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  • Getting default value for java primitive types

    - by ripper234
    I have a java primitive type at hand: Class c = int.class; // or long.class, or boolean.class I'd like to get a 'default value' for this class - specifically the value is assigned to fields of this type if they are not initialized. E.g., '0' for a number, 'false' for a boolean. Is there a generic way to do this? I tried c.newInstance() But I'm getting an InstantiationException, and not a default instance.

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  • Dilemma with two types and operator +

    - by user35443
    I have small problem with operators. I have this code: public class A { public string Name { get; set; } public A() { } public A(string Name) { this.Name = Name; } public static implicit operator B(A a) { return new B(a.Name); } public static A operator+(A a, A b) { return new A(a.Name + " " + b.Name); } } public class B { public string Name { get; set; } public B() { } public B(string Name) { this.Name = Name; } public static implicit operator A(B b) { return new A(b.Name); } public static B operator +(B b, B a) { return new B(b.Name + " " + a.Name); } } Now I want to know, which's conversion operator will be called and which's addition operator will be called in this operation: new A("a") + new B("b"); Will it be operator of A, or of B? (Or both?) Thanks....

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  • Compiler error when using abstract types

    - by Dylan
    I'm trying to implement a "protocol helper" trait that is responsible for matching up Prompts and Responses. The eventual goal is to have an object that defines the various Prompt and Response classes as subclasses of a sealed trait, then have a class that mixes in the ProtocolSupport trait for that Protocol object. The problem is that my current approach won't compile, even though I'm fairly sure it should. Here's a distilled version of what I've got: trait Protocol { type Response type Prompt <: BasePrompt trait BasePrompt { type Data def validate(response: Response): Validated[Data] } } trait ProtocolSupport[P <: Protocol] { def foo(prompt: P#Prompt, response: P#Response) = { // compiler error prompt.validate(response) } } The compiler doesn't like the response as an argument to prompt.validate: [error] found : response.type (with underlying type P#Response) [error] required: _4.Response where val _4: P [error] prompt.validate(response) [error] ^ This isn't very helpful.. it seems to say that it wants a P.Response but that's exactly what I'm giving it, so what's the problem?

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  • C# 3.5 Merge 2 lists of 2 different types

    - by Ehsan
    I have 2 generic Lists List<type1> L1 , List<type2> L2 in C# 3.5 Problem: type1 has an attribute called "key1" and type2 has an attribute called "key2" How to merge L1 and L2 on key1 = key2. Both lists are unsorted but I'm welcome to any ideas on how to sort the lists based on the attribute. I'd like to be able to merge the two lists on a key. I know it's not a dictionary and it would've been nice if it was but there is a very specific reason why they are lists which I will not get in to because that is irrelevant.

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  • Design classes/interface to support methods returning different types

    - by Nayn
    Hi, I have classes as below. public interface ITest <T> { public T MethodHere(); } public class test1 implements ITest<String> { String MethodHere(){ return "Bla"; } } public class test2 implements ITest<Integer> { Integer MethodHere(){ return Integer.valueOf(2); } } public class ITestFactory { public static ITest getInstance(int type) { if(type == 1) return new test1(); else if(type == 2) return new test2(); } } There is a warning in the factory class that ITest is used as raw type. What modification should I do to get rid of it? Thanks Nayn

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  • How to reference/link another project in grails workspace without using jar files?

    - by Ivan Alagenchev
    I have a Grails website that references a java core application. I have been successful in adding a .jar dependency to that project; however the java project is in the same workspace as my grails project and I would ultimately like to reference that project directly. I don't want to deal with the added step of creating a new jar file every time that there is a modification to the java project, cleaning and updating my dependencies. I added the java project to my grails' project "Java Build Path" and at first everything seemed to work fine, but when I run grailscompile, the compiler fails to resolve all imports that point to the java project. I am using Spring Source Toolsuite as my IDE.

