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  • What is a "context bound" in Scala?

    - by Jesper
    One of the new features of Scala 2.8 are context bounds. What is a context bound and where is it useful? Of course I searched first (and found for example this) but I couldn't find any really clear and detailed information.

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  • How to get Ponter/Reference semantics in Scala.

    - by Lukasz Lew
    In C++ I would just take a pointer (or reference) to arr[idx]. In Scala I find myself creating this class to emulate a pointer semantic. class SetTo (val arr : Array[Double], val idx : Int) { def apply (d : Double) { arr(idx) = d } } Isn't there a simpler way? Doesn't Array class have a method to return some kind of reference to a particular field?

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  • Scala - learning by doing

    - by wrecked
    coming from the PHP-framework symfony (with Apache/MySQL) I would like to dive into the Scala programming language. I already followed a few tutorials and had a look at Lift and Play. Java isn't a stranger to me either. However the past showed that it's easiest to learn things by just doing them. Currently we have a little - mostly ajax-driven - application build on symfony at my company. My idea is to just build a little project similar to this one which might gets into production in the future. The app should feature: High scalability and performance a backend-server web-interface and a GUI-client There are plenty of questions arising when I think of this. First of all: Whats the best way to accomplish a easy to maintain, structured base for it? Would it be best to establish a socket based communication between pure-scala server/client and accessing that data via Lift or is building a Lift-app serving as a server and connecting the gui-client (via REST?) the better way? Furthermore I wounder which database to choose. I'm familiar with (My)SQL but feel like a fool beeing confronted with all these things like NoSQL, MongoDB and more. Thanks in advance!

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  • What is in scala-android.jar?

    - by synic
    I've been trying to experiment with developing Android applications with Scala. I've gotten to the point where I can get the app to compile, but there are no helper functions for things like: button.setOnClickListener( () => { text.setText("test") }) (I'm talking about the closure there) I see lots of references to scala-android.jar, and have this file in my project, but I'm not sure what it does, or how to use it. I get the feeling it has these helper conversion functions, but I'm not sure. Running jar -tvf scala-android.jar on the file gives me this: 401 Sun Jun 06 10:06:02 MDT 2010 scala/Function0$class.class 431 Sun Jun 06 10:06:02 MDT 2010 scala/Function0.class 572 Sun Jun 06 10:06:02 MDT 2010 scala/Function1.class 282 Sun Jun 06 10:06:02 MDT 2010 scala/ScalaObject$class.class 271 Sun Jun 06 10:06:02 MDT 2010 scala/ScalaObject.class 458 Sun Jun 06 10:06:02 MDT 2010 scala/runtime/BoxedUnit.class If this isn't what I'm looking for, is there a simple library that'll do conversions for this kind of stuff?

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  • Diving into Scala with Cay Horstmann

    - by Janice J. Heiss
    A new interview with Java Champion Cay Horstmann, now up on otn/java, titled  "Diving into Scala: A Conversation with Java Champion Cay Horstmann," explores Horstmann's ideas about Scala as reflected in his much lauded new book,  Scala for the Impatient.  None other than Martin Odersky, the inventor of Scala, called it "a joy to read" and the "best introduction to Scala". Odersky was so enthused by the book that he asked Horstmann if the first section could be made available as a free download on the Typesafe Website, something Horstmann graciously assented to. Horstmann acknowledges that some aspects of Scala are very complex, but he encourages developers to simply stay away from those parts of the language. He points to several ways Java developers can benefit from Scala: "For example," he says, " you can write classes with less boilerplate, file and XML handling is more concise, and you can replace tedious loops over collections with more elegant constructs. Typically, programmers at this level report that they write about half the number of lines of code in Scala that they would in Java, and that's nothing to sneeze at. Another entry point can be if you want to use a Scala-based framework such as Akka or Play; you can use these with Java, but the Scala API is more enjoyable. " Horstmann observes that developers can do fine with Scala without grasping the theory behind it. He argues that most of us learn best through examples and not through trying to comprehend abstract theories. He also believes that Scala is the most attractive choice for developers who want to move beyond Java and C++.  When asked about other choices, he comments: "Clojure is pretty nice, but I found its Lisp syntax a bit off-putting, and it seems very focused on software transactional memory, which isn't all that useful to me. And it's not statically typed. I wanted to like Groovy, but it really bothers me that the semantics seems under-defined and in flux. And it's not statically typed. Yes, there is Groovy++, but that's in even sketchier shape. There are a couple of contenders such as Kotlin and Ceylon, but so far they aren't real. So, if you want to do work with a statically typed language on the JVM that exists today, Scala is simply the pragmatic choice. It's a good thing that it's such a nice choice." Learn more about Scala by going to the interview here.

