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• Higher-order type constructors and functors in Ocaml

  - by sdcvvc

  Can the following polymorphic functions let id x = x;; let compose f g x = f (g x);; let rec fix f = f (fix f);; (*laziness aside*) be written for types/type constructors or modules/functors? I tried type 'x id = Id of 'x;; type 'f 'g 'x compose = Compose of ('f ('g 'x));; type 'f fix = Fix of ('f (Fix 'f));; for types but it doesn't work. Here's a Haskell version for types: data Id x = Id x data Compose f g x = Compose (f (g x)) data Fix f = Fix (f (Fix f)) -- examples: l = Compose [Just 'a'] :: Compose [] Maybe Char type Natural = Fix Maybe -- natural numbers are fixpoint of Maybe n = Fix (Just (Fix (Just (Fix Nothing)))) :: Natural -- n is 2 -- up to isomorphism composition of identity and f is f: iso :: Compose Id f x -> f x iso (Compose (Id a)) = a

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• C++: Dependency injection, circular dependency and callbacks

  - by Jonathan

  Consider the (highly simplified) following case: class Dispatcher { public: receive() {/*implementation*/}; // callback } class CommInterface { public: send() = 0; // call } class CommA : public CommInterface { public: send() {/*implementation*/}; } Various classes in the system send messages via the dispatcher. The dispatcher uses a comm to send. Once an answer is returned, the comm relays it back to the dispatcher which dispatches it back to the appropriate original sender. Comm is polymorphic and which implementation to choose can be read from a settings file. Dispatcher has a dependency on the comm in order to send. Comm has a dependency on dispatcher in order to callback. Therefor there's a circular dependency here and I can't seem to implement the dependency injection principle (even after encountering this nice blog post).

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• Is it legal to stub the #class method of a Mock object when using RSpec in a Ruby on Rails applicati

  - by MiniQuark

  I would like to stub the #class method of a mock object: describe Letter do before(:each) do @john = mock("John") @john.stub!(:id).and_return(5) @john.stub!(:class).and_return(Person) # is this ok? @john.stub!(:name).and_return("John F.") Person.stub!(:find).and_return(@john) end it.should "have a valid #to field" do letter = Letter.create!(:to=>@john, :content => "Hello John") letter.to_type.should == @john.class.name letter.to_id.should == @john.id end [...] end On line 5 of this program, I stub the #class method, in order to allow things like @john.class.name. Is this the right way to go? Will there be any bad side effect? Edit: The Letter class looks like this: class Letter < ActiveRecord::Base belongs_to :to, :polymorphic => true [...] end I wonder whether ActiveRecord gets the :to field's class name with to.class.name or by some other means. Maybe this is what the class_name method is ActiveRecord::Base is for?

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• How to create a container that holds different types of function pointers in C++?

  - by Alex

  I'm doing a linear genetic programming project, where programs are bred and evolved by means of natural evolution mechanisms. Their "DNA" is basically a container (I've used arrays and vectors successfully) which contain function pointers to a set of functions available. Now, for simple problems, such as mathematical problems, I could use one type-defined function pointer which could point to functions that all return a double and all take as parameters two doubles. Unfortunately this is not very practical. I need to be able to have a container which can have different sorts of function pointers, say a function pointer to a function which takes no arguments, or a function which takes one argument, or a function which returns something, etc (you get the idea)... Is there any way to do this using any kind of container ? Could I do that using a container which contains polymorphic classes, which in their turn have various kinds of function pointers? I hope someone can direct me towards a solution because redesigning everything I've done so far is going to be painful.

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• Referring to the type of an inner class in Scala

  - by saucisson

  The following code tries to mimic Polymorphic Embedding of DSLs: rather than giving the behavior in Inner, it is encoded in the useInner method of its enclosing class. I added the enclosing method so that user has only to keep a reference to Inner instances, but can always get their enclosing instance. By doing this, all Inner instances from a specific Outer instance are bound to only one behavior (but it is wanted here). abstract class Outer { sealed class Inner { def enclosing = Outer.this } def useInner(x:Inner) : Boolean } def toBoolean(x:Outer#Inner) : Boolean = x.enclosing.useInner(x) It does not compile and scala 2.8 complains about: type mismatch; found: sandbox.Outer#Inner required: _81.Inner where val _81:sandbox.Outer From Programming Scala: Nested classes and A Tour of Scala: Inner Classes, it seems to me that the problem is that useInnerexpects as argument an Inner instance from a specific Outer instance. What is the true explanation and how to solve this problem ?

