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• Do Scala and Erlang use green threads?

  - by CHAPa

  I've been reading a lot about how Scala and Erlang does lightweight threads and their concurrency model (actors). However, I have my doubts. Do Scala and Erlang use an approach similar to the old thread model used by Java (green threads) ? For example, suppose that there is a machine with 2 cores, so the Scala/Erlang environment will fork one thread per processor? The other threads will be scheduled by user-space (Scala VM / Erlang VM ) environment. Is this correct? Under the hood, how does this really work?

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• Scala/Erlang use something like greenThread or not ?

  - by CHAPa

  Hi all, Im reading a lot about how scala/Erlang does lightweight threads and your concurrency model ( Actor Model ). Off course, some doubts appear in my head. Scala/Erlang use a approach similar to the old thread model used by java (greenThread) ? for example, suppose that there is a machine with 2 cores, so the scala/erlang environment will fork one thread per processor ? The other threads will be scheduled by user-space( scala VM / erlang vm ) environment. is it correct ? how under the hood that really work ? thanks a lot.

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• What are the biggest differences between Scala 2.8 and Scala 2.7?

  - by André Laszlo

  I've written a rather large program in Scala 2.75, and now I'm looking forward to version 2.8. But I'm curious about how this big leap in the evolution of Scala will affect me. What will be the biggest differences between these two versions of Scala? And perhaps most importantly: Will I need to rewrite anything? Do I want to rewrite anything just to take advantage of some cool new feature? What exactly are the new features of Scala 2.8 in general?

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• Common programming mistakes for Scala developers to avoid

  - by jelovirt

  In the spirit of Common programming mistakes for Java developers to avoid? Common programming mistakes for JavaScript developers to avoid? Common programming mistakes for .NET developers to avoid? Common programming mistakes for Haskell developers to avoid? Common programming mistakes for Python developers to avoid? Common Programming Mistakes for Ruby Developers to Avoid Common programming mistakes for PHP developers to avoid? what are some common mistakes made by Scala developers, and how can we avoid them? Also, as the biggest group of new Scala developers come from Java, what specific pitfalls they have to be aware of? For example, one often cited problem Java programmers moving to Scala make is use a procedural approach when a functional one would be more suitable in Scala. What other mistakes e.g. in API design newcomers should try to avoid.

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• Java->Scala Remove Iterator<T>-Element from a JavaConversions-wrapped Iterable

  - by ifischer

  I have to translate the following code from Java to Scala: for (Iterator<ExceptionQueuedEvent> i = getUnhandledExceptionQueuedEvents().iterator(); i.hasNext();) { ExceptionQueuedEvent event = i.next(); try { //do something } finally { i.remove(); } } I'm using the JavaConversions library to wrap the Iterable. But as i'm not using the original Iterator, i don't know how to remove the current element correctly from the collection the same way as i did in Java: import scala.collection.JavaConversions._ (...) for (val event <- events) { try { //do something } finally { //how can i remove the current event from events? } } Can someone help me? I guess it's easy, but i'm still kinda new to Scala and don't understand what's going on when Scala wraps something of Java.

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• Launch Scala REPL programatically?

  - by David Crawshaw

  I would like to launch a Scala Swing application from the command line, then after the application is started, drop into the Scala REPL to use as a control interface. Ideally I would also like to pre-bind some variable names. Even better would be using a Java2D terminal emulator for the REPL, but I couldn't find anything appropriate. Does the Scala REPL have a public API?

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• Scala 2.8: use Java annotation with an array parameter

  - by yournamehere

  I'm trying to implement an JavaEE Session Bean with Scala 2.8. Because it's a Remote Session Bean, i have to annotate it with the following Java Annotation: @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) public @interface Remote { Class[] value() default {}; } I only found this example for scala 2.7. In Scala 2.7, its possible to define the session bean like this: @Remote {val value = Array(classOf[ITest])} class MyEJB ... How can i use this annotation the same way with Scala 2.8? I already tried many different versions, all resulting in "annotation argument needs to be a constant", "illegal start of simple expression". All of these definitions don't work: @Remote{val value = Array(classOf[PersonScalaEJB])} @Remote(val value = Array(classOf[PersonScalaEJB])) @Remote(Array(classOf[PersonScalaEJB]))

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• Scala in image procesing

  - by Mayank Sinha

  I am new to scala and keep on researching and compiling programs , But i am more interested in Image processing in scala. Iam doing project in eclipse environment from http://www.scala-lang.org/download/2.11.0-M5.html but i couldn't find any resource to do image processing in scala . please provide me all information i.e How to install image processing package till manuals of the package to read and write image. I have tried Maven ,jmagic,opencv ,javacv etc but couldn't succeeded. Please reply with in hours my job is at stake. Mayank

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• Scala on the CLR

  - by Michal Bendowski

  The Scala homepage says that Scala 1.4 was runnable on the .NET framework - what is the status of Scala on the CLR now? Is anyone working on it? I think it would make a great GUI tool combined with GTK# and Mono...

