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  • Does C# have an equivalent to Scala's structural typing?

    - by Tom Morris
    In Scala, I can define structural types as follows: type Pressable { def press(): Unit } This means that I can define a function or method which takes as an argument something that is Pressable, like this: def foo(i: Pressable) { // etc. The object which I pass to this function must have defined for it a method called press() that matches the type signature defined in the type - takes no arguments, returns Unit (Scala's version of void). I can even use the structural type inline: def foo(i: { def press(): Unit }) { // etc. It basically allows the programmer to have all the benefits of duck typing while still having the benefit of compile-time type checking. Does C# have something similar? I've Googled but can't find anything, but I'm not familiar with C# in any depth. If there aren't, are there any plans to add this?

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  • Scala: Can I nudge a combinator parser to be locally greedy?

    - by eed3si9n
    Suppose I have an ambiguous language expressed in combinator parser. Is there a way to make certain expressions locally greedy? Here's an example of what I mean. import scala.util.parsing.combinator._ object Example extends JavaTokenParsers { def obj: Parser[Any] = (shortchain | longchain) ~ anyrep def longchain: Parser[Any] = zero~zero~one~one def shortchain: Parser[Any] = zero~zero def anyrep: Parser[Any] = rep(any) def any: Parser[Any] = zero | one def zero: Parser[Any] = "0" def one: Parser[Any] = "1" def main(args: Array[String]) { println(parseAll(obj, args(0) )) } } After compiling, I can run it as follows: $ scala Example 001111 [1.7] parsed: ((0~0)~List(1, 1, 1, 1)) I would like to somehow instruct the first part of obj to be locally greedy and match with longchain. If I switch the order around, it matches the longchain, but that's not because of the greediness. def obj: Parser[Any] = (longchain | shortchain) ~ anyrep

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  • When is a scala partial function not a partial function?

    - by Fred Haslam
    While creating a map of String to partial functions I ran into unexpected behavior. When I create a partial function as a map element it works fine. When I allocate to a val it invokes instead. Trying to invoke the check generates an error. Is this expected? Am I doing something dumb? Comment out the check() to see the invocation. I am using scala 2.7.7 def PartialFunctionProblem() = { def dream()() = { println("~Dream~"); new Exception().printStackTrace() } val map = scala.collection.mutable.HashMap[String,()=>Unit]() map("dream") = dream() // partial function map("dream")() // invokes as expected val check = dream() // unexpected invocation check() // error: check of type Unit does not take parameters }

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  • Scala and HttpClient: How do I resolve this error?

    - by Benjamin Metz
    I'm using scala with Apache HttpClient, and working through examples. I'm getting the following error: /Users/benjaminmetz/IdeaProjects/JakartaCapOne/src/JakExamp.scala Error:Error:line (16)error: overloaded method value execute with alternatives (org.apache.http.HttpHost,org.apache.http.HttpRequest)org.apache.http.HttpResponse <and> (org.apache.http.client.methods.HttpUriRequest,org.apache.http.protocol.HttpContext)org.apache.http.HttpResponse cannot be applied to (org.apache.http.client.methods.HttpGet,org.apache.http.client.ResponseHandler[String]) val responseBody = httpclient.execute(httpget, responseHandler) Here is the code with the error and line in question highlighted: import org.apache.http.client.ResponseHandler import org.apache.http.client.HttpClient import org.apache.http.client.methods.HttpGet import org.apache.http.impl.client.BasicResponseHandler import org.apache.http.impl.client.DefaultHttpClient object JakExamp { def main(args : Array[String]) : Unit = { val httpclient: HttpClient = new DefaultHttpClient val httpget: HttpGet = new HttpGet("www.google.com") println("executing request..." + httpget.getURI) val responseHandler: ResponseHandler[String] = new BasicResponseHandler val responseBody = httpclient.execute(httpget, responseHandler) // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ println(responseBody) client.getConnectionManager.shutdown } } I can successfully run the example in java...

