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• ensime scala errors (class scala.Array not found, object scala not found)

  - by Jeff Bowman

  I've installed ensime according to the README.md file, however, I get errors in the inferior-ensime-server buffer with the following: INFO: Fatal Error: scala.tools.nsc.MissingRequirementError: object scala not found. scala.tools.nsc.MissingRequirementError: object scala not found. at scala.tools.nsc.symtab.Definitions$definitions$.getModuleOrClass(Definitions.scala:516) at scala.tools.nsc.symtab.Definitions$definitions$.ScalaPackage(Definitions.scala:43) at scala.tools.nsc.symtab.Definitions$definitions$.ScalaPackageClass(Definitions.scala:44) at scala.tools.nsc.symtab.Definitions$definitions$.UnitClass(Definitions.scala:89) at scala.tools.nsc.symtab.Definitions$definitions$.init(Definitions.scala:786) at scala.tools.nsc.Global$Run.(Global.scala:593) at scala.tools.nsc.interactive.Global$TyperRun.(Global.scala:473) at scala.tools.nsc.interactive.Global.newTyperRun(Global.scala:535) at scala.tools.nsc.interactive.Global.reloadSources(Global.scala:289) at scala.tools.nsc.interactive.Global$$anonfun$reload$1.apply(Global.scala:300) at scala.tools.nsc.interactive.Global$$anonfun$reload$1.apply(Global.scala:300) at scala.tools.nsc.interactive.Global.respond(Global.scala:276) at scala.tools.nsc.interactive.Global.reload(Global.scala:300) at scala.tools.nsc.interactive.CompilerControl$$anon$1.apply$mcV$sp(CompilerControl.scala:81) at scala.tools.nsc.interactive.Global.pollForWork(Global.scala:132) at scala.tools.nsc.interactive.Global$$anon$2.run(Global.scala:192) also: INFO: Fatal Error: scala.tools.nsc.MissingRequirementError: class scala.Array not found. scala.tools.nsc.MissingRequirementError: class scala.Array not found. at scala.tools.nsc.symtab.Definitions$definitions$.getModuleOrClass(Definitions.scala:516) at scala.tools.nsc.symtab.Definitions$definitions$.getClass(Definitions.scala:474) at scala.tools.nsc.symtab.Definitions$definitions$.ArrayClass(Definitions.scala:217) at scala.tools.nsc.backend.icode.TypeKinds$REFERENCE.(TypeKinds.scala:258) at scala.tools.nsc.backend.icode.GenICode$ICodePhase.(GenICode.scala:55) at scala.tools.nsc.backend.icode.GenICode.newPhase(GenICode.scala:43) at scala.tools.nsc.backend.icode.GenICode.newPhase(GenICode.scala:25) at scala.tools.nsc.Global$Run$$anonfun$4.apply(Global.scala:606) at scala.tools.nsc.Global$Run$$anonfun$4.apply(Global.scala:605) at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:62) at scala.collection.immutable.List.foreach(List.scala:46) at scala.tools.nsc.Global$Run.(Global.scala:605) at scala.tools.nsc.interactive.Global$TyperRun.(Global.scala:473) at scala.tools.nsc.interactive.Global.newTyperRun(Global.scala:535) at scala.tools.nsc.interactive.Global.reloadSources(Global.scala:289) at scala.tools.nsc.interactive.Global.typedTreeAt(Global.scala:309) at scala.tools.nsc.interactive.Global$$anonfun$getTypedTreeAt$1.apply(Global.scala:326) at scala.tools.nsc.interactive.Global$$anonfun$getTypedTreeAt$1.apply(Global.scala:326) at scala.tools.nsc.interactive.Global.respond(Global.scala:276) at scala.tools.nsc.interactive.Global.getTypedTreeAt(Global.scala:326) at scala.tools.nsc.interactive.CompilerControl$$anon$2.apply$mcV$sp(CompilerControl.scala:89) at scala.tools.nsc.interactive.Global.pollForWork(Global.scala:132) at scala.tools.nsc.interactive.Global$$anon$2.run(Global.scala:192) Also none of the type identification works for me, I get 'NA' if I get anything at all. C-c t causes emacs to lock up. I'm running: Ubuntu 10.04 (64bit version) emacs 23.1.50.1 ensime from git (as of 3 May 2010) scala is version 2.8.0.RC1 java is 1.6.0_20 (from sun) here is a copy of the log: http://dl.dropbox.com/u/5309017/ensime.log Thanks! Jeff

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• How to use Scala in IntelliJ IDEA (or: why is it so difficult to get a working IDE for Scala)?