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• What ORMs work well with Scala?

  - by Clinton R. Nixon

  I'm about to write a Scala command-line application that relies on a MySQL database. I've been looking around for ORMs, and am having trouble finding one that will work well. The Lift ORM looks nice, but I'm not sure it can be decoupled from the entire Lift web framework. ActiveObjects also looks OK, but the author says that it may not work well with Scala. I'm not coming to Scala from Java, so I don't know all the options. Has anyone used an ORM with Scala, and if so, what did you use and how well did it work?

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• Scala model-view-presenter, traits

  - by Ralph

  I am a fan of Martin Fowler's (deprecated) model-view-presenter pattern. I am writing a Scala view class containing several button classes. I would like to include methods to set the action properties of the buttons, to be called by the presenter. A typical code fragment looks like this: private val aButton = new JButton def setAButtonAction(action: Action): Unit = { aButton.setAction(action) } This code is repeated for each button. If Java/Scala had the C preprocessor, I would create a macro to generate this code, given the button name (no lectures on the evils of the C preprocessor, please). This code is obviously very verbose and repetitive. Is there any better way way to do this in Scala, perhaps using traits? Please hold the lectures about scala.swing. I looking for a general pattern here.

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• IS classOf[] in scala 2.8 different from 2.7?

  - by redtank

  I have an interface from java public class IJava { ... public java.lang.Class getType(); ... } It is inherited in Scala class CScala { def getType() = classOf[Foo] } it worked in scala 2.7.7. But in 2.8.0.RC1, i get type mismatch; found : java.lang.ClassFoo required: java.lang.Class How do i get java.langClass in Scala 2.8

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• Practical examples of using symbols in Scala?

  - by Jesper

  Scala has symbols - names that start with a single quote ' and which are a kind of string constants. I know symbols from Ruby (where they start with a colon). In Ruby they are used for some meta-programming tasks, like generating getters and setters for member variables (for example attr_reader :name to generate a getter for name). I haven't seen a lot of use of symbols in Scala code yet. What are practical uses for symbols in Scala?

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• Actors in Scala.net

  - by weijiajun

  I have recently completed some study of erlang, and was intrigued by scala for its feature set and the ease of interpolating with java (and possibly .net) applications. I am finally studying actors and was wondering if there is an actor mechanism that currently works in .net. I have looked at the libararies that come down with sbaz and have found that there is a scala.Concurrent but no scala.actors.Actor. I tried to use the scala.Concurrent.Channel but was unable to use the ! to send messages. I was just wondering if this is something that is currently available and if so how do you go about setting it up.

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• Scala project does not automatically build in Eclipse

  - by stacker

  I copied the examples folder from scala-2.7.7.final-devel-docs to the src folder of a scala project. But the source files will not compiled unless I change them manually. "Project/Build automatically" is checked. I'm using the Scala Eclipse Plugin 2.7.7-final How can I achieve that this works like in java projects?

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• Scala println in a for loop

  - by random459

  The following Scala code does just what I expect it to - it prints each line of some_file.txt. import scala.io.Source val lines = Source.fromPath("some_file.txt").mkString for (line <- lines) print(line) If I use println instead of print, I expect to see some_file.txt printed out with double-spacing. Instead, the program prints a newline after every character of some_file.txt. Could someone explain this to me? I'm using Scala 2.8.0 Beta 1.

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• Scala 2.8.1 implicitly convert to java.util.List<java.util.Map<String, Object>>

  - by Ralph

  I have a Scala data structure created with the following: List(Map[String, Anyref]("a" -> someFoo, "b" -> someBar)) I would like to implicitly convert it (using scala.collection.JavaConversions or scala.collection.JavaConverters) to a java.util.List<java.util.Map<String, Object>> to be passed the a Java method that expects the latter. Is this possible? I have already created the following method that does it, but was wondering if it can be done automatically by the compiler? import scala.collection.JavaConversions._ def convertToJava(listOfMaps: List[Map[String, AnyRef]]): java.util.List[java.util.Map[String, Object]] = { asJavaList(listOfMaps.map(asJavaMap(_))) }

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• What are the best features of Scala?