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  • Class templating std::set key types

    - by TomFLuff
    I have a class to evaluate set algebra but wish to template it. At the minute it looks a bit like this set.h: template<typename T> class SetEvaluation { public: SetEvaluation<T>(); std::set<T> evaluate(std::string in_expression); } set.cpp template<typename T> std::set<T> SetEvaluation<T>::evaluate(std::string expression) { std::set<T> result; etc etc... } But i'm getting undefined reference errors when compiling. Is it possible to declare the return type as std::set<T> and then pass std::string as the class template param. There are no errors in the class but only when I try to instantiate SetEvaluation<std::string> Can anyone shed light on this problem? thanks

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  • Determining unknown content-types with the Html5 file api

    - by Jesse
    I'm working through a small file upload script (learning experience) and I noticed that when selecting microsoft office related files (.doc or .docx for example) the file objects do not have a type specified: For .doc files I had expected the type to be "application/msword" and along the same train of thought .docx to be "application/vnd.openxmlformats-officedocument.wordprocessingml.document". In the cases when the type cannot be determined is the correct course of action to look at the file extension and match that to the "expected" content / mime type? Sample script: <div id="fileUpload"> <input type="file" id="fileElem" style="display:none;" onchange="handleFiles(this.files)"/> <a href="#" id="fileSelect">Select some files</a> </div> <script type="text/javascript"> var fileSelect = document.getElementById("fileSelect"), fileElem = document.getElementById("fileElem"); fileSelect.addEventListener("click", function (e) { if (fileElem) { fileElem.click(); } e.preventDefault(); }, false); function handleFiles(files) { console.log(files); } </script>

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  • How to improve my software project's speed?

    - by Blitzkr1eg
    I'm doing a school software project with my class mates in Java. We store the info on a remote db. When we start the application we pull all the information from the database and transform it into objects to use in our application (using java sql statemens). In the application we edit some of these objects and then when we exit the application we save or update information in the database using Hibernate. As you see we dont use Hibernate for pulling in information, we use it just for saving and updating. We have 2, but very similar problems. The loading of object(when we start the app) and the saving of objects(with Hibernate) in the db(when closing the app) is taking too much time. And our project its not a huge enterprise application, its a quite small app, we just manage some students, teachers, homeworks and tests. So our db is also very very small. How could we increase performance ? later edit: if we use a local database it runs very quick, it just runs slow on remote databases

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  • What is the most challenging part of a project like MyLifeBits

    - by kennyzx
    I have made a small utility to keep track of my daily expenses, and my coffee consumption (I always want to quit coffee but never succeed), and of course, this kind of utility is quite simple, just involving an xml file and a few hundred lines of code to manipulate the file. Then I find this project MyLifeBits, it is very intesting, but I think it should require a lot of effort to achieve its goal- that is, to record everything about a person that can be digitally record. So I wonder, it is possible to write an advanced version of my own utility - but a tiny version of MyLifeBits, that can capture: Every webpage I've read, no matter what browser I am using, just download its contents for offline reading, Auto archive emails/documents/notes that I edited, Auto archive Codes that I run/written. Well, these are basically what I do on my PC. And the captured records can be searched easily. My question is, What do you think is the most challenging part? Interoperating with Visual Studio/Office/Lotes Notes/Web browsers is one thing, Database is another thing given that "everything" is record, Advanced programming patterns since it is not a "toy project"? And others that I have overlooked but can be very difficult to handle?

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  • Nested/Sub data types in haskell

    - by Tom Carstens
    So what would be nice is if you could do something like the following (not necessarily with this format, just the general idea): data Minor = MinorA | MinorB data Major = Minor | MajorB isMinor :: Major -> Bool isMinor Minor = True isMinor _ = False So isMinor MinorA would report True (instead of an error.) At the moment you might do something like: data Major = MinorA | MinorB | MajorB isMinor :: Major -> Bool isMinor MinorA = True isMinor MinorB = True isMinor _ = False It's not terrible or anything, but it doesn't expand nicely (as in if Minor when up to MinorZ this would be terribly clunky). To avoid that problem you can wrap Minor: data Minor = MinorA | MinorB data Major = MajorA Minor | MajorB isMinor :: Major -> Bool isMinor (MajorA _) = True isMinor _ = False But now you have to make sure to wrap your Minors to use them as a Major... again not terrible; just doesn't really express the semantics I'd like very well (i.e. Major can be any Minor or MajorB). The first (legal) example is "Major can be MinorA..." but doesn't have any knowledge of Minor and the second is "Major can be MajorA that takes a Minor..." p.s. No, this isn't really about anything concrete.