  - by Alex R

  I recently gave up trying to use Scala in Eclipse (basic stuff like completion doesn't work). So now I'm trying IntelliJ. I'm not getting very far. I've been able to edit programs (within syntax highlighting and completion... yay!). But I'm unable to run even the simplest "Hello World". This was the original error: Scala signature Predef has wrong version Expected 5.0 found: 4.1 in .... scala-library.jar But that was yesterday with IDEA 9.0.1. See below... UPDATE Today I uninstalled IntelliJ 9.0.1, and installed 9.0.2 Early Availability, with the 4/14 stable version of the Scala plug-in. Then I setup a project from scratch through the wizards: new project from scratch JDK is 1.6.u20 accept the default (project) instead of global / module accept the download of Scala 2.8.0beta1 into project's lib folder Created a new class: object hello { def main(args: Array[String]) { println("hello: " + args); } } For my efforts, I now have a brand-new error :) Here it is: Scalac internal error: class java.lang.ClassNotFoundException [java.net.URLClassLoader$1.run(URLClassLoader.java:202), java.security.AccessController.doPrivileged(Native Method), java.net.URLClassLoader.findClass(URLClassLoader.java:190), java.lang.ClassLoader.loadClass(ClassLoader.java:307), sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301), java.lang.ClassLoader.loadClass(ClassLoader.java:248), java.lang.Class.forName0(Native Method), java.lang.Class.forName(Class.java:169), org.jetbrains.plugins.scala.compiler.rt.ScalacRunner.main(ScalacRunner.java:72)] FINAL UPDATE I uninstalled 9.0.2 EA and reinstalled 9.0.1, but this time went with the 2.7.3 version of Scala rather than the default 2.7.6, because 2.7.3 is the one shown in the screen-shots at the IntelliJ website (I guess the screen-shots prove that they actually tested this version!). Now everything works!!!

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• More issues with IntelliJ 9.0.1 "Hello World" in Scala - Predef version 5.0 vs 4.1

  - by Alex R

  Any ideas what could cause this? Scala signature Predef has wrong version Expected 5.0 found: 4.1 in .... scala-library.jar I tried both versions 2.7.6 and 2.8 RC1 of scala-*.jar, the result was the same. JDK is 1.6.u20. UPDATE Today uninstalled IntelliJ 9.0.1, and installed 9.0.2 Early Availability, with the 4/14 stable version of the Scala plug-in. Then I setup a project from scratch through the wizards: new project from scratch JDK is 1.6.u20 accept the default (project) instead of global / module accept the download of Scala 2.8.0beta1 into project's lib folder Created a new class: object hello { def main(args: Array[String]) { println("hello: " + args); } } For my efforts, I now have a brand-new error :) Here it is: Scalac internal error: class java.lang.ClassNotFoundException [java.net.URLClassLoader$1.run(URLClassLoader.java:202), java.security.AccessController.doPrivileged(Native Method), java.net.URLClassLoader.findClass(URLClassLoader.java:190), java.lang.ClassLoader.loadClass(ClassLoader.java:307), sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301), java.lang.ClassLoader.loadClass(ClassLoader.java:248), java.lang.Class.forName0(Native Method), java.lang.Class.forName(Class.java:169), org.jetbrains.plugins.scala.compiler.rt.ScalacRunner.main(ScalacRunner.java:72)] Thanks

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• How to resolve java.nio.charset.UnmappableCharacterException in Scala 2.8.0?