  - by Pekka Mattila

  I can say that I really love Scala but now I would like to know the features you cannot live without when working with Scala? What about Scala 2.8?

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• scala jrebel superclass change

  - by coubeatczech

  hi, I'm using JRebel with Scala and I'm quite frequently experiencing the need for restart of server due to the fact that JRebel is unable to load a class if the superclass was changed. This is done mainly when I change anonymous functions as I can deduce from the JRebel error desription: Class 'mypackage.NewBook$$anonfun$2' superclass was changed from 'scala.runtime.AbstractFunction1' to 'scala.runtime.AbstractFunction2' and could not be reloaded. Is there any way, how can I design my code to avoid this? Does scala compiler take the functions, numbers them from one as they appear in source code?

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• How to deploy a Scala project from Eclipse?

  - by lach

  I have a Scala project in Eclipse that I need to package up so I can deploy it to a server. It's based on Jetty but it runs as a standalone application. It contains Scala classes, Java classes and a number of 3rd party jars. I assumed there would be some kind of deployment option in the Scala Eclipse plugin but I've drawn a blank. What is the simplest way to package the Scala project into a runnable file so it can be deployed? Any help greatly appreciated. Cheers.

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• Odd behaviour with scala method syntax

  - by Ceilingfish

  Hi chaps, I hit a bit of a quirk of scala's syntax I don't really understand object Board { def getObjectAt(x:Int, y:Int):Placeable = return locations(x)(y) } works fine. But object Board { def getObjectAt(x:Int, y:Int):Placeable { return locations(x)(y) } } returns the error Board.scala:8: error: illegal start of declaration return locations(x)(y) I found some stuff that says the second form convinces the scala compiler you're trying to specify an expansion to the return type Placeable. Is there a way I can fix this, or should I just avoid specifying a return type here?

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• Generate a sequence of Fibonacci number in Scala

  - by qin

  def fibSeq(n: Int): List[Int] = { var ret = scala.collection.mutable.ListBuffer[Int](1, 2) while (ret(ret.length - 1) < n) { val temp = ret(ret.length - 1) + ret(ret.length - 2) if (temp >= n) { return ret.toList } ret += temp } ret.toList } So the above is my code to generate a Fibonacci sequence using Scala to a value n. I am wondering if there is a more elegant way to do this in Scala?

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• Scala : cleanest way to recursively parse files checking for multiple strings

  - by fred basset

  Hi All, I want to write a Scala script to recursively process all files in a directory. For each file I'd like to see if there are any cases where a string occurs at line X and line X - 2. If a case like that occurs I'd like to stop processing that file, and add that filename to a map of filenames to occurrence counts. I just started learning Scala today, I've got the file recurse code working, and need some help with the string searching, here's what I have so far: import java.io.File import scala.io.Source val s1= "CmdNum = 506" val s2 = "Data = [0000,]" def processFile(f: File) { val lines = scala.io.Source.fromFile(f).getLines.toArray for (i = 0 to lines.length - 1) { // want to do string searches here, see if line contains s1 and line two lines above also contains s1 //println(lines(i)) } } def recursiveListFiles(f: File): Array[File] = { val these = f.listFiles if (these != null) { for (i = 0 to these.length - 1) { if (these(i).isFile) { processFile(these(i)) } } these ++ these.filter(_.isDirectory).flatMap(recursiveListFiles) } else { Array[File]() } } println(recursiveListFiles(new File(args(0))))

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• Can Scala be used to write GWT applications?

  - by jldupont

  Can Scala be used to write GWT applications? (NOTE: Java/Scala novice here...)

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• Scala's tuple unwrapping nuance

  - by Paul Milovanov

  I've noticed the following behavior in scala when trying to unwrap tuples into vals: scala> val (A, B, C) = (1, 2, 3) <console>:5: error: not found: value A val (A, B, C) = (1, 2, 3) ^ <console>:5: error: not found: value B val (A, B, C) = (1, 2, 3) ^ <console>:5: error: not found: value C val (A, B, C) = (1, 2, 3) ^ scala> val (u, v, w) = (1, 2, 3) u: Int = 1 v: Int = 2 w: Int = 3 Is that because scala's pattern matching mechanism automatically presumes that all identifiers starting with capitals within patterns are constants, or is that due to some other reason? Thanks!

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