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  • Question on Scala Closure (From "Programming in Scala")

    - by Ekkmanz
    I don't understand why authors said that Code Listing 9.1 from "Programming in Scala" use closure. In chapter 9, they show how to refactor code into more less duplicated form, from this original code: object FileMatcher { private def filesHere = (new java.io.File(".")).listFiles def filesEnding(query: String) = for (file <- filesHere; if file.getName.endsWith(query)) yield file def filesContaining(query: String) = for (file <- filesHere; if file.getName.contains(query)) yield file def filesRegex(query: String) = for (file <- filesHere; if file.getName.matches(query)) yield file } To the second version: object FileMatcher { private def filesHere = (new java.io.File(".")).listFiles def filesMatching(query: String, matcher: (String, String) => Boolean) = { for (file <- filesHere; if matcher(file.getName, query)) yield file } def filesEnding(query: String) = filesMatching(query, _.endsWith(_)) def filesContaining(query: String) = filesMatching(query, _.contains(_)) def filesRegex(query: String) = filesMatching(query, _.matches(_)) } Which they said that there is no use of closure here. Now I understand until this point. However they introduced the use of closure to refactor even some more, shown in Listing 9.1: object FileMatcher { private def filesHere = (new java.io.File(".")).listFiles private def filesMatching(matcher: String => Boolean) = for (file <- filesHere; if matcher(file.getName)) yield file def filesEnding(query: String) = filesMatching(_.endsWith(query)) def filesContaining(query: String) = filesMatching(_.contains(query)) def filesRegex(query: String) = filesMatching(_.matches(query)) } Now they said that query is a free variable but I don't really understand why they said so? Since ""query"" seems to be passed from top method down to string matching function explicitly.

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  • Scala method where type of second parameter equals part of generic type from first parameter

    - by ifischer
    I want to create a specific generic method in Scala. It takes two parameters. The first is of the type of a generic Java Interface (it's from the JPA criteria query). It currently looks like this: def genericFind(attribute:SingularAttribute[Person, _], value:Object) { ... } // The Java Interface which is the type of the first parameter in my find-method: public interface SingularAttribute<X, T> extends Attribute<X, T>, Bindable<T> Now i want to achieve the following: value is currently of type java.lang.Object. But I want to make it more specific. Value has to be the of the same type as the placeholder "_" from the first parameter (and so represents the "T" in the Java interface). Is that somehow possible, and how? BTW Sorry for the stupid question title (any suggestions?)

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  • Scala, make my loop more functional

    - by Pengin
    I'm trying to reduce the extent to which I write Scala (2.8) like Java. Here's a simplification of a problem I came across. Can you suggest improvements on my solutions that are "more functional"? Transform the map val inputMap = mutable.LinkedHashMap(1->'a',2->'a',3->'b',4->'z',5->'c') by discarding any entries with value 'z' and indexing the characters as they are encountered First try var outputMap = new mutable.HashMap[Char,Int]() var counter = 0 for(kvp <- inputMap){ val character = kvp._2 if(character !='z' && !outputMap.contains(character)){ outputMap += (character -> counter) counter += 1 } } Second try (not much better, but uses an immutable map and a 'foreach') var outputMap = new immutable.HashMap[Char,Int]() var counter = 0 inputMap.foreach{ case(number,character) => { if(character !='z' && !outputMap.contains(character)){ outputMap2 += (character -> counter) counter += 1 } } }

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  • Can Scala be considered a functional superset of Java?

    - by Giorgio
    Apart from the differences in syntax, can Scala be considered a superset of Java that adds the functional paradigm to the object-oriented paradigm? Or are there any major features in Java for which there is no direct Scala equivalent? With major features I mean program constructs that would force me to heavily rewrite / restructure my code, e.g., if I had to port a Java program to Scala. Or can I expect that, given a Java program, I can port it to Scala almost line-by-line?