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  • Can I create a collection in Scala that uses different equals/hashCode/compare implementations?

    - by Willis Blackburn
    I'm looking for as simple way to create an identity set. I just want to be able to keep track of whether or not I've "seen" a particular object while traversing a graph. I can't use a regular Set because Set uses "==" (the equals method in Scala) to compare elements. What I want is a Set that uses "eq." Is there any way to create a Set in Scala that uses some application-specified method for testing equality rather than calling equals on the set elements? I looked for some kind of "wrapEquals" method that I could override but did not find it. I know that I could use Java's IdentityHashMap, but I'm looking for something more general-purpose. Another idea I had was to just wrap each set element in another object that implements equals in terms of eq, but it's wasteful to generate tons of new objects just to get a new equals implementation. Thanks!

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  • Can i have a negative value as constant expression in Scala?

    - by Klinke
    I have an Java-Annotation that return a double value: @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface DoubleValue { double value(); } When i try to attach the annotation to a field in a scala class and the value is negativ like here: class Test { @DoubleValue(-0.05) var a = _ } i get an compiler error with the message: "annotation argument needs to be a constant; found: 0.05.unary_-". I understood that i need a numerical literal and i looked into the Scala Language Specification and it seems, that the - sign is only used for the exponent but not for the mantissa. Does someone has an idea how i can have a negative value as runtime information using annotations? Thanks, Klinke

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  • How to make a Scala Applet whose Applet class is a singleton?

    - by Jamie
    Hi, I don't know if a solution exists but it would be highly desirable. I'm making a Scala Applet, and I want the main Applet class to be a singleton so it can be accessed elsewhere in the applet, sort of like: object App extends Applet { def init { // do init here } } Instead I have to make the App class a normal instantiatable class otherwise it complains because the contructor is private. So the ugly hack I have is to go: object A { var pp: App = null } class App extends Applet { A.pp = this def init { // do init here } } I really hate this, and is one of the reasons I don't like making applets in Scala right now. Any better solution? It would be nice...

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  • Overloading on return type ???

    - by Green Hyena
    scala> val shares = Map("Apple" -> 23, "MicroSoft" -> 50, "IBM" -> 17) shares: scala.collection.immutable.Map[java.lang.String,Int] = Map(Apple -> 23, MicroSoft -> 50, IBM -> 17) scala> val shareholders = shares map {_._1} shareholders: scala.collection.immutable.Iterable[java.lang.String] = List(Apple, MicroSoft, IBM) scala> val newShares = shares map {case(k, v) => (k, 1.5 * v)} newShares: scala.collection.immutable.Map[java.lang.String,Double] = Map(Apple -> 34.5, MicroSoft -> 75.0, IBM -> 25.5) From this example it seems like the map method is overloaded on return type. Overloading on return type is not possible right? Would somebody please explain what's going on here?

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  • Why does the Scala compiler disallow overloaded methods with default arguments?

    - by soc
    While there might be valid cases where such method overloadings could become ambiguous, why does the compiler disallow code which is neither ambiguous at compile time nor at run time? Example: // This fails: def foo(a: String)(b: Int = 42) = a + b def foo(a: Int) (b: Int = 42) = a + b // This fails, too. Even if there is no position in the argument list, // where the types are the same. def foo(a: Int) (b: Int = 42) = a + b def foo(a: String)(b: String = "Foo") = a + b // This is OK: def foo(a: String)(b: Int) = a + b def foo(a: Int) (b: Int = 42) = a + b // Even this is OK. def foo(a: Int)(b: Int) = a + b def foo(a: Int)(b: String = "Foo") = a + b val bar = foo(42)_ // This complains obviously ... Are there any reasons why these restrictions can't be loosened a bit? Especially when converting heavily overloaded Java code to Scala default arguments are a very important and it isn't nice to find out after replacing plenty of Java methods by one Scala methods that the spec/compiler imposes arbitrary restrictions.

