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  • Scala :: operator, how it works?

    - by Felix
    Hello Guys, in Scala, I can make a caseclass case class Foo(x:Int) and then put it in a list like so: List(Foo(42)) Now, nothing strange here. The following is strange to me. The operator :: is a function on a list, right? With any function with 1 argument in Scala, I can call it with infix notation. An example is 1 + 2 is a function (+) on the object Int. The class Foo I just defined does not have the :: operator, so how is the following possible: Foo(40) :: List(Foo(2)) ? In scala 2.8 rc1, I get the following output from the interactive prompt: scala> case class Foo(x:Int) defined class Foo scala> Foo(40) :: List(Foo(2)) res2: List[Foo] = List(Foo(40), Foo(2)) scala> I can go on and use it, but if someone can explain it I will be glad :)

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  • Comparing two collections for equality

    - by Crossbrowser
    I would like to compare two collections (in C#), but I'm not sure of the best way to implement this efficiently. I've read the other thread about Enumerable.SequenceEqual, but it's not exactly what I'm looking for. In my case, two collections would be equal if they both contain the same items (no matter the order). Example: collection1 = {1, 2, 3, 4}; collection2 = {2, 4, 1, 3}; collection1 == collection2; // true What I usually do is to loop through each item of one collection and see if it exists in the other collection, then loop through each item of the other collection and see if it exists in the first collection. (I start by comparing the lengths). if (collection1.Count != collection2.Count) return false; // the collections are not equal foreach (Item item in collection1) { if (!collection2.Contains(item)) return false; // the collections are not equal } foreach (Item item in collection2) { if (!collection1.Contains(item)) return false; // the collections are not equal } return true; // the collections are equal However, this is not entirely correct, and it's probably not the most efficient way to do compare two collections for equality. An example I can think of that would be wrong is: collection1 = {1, 2, 3, 3, 4} collection2 = {1, 2, 2, 3, 4} Which would be equal with my implementation. Should I just count the number of times each item is found and make sure the counts are equal in both collections? The examples are in some sort of C# (let's call it pseudo-C#), but give your answer in whatever language you wish, it does not matter. Note: I used integers in the examples for simplicity, but I want to be able to use reference-type objects too (they do not behave correctly as keys because only the reference of the object is compared, not the content).

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  • Simple Scala syntax - trying to define "==" operator - what am I missing?

    - by Alex R
    While experimenting with some stuff on the REPL, I got to a point where I needed something like this: scala class A(x:Int) { println(x); def ==(a:A) : Boolean = { this.x == a.x; } } Just a simple class with an "==" operator. Why doesn't it work??? Here's the result: :10: error: type mismatch; found : A required: ?{val x: ?} Note that implicit conversions are not applicable because they are ambiguous: both method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A] and method any2Ensuring in object Predef of type [A](x: A)Ensuring[A] are possible conversion functions from A to ?{val x: ?} class A(x:Int) { println(x); def ==(a:A) : Boolean = { this.x == a.x; } } ^ This is scala 2.8 RC1. Thanks

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  • Why is this Scala example of implicit paremeter not working?

    - by Alex R
    simple REPL test... def g(a:Int)(implicit b:Int) = {a+b} Why do neither of these attempted usages work? 1. scala class A { var b:Int =8; var c = g(2) } :6: error: could not find implicit value for parameter b: Int class A { var b:Int =8; var c = g(2) } 2. scala class A(var b:Int) { var c = g(2) } :6: error: could not find implicit value for parameter b: Int class A(var b:Int) { var c = g(2) } ^ Thanks

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  • Type-safe, generic, empty Collections with static generics

    - by Droo
    I return empty collections vs. null whenever possible. I switch between two methods for doing so using java.util.Collections: return Collections.EMPTY_LIST; return Collections.emptyList(); where emptyList() is supposed to be type-safe. But I recently discovered: return Collections.<ComplexObject> emptyList(); return Collections.<ComplexObject> singletonList(new ComplexObject()); etc. I see this method in Eclipse Package Explorer: <clinit> () : void but I don't see how this is done in the source code (1.5). How is this magic tomfoolerie happening!!