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• boost::dynamic_pointer_cast with const pointer not working ?

  - by ereOn

  Hi, Let's say I have two classes, A and B, where B is a child class of A. I also have the following function: void foo(boost::shared_ptr<const A> a) { boost::shared_ptr<const B> b = boost::dynamic_pointer_cast<const B>(a); // Error ! } Compilation with gcc gives me the following errors: C:\Boost\include/boost/smart_ptr/shared_ptr.hpp: In constructor 'boost::shared_ptr< <template-parameter-1-1> >::shared_ptr(const boost::shared_ptr<Y>&, boost::detail::dynamic_cast_tag) [with Y = const A, T = const B]': C:\Boost\include/boost/smart_ptr/shared_ptr.hpp:522: instantiated from 'boost::shared_ptr<X> boost::dynamic_pointer_cast(const boost::shared_ptr<U>&) [with T = const B, U = const A]' src\a.cpp:10: instantiated from here C:\Boost\include/boost/smart_ptr/shared_ptr.hpp:259: error: cannot dynamic_cast 'r->boost::shared_ptr<const A>::px' (of type 'const class A* const') to type 'const class B*' (source type is not polymorphic) What could possibly be wrong ? Thank you.

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• how do I join and include the association

  - by Mark

  Hi All, How do I use both include and join in a named scope? Post is polymorphic class Post has_many :approved_comments, :class_name => 'Comment' end class Comment belongs_to :post end Comment.find(:all, :joins => :post, :conditions => ["post.approved = ? ", true], :include => :post) This does not work as joins does an inner join, and include does a left out join. The database throws an error as both joins can't be there in same query.

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• C# generics method invocation

  - by Firat KÜÇÜK

  Hi, i have some polymorphic methods and i want to call via using an intermediate method. Following class is the simplified version of my program. class Program { public class A { } public class B { } public class C { } public void SomeMethod(A value) { Console.WriteLine("A value"); } public void SomeMethod(B value) { Console.WriteLine("B value"); } public void SomeMethod(C value) { Console.WriteLine("C value"); } static void Main(string[] args) { Program p = new Program(); // code block p.IntermediateMethod<A>(new A()); p.IntermediateMethod<B>(new B()); p.IntermediateMethod<C>(new C()); } public void IntermediateMethod<T>(T value) { // code block SomeMethod(value); // code block } }

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• Rails / JBuilder - Entity array with has_many attributes

  - by seufagner

  I have two models, Person and Image and I want return an json array of Persons with your Images. But I dont want return all Image attributes, but produces a different result. Code below: class Person < ActiveRecord::Base has_many :images, as: :imageable validates :name, presence: true accepts_nested_attributes_for :images, :reject_if => lambda { |img| img['asset'].blank? } end class Image < ActiveRecord::Base belongs_to :imageable, polymorphic: true mount_uploader :asset, ImageUploader validates :asset, presence: true end zzz.jbuilder.json template json.persons(@rodas, :id, :name, :images) json produced: { "rodas": [{ "id": 4, "name": "John", "images": [ { "asset": { "url": "/uploads/image/xxxx.png" } }, { "asset": { "url": "/uploads/image/yyyyy.jpeg" } } ]}, { "id": 19, "name": "Mary", "images": [ { "asset": { "url": "/uploads/image/kkkkkkk.png" } } ] }] } I want something like: { "rodas": [ { "id": 4, "name": "John", "images": [ "/uploads/image/xxxx.png" , "/uploads/image/yyyy.jpeg" ] }, { "id": 10, "name": "Mary", "images": [ "/uploads/image/dddd.png" , "/uploads/image/xxxx.jpeg" ] } ]}