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  • Is it possible in Scala to force the caller to specify a type parameter for a polymorphic method ?

    - by Alex Kravets
    //API class Node class Person extends Node object Finder { def find[T <: Node](name: String): T = doFind(name).asInstanceOf[T] } //Call site (correct) val person = find[Person]("joe") //Call site (dies with a ClassCast inside b/c inferred type is Nothing) val person = find("joe") In the code above the client site "forgot" to specify the type parameter, as the API writer I want that to mean "just return Node". Is there any way to define a generic method (not a class) to achieve this (or equivalent). Note: using a manifest inside the implementation to do the cast if (manifest != scala.reflect.Manifest.Nothing) won't compile ... I have a nagging feeling that some Scala Wizard knows how to use Predef.<:< for this :-) Ideas ?

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  • Poll: What is stopping you from switching (from Java) to Scala ?

    - by Lukasz Lew
    What would make you to switch to Scala ? If you are negative on the switching to Scala, please state the reason as well (or upvote). As with all StackOverflow Poll type Q&As, please make certain your answer is NOT listed already before adding a new answer If it already exists, vote that one up so we see what the most popular answer is, rather than duplicating an existing entry. If you see a duplicate, vote it down. If you have interesting or additional information to add, use a comment or edit the original entry rather than creating a duplicate.

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  • File Adapter FileName Macros

    - by IntegrationOverload
    I can never find these when I need them...   Macro name Substitute value %datetime% Coordinated Universal Time (UTC) date time in the format YYYY-MM-DDThhmmss (for example, 1997-07-12T103508). %datetime_bts2000% UTC date time in the format YYYYMMDDhhmmsss, where sss means seconds and milliseconds (for example, 199707121035234 means 1997/07/12, 10:35:23 and 400 milliseconds). %datetime.tz% Local date time plus time zone from GMT in the format YYYY-MM-DDThhmmssTZD, (for example, 1997-07-12T103508+800). %DestinationParty% Name of the destination party. The value comes from the message context property BTS.DestinationParty. %DestinationPartyQualifier% Qualifier of the destination party. The value comes from the message context property BTS.DestinationPartyQualifier. %MessageID% Globally unique identifier (GUID) of the message in BizTalk Server. The value comes directly from the message context property BTS.MessageID. %SourceFileName% Name of the file from where the File adapter read the message. The file name includes the extension and excludes the file path, for example, Sample.xml. When substituting this property, the File adapter extracts the file name from the absolute file path stored in the FILE.ReceivedFileName context property. If the context property does not have a value, for example, if a message was received on an adapter other than the File adapter, the macro will not be substituted and will remain in the file name as is (for example, C:\Drop\%SourceFileName%). %SourceParty% Name of the source party from which the File adapter received the message. %SourcePartyQualifier% Qualifier of the source party from which the File adapter received the message. %time% UTC time in the format hhmmss. %time.tz% Local time plus time zone from GMT in the format hhmmssTZD (for example, 124525+530).

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  • Purpose of IF, ELSE, FOR macros ?

    - by psihodelia
    I have a source code of a library which has a lot of strange IF, ELSE, FOR, etc. macros for all common C-keywords instead of using just usual if,else,for,while keywords. These macros are defined like this: #define IF( a) if( increment_if(), a) where increment_if() function is defined so: static __inline void increment_if( void) { // If the "IF" operator comes just after an "ELSE", its counter // must not be incremented. ... //implementation } I don't really understand, what is the purpose of such macros? This library is for a real-time application and I suppose that using such macros must slow-down an application.

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  • What is the logic behind defining macros inside a struct?

