, Serbia Tuesday, October 22nd: Excel 2013 Analytics to the Max – another presentation focusing on the most advanced analytics you can get in Excel. SQL Rally Amsterdam, Netherlands Thursday, November 7th: Advanced Analytics in Excel 2013 – and again I am presenting about data mining in Excel. Why three different titles for the same presentation? I don’t know, I guess I forgot the name I proposed every time right after I sent the proposal. Courses Data Mining with SQL Server 2012 – I wrote a 3-day course for SolidQ. If you are interested in this course, which I could also deliver in a shorter seminar way, you can contact your closes SolidQ subsidiary, or, of course, me directly on addresses [email protected] or [email protected]. This course could also complement the existing courseware portfolio of training providers, which are welcome to contact me as well. OK, now you know: no more excuses, start learning data mining, get the most out of your data

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  • Why you shouldn't add methods to interfaces in APIs

    - by Simon Cooper
    It is an oft-repeated maxim that you shouldn't add methods to a publically-released interface in an API. Recently, I was hit hard when this wasn't followed. As part of the work on ApplicationMetrics, I've been implementing auto-reporting of MVC action methods; whenever an action was called on a controller, ApplicationMetrics would automatically report it without the developer needing to add manual ReportEvent calls. Fortunately, MVC provides easy hook when a controller is created, letting me log when it happens - the IControllerFactory interface. Now, the dll we provide to instrument an MVC webapp has to be compiled against .NET 3.5 and MVC 1, as the lowest common denominator. This MVC 1 dll will still work when used in an MVC 2, 3 or 4 webapp because all MVC 2+ webapps have a binding redirect redirecting all references to previous versions of System.Web.Mvc to the correct version, and type forwards taking care of any moved types in the new assemblies. Or at least, it should. IControllerFactory In MVC 1 and 2, IControllerFactory was defined as follows: public interface IControllerFactory { IController CreateController(RequestContext requestContext, string controllerName); void ReleaseController(IController controller); } So, to implement the logging controller factory, we simply wrap the existing controller factory: internal sealed class LoggingControllerFactory : IControllerFactory { private readonly IControllerFactory m_CurrentController; public LoggingControllerFactory(IControllerFactory currentController) { m_CurrentController = currentController; } public IController CreateController( RequestContext requestContext, string controllerName) { // log the controller being used FeatureSessionData.ReportEvent("Controller used:", controllerName); return m_CurrentController.CreateController(requestContext, controllerName); } public void ReleaseController(IController controller) { m_CurrentController.ReleaseController(controller); } } Easy. This works as expected in MVC 1 and 2. However, in MVC 3 this type was throwing a TypeLoadException, saying a method wasn't implemented. It turns out that, in MVC 3, the definition of IControllerFactory was changed to this: public interface IControllerFactory { IController CreateController(RequestContext requestContext, string controllerName); SessionStateBehavior GetControllerSessionBehavior( RequestContext requestContext, string controllerName); void ReleaseController(IController controller); } There's a new method in the interface. So when our MVC 1 dll was redirected to reference System.Web.Mvc v3, LoggingControllerFactory tried to implement version 3 of IControllerFactory, was missing the GetControllerSessionBehaviour method, and so couldn't be loaded by the CLR. Implementing the new method Fortunately, there was a workaround. Because interface methods are normally implemented implicitly in the CLR, if we simply declare a virtual method matching the signature of the new method in MVC 3, then it will be ignored in MVC 1 and 2 and implement the extra method in MVC 3: internal sealed class LoggingControllerFactory : IControllerFactory { ... public virtual SessionStateBehaviour GetControllerSessionBehaviour( RequestContext requestContext, string controllerName) {} ... } However, this also has problems - the SessionStateBehaviour type only exists in .NET 4, and we're limited to .NET 3.5 by support for MVC 1 and 2. This means that the only solutions to support all MVC versions are: Construct the LoggingControllerFactory type at runtime using reflection Produce entirely separate dlls for MVC 1&2 and MVC 3. Ugh. And all because of that blasted extra method! Another solution? Fortunately, in this case, there is a third option - System.Web.Mvc also provides a DefaultControllerFactory type that can provide the implementation of GetControllerSessionBehaviour for us in MVC 3, while still allowing us to override CreateController and ReleaseController. However, this does mean that LoggingControllerFactory won't be able to wrap any calls to GetControllerSessionBehaviour. This is an acceptable bug, given the other options, as very few developers will be overriding GetControllerSessionBehaviour in their own custom controller factory. So, if you're providing an interface as part of an API, then please please please don't add methods to it. Especially if you don't provide a 'default' implementing type. Any code compiled against the previous version that can't be updated will have some very tough decisions to make to support both versions.