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• correct way to store an exception in a variable

  - by Evan Teran

  I have an API which internally has some exceptions for error reporting. The basic structure is that it has a root exception object which inherits from std::exception, then it will throw some subclass of that. Since catching an exception thrown in one library and catching it in another can lead to undefined behavior (at least Qt complains about it and disallows it in many contexts). I would like to wrap the library calls in functions which will return a status code, and if an exception occurred, a copy of the exception object. What is the best way to store an exception (with it's polymorphic behavior) for later use? I believe that the c++0x futures API makes use of something like this. So what is the best approach? The best I can think of is to have a clone() method in each exception class which will return a pointer to an exception of the same type. But that's not very generic and doesn't deal with standard exceptions at all. Any thoughts?

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• Casting to specific class in HQL

  - by bungrudi

  My situation is like this.. (note: for those who work with JBPM might already familiar with following data structures and HB mapping) Class LongInstance extends from VariableInstance, with the mapping for field "value" overridden in LongInstance. The mapping for VariableInstance is here and for LongInstance here. VariableInstance is polymorphically mapped to a collection in TokenVariableMap, the mapping is here. The question: how can I query the polymorphic collection using specific/overridden property of the member class? I'm looking for something like this "... from TokenVariableMaps tvm left join fetch tvm.variableInstances tvi where cast(tvi as LongInstance).value in(:vars)"

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• Shared static classes between AppDomains in loaded library code

  - by Christian Stewart

  I'm working on a program in which I want to do something similar to what the Photon Server system does: Offer a common "API" class library, which contains common data classes, enumerations, and interfaces for working with the host program. Have client programs (class libraries) reference this DLL and implement interfaces listed within it. Have the "host" application load built DLL client libraries into separate AppDomains and reference the interfaces that lie within to have polymorphic client code from within a dll file. I have something like this worked out: a class library that contains common code, but I've run into the following question How should I handle static classes? Should I add a method that is called by the host program to synchronize data? How do I keep a static class the same between AppDomains? Should I discard these classes in favor of better interfaces between the code levels? And in general, how do I share data between these loaded AppDomains?

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• Rails 3 Join Question for Votes Table

  - by Dex

  I have a table posts and a polymorphic table votes. The votes table looks like this: create_table :votes do |t| t.references :user # user_id t.vote # the vote value t.references :votable # votable_type and votable_id end I want to list all posts that the user has not yet voted on. Right now I'm basically taking all the posts they've already voted on and subtracting that from the entire set of posts. It works but it's not very convenient as I currently have it. def self.where_not_voted_on_by(user) sql = "SELECT P.* FROM posts P LEFT OUTER JOIN (" sql << where_voted_on_by(user).to_sql sql << ") ALREADY_VOTED_FOR ON P.id = ALREADY_VOTED_FOR.id WHERE (user_id is null)" puts sql resultset = connection.select_all(sql) results = [] resultset.each do |r| results << Post.new(r) end results end def self.where_voted_on_by(user) joins(:votes.outer).where("user_id = #{user.id}").select("posts.*, votes.user_id") end

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• Does dynamic_cast work inside overloaded operator delete ?

  - by iammilind

  I came across this: struct Base { void* operator new (size_t); void operator delete (void*); virtual ~Base () {} // <--- polymorphic }; struct Derived : Base {}; void Base::operator delete (void *p) { Base *pB = static_cast<Base*>(p); if(dynamic_cast<Derived*>(pB) != 0) { /* ... NOT reaching here ? ... */ } free(p); } Now if we do, Base *p = new Derived; delete p; Surprisingly, the condition inside the Base::delete is not satisfied Am I doing anything wrong ? Or casting from void* looses the information of Derived* ?

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• rails: has_many :through + polymorphism validation?