    - by systemsfault
    As apparent in the title, I'm questioning the reason behind defining the macros inside a struct. I frequently see this approach in network programming for instance following snippet: struct sniff_tcp { u_short th_sport; /* source port */ u_short th_dport; /* destination port */ tcp_seq th_seq; /* sequence number */ tcp_seq th_ack; /* acknowledgement number */ u_char th_offx2; /* data offset, rsvd */ #define TH_OFF(th) (((th)->th_offx2 & 0xf0) >> 4) u_char th_flags; #define TH_FIN 0x01 #define TH_SYN 0x02 #define TH_RST 0x04 #define TH_PUSH 0x08 #define TH_ACK 0x10 #define TH_URG 0x20 #define TH_ECE 0x40 #define TH_CWR 0x80 #define TH_FLAGS (TH_FIN|TH_SYN|TH_RST|TH_ACK|TH_URG|TH_ECE|TH_CWR) u_short th_win; /* window */ u_short th_sum; /* checksum */ u_short th_urp; /* urgent pointer */ }; This example is from sniffex.c code in tcpdump's web site. Is this for enhancing readability and making code clearer.

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  • Is it a good idea to apply some basic macros to simplify code in a large project?

    - by DoctorT
    I've been working on a foundational c++ library for some time now, and there are a variety of ideas I've had that could really simplify the code writing and managing process. One of these is the concept of introducing some macros to help simplify statements that appear very often, but are a bit more complicated than should be necessary. For example, I've come up with this basic macro to simplify the most common type of for loop: #define loop(v,n) for(unsigned long v=0; v<n; ++v) This would enable you to replace those clunky for loops you see so much of: for (int i = 0, i < max_things; i++) With something much easier to write, and even slightly more efficient: loop (i, max_things) Is it a good idea to use conventions like this? Are there any problems you might run into with different types of compilers? Would it just be too confusing for someone unfamiliar with the macro(s)?

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  • Modern Java alternatives

    - by Ralph
    I'm not sure if stackoverflow is the best forum for this discussion. I have been a Java developer for 14 years and have written an enterprise-level (~500,000 line) Swing application that uses most of the standard library APIs. Recently, I have become disappointed with the progress that the language has made to "modernize" itself, and am looking for an alternative for ongoing development. I have considered moving to the .NET platform, but I have issues with using something the only runs well in Windows (I know about Mono, but that is still far behind Microsoft). I also plan on buying a new Macbook Pro as soon as Apple releases their new rumored Arrandale-based machines and want to develop in an environment that will feel "at home" in Unix/Linux. I have considered using Python or Ruby, but the standard Java library is arguably the largest of any modern language. In JVM-based languages, I looked at Groovy, but am disappointed with its performance. Rumor has it that with the soon-to-be released JDK7, with its InvokeDynamic instruction, this will improve, but I don't know how much. Groovy is also not truly a functional language, although it provides closures and some of the "functional" features on collections. It does not embrace immutability. I have narrowed my search down to two JVM-based alternatives: Scala and Clojure. Each has its strengths and weaknesses. I am looking for the stackoverflow readerships' opinions. I am not an expert at either of these languages; I have read 2 1/2 books on Scala and am currently reading Stu Halloway's book on Clojure. Scala is strongly statically typed. I know the dynamic language folks claim that static typing is a crutch for not doing unit testing, but it does provide a mechanism for compile-time location of a whole class of errors. Scala is more concise than Java, but not as much as Clojure. Scala's inter-operation with Java seems to be better than Clojure's, in that most Java operations are easier to do in Scala than in Clojure. For example, I can find no way in Clojure to create a non-static initialization block in a class derived from a Java superclass. For example, I like the Apache commons CLI library for command line argument parsing. In Java and Scala, I can create a new Options object and add Option items to it in an initialization block as follows (Java code): final Options options = new Options() { { addOption(new Option("?", "help", false, "Show this usage information"); // other options } }; I can't figure out how to the same thing in Clojure (except by using (doit...)), although that may reflect my lack of knowledge of the language. Clojure's collections are optimized for immutability. They rarely require copy-on-write semantics. I don't know if Scala's immutable collections are implemented using similar algorithms, but Rich Hickey (Clojure's inventor) goes out of his way to explain how that language's data structures are efficient. Clojure was designed from the beginning for concurrency (as was Scala) and with modern multi-core processors, concurrency takes on more importance, but I occasionally need to write simple non-concurrent utilities, and Scala code probably runs a little faster for these applications since it discourages, but does not prohibit, "simple" mutability. One could argue that one-off utilities do not have to be super-fast, but sometimes they do tasks that take hours or days to complete. I know that there is no right answer to this "question", but I thought I would open it up for discussion. If anyone has a suggestion for another JVM-based language that can be used for enterprise level development, please list it. Also, it is not my intent to start a flame war. Thanks, Ralph