  - by Roman Kagan

  I'm using Scala 2.8.0 and trying to read pipe delimited file like in code snipped below: object Main { def main(args: Array[String]) :Unit = { if (args.length > 0) { val lines = scala.io.Source.fromPath("QUICK!LRU-2009-11-15.psv") for (line <-lines) print(line) } } } Here's the error: Exception in thread "main" java.nio.charset.UnmappableCharacterException: Input length = 1 at java.nio.charset.CoderResult.throwException(CoderResult.java:261) at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:319) at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158) at java.io.InputStreamReader.read(InputStreamReader.java:167) at java.io.BufferedReader.fill(BufferedReader.java:136) at java.io.BufferedReader.read(BufferedReader.java:157) at scala.io.BufferedSource$$anonfun$1$$anonfun$apply$1.apply(BufferedSource.scala:29) at scala.io.BufferedSource$$anonfun$1$$anonfun$apply$1.apply(BufferedSource.scala:29) at scala.io.Codec.wrap(Codec.scala:65) at scala.io.BufferedSource$$anonfun$1.apply(BufferedSource.scala:29) at scala.io.BufferedSource$$anonfun$1.apply(BufferedSource.scala:29) at scala.collection.Iterator$$anon$14.next(Iterator.scala:149) at scala.collection.Iterator$$anon$2.next(Iterator.scala:745) at scala.collection.Iterator$$anon$2.head(Iterator.scala:732) at scala.collection.Iterator$$anon$24.hasNext(Iterator.scala:405) at scala.collection.Iterator$$anon$20.hasNext(Iterator.scala:320) at scala.io.Source.hasNext(Source.scala:209) at scala.collection.Iterator$class.foreach(Iterator.scala:534) at scala.io.Source.foreach(Source.scala:143) ... at infillreports.Main$.main(Main.scala:8) at infillreports.Main.main(Main.scala) Java Result: 1

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• idiomatic property changed notification in scala?

  - by Jeremy Bell

  I'm trying to find a cleaner alternative (that is idiomatic to Scala) to the kind of thing you see with data-binding in WPF/silverlight data-binding - that is, implementing INotifyPropertyChanged. First, some background: In .Net WPF or silverlight applications, you have the concept of two-way data-binding (that is, binding the value of some element of the UI to a .net property of the DataContext in such a way that changes to the UI element affect the property, and vise versa. One way to enable this is to implement the INotifyPropertyChanged interface in your DataContext. Unfortunately, this introduces a lot of boilerplate code for any property you add to the "ModelView" type. Here is how it might look in Scala: trait IDrawable extends INotifyPropertyChanged { protected var drawOrder : Int = 0 def DrawOrder : Int = drawOrder def DrawOrder_=(value : Int) { if(drawOrder != value) { drawOrder = value OnPropertyChanged("DrawOrder") } } protected var visible : Boolean = true def Visible : Boolean = visible def Visible_=(value: Boolean) = { if(visible != value) { visible = value OnPropertyChanged("Visible") } } def Mutate() : Unit = { if(Visible) { DrawOrder += 1 // Should trigger the PropertyChanged "Event" of INotifyPropertyChanged trait } } } For the sake of space, let's assume the INotifyPropertyChanged type is a trait that manages a list of callbacks of type (AnyRef, String) = Unit, and that OnPropertyChanged is a method that invokes all those callbacks, passing "this" as the AnyRef, and the passed-in String). This would just be an event in C#. You can immediately see the problem: that's a ton of boilerplate code for just two properties. I've always wanted to write something like this instead: trait IDrawable { val Visible = new ObservableProperty[Boolean]('Visible, true) val DrawOrder = new ObservableProperty[Int]('DrawOrder, 0) def Mutate() : Unit = { if(Visible) { DrawOrder += 1 // Should trigger the PropertyChanged "Event" of ObservableProperty class } } } I know that I can easily write it like this, if ObservableProperty[T] has Value/Value_= methods (this is the method I'm using now): trait IDrawable { // on a side note, is there some way to get a Symbol representing the Visible field // on the following line, instead of hard-coding it in the ObservableProperty // constructor? val Visible = new ObservableProperty[Boolean]('Visible, true) val DrawOrder = new ObservableProperty[Int]('DrawOrder, 0) def Mutate() : Unit = { if(Visible.Value) { DrawOrder.Value += 1 } } } // given this implementation of ObservableProperty[T] in my library // note: IEvent, Event, and EventArgs are classes in my library for // handling lists of callbacks - they work similarly to events in C# class PropertyChangedEventArgs(val PropertyName: Symbol) extends EventArgs("") class ObservableProperty[T](val PropertyName: Symbol, private var value: T) { protected val propertyChanged = new Event[PropertyChangedEventArgs] def PropertyChanged: IEvent[PropertyChangedEventArgs] = propertyChanged def Value = value; def Value_=(value: T) { if(this.value != value) { this.value = value propertyChanged(this, new PropertyChangedEventArgs(PropertyName)) } } } But is there any way to implement the first version using implicits or some other feature/idiom of Scala to make ObservableProperty instances function as if they were regular "properties" in scala, without needing to call the Value methods? The only other thing I can think of is something like this, which is more verbose than either of the above two versions, but is still less verbose than the original: trait IDrawable { private val visible = new ObservableProperty[Boolean]('Visible, false) def Visible = visible.Value def Visible_=(value: Boolean): Unit = { visible.Value = value } private val drawOrder = new ObservableProperty[Int]('DrawOrder, 0) def DrawOrder = drawOrder.Value def DrawOrder_=(value: Int): Unit = { drawOrder.Value = value } def Mutate() : Unit = { if(Visible) { DrawOrder += 1 } } }

