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• Canonical pattern reference in Actors programming model.

  - by Bubba88

  Hello! Is there a source, which I could use to learn some of the most used and popular practices regarding Actor-/Agent-oriented programming. My primary concern is about parallelism and distribution limited to the mentioned scheme - Actors, message passing. Should I begin with Erlang documentation or maybe there is any kind of book that describes the most important building blocks when programming Actor-oriented? Thank you! (Most useful examples would be in Scala or F#)

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• Can't declare unused exception variable when using catch-all pattern

  - by b0x0rz

  what is a best practice in cases such as this one: try { // do something } catch (SpecificException ex) { Response.Redirect("~/InformUserAboutAn/InternalException/"); } the warning i get is that ex is never used. however all i need here is to inform the user, so i don't have a need for it. do i just do: try { // do something } catch { Response.Redirect("~/InformUserAboutAn/InternalException/"); } somehow i don't like that, seems strange!!? any tips? best practices? what would be the way to handle this. thnx

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• Haskell: Pattern Matching with Lists

  - by user1670032

  I'm trying to make a function that takes in a list, and if one of the elements is negative, then any elements in that list that are equal to its positive counterpart should be changed to 0. Eg, if there is a -2 in a list, then all 2's in that list should be changed to 0. Any ideas why it only works for some cases and not others? I'm not understanding why this is, I've looked it over several times. changeToZero [] = [] changeToZero [x] = [x] changeToZero (x:zs:y:ws) | (x < 0) && ((-1)*(x) == y) = x : zs : 0 : changeToZero ws changeToZero (x:xs) = x : changeToZero xs *Main changeToZero [-1,1,-2,2,-3,3] [-1,1,-2,2,-3,3] *Main changeToZero [-2,1,2,3] [-2,1,0,3] *Main changeToZero [-2,1,2,3,2] [-2,1,0,3,2] *Main changeToZero [1,-2,2,2,1] [1,-2,2,0,1]

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• Trouble implementing Singleton pattern in Tomcat web application due to Class Loader

  - by jwegan

  I'm trying to implement a Singleton in Tomcat 6.24 on Linux with x86_64 OpenJDK 1.6. My application is just a bunch of JSPs and some static content and the JSPs make calls to my Java code. Currently the web.xml just looks like this: <web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5"> <description> App Name </description> <display-name>App Name</display-name> <!-- The Usual Welcome File List --> <welcome-file-list> <welcome-file>pages/index.jsp</welcome-file> </welcome-file-list> </web-app> Before when I was trying to load my Singleton it was getting instantiated twice since the class was getting loaded by two different class loaders (I'm not sure why) and each loader would create an instance of the singleton which is not acceptable for my application. I finally figured out if I exported my code as a jar and put it in $CATALINA_HOME/lib then there was only one instance, but this is not an elegant solution. I've been googling for hours, but I haven't come up with anything yet. I'm wondering if there is some other solution. Currently I'm not precompling my JSPs, could this be part of the problem? Could I write a servlet to ensure the singleton is created? If so how do I do that?

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• Count subset of binary pattern ..

  - by mr.bio

  Hi there . I have a A=set of strings and a B=seperate string. I want to count the number of occurences in from B in A. Example : A: 10001 10011 11000 10010 10101 B: 10001 result would be 3.(10001 is a subset of 10001,10011,10101) So i need a function that takes a set and string and returns an int. int myfunc(set<string> , string){ int result; // My Brain is melting return result ; }

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• Is it possible in .NET, using C#, to achieve event based asynchronous pattern without multithreading

  - by Jeffrey

  I am amazed by the architectural design of node.js and was wondering if C# is capable of such design: Asynchronous, event based / event loop, non-blocking IO without multithreading.