  - by ramonrails

  I am trying to achieve this. Any hints? A project has many users through join model A user has many projects through join model Admin class inherits User class. It also has some Admin specific stuff. Admin like inheritance for Supervisor and Operator Project has one Admin, One supervisor and many operators. Now I want to 1. submit data for project, admin, supervisor and operator in a single project form 2. validate all and show errors on the project form. Project has_many :users, :through = :projects_users User has_many :projects, :through = :projects_users ProjectsUser = :id integer, :user_id :integer, :project_id :integer, :user_type :string ProjectUser belongs_to :project, belongs_to :user, :polymorphic = true Admin < User Supervisor < User Operator < User Is the approach correct? Any and all suggestions are welcome.

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• save object associate to another object automatically

  - by Luca Romagnoli

  Hi i have these classes: class Core < ActiveRecord::Base belongs_to :resource, :polymorphic => true belongs_to :image, :class_name => 'Multimedia', :foreign_key => 'image_id' end class Place < ActiveRecord::Base has_one :core, :as => :resource end If i try do launch this: a = Place.find(5) a.name ="a" a.core.image_id = 24 a.save name is saved. image_id no i want save automatically all changes in records in relationship with place class at a.save command. is possible? thanks

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• Define a method in base class that returns the name of itself (using reflection) - subclasses inheri

  - by Khnle

  In C#, using reflection, is it possible to define method in the base class that returns its own name (in the form of a string) and have subclasses inherit this behavior in a polymorphic way? For example: public class Base { public string getClassName() { //using reflection, but I don't want to have to type the word "Base" here. //in other words, DO NOT WANT get { return typeof(Base).FullName; } return className; //which is the string "Base" } } public class Subclass : Base { //inherits getClassName(), do not want to override } Subclass subclass = new Subclass(); string className = subclass.getClassName(); //className should be assigned "Subclass"

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• What's the best way to return something like a collection of `std::auto_ptr`s in C++03?

  - by Billy ONeal

  std::auto_ptr is not allowed to be stored in an STL container, such as std::vector. However, occasionally there are cases where I need to return a collection of polymorphic objects, and therefore I can't return a vector of objects (due to the slicing problem). I can use std::tr1::shared_ptr and stick those in the vector, but then I have to pay a high price of maintaining separate reference counts, and object that owns the actual memory (the container) no longer logically "owns" the objects because they can be copied out of it without regard to ownership. C++0x offers a perfect solution to this problem in the form of std::vector<std::unique_ptr<t>>, but I don't have access to C++0x. Some other notes: I don't have access to C++0x, but I do have TR1 available. I would like to avoid use of Boost (though it is available if there is no other option) I am aware of boost::ptr_container containers (i.e. boost::ptr_vector), but I would like to avoid this because it breaks the debugger (innards are stored in void *s which means it's difficult to view the object actually stored inside the container in the debugger)

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• This is right way for database?

  - by ciss

  Hello, i have some issue with database. Okay, i have two models - Page and Item. Page for displaying some content. Item - this is item discription. So, i work on small ecommerce shop. Okay, all of this models can have some comments. So, this is my Comments model at this moment: Comments - string : id text : body integer : page_id integer : item_id So when some one add comment to page - page_id will be filled with current Page id. And if some one add comment to item - item_id will be filled. Okay, i know what the best way is to create STI or Polymorphic assoc, but does i really need this way for my situation? Sorry for my bad english, i'm from Russia.=)

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• Polymorphism in Rails

  - by Newy

  Say I have two models, Apples and Oranges, and they are both associated with a description in a Text model. Text is a separate class as I'd like to keep track of the different revisions. Is the following correct? Is there a better way to do this? [Apple] has_one :text, :as => :targit, :order => 'id DESC' has_many :revisions, :class_name => 'Text', :as => :targit, :order => 'id', :dependent => :destroy [Text] belongs_to :targit, :polymorphic => true

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• Frederick .NET User Group April 2010 Meeting