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• Adding two Set[Any]

  - by Alex Boisvert

  Adding two Set[Int] works: Welcome to Scala version 2.8.1.final (Java HotSpot(TM) Server VM, Java 1.6.0_23). Type in expressions to have them evaluated. Type :help for more information. scala> Set(1,2,3) ++ Set(4,5,6) res0: scala.collection.immutable.Set[Int] = Set(4, 5, 6, 1, 2, 3) But adding two Set[Any] doesn't: scala> Set[Any](1,2,3) ++ Set[Any](4,5,6) <console>:6: error: ambiguous reference to overloaded definition, both method ++ in trait Addable of type (xs: scala.collection.TraversableOnce[Any])scala.collection.immutable.Set[Any] and method ++ in trait TraversableLike of type [B >: Any,That](that: scala.collection.TraversableOnce[B])(implicit bf: scala.collection.generic.CanBuildFrom[scala.collection.immutable.Set[Any],B,That])That match argument types (scala.collection.immutable.Set[Any]) Set[Any](1,2,3) ++ Set[Any](4,5,6) ^ Any suggestion to work around this error?

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• Why is it so difficult to get a working IDE for Scala?

  - by Alex R

  I recently gave up trying to use Scala in Eclipse (basic stuff like completion doesn't work). So now I'm trying IntelliJ. I'm not getting very far. This was the original error. See below for update: Scala signature Predef has wrong version Expected 5.0 found: 4.1 in .... scala-library.jar I tried both versions 2.7.6 and 2.8 RC1 of scala-*.jar, the result was the same. JDK is 1.6.u20. UPDATE Today I uninstalled IntelliJ 9.0.1, and installed 9.0.2 Early Availability, with the 4/14 stable version of the Scala plug-in. Then I setup a project from scratch through the wizards: new project from scratch JDK is 1.6.u20 accept the default (project) instead of global / module accept the download of Scala 2.8.0beta1 into project's lib folder Created a new class: object hello { def main(args: Array[String]) { println("hello: " + args); } } For my efforts, I now have a brand-new error :) Here it is: Scalac internal error: class java.lang.ClassNotFoundException [java.net.URLClassLoader$1.run(URLClassLoader.java:202), java.security.AccessController.doPrivileged(Native Method), java.net.URLClassLoader.findClass(URLClassLoader.java:190), java.lang.ClassLoader.loadClass(ClassLoader.java:307), sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301), java.lang.ClassLoader.loadClass(ClassLoader.java:248), java.lang.Class.forName0(Native Method), java.lang.Class.forName(Class.java:169), org.jetbrains.plugins.scala.compiler.rt.ScalacRunner.main(ScalacRunner.java:72)] Thanks