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  • Writing functions of tuples conveniently in Scala

    - by Alexey Romanov
    Quite a few functions on Map take a function on a key-value tuple as the argument. E.g. def foreach(f: ((A, B)) ? Unit): Unit. So I looked for a short way to write an argument to foreach: > val map = Map(1 -> 2, 3 -> 4) map: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 3 -> 4) > map.foreach((k, v) => println(k)) error: wrong number of parameters; expected = 1 map.foreach((k, v) => println(k)) ^ > map.foreach({(k, v) => println(k)}) error: wrong number of parameters; expected = 1 map.foreach({(k, v) => println(k)}) ^ > map.foreach(case (k, v) => println(k)) error: illegal start of simple expression map.foreach(case (k, v) => println(k)) ^ I can do > map.foreach(_ match {case (k, v) => println(k)}) 1 3 Any better alternatives?

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  • Minimal framework in Scala for collections with inheriting return type

    - by Rex Kerr
    Suppose one wants to build a novel generic class, Novel[A]. This class will contain lots of useful methods--perhaps it is a type of collection--and therefore you want to subclass it. But you want the methods to return the type of the subclass, not the original type. In Scala 2.8, what is the minimal amount of work one has to do so that methods of that class will return the relevant subclass, not the original? For example, class Novel[A] /* What goes here? */ { /* Must you have stuff here? */ def reverse/* What goes here instead of :Novel[A]? */ = //... def revrev/*?*/ = reverse.reverse } class ShortStory[A] extends Novel[A] /* What goes here? */ { override def reverse: /*?*/ = //... } val ss = new ShortStory[String] val ss2 = ss.revrev // Type had better be ShortStory[String], not Novel[String] Does this minimal amount change if you want Novel to be covariant? (The 2.8 collections do this among other things, but they also play with return types in more fancy (and useful) ways--the question is how little framework one can get away with if one only wants this subtypes-always-return-subtypes feature.)

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  • Using XFCE4 with awesome

    - by whitequark
    When I got tired of using the standard XFCE window manager, xfwm, I simply installed openbox and selected "XFCE/Openbox" session in the GDM login screen. This was very convenient because I didn't want the plain Openbox session in which there are no network-manager, no panels, nothing. Now I have installed awesome: I really like the idea of tiling window managers. But after I launched it, everything went wrong. Absence of XFCE panels isn't anything bad, but subpixel-aliased fonts were somehow turned on, and that was really awful. I tried to login in XFCE, kill openbox/xfwm and start awesome, but that didn't worked: xfce4-session keeps restarting it's predefined WM, and killing it kills the whole X session too. And I didn't found the configuration for a dropdown list in GDM either. How can I start XFCE session with awesome as a WM?

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  • Chromium always starts as floating in awesome.wm

    - by xhochy
    I'm using awesome as window manager for a small surf&info terminal. Chromium is started directly after login on the first workspace and should be displayed fullscreen. I've set the layout of all workspaces to awful.layout.suit.max and followed Awesome FAQ so that Chromium and all other (automatically) started programs will be shown on the right workspace. All programs except Chromium will start correctly in fullscreen mode. I tried { rule = { class = "chromium-browser" }, properties = {floating = false, tag = tags[1][1]}} and { rule = { class = "chromium-browser" }, properties = {tag = tags[1][1]}} but Chromium will always start in floating mode. This is a bit annoying as you still see awesome's panel at the top.

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  • How do I use the awesome window manager?

    - by Jason Baker
    I've installed awesome on my Ubuntu laptop, and I like it. But I feel kind of lost. I don't know any keyboard shortcuts and the man pages aren't really any help (for instance, what does Mod4 mean?). Is there any kind of brief introduction to awesome I can read?

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  • running scala apps with java -jar

    - by paintcan
    Yo dawgs, I got some problems with the java. Check it out. sebastian@sebastian-desktop:~/scaaaaaaaaala$ java -cp /home/sebastian/.m2/repository/org/scala-lang/scala-library/2.8.0.RC3/scala-library-2.8.0.RC3.jar:target/scaaaaaaaaala-1.0.jar scaaalaaa.App Hello World! That's cool, right, but how bout this: sebastian@sebastian-desktop:~/scaaaaaaaaala$ java -cp /home/sebastian/.m2/repository/org/scala-lang/scala-library/2.8.0.RC3/scala-library-2.8.0.RC3.jar -jar target/scaaaaaaaaala-1.0.jar Exception in thread "main" java.lang.NoClassDefFoundError: scala/Application at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632) at java.lang.ClassLoader.defineClass(ClassLoader.java:616) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141) at java.net.URLClassLoader.defineClass(URLClassLoader.java:283) at java.net.URLClassLoader.access$000(URLClassLoader.java:58) at java.net.URLClassLoader$1.run(URLClassLoader.java:197) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:248) at scaaalaaa.App.main(App.scala) Caused by: java.lang.ClassNotFoundException: scala.Application at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:248) ... 13 more Wat the heck? Any idea why the first works and not the 2nd? How do I -jar my scala?? Thanks in advance bro.