  - by John Blumenauer

  FredNUG is pleased to announce that we have an excellent speaker lined up for April.  On April 20th, we’ll start with pizza and social networking at 6:30 PM.  Then, starting at 7 PM, Dane Morgridge will present “Getting Started with Entity Framework 4” The scheduled agenda is:   6:30 PM - 7:00 PM - Pizza/Social Networking/Announcements 7:00 PM - 8:30 PM - Main Topic: Getting Started with Entity Framework 4 with Dane Morgridge  Main Topic Description:  Getting Started with Entity Framework 4 With .Net 3.5 Microsoft release Linq to Sql and with .Net 3.5 SP1 came the Entity Framework, both powerful ORM tools leveraging Linq technology.   Entity Framework v1, while usable, was definitely lacking some important features and the Entity Framework team delivered with version 4 coming with Visual Studio 2010.  In this session we will look at Entity Framework 4 from the ground level and you will get a solid understanding of it basic principles.  We will also go through all of the new features in Entity Framework 4 and see how far it’s come since the initial release.  If you’ve never taken a look at Entity Framework, now is the time as version 4 is the real deal. Speaker Bio: Dane Morgridge has been a developer for 9+ years and has worked with .Net & C# since the first public beta. His current passions are Entity Framework, WPF, WCF, Silverlight and LINQ. He works mostly with C#, but is also a big fan of whatever new technology he happens to come across. In addition to software development, he is the host of the Community Megaphone Podcast and also enjoys dabbling in graphic design, video special effects and hockey. When not with his family he is usually learning some new technology or working on some side projects. He is currently working as the Development Manager & Architect at Roska Direct in Montgomeryville, PA.  He can be reached through is blog http://geekswithblogs.net/danemorgridge or on Twitter @danemorgridge.  8:30 PM - 8:45 PM – RAFFLE! Please join us and get involved in our .NET developers community!

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• AJI Report #19 | Scott K Davis and his son Tommy on Gamification and Programming for Kids

  - by Jeff Julian

  We are very excited about this show. John and Jeff sat down with Scott Davis and his son Tommy to talk about Gamification and Programming for Kids. Tommy is nine years old and the Iowa Code Camp was his second time presenting. Scott and Tommy introduce a package called Scratch that was developed by MIT to teach kids about logic and interacting with programming using sprites. Tommy's favorite experience with programming right now is Lego Mindstorms because of the interaction with the Legos and the development. Most adults when they get started with development also got started with interacting more with the physical machines. The next generation is given amazing tools, but the tools tend to be sealed and the physical interaction is not there. With some of these alternative hobby platforms like Legos, Arduino, and .NET Micro Framework, kids can write some amazing application and see their code work with physical movement and interaction with devices and sensors. In the second half of this podcast, Scott talks about how companies can us Gamification to prompt employees to interact with software and processes in the organization. We see gamification throughout the consumer space and you need to do is open up the majority of the apps on our phones or tablets and there is some interaction point to give the user a reward for using the tool. Scott gets into his product Qonqr which is described as the board game Risk and Foursquare together. Scott gets into the different mindsets of gamers (Bartle Index) and how you can use these mindsets to get the most out of your team through gamification techniques. Listen to the Show Site: http://scottkdavis.com/ Twitter: @ScottKDavis LinkedIn: ScottKDavis Scratch: http://scratch.mit.edu/ Lego Mindstorms: http://mindstorms.lego.com/ Bartle Test: Wikipedia Gamification: Wikipedia

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• Learn Domain-Driven Design

  - by Ben Griswold

  I just wrote about how I like to present on unfamiliar topics. With this said, Domain-Driven Design (DDD) is no exception. This is yet another area I knew enough about to be dangerous but I certainly was no expert.  As it turns out, researching this topic wasn’t easy. I could be wrong, but it is as if DDD is a secret to which few are privy. If you search the Interwebs, you will likely find little information about DDD until you start rolling over rocks to find that one great write-up, a handful of podcasts and videos and the Readers’ Digest version of the Blue Book which apparently you must read if you really want to get the complete, unabridged skinny on DDD.  Even Wikipedia’s write-up is skimpy which I didn’t know was possible…   Here’s a list of valuable resources.  If you, too, are interested in DDD, this is a good starting place.  Domain-Driven Design: Tackling Complexity in the Heart of Software by Eric Evans Domain-Driven Design Quickly, by Abel Avram & Floyd Marinescu An Introduction to Domain-Driven Design by David Laribee Talking Domain-Driven Design with David Laribee Part 1, Deep Fried Bytes Talking Domain-Driven Design with David Laribee Part 2, Deep Fried Bytes Eric Evans on Domain Driven Design, .NET Rocks Domain-Driven Design Community Eric Evans on Domain Driven Design Jimmy Nilsson on Domain Driven Design Domain-Driven Design Wikipedia What I’ve Learned About DDD Since the Book, Eric Evans Domain Driven Design, Alt.Net Podcast Applying Domain-Driven Design and Patterns: With Examples in C# and .NET, Jimmy Nilsson Domain-Driven Design Discussion Group DDD: Putting the Model to Work by Eric Evans The Official DDD Site