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• infinite loop shutting down ensime

  - by Jeff Bowman

  When I run M-X ensime-disconnect I get the following forever: string matching regex `\"((?:[^\"\\]|\\.)*)\"' expected but `^@' found and I see this exception when I use C-c C-c Uncaught exception in com.ensime.server.SocketHandler@769aba32 java.net.SocketException: Broken pipe at java.net.SocketOutputStream.socketWrite0(Native Method) at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:109) at java.net.SocketOutputStream.write(SocketOutputStream.java:153) at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:220) at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:290) at sun.nio.cs.StreamEncoder.implFlush(StreamEncoder.java:294) at sun.nio.cs.StreamEncoder.flush(StreamEncoder.java:140) at java.io.OutputStreamWriter.flush(OutputStreamWriter.java:229) at java.io.BufferedWriter.flush(BufferedWriter.java:253) at com.ensime.server.SocketHandler.write(server.scala:118) at com.ensime.server.SocketHandler$$anonfun$act$1$$anonfun$apply$mcV$sp$1.apply(server.scala:132) at com.ensime.server.SocketHandler$$anonfun$act$1$$anonfun$apply$mcV$sp$1.apply(server.scala:127) at scala.actors.Actor$class.receive(Actor.scala:456) at com.ensime.server.SocketHandler.receive(server.scala:67) at com.ensime.server.SocketHandler$$anonfun$act$1.apply$mcV$sp(server.scala:127) at com.ensime.server.SocketHandler$$anonfun$act$1.apply(server.scala:127) at com.ensime.server.SocketHandler$$anonfun$act$1.apply(server.scala:127) at scala.actors.Reactor$class.seq(Reactor.scala:262) at com.ensime.server.SocketHandler.seq(server.scala:67) at scala.actors.Reactor$$anon$3.andThen(Reactor.scala:240) at scala.actors.Combinators$class.loop(Combinators.scala:26) at com.ensime.server.SocketHandler.loop(server.scala:67) at scala.actors.Combinators$$anonfun$loop$1.apply(Combinators.scala:26) at scala.actors.Combinators$$anonfun$loop$1.apply(Combinators.scala:26) at scala.actors.Reactor$$anonfun$seq$1$$anonfun$apply$1.apply(Reactor.scala:259) at scala.actors.ReactorTask.run(ReactorTask.scala:36) at scala.actors.ReactorTask.compute(ReactorTask.scala:74) at scala.concurrent.forkjoin.RecursiveAction.exec(RecursiveAction.java:147) at scala.concurrent.forkjoin.ForkJoinTask.quietlyExec(ForkJoinTask.java:422) at scala.concurrent.forkjoin.ForkJoinWorkerThread.mainLoop(ForkJoinWorkerThread.java:340) at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:325) Is there something else I'm missing in my config or I should check on? Thanks, Jeff

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• scalablity of Scala over Java

  - by Marcus

  I read an article that says Scala handles concurrency better than Java. http://www.theserverside.com/feature/Solving-the-Scalability-Paradox-with-Scala-Clojure-and-Groovy ...the scalability limitation is confined specifically to the Java programming language itself, but it is not a limitation of the Java platform as a whole... The scalability issues with Java aren't a new revelation. In fact, plenty of work has been done to address these very issues, with two of the most successful projects being the programming languages named Scala and Clojure... ...Scala is finding ways around the problematic thread and locking paradigm of the Java language... How is this possible? Doesn't Scala use Java's core libraries which brings all the threading and locking issues from Java to Scala?

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• Generating scala AST for recursive method.