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  • problem with Expression not equal types

    - by user428547
    class first { private int? firstID; } class second { private int secondID; private int secondField; } public override Expression<Func<first, bool>> FirstFilter() { Contex db = new Contex(); List<second> list = (from p in db.second select p).ToList(); return b => list.Select(p => p.secondID).Contains(b.firstID); } and i have error: cannot convert from 'System.Collections.Generic.IEnumerable' to 'System.Collections.Generic.IEnumerable' i have tried many diferent ways, but i just don't know how can i fix it.

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  • Normalization two types of customers into one table

    - by JDewzy
    I am trying to model a sales situation where you can sell to a person or to a business with a contact person. I cannot figure out the proper way to do this. It seems like 2 tables would be incorrect. But how do I model a Customer table that can be a business or a person? Would I just have a boolean for "business" and an additional "business_name" field that would default to Null. But then I have to do an if/then on the columns and that seems like poor design. Any advice, direction, or links is appreciated.

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  • What is the evidence that an API has exceeded its orthogonality in the context of types?

    - by hawkeye
    Wikipedia defines software orthogonality as: orthogonality in a programming language means that a relatively small set of primitive constructs can be combined in a relatively small number of ways to build the control and data structures of the language. The term is most-frequently used regarding assembly instruction sets, as orthogonal instruction set. Jason Coffin has defined software orthogonality as Highly cohesive components that are loosely coupled to each other produce an orthogonal system. C.Ross has defined software orthogonality as: the property that means "Changing A does not change B". An example of an orthogonal system would be a radio, where changing the station does not change the volume and vice-versa. Now there is a hypothesis published in the the ACM Queue by Tim Bray - that some have called the Bánffy Bray Type System Criteria - which he summarises as: Static typings attractiveness is a direct function (and dynamic typings an inverse function) of API surface size. Dynamic typings attractiveness is a direct function (and static typings an inverse function) of unit testing workability. Now Stuart Halloway has reformulated Banfy Bray as: the more your APIs exceed orthogonality, the better you will like static typing My question is: What is the evidence that an API has exceeded its orthogonality in the context of types? Clarification Tim Bray introduces the idea of orthogonality and APIs. Where you have one API and it is mainly dealing with Strings (ie a web server serving requests and responses), then a uni-typed language (python, ruby) is 'aligned' to that API - because the the type system of these languages isn't sophisticated, but it doesn't matter since you're dealing with Strings anyway. He then moves on to Android programming, which has a whole bunch of sensor APIs, which are all 'different' to the web server API that he was working on previously. Because you're not just dealing with Strings, but with different types, the API is non-orthogonal. Tim's point is that there is a empirical relationship between your 'liking' of types and the API you're programming against. (ie a subjective point is actually objective depending on your context).

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  • HttpPost works in Java project, not in Android