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  • Ubuntu server or Debian server (to run C++ apps developed on Ubuntu)

    - by skyeagle
    I have written a number of C++ server side daemons for my website, using my Ubuntu 9.10 dev machine. I am now about to venture out to look for a hosting provider etc. This is my problem: I have read on many posts (admittedly old posts) that Debian server is much more robust than Ubuntu server - is this till the case?. In particular, I am constantly "raising elephants" with my Ubuntu 9.10 - this is "ok" for home use, but for a website server, I would not be so forgiving. Also, there seems to be a new "patch" every few weeks - which I would not like on a server (I want to leave the server well alone, and let it get on with its business of serving pages). So in this instance Debian looks a more attractive proposition. I am worried that the C++ apps I have developed on Ubuntu may not be binary compatable with Debian (or I may need to install additional libraries/packages etc to get things to work), and I have zero experience with Debian. Additionally, I dont want to be grappling with the learning curve associated with a new OS, whilst trying to launch a new web site (I am assuming Debian UI is quite different from Ubuntu). In this case, the maxim "the devil you know is better than the one you dont", seems appropriate - and I find Ubuntu a more attractive proposition (atleast I know my apps will run without any probs etc). Can anyone provide some rational advice (based on actual experience), to help me decide which route to take - given the two (conflicting) trains of thought outlined above?

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  • An Alphabet of Eponymous Aphorisms, Programming Paradigms, Software Sayings, Annoying Alliteration

    - by Brian Schroer
    Malcolm Anderson blogged about “Einstein’s Razor” yesterday, which reminded me of my favorite software development “law”, the name of which I can never remember. It took much Wikipedia-ing to find it (Hofstadter’s Law – see below), but along the way I compiled the following list: Amara’s Law: We tend to overestimate the effect of a technology in the short run and underestimate the effect in the long run. Brook’s Law: Adding manpower to a late software project makes it later. Clarke’s Third Law: Any sufficiently advanced technology is indistinguishable from magic. Law of Demeter: Each unit should only talk to its friends; don't talk to strangers. Einstein’s Razor: “Make things as simple as possible, but not simpler” is the popular paraphrase, but what he actually said was “It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience”, an overly complicated quote which is an obvious violation of Einstein’s Razor. (You can tell by looking at a picture of Einstein that the dude was hardly an expert on razors or other grooming apparati.) Finagle's Law of Dynamic Negatives: Anything that can go wrong, will—at the worst possible moment. - O'Toole's Corollary: The perversity of the Universe tends towards a maximum. Greenspun's Tenth Rule: Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp. (Morris’s Corollary: “…including Common Lisp”) Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law. Issawi’s Omelet Analogy: One cannot make an omelet without breaking eggs - but it is amazing how many eggs one can break without making a decent omelet. Jackson’s Rules of Optimization: Rule 1: Don't do it. Rule 2 (for experts only): Don't do it yet. Kaner’s Caveat: A program which perfectly meets a lousy specification is a lousy program. Liskov Substitution Principle (paraphrased): Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it Mason’s Maxim: Since human beings themselves are not fully debugged yet, there will be bugs in your code no matter what you do. Nils-Peter Nelson’s Nil I/O Rule: The fastest I/O is no I/O.    Occam's Razor: The simplest explanation is usually the correct one. Parkinson’s Law: Work expands so as to fill the time available for its completion. Quentin Tarantino’s Pie Principle: “…you want to go home have a drink and go and eat pie and talk about it.” (OK, he was talking about movies, not software, but I couldn’t find a “Q” quote about software. And wouldn’t it be cool to write a program so great that the users want to eat pie and talk about it?) Raymond’s Rule: Computer science education cannot make anybody an expert programmer any more than studying brushes and pigment can make somebody an expert painter.  Sowa's Law of Standards: Whenever a major organization develops a new system as an official standard for X, the primary result is the widespread adoption of some simpler system as a de facto standard for X. Turing’s Tenet: We shall do a much better programming job, provided we approach the task with a full appreciation of its tremendous difficulty, provided that we respect the intrinsic limitations of the human mind and approach the task as very humble programmers.  Udi Dahan’s Race Condition Rule: If you think you have a race condition, you don’t understand the domain well enough. These rules didn’t exist in the age of paper, there is no reason for them to exist in the age of computers. When you have race conditions, go back to the business and find out actual rules. Van Vleck’s Kvetching: We know about as much about software quality problems as they knew about the Black Plague in the 1600s. We've seen the victims' agonies and helped burn the corpses. We don't know what causes it; we don't really know if there is only one disease. We just suffer -- and keep pouring our sewage into our water supply. Wheeler’s Law: All problems in computer science can be solved by another level of indirection... Except for the problem of too many layers of indirection. Wheeler also said “Compatibility means deliberately repeating other people's mistakes.”. The Wrong Road Rule of Mr. X (anonymous): No matter how far down the wrong road you've gone, turn back. Yourdon’s Rule of Two Feet: If you think your management doesn't know what it's doing or that your organisation turns out low-quality software crap that embarrasses you, then leave. Zawinski's Law of Software Envelopment: Every program attempts to expand until it can read mail. Zawinski is also responsible for “Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems.” He once commented about X Windows widget toolkits: “Using these toolkits is like trying to make a bookshelf out of mashed potatoes.”