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  • Clojure Protocols vs Scala Structural Types

    - by Vasil Remeniuk
    After watching the interview with Rich Hickey on Protocols in Clojure 1.2, and knowing very little about Clojure, I have some questions on Clojure Protocols: Are they intended to do the same thing as Structural Types in Scala? What benefits do Protocols have over Structural Types (performance, flexibility, code clarity, etc.)? Are they implemented through reflections? Questions on interoperability with Scala: Can Protocols be used instead of Structural Types in Scala? Can they be extended (if 'extension' term can be applied to Protocols) in Scala?

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  • Scala command line parameters in eclipse?

    - by Mitch Blevins
    Scala includes the continuations plugin now (yay), but must be enabled by passing "-P:continuations:enable" to the scala compiler. Is there a way to pass arbitrary arguments to scalac for the eclipse scala plugin? From: http://permalink.gmane.org/gmane.comp.lang.scala/19439 the plugin is loaded by default, but it must be enabled by the command line argument -P:continuations:enable

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  • Scala actors: receive vs react

    - by jqno
    Let me first say that I have quite a lot of Java experience, but have only recently become interested in functional languages. Recently I've started looking at Scala, which seems like a very nice language. However, I've been reading about Scala's Actor framework in Programming in Scala, and there's one thing I don't understand. In chapter 30.4 it says that using react instead of receive makes it possible to re-use threads, which is good for performance, since threads are expensive in the JVM. Does this mean that, as long as I remember to call react instead of receive, I can start as many Actors as I like? Before discovering Scala, I've been playing with Erlang, and the author of Programming Erlang boasts about spawning over 200,000 processes without breaking a sweat. I'd hate to do that with Java threads. What kind of limits am I looking at in Scala as compared to Erlang (and Java)? Also, how does this thread re-use work in Scala? Let's assume, for simplicity, that I have only one thread. Will all the actors that I start run sequentially in this thread, or will some sort of task-switching take place? For example, if I start two actors that ping-pong messages to each other, will I risk deadlock if they're started in the same thread? According to Programming in Scala, writing actors to use react is more difficult than with receive. This sounds plausible, since react doesn't return. However, the book goes on to show how you can put a react inside a loop using Actor.loop. As a result, you get loop { react { ... } } which, to me, seems pretty similar to while (true) { receive { ... } } which is used earlier in the book. Still, the book says that "in practice, programs will need at least a few receive's". So what am I missing here? What can receive do that react cannot, besides return? And why do I care? Finally, coming to the core of what I don't understand: the book keeps mentioning how using react makes it possible to discard the call stack to re-use the thread. How does that work? Why is it necessary to discard the call stack? And why can the call stack be discarded when a function terminates by throwing an exception (react), but not when it terminates by returning (receive)? I have the impression that Programming in Scala has been glossing over some of the key issues here, which is a shame, because otherwise it's a truly excellent book.

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  • Using Scala array from java

    - by Grégoire
    Hi I'm trying to use some library code written in scala from a java program. I have a function that returns an Array (a scala Array) and I thought it would be possible to do Tree[] = ScalaObject.myScalaFunction() But the I get this error : [error] found : scala.runtime.BoxedArray [error] required: org.grammaticalframework.Trees.Absyn.Tree[] What is the correct way to use a scala array in java ?

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  • Samples of Scala and Java code where Scala code looks simpler/has fewer lines?

    - by Roman
    I need some code samples (and I also really curious about them) of Scala and Java code which show that Scala code is more simple and concise then code written in Java (of course both samples should solve the same problem). If there is only Scala sample with comment like "this is abstract factory in Scala, in Java it will look much more cumbersome" then this is also acceptable. Thanks!

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  • Parser that accepts Scala Identifiers?