    - by dave.c
    I've written some code for my Android device to login to a web site over https and parse some data out of the resulting pages. An HttpGet happens first to get some info needed for login, then an HttpPost to do the actual login process. The code below works great in a Java project within Eclipse which has the following Jar files on the build path: httpcore-4.1-beta2.jar, httpclient-4.1-alpha2.jar, httpmime-4.1-alpha2.jar, commons-logging-1.1.1.jar. public static MyBean gatherData(String username, String password) { MyBean myBean = new MyBean(); try { HttpResponse response = doHttpGet(URL_PAGE_LOGIN, null, null); System.out.println("Got login page"); String content = EntityUtils.toString(response.getEntity()); String token = ContentParser.getToken(content); String cookie = getCookie(response); System.out.println("Performing login"); System.out.println("token = "+token +" || cookie = "+cookie); response = doLoginPost(username,password,cookie, token); int respCode = response.getStatusLine().getStatusCode(); if (respCode != 302) { System.out.println("ERROR: not a 302 redirect!: code is \""+ respCode+"\""); if (respCode == 200) { System.out.println(getHeaders(response)); System.out.println(EntityUtils.toString(response.getEntity()).substring(0, 500)); } } else { System.out.println("Logged in OK, loading account home"); // redirect handler and rest of parse removed } }catch (Exception e) { System.out.println("ERROR in gatherdata: "+e.toString()); e.printStackTrace(); } return myBean; } private static HttpResponse doHttpGet(String url, String cookie, String referrer) { try { HttpClient client = new DefaultHttpClient(); client.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, HttpVersion.HTTP_1_1); client.getParams().setParameter(CoreProtocolPNames.HTTP_CONTENT_CHARSET, "UTF-8"); HttpGet httpGet = new HttpGet(url); httpGet.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, HttpVersion.HTTP_1_1); httpGet.setHeader(HEADER_USER_AGENT,HEADER_USER_AGENT_VALUE); if (referrer != null && !referrer.equals("")) httpGet.setHeader(HEADER_REFERER,referrer); if (cookie != null && !cookie.equals("")) httpGet.setHeader(HEADER_COOKIE,cookie); return client.execute(httpGet); } catch (Exception e) { e.printStackTrace(); throw new ConnectException("Failed to read content from response"); } } private static HttpResponse doLoginPost(String username, String password, String cookie, String token) throws ClientProtocolException, IOException { try { HttpClient client = new DefaultHttpClient(); client.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, HttpVersion.HTTP_1_1); client.getParams().setParameter(CoreProtocolPNames.HTTP_CONTENT_CHARSET, "UTF-8"); HttpPost post = new HttpPost(URL_LOGIN_SUBMIT); post.getParams().setParameter(CoreProtocolPNames.PROTOCOL_VERSION, HttpVersion.HTTP_1_1); post.setHeader(HEADER_USER_AGENT,HEADER_USER_AGENT_VALUE); post.setHeader(HEADER_REFERER, URL_PAGE_LOGIN); post.setHeader(HEADER_COOKIE, cookie); post.setHeader("Content-Type","application/x-www-form-urlencoded"); List<NameValuePair> formParams = new ArrayList<NameValuePair>(); formParams.add(new BasicNameValuePair("org.apache.struts.taglib.html.TOKEN", token)); formParams.add(new BasicNameValuePair("showLogin", "true")); formParams.add(new BasicNameValuePair("upgrade", "")); formParams.add(new BasicNameValuePair("username", username)); formParams.add(new BasicNameValuePair("password", password)); formParams.add(new BasicNameValuePair("submit", "Secure+Log+in")); UrlEncodedFormEntity entity = new UrlEncodedFormEntity(formParams,HTTP.UTF_8); post.setEntity(entity); return client.execute(post); } catch (Exception e) { e.printStackTrace(); throw new ConnectException("ERROR in doLoginPost(): "+e.getMessage()); } } The server (which is not under my control) returns a 302 redirect when the login was successful, and 200 if it fails and re-loads the login page. When run with the above Jar files I get the 302 redirect, however if I run the exact same code from an Android project with the 1.6 Android Jar file on the build path I get the 200 response from the server. I get the same 200 response when running the code on my 2.2 device. My android application has internet permissions, and the HttpGet works fine. I'm assuming that the problem lies in the fact that HttpPost (or some other class) is different in some significant way between the Android Jar version and the newer Apache versions. I've tried adding the Apache libraries to the build path of the Android project, but due to the duplicate classes I get messages like: INFO/dalvikvm(390): DexOpt: not resolving ambiguous class 'Lorg/apache/http/impl/client/DefaultHttpClient;' in the log. I've also tried using a MultipartEntity instead of the UrlEncodedFormEntity but I get the same 200 result. So, I have a few questions: - Can I force the code running under android to use the newer Apache libraries in preference to the Android versions? - If not, does anyone have any ideas how can I alter my code so that it works with the Android Jar? - Are there any other, totally different approaches to doing an HttpPost in Android? - Any other ideas? I've read a lot of posts and code but I'm not getting anywhere. I've been stuck on this for a couple of days and I'm at a loss how to get the thing to work, so I'll try anything at this point. Thanks in advance.

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  • Cryptographic Validation Explained