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  • Subterranean IL: Generics and array covariance

    - by Simon Cooper
    Arrays in .NET are curious beasts. They are the only built-in collection types in the CLR, and SZ-arrays (single dimension, zero-indexed) have their own commands and IL syntax. One of their stranger properties is they have a kind of built-in covariance long before generic variance was added in .NET 4. However, this causes a subtle but important problem with generics. First of all, we need to briefly recap on array covariance. SZ-array covariance To demonstrate, I'll tweak the classes I introduced in my previous posts: public class IncrementableClass { public int Value; public virtual void Increment(int incrementBy) { Value += incrementBy; } } public class IncrementableClassx2 : IncrementableClass { public override void Increment(int incrementBy) { base.Increment(incrementBy); base.Increment(incrementBy); } } In the CLR, SZ-arrays of reference types are implicitly convertible to arrays of the element's supertypes, all the way up to object (note that this does not apply to value types). That is, an instance of IncrementableClassx2[] can be used wherever a IncrementableClass[] or object[] is required. When an SZ-array could be used in this fashion, a run-time type check is performed when you try to insert an object into the array to make sure you're not trying to insert an instance of IncrementableClass into an IncrementableClassx2[]. This check means that the following code will compile fine but will fail at run-time: IncrementableClass[] array = new IncrementableClassx2[1]; array[0] = new IncrementableClass(); // throws ArrayTypeMismatchException These checks are enforced by the various stelem* and ldelem* il instructions in such a way as to ensure you can't insert a IncrementableClass into a IncrementableClassx2[]. For the rest of this post, however, I'm going to concentrate on the ldelema instruction. ldelema This instruction pops the array index (int32) and array reference (O) off the stack, and pushes a pointer (&) to the corresponding array element. However, unlike the ldelem instruction, the instruction's type argument must match the run-time array type exactly. This is because, once you've got a managed pointer, you can use that pointer to both load and store values in that array element using the ldind* and stind* (load/store indirect) instructions. As the same pointer can be used for both input and output to the array, the type argument to ldelema must be invariant. At the time, this was a perfectly reasonable restriction, and maintained array type-safety within managed code. However, along came generics, and with it the constrained callvirt instruction. So, what happens when we combine array covariance and constrained callvirt? .method public static void CallIncrementArrayValue() { // IncrementableClassx2[] arr = new IncrementableClassx2[1] ldc.i4.1 newarr IncrementableClassx2 // arr[0] = new IncrementableClassx2(); dup newobj instance void IncrementableClassx2::.ctor() ldc.i4.0 stelem.ref // IncrementArrayValue<IncrementableClass>(arr, 0) // here, we're treating an IncrementableClassx2[] as IncrementableClass[] dup ldc.i4.0 call void IncrementArrayValue<class IncrementableClass>(!!0[],int32) // ... ret } .method public static void IncrementArrayValue<(IncrementableClass) T>( !!T[] arr, int32 index) { // arr[index].Increment(1) ldarg.0 ldarg.1 ldelema !!T ldc.i4.1 constrained. !!T callvirt instance void IIncrementable::Increment(int32) ret } And the result: Unhandled Exception: System.ArrayTypeMismatchException: Attempted to access an element as a type incompatible with the array. at IncrementArrayValue[T](T[] arr, Int32 index) at CallIncrementArrayValue() Hmm. We're instantiating the generic method as IncrementArrayValue<IncrementableClass>, but passing in an IncrementableClassx2[], hence the ldelema instruction is failing as it's expecting an IncrementableClass[]. On features and feature conflicts What we've got here is a conflict between existing behaviour (ldelema ensuring type safety on covariant arrays) and new behaviour (managed pointers to object references used for every constrained callvirt on generic type instances). And, although this is an edge case, there is no general workaround. The generic method could be hidden behind several layers of assemblies, wrappers and interfaces that make it a requirement to use array covariance when calling the generic method. Furthermore, this will only fail at runtime, whereas compile-time safety is what generics were designed for! The solution is the readonly. prefix instruction. This modifies the ldelema instruction to ignore the exact type check for arrays of reference types, and so it lets us take the address of array elements using a covariant type to the actual run-time type of the array: .method public static void IncrementArrayValue<(IncrementableClass) T>( !!T[] arr, int32 index) { // arr[index].Increment(1) ldarg.0 ldarg.1 readonly. ldelema !!T ldc.i4.1 constrained. !!T callvirt instance void IIncrementable::Increment(int32) ret } But what about type safety? In return for ignoring the type check, the resulting controlled mutability pointer can only be used in the following situations: As the object parameter to ldfld, ldflda, stfld, call and constrained callvirt instructions As the pointer parameter to ldobj or ldind* As the source parameter to cpobj In other words, the only operations allowed are those that read from the pointer; stind* and similar that alter the pointer itself are banned. This ensures that the array element we're pointing to won't be changed to anything untoward, and so type safety within the array is maintained. This is a typical example of the maxim that whenever you add a feature to a program, you have to consider how that feature interacts with every single one of the existing features. Although an edge case, the readonly. prefix instruction ensures that generics and array covariance work together and that compile-time type safety is maintained. Tune in next time for a look at the .ctor generic type constraint, and what it means.

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