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• Don’t be a dinosaur. Use Calendar Tree!

  - by jamiet

  If one spends long enough in my company one will likely eventually have to listen to me bark on about subscribable calendars. I was banging on about them way back in 2009, I’ve cajoled SQLBits into providing one, provided one myself for the World Cup, and opined that they could be transformative for the delivery of BI. I believe subscribable calendars can change the world but have never been good at elucidating why I thought so, for that reason I always direct people to read a blog by Scott Adams (yes, the guy who draws Dilbert) entitled Calendar as Filter. In that blog post Scott writes: I think the family calendar is the organizing principle into which all external information should flow. I want the kids' school schedules for sports and plays and even lunch choices to automatically flow into the home calendar. Everything you do has a time dimension. If you are looking for a new home, the open houses are on certain dates, and certain houses that fit your needs are open at certain times. If you are shopping for some particular good, you often need to know the store hours. Your calendar needs to know your shopping list and preferences so it can suggest good times to do certain things I think the biggest software revolution of the future is that the calendar will be the organizing filter for most of the information flowing into your life. You think you are bombarded with too much information every day, but in reality it is just the timing of the information that is wrong. Once the calendar becomes the organizing paradigm and filter, it won't seem as if there is so much. I wholly agree and hence was delighted to discover (via the Hanselminutes podcast) that Scott has a startup called CalendarTree.com whose raison d’etre is to solve this very problem. What better way to describe a Scott Adams startup than with a Scott Adams comic: I implore you to check out Calendar Tree and make the world a tiny bit better by using it to share any information that has a time dimension to it. Don’t be a dinosaur, use Calendar tree! @Jamiet

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• What’s New in JD Edwards EnterpriseOne Release 9.1

  - by Breanne Cooley

  Oracle JD Edwards EnterpriseOne 9.1 offers customers significant updates to usability and business processes to improve productivity and bolster business value. This release addresses critical user needs, while delivering key enhancements in several areas, including:  New User Experience Release 9.1 offers significant enhancements to the user experience. New Web 2.0 features reduce task time and enable you to access meaningful information. Enhanced Reporting Oracle’s JD Edwards EnterpriseOne One View Reporting is a breakthrough solution that allows business users to create interactive reports - without IT support. Industry Specific Functionality This new release delivers key enhancements for the consumer goods, real estate management and manufacturing and distribution industries. Enhanced Support for Global Operations JD Edwards EnterpriseOne 9.1 supports global operations with several new features, including enhancements that consider the entire ERP business process associated with managing country of origin requirements. Productivity Features This new release offers new more tightly integrated business processes and other productivity advancements like improved data access and enhanced financial controls. Want to find out more? ü Bookmark the JD Edwards EnterpriseOne web page ü Listen to the Podcast: Announcing JD Edwards EnterpriseOne 9.1  ü Watch the Demo: JD Edwards EnterpriseOne 9.1 Features Demo ü Watch the Demo: JD Edwards EnterpriseOne One View Reporting Demo  ü Review the Data Sheet: JD Edwards EnterpriseOne Tools 9.1  New Training JD Edwards EnterpriseOne 9.1 training through Oracle University is designed to help you leverage these usability and productivity enhancements. The curriculum is aligned to the JD Edwards EnterpriseOne products and will teach your team how to efficiently and effectively implement and use your applications. Get started today! View available training and schedules.   -Jim Vonick, Oracle University Market Development Manager 

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