    - by MarkPearl
    We have been using LogicNP’s CryptoLicensing for some of our software and I was battling to understand how exactly the whole process worked. I was sent the following document which really helped explain it – so if you ever use the same tool it is well worth a read. Licensing Basics LogicNP CryptoLicensing For .Net is the most advanced and state-of-the art licensing and copy protection system you can use for your software. LogicNP CryptoLicensing System uses the latest cryptographic technology to generate and validate licenses. The cryptographic algorithm used is the RSA algorithm which consists of a pair of keys called as the generation key and the validation key. Data encrypted using the generation key can only be decrypted using the corresponding validation key. How does cryptographic validation work? When a new license project is created, a unique validation-generation key pair is created for the project. When LogicNP CryptoLicensing For .Net generates licenses, it encrypts the license settings using the generation key. The validation key can be safely distributed with your software and is used during validation. During license validation, LogicNP CryptoLicensing For .Net attempts to decrypt the encrypted license code using the validation key. If the decryption is successful, this means that the data was encrypted using the generation key, since only the corresponding validation key can decrypt data encrypted with the generation key. This further means that not only is the license valid but that it was generated by you and only you since nobody else has access to the generation key. Generation Key This key is used by CryptoLicensing Generator to generate encrypted license codes. This key is stored in the license project file, so the license project file must be kept secure and confidential and must be accorded the same care as any other critical asset such as source code. Validation Key This key is used for validating generated license codes. It is the same key displayed in the 'Get Validation Key And Code' dialog (Ctrl+K) and is used by your software when validating license codes (using LogicNP.CryptoLicensing.dll). Unlike the generation key, it is not necessary to keep this key secure and confidential. Note that the generation key pair is stored in the project file created by LogicNP CryptoLicensing For .Net, so it is very important to backup this file and to keep it secure. Once the file is lost, it is not possible to retrieve the key pair. FAQ Do I use the same validation key to validate all license codes? Yes, the validation key (and generation key) for the project remains the same; you use the same key to validate all license codes generated using the project. You can retrieve the validation key using the "Project" menu --> "Get Validation Key & Code" menu item. Can license codes generated using generation key from one project be validated using validation key of another project? No! Q. Is every generated license code unique? A. Yes, every license code generated by CryptoLicensing is guaranteed to be unique, even if you generate thousands of codes at a time. Q. What makes CryptoLicensing so secure? A. CryptoLicensing uses the latest cryptographic technology to generate and validate licenses. The cryptographic algorithm used is the RSA asymmetric key algorithm which can use upto 3072-bit keys. Given current computing power, it takes years to break a 3072-bit key. Q. Is is possible for a hacker to develop a keygen for my software? A. Impossible. The cryptographic algorithm used by CryptoLicensing consists of a pair of keys called as the generation key and the validation key. Data encrypted with one key can only be decrypted by the other key and vice versa. Licenses are generated using the generation key and validated using the validation key. Without the generation key, it is impossible to generate valid licenses. Q. What is the difference between validation key and generation key? Generation Key This key is used by CryptoLicensing Generator to generate encrypted license codes. This key is stored in the license project file, so the license project file must be kept secure and confidential and must be accorded the same care as any other critical asset such as source code. Validation Key This key is used for validating generated license codes. It is the same key displayed in the 'Get Validation Key And Code' dialog (Ctrl+K) and is used by your software when validating license codes (using LogicNP.CryptoLicensing.dll). Unlike the generation key, it is not necessary to keep this key secure and confidential. Q. Do I have to include the license project file (.licproj) with my software? A. No!!! This goes against the very essence of the security of the asymmetric cryptographic scheme because the project file contains both the validation and generation key. With your software, you only need to include the validation key which will be used to validate licenses generated by CryptoLicensing using the generation key. The license project file should be treated as any other valuable and confidential asset such as your source code. Q. Does the license service need the license project file? A. Yes. The license project file is needed whenever new licenses are generated (via the UI, via the API or via the license service). As just one example, the license service generates new machine-locked licenses when activated licenses are presented to it for activation, therefore the license service needs the license project file. Q. Is it possible to embed my own data in the generated licenses? A. Yes. You can embed any amount of additional data in the licenses. This data will have the same amount of security as the license code itself and will be tamper-proof. The embedded user data can be retrieved from your software. Q. What additional steps can I take to ensure that my software does not get cracked? A. There are many methods and techniques which can make it extremely difficult for a hacker to crack your software. See Writing Effective License Checking Code And Designing Effective Licenses for more information. Q. Why is the license service not working? A. The most common cause is not setting the CryptoLicense.LicenseServiceURL property before trying to validate a license. Make sure that this property is set to the correct URL where your license service is hosted. The most common cause after this is that the license project file on the web server where your license service is hosted is not the latest. This happens if you make changes to the license project (for example, set the 'Enable With Serials' setting for a profile), but don't upload the updated project file to your web server. Q. Why are my serials not working? Serial codes require the user of a license service. See Using Serial Codes for more details. Also see the earlier question 'Why is the license service not working?' Q. Is the same validation key used to validate license codes generated from different profiles. A. Yes. Profiles are just pre specified license settings for quickly generating licenses having those settings. The actual license code is still generated using the license project's cryptographic generation key and thus, can be validated using the project's validation key. Q. Why are changes made to a profile not getting saved? A. Simply changing license settings via UI and saving the license project does not save those license settings to the active profile. You must first save the license settings to a profile using the Save/Save As command from the Profiles menu (see above). Q. Why is validation of activated licenses failing from CryptoLicensing Generator, but works from my software? A. Make sure that you have specified the URL of the license service using the Project Properties Dialog. Also see the earlier question 'Why is the license service not working?' Q. How can I extend the trial period of my customer? A. To extend the evaluation period of the customer, simply send him a new license code specifying the desired evaluation limits. Evaluation information such as the current used days, executions, etc are stored in garbled form in a registry location which is derived from the license code. Therefore, when a new license code is used, the old evaluation information will not be used and a new evaluation period will be started.