  - by scout

  I am generating the scala AST using the following code: val setting = new Settings(error) val reporter = new ConsoleReporter(setting, in, out) { override def displayPrompt = () } val compiler = new Global(setting, reporter) with ASTExtractor{ override def onlyPresentation = true } //setting.PhasesSetting("parser", "parserPhase") val run = new compiler.Run val sourceFiles:List[String] = List("Test.scala") run.compile(sourceFiles.toList) I guess this is the standard code used to run the compiler in the code and generate the AST to work with. The above code worked fine for any valid scala code in Test.scala till now. When I use a recursive function in Test.scala, like def xMethod(x:Int):Int = if(x == 0) -1 else xMethod(x-1) It gives me a java.lang.NullPointerException. The top few lines of the stack trace look like this at scala.tools.nsc.typechecker.Typers$Typer.checkNoDoubleDefsAndAddSynthetics$1(Typers.scala:2170) at scala.tools.nsc.typechecker.Typers$Typer.typedStats(Typers.scala:2196) at scala.tools.nsc.typechecker.Typers$Typer.typedBlock(Typers.scala:1951) at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:3815) at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:4124) at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:4177) at scala.tools.nsc.transform.TailCalls$TailCallElimination.transform(TailCalls.scala:199) The code works fine for a method like def aMethod(c:Int):Int = { bMethod(c) } def bMethod(x:Int):Int = aMethod(x) Please let me know if recursive functions need any other setting.

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• Scala 2.8 TreeMap and custom Ordering

  - by Dave

  I'm switching from scala 2.7 and ordered to scala 2.8 and using ordering. It looks quite straight forward but I was wondering could I make it a little less verbose. For example: scala> case class A(i: Int) defined class A scala> object A extends Ordering[A] { def compare(o1: A, o2: A) = o1.i - o2.i} defined module A If I then try to create a TreeMap I get an error scala> new collection.immutable.TreeMap[A, String]() <console>:10: error: could not find implicit value for parameter ordering: Ordering[A] new collection.immutable.TreeMap[A, String]() ^ However if I explicitly specify the object A as the ordering it works fine. scala> new collection.immutable.TreeMap[A, String]()(A) res34: scala.collection.immutable.TreeMap[A,String] = Map() Do I always have to explicitly specify the ordering or is there a shorter format? Thanks

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• Targeting Android with Scala 2.8 Trunk builds

  - by Kevin Wright

  The definitive reference for using Scala on android seems to be here: http://www.scala-lang.org/node/160 Unfortunately, all the references on using scala with android are based around Scala 2.7 and refer to a custom build android-library.jar, with a couple of cryptic references suggesting that this custom build isn't needed for later versions of android (I'm using 2.1 / API v7) So... What are the steps needed to use Scala 2.8 in an android project? Preferably using eclipse and the Android tools that Google supplies for that IDE.

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• First impressions of Scala

  - by Scott Weinstein

  I have an idea that it may be possible to predict build success/failure based on commit data. Why Scala? It’s a JVM language, has lots of powerful type features, and it has a linear algebra library which I’ll need later. Project definition and build Neither maven or the scala build tool (sbt) are completely satisfactory. This maven **archetype** (what .Net folks would call a VS project template) mvn archetype:generate `-DarchetypeGroupId=org.scala-tools.archetypes `-DarchetypeArtifactId=scala-archetype-simple `-DremoteRepositories=http://scala-tools.org/repo-releases `-DgroupId=org.SW -DartifactId=BuildBreakPredictor gets you started right away with “hello world” code, unit tests demonstrating a number of different testing approaches, and even a ready made `.gitignore` file - nice! But the Scala version is behind at v2.8, and more seriously, compiling and testing was painfully slow. So much that a rapid edit – test – edit cycle was not practical. So Lab49 colleague Steve Levine tells me that I can either adjust my pom to use fsc – the fast scala compiler, or use sbt. Sbt has some nice features It’s fast – it uses fsc by default It has a continuous mode, so  `> ~test` will compile and run your unit test each time you save a file It’s can consume (and produce) Maven 2 dependencies the build definition file can be much shorter than the equivalent pom (about 1/5 the size, as repos and dependencies can be declared on a single line) And some real limitations Limited support for 3rd party integration – for instance out of the box, TeamCity doesn’t speak sbt, nor does IntelliJ IDEA Steeper learning curve for build steps outside the default Side note: If a language has a fast compiler, why keep the slow compiler around? Even worse, why make it the default? I choose sbt, for the faster development speed it offers. Syntax Scala APIs really like to use punctuation – sometimes this works well, as in the following map1 |+| map2 The `|+|` defines a merge operator which does addition on the `values` of the maps. It’s less useful here: http(baseUrl / url >- parseJson[BuildStatus] sure you can probably guess what `>-` does from the context, but how about `>~` or `>+`? Language features I’m still learning, so not much to say just yet. However case classes are quite usefull, implicits scare me, and type constructors have lots of power. Community A number of projects, such as https://github.com/scalala and https://github.com/scalaz/scalaz are split between github and google code – github for the src, and google code for the docs. Not sure I understand the motivation here.