    - by Mirko Stocker
    I was wondering whether the standard Scala parser combinators contain a parser that accepts the same identifiers that the Scala language itself also accepts (as specified in the Scala Language Specification, Section 1.1). The StdTokenParsers trait has an ident parser, but it rejects identifiers like empty_?. (If there is indeed no such parser, I could also just instantiate the Scala parser itself, but that wouldn't be as lightweight anymore.)

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  • Samples of Scala and Java code where Scala code looks simpler/has less lines?

    - by Roman
    I need some code samples (and I also really curious about them) of Scala and Java code which show that Scala code is more simple and concise then code written in Java (of course both samples should solve the same problem). If there is only Scala sample with comment like "this is abstract factory in Scala, in Java it will look much more cumbersome" then this is also acceptable. Thanks!

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  • In Scala, when is (A,A)=>R not equivalent to Function2 ?

    - by Alex R
    I'm trying to define and use my own foreach function. This is in the middle of a larger block of code. But the essence of the error I'm getting is this: test_2.scala:32: error: type mismatch; found : (A, A) = Unit required: Function2 $amount.foreach( (k:A,v:A) = { How is this error conceivably possible? Isn't (A, A) => Unit always a subtype of Function2 regardless of what else might be going on in the code?

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  • How to write Tetris in Scala? (code review)

    - by eed3si9n
    Today's the 25th birthday of Tetris. I believe writing Tetris clone is one of the best ways to familiarize oneself to a new language or a platform. It's not completely trivial and it lends itself well to learning language specific constructs like iterators and closures. I've been hearing about Scala, and finally decided to read some docs and write a Tetris clone. So, this is my first Scala code. I did try to use functional constructs, but am sure there are lots of things I can improve to do it more Scala way. Please give me suggestions using comment. Also other submissions of Tetris clone in Scala are welcome too. I'm aware that the actual question itself is somewhat subjective, but I think this is of some value since others can use this as example (or anti-example) code. Edit: Let me rephrase the question. What can I do to make the code more Scala-ish?

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  • How to use @PersistentCapable annotation in Scala 2.8

    - by Gero
    Hi, I'm switching from Scala 2.7.7 to Scala 2.8.0RC3 and now a few of my classes don't compile anymore. The problem is in the @PersistentCapable annotation: import javax.jdo.annotations._ import java.util.Date @PersistenceCapable{identityType=IdentityType.APPLICATION} class Counter(dt: Date, cName: String, vl: int) { <.. snip ..> } This code results in the following compilation errors: [ERROR] /Users/gero/prive/kiva/kivanotify-gae/src/main/scala/net/vermaas/kivanotify/model/LoanProcessed.scala:7: error: expected start of definition [INFO] @PersistenceCapable{val identityType = IdentityType.APPLICATION} I already tried a couple of variations, did some Googling but without luck. Any ideas on how I can use the @PersistentCapable annotation with Scala 2.8.0 RC3? Thanks, Gero

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  • Catch all exceptions in Scala 2.8 RC1

    - by Michel Krämer
    I have the following dummy Scala code in the file test.scala: class Transaction { def begin() {} def commit() {} def rollback() {} } object Test extends Application { def doSomething() {} val t = new Transaction() t.begin() try { doSomething() t.commit() } catch { case _ => t.rollback() } } If I compile this on Scala 2.8 RC1 with scalac -Xstrict-warnings test.scala I'll get the following warning: test.scala:16: warning: catch clause swallows everything: not advised. case _ => t.rollback() ^ one warning found So, if catch-all expressions are not advised, how am I supposed to implement such a pattern instead? And apart from that why are such expressions not advised anyhow?

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  • How well does Scala Perform Comapred to Java?

    - by Teja Kantamneni
    The Question actually says it all. The reason behind this question is I am about to start a small side project and want to do it in Scala. I am learning scala for the past one month and now I am comfortable working with it. The scala compiler itself is pretty slow (unless you use fsc). So how well does it perform on JVM? I previously worked on groovy and I had seen sometimes over performed than java. My Question is how well scala perform on JVM compared to Java. I know scala has some very good features(FP, dynamic lang, statically typed...) but end of the day we need the performance...