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  • New vs Clone Git in Eclipse with EGit

    - by Matty F
    I'm not sure I have either my new repo or clone repo workflow, or both, setup correctly. When I create a new project I create a repo on github, can't clone from it as it's empty so I create a new project which goes into my workspace and then the git init runs on the workspace copy. So I end up with everything in workspace\project-name. However, when I clone from github first I need to clone the repo and this goes into my default git directory (C:\git) as git\cloned-project-name, I then need to import this Git repo as a project into my workspace and I end up with workspace\cloned-project-name effectively duplicating the project folder in the git area. I've tried to clone to workspace\cloned-project-name but then it asks to import the git project and if I try to use workspace\cloned-project-name again, it errs. What am I doing wrong? Thanks, Matt.

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  • Dynamic Post-build event in Visual Studio

    - by SSumner
    I am building a video server application that has multiple projects in Visual Studio. One project, the video server project, needs to call a shell script to generate documentation. This works fine when you build the video server project, because the script is simply cd "$(SolutionDir)" start documentationgenerator However, there is also an SDK project that, when built, also builds the video server project. However, when it does this, it does not know where the shell script is, since it tries to use the SDK Project's Solution Directory. SDK Project Video Server Project shell script So the question is: how do I make the SDK Project find the Video Server Project? I checked the MSBuild properties and there are no properties that seem to deal with 'nested' projects.

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  • Why is this code invalid in C#?

    - by mmattax
    The following code will not compile: string foo = "bar"; Object o = foo == null ? DBNull.Value : foo; I get: Error 1 Type of conditional expression cannot be determined because there is no implicit conversion between 'System.DBNull' and 'string' To fix this, I must do something like this: string foo = "bar"; Object o = foo == null ? DBNull.Value : (Object)foo; This cast seems pointless as this is certainly legal: string foo = "bar"; Object o = foo == null ? "gork" : foo; It seems to me that when the ternary branches are of different types, the compiler will not autobox the values to the type object...but when they are of the same type then the autoboxing is automatic. In my mind the first statement should be legal... Can anyone describe why the compiler does not allow this and why the designers of C# chose to do this? I believe this is legal in Java...Though I have not verified this. Thanks. EDIT: I am asking for an understanding of why Java and C# handle this differently, what is going on underneath the scenes in C# that make this invalid. I know how to use ternary, and am not looking for a "better way" to code the examples. I understand the rules of ternary in C#, but I want to know WHY... EDIT (Jon Skeet): Removed "autoboxing" tag as no boxing is involved in this question.

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  • ADT-like polymorphism in Java (without altering class)

    - by ffriend
    In Haskell I can define following data type: data Tree = Empty | Leaf Int | Node Tree Tree and then write polymorphic function like this: depth :: Tree -> Int depth Empty = 0 depth (Leaf n) = 1 depth (Node l r) = 1 + max (depth l) (depth r) In Java I can emulate algebraic data types with interfaces: interface Tree {} class Empty implements Tree {} class Leaf implements Tree { int n; } class Node implements Tree { Tree l; Tree r; } But if I try to use Haskell-like polymorphism, I get an error: int depth(Empty node) { return 0; } int depth(Leaf node) { return 1; } int depth(Node node) { return 1 + Math.max(depth(node.l), depth(node.r)); // ERROR: Cannot resolve method 'depth(Tree)' } Correct way to overcome this is to put method depth() to each class. But what if I don't want to put it there? For example, method depth() may be not directly related to Tree and adding it to class would break business logic. Or, even worse, Tree may be written in 3rd party library that I don't have access to. In this case, what is the simplest way to implement ADT-like polymorpism? Just in case, for the moment I'm using following syntax, which is obviously ill-favored: int depth(Tree tree) { if (tree instanceof Empty) depth((Empty)tree) if (tree instanceof Leaf) depth((Leaf)tree); if (tree instanceof Node) depth((Node)tree); else throw new RuntimeException("Don't know how to find depth of " + tree.getClass()); }