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• SubCut Scala Dependency Injection Framework

  - by kerry

  It’s no secret I am a fan of dependency injection.  So I was happy to hear that Dick Wall of the Java Posse recently released a dependency injection framework for scala.  Called SubCut, or Scala Uniquely Bound Classes Under Traits, the project is a ‘mix of service locator and dependency injection patterns designed to provide an idiomatic way of providing configured dependencies to scala applications’. It’s hosted on github, so ‘git’ (rimshot) over there and try it out: Dependency injection framework for Scala

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• Do you need to know Java before trying Scala

  - by gizgok

  I'm interested in learning Scala. I've been reading a lot about it, but a lot of people value it because it has an actor model which is better for concurrency, it handles xml in a much better way, solves the problem of first class functions. My question is do you need to know Java to understand/appreciate the way things work in Scala? Is it better to first take a stab at Java and then try Scala or you can start Scala with absolutely no java backround?

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• What is a good use case for scala?

  - by Usman Ismail

  In a current project we have setup the build so that we could mix Java and Scala. I would like to use more Scala in our code base to make the code more readable and concise. In the process also learn the language by handing over real features. So I plan to use Scala for some classes to showcase its benefits and convince other devs to look into using Scala too. For a rest based web server or a program in general what kind of code structures lend themselves to Scala's functional programming style.

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• Why scala not allowing '$' identifier in case statement?

  - by Alex R

  this works as expected scala 3 match { case x:Int = 2*x } res1: Int = 6 why does this fail? scala 3 match { case $x:Int = 2*$x } :1: error: '=' expected but ':' found. 3 match { case $x:Int = 2*$x } ^ scala 3 match { case `$x`:Int = 2*$x } :1: error: '=' expected but ':' found. 3 match { case `$x`:Int = 2*$x } ^ scala 3 match { case `$x` : Int = 2*$x } :1: error: '=' expected but ':' found. 3 match { case `$x` : Int = 2*$x } '$' is supposed to be a valid identifier character, as demonstrated here: scala var y = 1 y: Int = 1 scala var $y = 2 $y: Int = 2 Thanks

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• How do I initialize a Scala map with more than 4 initial elements in Java?