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• Observer pattern and violation of Single Responsibility Principle

  - by Devil Jin

  I have an applet which repaints itself once the text has changed Design 1: //MyApplet.java public class MyApplet extends Applet implements Listener{ private DynamicText text = null; public void init(){ text = new DynamicText("Welcome"); } public void paint(Graphics g){ g.drawString(text.getText(), 50, 30); } //implement Listener update() method public void update(){ repaint(); } } //DynamicText.java public class DynamicText implements Publisher{ // implements Publisher interface methods //notify listeners whenever text changes } Isn't this a violation of Single Responsibility Principle where my Applet not only acts as Applet but also has to do Listener job. Same way DynamicText class not only generates the dynamic text but updates the registered listeners. Design 2: //MyApplet.java public class MyApplet extends Applet{ private AppletListener appLstnr = null; public void init(){ appLstnr = new AppletListener(this); // applet stuff } } // AppletListener.java public class AppletListener implements Listener{ private Applet applet = null; public AppletListener(Applet applet){ this.applet = applet; } public void update(){ this.applet.repaint(); } } // DynamicText public class DynamicText{ private TextPublisher textPblshr = null; public DynamicText(TextPublisher txtPblshr){ this.textPblshr = txtPblshr; } // call textPblshr.notifyListeners whenever text changes } public class TextPublisher implments Publisher{ // implements publisher interface methods } Q1. Is design 1 a SPR violation? Q2. Is composition a better choice here to remove SPR violation as in design 2.

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• Choosing a design pattern for a class that might change it's internal attributes

  - by the_drow

  I have a class that holds arbitrary state and it's defined like this: class AbstractFoo { }; template <class StatePolicy> class Foo : public StatePolicy, public AbstractFoo { }; The state policy contains only protected attributes that represent the state. The state might be the same for multiple behaviors and they can be replaced at runtime. All Foo objects have the same interface to abstract the state itself and to enable storing Foo objects in containers. I would like to find the least verbose and the most maintainable way to express this.

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• C#: Preferred pattern for functions requiring arguments that implement two interfaces

  - by JS Bangs

  The argument to my function f() must implement two different interfaces that are not related to each other by inheritance, IFoo and IBar. I know of two different ways of doing this. The first is to declare an empty interface that inherits from both: public interface IFooBar : IFoo, IBar { // nothing to see here } public int f(IFooBar arg) { // etc. } This, of course, requires that the classes declare themselves as implementing IFooBar rather than IFoo and IBar separately. The second way is to make f() generic with a constraint: public int f<T>(T arg) where T : IFoo, IBar { // etc. } Which of these do you prefer, and why? Are there any non-obvious advantages or disadvantages to each?

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• Client-server synchronization pattern / algorithm?

  - by tm_lv

  I have a feeling that there must be client-server synchronization patterns out there. But i totally failed to google up one. Situation is quite simple - server is the central node, that multiple clients connect to and manipulate same data. Data can be split in atoms, in case of conflict, whatever is on server, has priority (to avoid getting user into conflict solving). Partial synchronization is preferred due to potentially large amounts of data. Are there any patterns / good practices for such situation, or if you don't know of any - what would be your approach? Below is how i now think to solve it: Parallel to data, a modification journal will be held, having all transactions timestamped. When client connects, it receives all changes since last check, in consolidated form (server goes through lists and removes additions that are followed by deletions, merges updates for each atom, etc.). Et voila, we are up to date. Alternative would be keeping modification date for each record, and instead of performing data deletes, just mark them as deleted. Any thoughts?

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• wpf command pattern

  - by evan

  I have a wpf gui which displays a list of information in separate window and in a separate thread from the main application. As the user performs actions in the main window the side window is updated. (For example if you clicked page down in the main window a listbox in the side window would page down). Right now the architecture for this application feels very messy and I'm sure there is a cleaner way to do it. It looks like this: Main Window contains a singleton SideWindowControl which communicates with an instance of the SideWindowDisplay using events - so, for example, the pagedown button would work like: 1) the event handler of the button on the main window calls SideWindowControl.PageDown() 2) in the PageDown() function a event is created and thrown. 3) finally the gui, ShowSideWindowDisplay is subscribing to the SideWindowControl.Actions event handles the event and actually scrolls the listbox down - note because it is in a different thread it has to do that by running the command via Dispatcher.Invoke() This just seems like a very messy way to this and there must be a clearer way (The only part that can't change is that the main window and the side window must be on different threads). Perhaps using WPF commands? I'd really appreciate any suggestions!! Thanks

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• Override Maven's default resource filter replacement pattern.