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  • "Pattern matching" of algebraic type data constructors

    - by jetxee
    Let's consider a data type with many constructors: data T = Alpha Int | Beta Int | Gamma Int Int | Delta Int I want to write a function to check if two values are produced with the same constructor: sameK (Alpha _) (Alpha _) = True sameK (Beta _) (Beta _) = True sameK (Gamma _ _) (Gamma _ _) = True sameK _ _ = False Maintaining sameK is not much fun, it is potentially buggy. For example, when new constructors are added to T, it's easy to forget to update sameK. I omitted one line to give an example: -- it’s easy to forget: -- sameK (Delta _) (Delta _) = True The question is how to avoid boilerplate in sameK? Or how to make sure it checks for all T constructors? The workaround I found is to use separate data types for each of the constructors, deriving Data.Typeable, and declaring a common type class, but I don't like this solution, because it is much less readable and otherwise just a simple algebraic type works for me: {-# LANGUAGE DeriveDataTypeable #-} import Data.Typeable class Tlike t where value :: t -> t value = id data Alpha = Alpha Int deriving Typeable data Beta = Beta Int deriving Typeable data Gamma = Gamma Int Int deriving Typeable data Delta = Delta Int deriving Typeable instance Tlike Alpha instance Tlike Beta instance Tlike Gamma instance Tlike Delta sameK :: (Tlike t, Typeable t, Tlike t', Typeable t') => t -> t' -> Bool sameK a b = typeOf a == typeOf b

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  • Accessing type members outside the class in Scala

    - by Pekka Mattila
    Hi, I am trying to understand type members in Scala. I wrote a simple example that tries to explain my question. First, I created two classes for types: class BaseclassForTypes class OwnType extends BaseclassForTypes Then, I defined an abstract type member in trait and then defined the type member in a concerete class: trait ScalaTypesTest { type T <: BaseclassForTypes def returnType: T } class ScalaTypesTestImpl extends ScalaTypesTest { type T = OwnType override def returnType: T = { new T } } Then, I want to access the type member (yes, the type is not needed here, but this explains my question). Both examples work. Solution 1. Declaring the type, but the problem here is that it does not use the type member and the type information is duplicated (caller and callee). val typeTest = new ScalaTypesTestImpl val typeObject:OwnType = typeTest.returnType // declare the type second time here true must beTrue Solution 2. Initializing the class and using the type through the object. I don't like this, since the class needs to be initialized val typeTest = new ScalaTypesTestImpl val typeObject:typeTest.T = typeTest.returnType // through an instance true must beTrue So, is there a better way of doing this or are type members meant to be used only with the internal implementation of a class?

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  • An unusual type signature

    - by Travis Brown
    In Monads for natural language semantics, Chung-Chieh Shan shows how monads can be used to give a nicely uniform restatement of the standard accounts of some different kinds of natural language phenomena (interrogatives, focus, intensionality, and quantification). He defines two composition operations, A_M and A'_M, that are useful for this purpose. The first is simply ap. In the powerset monad ap is non-deterministic function application, which is useful for handling the semantics of interrogatives; in the reader monad it corresponds to the usual analysis of extensional composition; etc. This makes sense. The secondary composition operation, however, has a type signature that just looks bizarre to me: (<?>) :: (Monad m) => m (m a -> b) -> m a -> m b (Shan calls it A'_M, but I'll call it <?> here.) The definition is what you'd expect from the types; it corresponds pretty closely to ap: g <?> x = g >>= \h -> return $ h x I think I can understand how this does what it's supposed to in the context of the paper (handle question-taking verbs for interrogatives, serve as intensional composition, etc.). What it does isn't terribly complicated, but it's a bit odd to see it play such a central role here, since it's not an idiom I've seen in Haskell before. Nothing useful comes up on Hoogle for either m (m a -> b) -> m a -> m b or m (a -> b) -> a -> m b. Does this look familiar to anyone from other contexts? Have you ever written this function?

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