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  • Modern alternatives to Java

    - by Ralph
    I have been a Java developer for 14 years and have written an enterprise-level (~500 kloc) Swing application that uses most of the standard library APIs. Recently, I have become disappointed with the progress that the language has made to "modernize" itself, and am looking for an alternative for ongoing development. I have considered moving to the .NET platform, but I have issues with using something the only runs well in Windows (I know about Mono, but that is still far behind Microsoft). I also plan on buying a new Macbook Pro as soon as Apple releases their new rumored Arrandale-based machines and want to develop in an environment that will feel "at home" in Unix/Linux. I have considered using Python or Ruby, but the standard Java library is arguably the largest of any modern language. In JVM-based languages, I looked at Groovy, but am disappointed with its performance. Rumor has it that with the soon-to-be released JDK7, with its InvokeDynamic instruction, this will improve, but I don't know how much. Groovy is also not truly a functional language, although it provides closures and some of the "functional" features on collections. It does not embrace immutability. I have narrowed my search down to two JVM-based alternatives: Scala and Clojure. Each has its strengths and weaknesses. I am looking for opinions. I am not an expert at either of these languages; I have read 2 1/2 books on Scala and am currently reading Stu Halloway's book on Clojure. Scala is strongly statically typed. I know the dynamic language folks claim that static typing is a crutch for not doing unit testing, but it does provide a mechanism for compile-time location of a whole class of errors. Scala is more concise than Java, but not as much as Clojure. Scala's inter-operation with Java seems to be better than Clojure's, in that most Java operations are easier to do in Scala than in Clojure. For example, I can find no way in Clojure to create a non-static initialization block in a class derived from a Java superclass. For example, I like the Apache commons CLI library for command line argument parsing. In Java and Scala, I can create a new Options object and add Option items to it in an initialization block as follows (Java code): final Options options = new Options() { { addOption(new Option("?", "help", false, "Show this usage information"); // other options } }; I can't figure out how to the same thing in Clojure (except by using (doit...)), although that may reflect my lack of knowledge of the language. Clojure's collections are optimized for immutability. They rarely require copy-on-write semantics. I don't know if Scala's immutable collections are implemented using similar algorithms, but Rich Hickey (Clojure's inventor) goes out of his way to explain how that language's data structures are efficient. Clojure was designed from the beginning for concurrency (as was Scala) and with modern multi-core processors, concurrency takes on more importance, but I occasionally need to write simple non-concurrent utilities, and Scala code probably runs a little faster for these applications since it discourages, but does not prohibit, "simple" mutability. One could argue that one-off utilities do not have to be super-fast, but sometimes they do tasks that take hours or days to complete. I know that there is no right answer to this "question", but I thought I would open it up for discussion. Are there other JVM-based languages that can be used for enterprise level development?