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  • Setting Scala "Platform" for NetBeans 6.8 on Ubuntu Lucid Lynx

    - by David
    I'm trying to use NetBeans 6.8 with Scala, and it can't find the "Scala Platform" (whatever that is supposed to be). I'm using Ubuntu Lucid Lynx (fully updated). The libraries are in /usr/share/java, the binaries in /usr/bin, the docs in /usr/share/doc/scala-doc/, and the sources are uninstalled. I think that NetBeans is looking for Scala to be in one single directory (with bin, lib, etc.). I created /usr/share/scala and placed links to the other directories, but it still didn't quite make it. Can someone straighten me out? (And why is it a "platform"? More needless jargon...)

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  • Print the next X number of lines in Scala

    - by soulesschild
    Trying to learn Scala using the Programming in Scala book and they have a very basic example for reading lines from a file. I'm trying to expand on it and read a file line by line, look for a certain phrase, then print the next 6 lines following that line if it finds the line. I can write the script easily enough in something like java or Perl but I have no idea how to do it in Scala (probably because I'm not very familiar with the language yet...) Here's the semi adapted sample code from the Programming in Scala book, import scala.io.Source if(args.length>0) { val lines = Source.fromFile(args(0)).getLines().toList for(line<-lines) { if(line.contains("secretPhrase")) { println(line) //How to get the next lines here? } } } else Console.err.println("Pleaseenterfilename")

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  • Common practice for higher-order-polymorphism in scala

    - by raichoo
    Hi, I'm trying to grasp higher-order-polymophism in scala by implementing a very basic interface that describes a monad but I come across a problem that I don't really understand. I implemented the same with C++ and the code looks like this: #include <iostream> template <typename T> class Value { private: T value; public: Value(const T& t) { this->value = t; } T get() { return this->value; } }; template < template <typename> class Container > class Monad { public: template <typename A> Container<A> pure(const A& a); }; template <template <typename> class Container> template <typename A> Container<A> Monad<Container>::pure(const A& a) { return Container<A>(a); } int main() { Monad<Value> m; std::cout << m.pure(1).get() << std::endl; return 0; } When trying to do the same with scala I fail: class Value[T](val value: T) class Monad[Container[T]] { def pure[A](a: A): Container[A] = Container[A](a) } object Main { def main(args: Array[String]): Unit = { val m = new Monad[Value] m.pure(1) } } The compiler complains about: [raichoo@lain:Scala]:434> scalac highorder.scala highorder.scala:5: error: not found: value Container Container[A](a) ^ one error found What am I doing wrong here? There seems to be a fundamental concept I don't seem to understand about scala typeconstructors. Regards, raichoo

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  • Why does this explicit call of a Scala method allow it to be implicitly resolved?

    - by Matt R
    Why does this code fail to compile, but compiles successfully when I uncomment the indicated line? (I'm using Scala 2.8 nightly). It seems that explicitly calling string2Wrapper allows it to be used implicitly from that point on. class A { import Implicits.string2Wrapper def foo() { //string2Wrapper("A") ==> "B" // <-- uncomment } def bar() { "A" ==> "B" "B" ==> "C" "C" ==> "D" } object Implicits { implicit def string2Wrapper(s: String) = new Wrapper(s) class Wrapper(s: String) { def ==>(s2: String) {} } } }

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  • Are Scala "continuations" just a funky syntax for defining and using Callback Functions?

    - by Alex R
    And I mean that in the same sense that a C/Java for is just a funky syntax for a while loop. I still remember when first learning about the for loop in C, the mental effort that had to go into understanding the execution sequence of the three control expressions relative to the loop statement. Seems to me the same sort of effort has to be applied to understand Continuations (in Scala and I guess probably other languages). And then there's the obvious follow-up question... if so, then what's the point? It seems like a lot of pain (language complexity, programmer errors, unreadable programs, etc) for no gain.

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