  - by Matt Campbell

  By default maven will filter resources like this: <properties> <replace.me>value</replace.me> </properties> <some-tag> <key>${replace.me}</key> </some-tag> will get you: <some-tag> <key>value</key> </some-tag> Is there a way to override the way maven selects the strings to replace? Specifically, I want to be able to use this: <some-tag> <key>@replace.me@</key> </some-tag> to get the same result as above.

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• Javascript Regex multiple group matches in pattern

  - by mummybot

  I have a question for a Javascript regex ninja: How could I simplify my variable creation from a string using regex? I currently have it working without using any grouping, but I would love to see a better way! The string is: var url = 'resources/css/main.css?detect=#information{width:300px;}'; The code that works is: var styleStr = /[^=]+$/.exec(url).toString(); var id = /[^\#][^\{]+/.exec(styleStr).toString(); var property = /[^\{]+/.exec(/[^\#\w][^\:]+/.exec(styleStr)).toString(); var value = /[^\:]+/.exec(/[^\#\w\{][^\:]+[^\;\}]/.exec(styleStr)).toString(); This gives: alert(id) //information alert(property) //width alert(value) //300px Any takers?

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• Design pattern to integrate Rails with a Comet server

  - by empire29

  I have a Ruby on Rails (2.3.5) application and an APE (Ajax Push Engine) server. When records are created within the Rails application, i need to push the new record out on applicable channels to the APE server. Records can be created in the rails app by the traditional path through the controller's create action, or it can be created by several event machines that are constantly monitoring various inputstream and creating records when they see data that meets a certain criteria. It seems to me that the best/right place to put the code that pushes the data out to the APE server (which in turn pushes it out to the clients) is in the Model's after_create hook (since not all record creations will flow through the controller's create action). The final caveat is I want to push a piece of formatted HTML out to the APE server (rather than a JSON representation of the data). The reason I want to do this is 1) I already have logic to produce the desired layout in existing partials 2) I don't want to create a javascript implementation of the partials (javascript that takes a JSON object and creates all the HTML around it for presentation). This would quickly become a maintenance nightmare. The problem with this is it would require "rendering" partials from within the Model (which im having trouble doing anyhow because they don't seem to have access to Helpers when they're rendered in this manner). Anyhow - Just wondering what the right way to go about organizing all of this is. Thanks

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• The simplest concurrency pattern

  - by Ilya Kogan

  Please, would you help me in reminding me of one of the simplest parallel programming techniques. How do I do the following in C#: Initial state: semaphore counter = 0 Thread 1: // Block until semaphore is signalled semaphore.Wait(); // wait until semaphore counter is 1 Thread 2: // Allow thread 1 to run: semaphore.Signal(); // increments from 0 to 1 It's not a mutex because there is no critical section, or rather you can say there is an infinite critical section. So what is it?

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• Is the a pattern for iterating over lists held by a class (dynamicly typed OO languages)

  - by Roman A. Taycher

  If I have a class that holds one or several lists is it better to allow other classes to fetch those lists(with a getter) or to implement a doXList/eachXList type method for that list that take a function and call that function on each element of the list contained by that object. I wrote a program that did a ton of this and I hated passing around all these lists sometimes with method in class a calling method in class B to return lists contained in class C, B contains a C or multiple C's (note question is about dynamically typed OO languages languages like ruby or smalltalk) ex. (that came up in my program) on a Person class containing scheduling preferences and a scheduler class needing to access them.

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• MSMQ - Message Queue Abstraction and Pattern

  - by Maxim Gershkovich

  Hi All, Let me define the problem first and why a messagequeue has been chosen. I have a datalayer that will be transactional and EXTREMELY insert heavy and rather then attempt to deal with these issues when they occur I am hoping to implement my application from the ground up with this in mind. I have decided to tackle this problem by using the Microsoft Message Queue and perform inserts as time permits asynchronously. However I quickly ran into a problem. Certain inserts that I perform may need to be recalled (ie: retrieved) immediately (imagine this is for POS system and what happens if you need to recall the last transaction - one that still hasn’t been inserted). The way I decided to tackle this problem is by abstracting the MessageQueue and combining it in my data access layer thereby creating the illusion of a single set of data being returned to the user of the datalayer (I have considered the other issues that occur in such a scenario (ie: essentially dirty reads and such) and have concluded for my purposes I can control these issues). However this is where things get a little nasty... I’ve worked out how to get the messages back and such (trivial enough problem) but where I am stuck is; how do I create a generic (or at least somewhat generic) way of querying my message queue? One where I can minimize the duplication between the SQL queries and MessageQueue queries. I have considered using LINQ (but have very limited understanding of the technology) and have also attempted an implementation with Predicates which so far is pretty smelly. Are there any patterns for such a problem that I can utilize? Am I going about this the wrong way? Does anyone have an of their own ideas about how I can tackle this problem? Does anyone even understand what I am talking about? :-) Any and ALL input would be highly appreciated and seriously considered… Thanks again.