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  • Functional Methods on Collections

    - by GlenPeterson
    I'm learning Scala and am a little bewildered by all the methods (higher-order functions) available on the collections. Which ones produce more results than the original collection, which ones produce less, and which are most appropriate for a given problem? Though I'm studying Scala, I think this would pertain to most modern functional languages (Clojure, Haskell) and also to Java 8 which introduces these methods on Java collections. Specifically, right now I'm wondering about map with filter vs. fold/reduce. I was delighted that using foldRight() can yield the same result as a map(...).filter(...) with only one traversal of the underlying collection. But a friend pointed out that foldRight() may force sequential processing while map() is friendlier to being processed by multiple processors in parallel. Maybe this is why mapReduce() is so popular? More generally, I'm still sometimes surprised when I chain several of these methods together to get back a List(List()) or to pass a List(List()) and get back just a List(). For instance, when would I use: collection.map(a => a.map(b => ...)) vs. collection.map(a => ...).map(b => ...) The for/yield command does nothing to help this confusion. Am I asking about the difference between a "fold" and "unfold" operation? Am I trying to jam too many questions into one? I think there may be an underlying concept that, if I understood it, might answer all these questions, or at least tie the answers together.

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  • Performance of concurrent software on multicore processors

    - by Giorgio
    Recently I have often read that, since the trend is to build processors with multiple cores, it will be increasingly important to have programming languages that support concurrent programming in order to better exploit the parallelism offered by these processors. In this respect, certain programming paradigms or models are considered well-suited for writing robust concurrent software: Functional programming languages, e.g. Haskell, Scala, etc. The actor model: Erlang, but also available for Scala / Java (Akka), C++ (Theron, Casablanca, ...), and other programming languages. My questions: What is the state of the art regarding the development of concurrent applications (e.g. using multi-threading) using the above languages / models? Is this area still being explored or are there well-established practices already? Will it be more complex to program applications with a higher level of concurrency, or is it just a matter of learning new paradigms and practices? How does the performance of highly concurrent software compare to the performance of more traditional software when executed on multiple core processors? For example, has anyone implemented a desktop application using C++ / Theron, or Java / Akka? Was there a boost in performance on a multiple core processor due to higher parallelism?

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  • 2011 Tech Goal Review

    - by kerry
    A year ago I wrote a post listing my professional goals for 2011.  I thought I would review them and see how I did. Release an Android app to the marketplace – Didn’t do it.  In fact, haven’t really touched Android much since I wrote that.  I still have some ideas but am not sure if I will get around to it. Contribute free software to the community – I did do this.  I have been collaborating with others via github more lately. Regularly attend a user group meetings outside of Java – Did not do this.  Family life being what it is makes this not that much of a priority right now. Obtain the Oracle Certified Web Developer Certification – Did not do this.  This is not much of a priority to me any more. Learn scala – I am about 50/50 on this one.  I read a few scala books but did not write an actual application. Write an app using JSF – Did not do this.  Still interested. Present at a user group meeting – I did a Maven presentation at the Java user group. Use git more, and more effectively – Definitely did this.  Using it on a daily basis now. Overall, I got about halfway on my goals.  It’s not too bad since I did do a few things that weren’t on my list. Learned to develop applications using GWT and deploy them to Google App Engine Converted one of my sites from PHP to Ruby / Sinatra (learning to use it in the process) Studied up on the HTML 5 features and did a lot of Javascript development

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  • Programming languages with a Lisp-like syntax extension mechanism

    - by Giorgio
    I have only a limited knowledge of Lisp (trying to learn a bit in my free time) but as far as I understand Lisp macros allow to introduce new language constructs and syntax by describing them in Lisp itself. This means that a new construct can be added as a library, without changing the Lisp compiler / interpreter. This approach is very different from that of other programming languages. E.g., if I wanted to extend Pascal with a new kind of loop or some particular idiom I would have to extend the syntax and semantics of the language and then implement that new feature in the compiler. Are there other programming languages outside the Lisp family (i.e. apart from Common Lisp, Scheme, Clojure (?), Racket (?), etc) that offer a similar possibility to extend the language within the language itself? EDIT Please, avoid extended discussion and be specific in your answers. Instead of a long list of programming languages that can be extended in some way or another, I would like to understand from a conceptual point of view what is specific to Lisp macros as an extension mechanism, and which non-Lisp programming languages offer some concept that is close to them.

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