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  • How can I use the Scala program schema2src?

    - by pr1001
    This perhaps more a Server Fault question... I installed schema2src via sbaz and now I would like to convert a DTD (Apple's plist schema) to Scala source. $ schema2src usage: schema2src [flags] --module mname arg* or schema2src dtd arg* or (experimental) schema2src xsd arg* (this doesn't work at all yet) where supported [flags] may be: --verbose prints some debugging information However, if I try give any argument, it appears it can't find Scala: $ schema2src --verbose Exception in thread "main" java.lang.NoClassDefFoundError: scala/runtime/BoxesUtility at schema2src.Main$.processArgs(Main.scala:56) at schema2src.Main$.main(Main.scala:25) at schema2src.Main.main(Main.scala) Caused by: java.lang.ClassNotFoundException: scala.runtime.BoxesUtility at java.net.URLClassLoader$1.run(URLClassLoader.java:200) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:188) at java.lang.ClassLoader.loadClass(ClassLoader.java:315) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:330) at java.lang.ClassLoader.loadClass(ClassLoader.java:250) at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:398) ... 3 more I have scala in my PATH... Any suggestions? Assuming that problem can be fixed, is this the correct syntax? $ schema2src PropertyList-1.0.dtd

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  • Bad class file error when using Scala 2.8.0-rc1 in Javafx 1.2

    - by aoprisan
    When trying to import scala.Option in a javafx script, I get the following javafxc error: bad class file: scala/Option$$anonfun$orNull$1.class(scala:Option$$anonfun$orNull$1.class) undeclared type variable: A1 Please remove or make sure it appears in the correct subdirectory of the classpath. import scala.Option; ^ I am using Scala 2.8.0-RC1, Javafxc 1.2.3_b36, JVM 1.6.0_18-b07, OS Ubuntu 9.10. The same code was working in Scala 2.7.7 .

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  • Why does Scala require functions to have explicit return type?

    - by garbage collection
    I recently began learning to program in Scala, and it's been fun so far. I really like the ability to declare functions within another function which just seems to intuitive thing to do. One pet peeve I have about Scala is the fact that Scala requires explicit return type in its functions. And I feel like this hinders on expressiveness of the language. Also it's just difficult to program with that requirement. Maybe it's because I come from Javascript and Ruby comfort zone. But for a language like Scala which will have tons of connected functions in an application, I cannot conceive how I brainstorm in my head exactly what type the particular function I am writing should return with recursions after recursions. This requirement of explicit return type declaration on functions, do not bother me for languages like Java and C++. Recursions in Java and C++, when they did happen, often were dealt with 2 to 3 functions max. Never several functions chained up together like Scala. So I guess I'm wondering if there is a good reason why Scala should have the requirement of functions having explicit return type?

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  • Assign method in Scala.

    - by Lukasz Lew
    When this code is executed: var a = 24 var b = Array (1, 2, 3) a = 42 b = Array (3, 4, 5) b (1) = 42 I see three (five?) assignments here. What is the name of the method call that is called in such circumstances? Is it operator overloading?

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  • How to access a field's value via reflection (Scala 2.8)

    - by soc
    Consider the following code: class Foo(var name: String = "bar") Now i try to get the value and the correct type of it via reflection: val foo = new Foo val field = foo.getDeclaredField("name") field.setAccessible(true) //This is where it doesn't work val value = field.get(????) I tried things like field.get(foo), but that just returns an java.lang.Object but no String. Basically I need the correct type, because I want to invoke a method on it (e. g. toCharArray). What is the suggested way to do that?

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  • Perfect hash in Scala.

    - by Lukasz Lew
    I have some class C: class C (...) { ... } I want to use it to index an efficient map. The most efficient map is an Array. So I add a "global" "static" counter in companion object to give each object unique id: object C { var id_counter = 0 } In primary constructor of C, with each creation of C I want to remember global counter value and increase it. Question 1: How to do it? Now I can use id in C objects as perfect hash to index array. But array does not preserve type information like map would, that a given array is indexed by C's id. Question 2: Is it possible to have it with type safety?

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