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• small scale web site - global javascript file style/format/pattern - improving maintainability

  - by yaya3

  I frequently create (and inherit) small to medium websites where I have the following sort of code in a single file (normally named global.js or application.js or projectname.js). If functions get big, I normally put them in a seperate file, and call them at the bottom of the file below in the $(document).ready() section. If I have a few functions that are unique to certain pages, I normally have another switch statement for the body class inside the $(document).ready() section. How could I restructure this code to make it more maintainable? Note: I am less interested in the functions innards, more so the structure, and how different types of functions should be dealt with. I've also posted the code here - http://pastie.org/999932 in case it makes it any easier var ProjectNameEnvironment = {}; function someFunctionUniqueToTheHomepageNotWorthMakingConfigurable () { $('.foo').hide(); $('.bar').click(function(){ $('.foo').show(); }); } function functionThatIsWorthMakingConfigurable(config) { var foo = config.foo || 700; var bar = 200; return foo * bar; } function globallyRequiredJqueryPluginTrigger (tooltip_string) { var tooltipTrigger = $(tooltip_string); tooltipTrigger.tooltip({ showURL: false ... }); } function minorUtilityOneLiner (selector) { $(selector).find('li:even').not('li ul li').addClass('even'); } var Lightbox = {}; Lightbox.setup = function(){ $('li#foo a').attr('href','#alpha'); $('li#bar a').attr('href','#beta'); } Lightbox.init = function (config){ if (typeof $.fn.fancybox =='function') { Lightbox.setup(); var fade_in_speed = config.fade_in_speed || 1000; var frame_height = config.frame_height || 1700; $(config.selector).fancybox({ frameHeight : frame_height, callbackOnShow: function() { var content_to_load = config.content_to_load; ... }, callbackOnClose : function(){ $('body').height($('body').height()); } }); } else { if (ProjectNameEnvironment.debug) { alert('the fancybox plugin has not been loaded'); } } } // ---------- order of execution ----------- $(document).ready(function () { urls = urlConfig(); (function globalFunctions() { $('.tooltip-trigger').each(function(){ globallyRequiredJqueryPluginTrigger(this); }); minorUtilityOneLiner('ul.foo') Lightbox.init({ selector : 'a#a-lightbox-trigger-js', ... }); Lightbox.init({ selector : 'a#another-lightbox-trigger-js', ... }); })(); if ( $('body').attr('id') == 'home-page' ) { (function homeFunctions() { someFunctionUniqueToTheHomepageNotWorthMakingConfigurable (); })(); } });

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• Removing a pattern from the beggining and end of a string in ruby

  - by seaneshbaugh

  So I found myself needing to remove <br /> tags from the beginning and end of strings in a project I'm working on. I made a quick little method that does what I need it to do but I'm not convinced it's the best way to go about doing this sort of thing. I suspect there's probably a handy regular expression I can use to do it in only a couple of lines. Here's what I got: def remove_breaks(text) if text != nil and text != "" text.strip! index = text.rindex("<br />") while index != nil and index == text.length - 6 text = text[0, text.length - 6] text.strip! index = text.rindex("<br />") end text.strip! index = text.index("<br />") while index != nil and index == 0 text = test[6, text.length] text.strip! index = text.index("<br />") end end return text end Now the "<br />" could really be anything, and it'd probably be more useful to make a general use function that takes as an argument the string that needs to be stripped from the beginning and end. I'm open to any suggestions on how to make this cleaner because this just seems like it can be improved.

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• .NET Deployment of Interface/Classes for Command Pattern Question

  - by Jonno

  In theory I would like to produce 2 projects: 1) Asp.net (Sever A) 2) DAL running (Server B) I would like to utilise command objects to comunicate with the DAL. ASP.net instantiates a command class e.g. CmdGetAllUsers which impliments IMyCommand interface and sends it to the DAL (using ASMX or WCF). My question is: Would the class definition of CmdGetAllUsers need to exist on the DAL server? Or would having the interface definition be enough? My goal is to reduce the need to redeploy the DAL code, and have it as a fairly simple pass-through layer. Many thanks for your time.

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• Storing millions of URLs in a database for fast pattern matching

  - by Paras Chopra

  I am developing a web analytics kind of system which needs to log referring URL, landing page URL and search keywords for every visitor on the website. What I want to do with this collected data is to allow end-user to query the data such as "Show me all visitors who came from Bing.com searching for phrase that contains 'red shoes'" or "Show me all visitors who landed on URL that contained 'campaign=twitter_ad'", etc. Because this system will be used on many big websites, the amount of data that needs to log will grow really, really fast. So, my question: a) what would be the best strategy for logging so that scaling the system doesn't become a pain; b) how to use that architecture for rapid querying of arbitrary requests? Is there a special method of storing URLs so that querying them gets faster? In addition to MySQL database that I use, I am exploring (and open to) other alternatives better suited for this task.

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• Lined Pattern in Flash Background

  - by Shonna

  So i have a flash site i am doing in as2, even if the solution can only be done in as3, I still want it. I am trying to accomplish lines through the background image like on this site http://larc-paris.com/#/fr/club I tried just putting the patten on the image itself, but when i scale my site, its all distorted and the lines does not look as crisp anymore, like theirs do, so I am assuming they did the lines themself in flash... any clue? I have the image just need the lines, dont need a slideshow.

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• user interface pattern for associating single or many objects to an entity

  - by Samuel

  Need suggestions on implementing associating single or many objects to an entity. All soccer team players are registered individually (e.g. they are part of 'players' table) A soccer team has many players. The click sequence is like this:- a] Soccer team owner provides a name and brief description of the soccer team. b] Now it wants to add players to this team. c] You have the following button 'Add players to team' which lets you navigate to the 'View Players' page and lets you multi select users from there. Assuming this is a paginated list of players, how do you handle the following:- Do you provide a check box against each player and let the manager do a multi selection. If you need to add more players, it doesn't make sense to show the players who have been already added to the team. Do you mark those entries as not selectable or you would adding showing these entries. If you need to filter, do you provide search filters at the top of this page. Am looking for ideas on how to implement this or sites which have already done something similar.

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• Design pattern: polymorphisim for list of objects

  - by ziang

  Suppose I have a class A, and A1, A2 inherits from A. There are 2 functions: List<A1> getListA1(){...} List<A2> getListA2(){...} Now I want to do something similar to both A1 and A2 in another function public void process(List<A>){...} If I want to pass the instance of either ListA1 or ListA2, of course the types doesn't match because the compiler doesn't allow the coercion from List< A1 to List< A. I can't do something like this: List<A1> listA1 = getListA1(); List<A> newList = (List<A>)listA1; //this is not allowed. So what is the best approach to the process()? Is there any way to do it in a universal way rather than write the similar code to both List and List?

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• C++ CRTP(template pattern) question

  - by aaa

  following piece of code does not compile, the problem is in T::rank not be inaccessible (I think) or uninitialized in parent template. Can you tell me exactly what the problem is? is passing rank explicitly the only way? or is there a way to query tensor class directly? Thank you #include <boost/utility/enable_if.hpp> template<class T, // size_t N, class enable = void> struct tensor_operator; // template<class T, size_t N> template<class T> struct tensor_operator<T, typename boost::enable_if_c< T::rank == 4>::type > { tensor_operator(T &tensor) : tensor_(tensor) {} T& operator()(int i,int j,int k,int l) { return tensor_.layout.element_at(i, j, k, l); } T &tensor_; }; template<size_t N, typename T = double> // struct tensor : tensor_operator<tensor<N,T>, N> { struct tensor : tensor_operator<tensor<N,T> > { static const size_t rank = N; }; I know the workaround, however am interested in mechanics of template instantiation for self-education

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