  - by GlenPeterson

  For 4 or fewer elements, something like this works (or at least compiles): import scala.collection.immutable.Map; Map<String,String> HAI_MAP = new Map4<>("Hello", "World", "Happy", "Birthday", "Merry", "XMas", "Bye", "For Now"); For a 5th element I could do this: Map<String,String> b = HAI_MAP.$plus(new Tuple2<>("Later", "Aligator")); But I want to know how to initialize an immutable map with 5 or more elements and I'm flailing in Type-hell. Partial Solution I thought I'd figure this out quickly by compiling what I wanted in Scala, then decompiling the resultant class files. Here's the scala: object JavaMapTest { def main(args: Array[String]) = { val HAI_MAP = Map(("Hello", "World"), ("Happy", "Birthday"), ("Merry", "XMas"), ("Bye", "For Now"), ("Later", "Aligator")) println("My map is: " + HAI_MAP) } } But the decompiler gave me something that has two periods in a row and thus won't compile (I don't think this is valid Java): scala.collection.immutable.Map HAI_MAP = (scala.collection.immutable.Map) scala.Predef..MODULE$.Map().apply(scala.Predef..MODULE$.wrapRefArray( scala.Predef.wrapRefArray( (Object[])new Tuple2[] { new Tuple2("Hello", "World"), new Tuple2("Happy", "Birthday"), new Tuple2("Merry", "XMas"), new Tuple2("Bye", "For Now"), new Tuple2("Later", "Aligator") })); I'm really baffled by the two periods in this: scala.Predef..MODULE$ I asked about it on #java on Freenode and they said the .. looked like a decompiler bug. It doesn't seem to want to compile, so I think they are probably right. I'm running into it when I try to browse interfaces in IntelliJ and am just generally lost. Based on my experimentation, the following is valid: Tuple2[] x = new Tuple2[] { new Tuple2<String,String>("Hello", "World"), new Tuple2<String,String>("Happy", "Birthday"), new Tuple2<String,String>("Merry", "XMas"), new Tuple2<String,String>("Bye", "For Now"), new Tuple2<String,String>("Later", "Aligator") }; scala.collection.mutable.WrappedArray<Tuple2> y = scala.Predef.wrapRefArray(x); There is even a WrappedArray.toMap() method but the types of the signature are complicated and I'm running into the double-period problem there too when I try to research the interfaces from Java.

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• Do you think Scala will be the dominant JVM langauge, ie be the next Java? [on hold]

  - by user1037729

  From what I've read about Scala do far I think it has some nice features but I do not think it should be "the next Java". It might however end up being the next Java (due to fashion rather than fact) but lets not hope it does not... To me adds a lot of complexity over Java which is a simple and scalable language. Scala Pattern matching allows you to perform some type/value checking in a more concise way, this is possible in Java, Scala's pattern matching has a limit to it, you cannot continuously match deeper and deeper down the object graph, so why not just stick to Java and use decent invariants? Scala provides tuples, easy enough to make in Java, create a static factory method and it all reads nicely too. Scala provides mixins, why not just use composition? I believe Scala implicit's are bad, they can lead to code becoming complex and hard to maintain, explicitness is good. Scala provides closures, well they will be in Java 8 too. Scala has lazy keyword for lazy instantiation, this is easy enough to do in Java by calling a getter which creates the instance when needed, no hidden magic here. Scala can be used with AKKA, well so can Java, there is an Java AKKA implementation. Scala offers addition functional features but these can all be created in Java, there are many frameworks with have implemented functional features in Java. All in all Scala seems to offer is addition complexity and thats it...

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• a scala remote actor exception

  - by kula

  hi all i with a scala code like this for echo service. import scala.actors.Actor import scala.actors.Actor._ import scala.actors.remote.RemoteActor._ class Echo extends Actor { def act() { alive(9010) register('myName, self) loop { react { case msg = println(msg) } } } } object EchoServer { def main(args: Array[String]): unit = { val echo = new Echo echo.start println("Echo server started") } } EchoServer.main(null) but there has some exception. java.lang.NoClassDefFoundError: Main$$anon$1$Echo$$anonfun$act$1 at Main$$anon$1$Echo.act((virtual file):16) at scala.actors.Reaction.run(Reaction.scala:76) at scala.actors.Actor$$anonfun$start$1.apply(Actor.scala:785) at scala.actors.Actor$$anonfun$start$1.apply(Actor.scala:783) at scala.actors.FJTaskScheduler2$$anon$1.run(FJTaskScheduler2.scala:160) at scala.actors.FJTask$Wrap.run(Unknown Source) at scala.actors.FJTaskRunner.scanWhileIdling(Unknown Source) at scala.actors.FJTaskRunner.run(Unknown Source) Caused by: java.lang.ClassNotFoundException: Main$$anon$1$Echo$$anonfun$act$1 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:307) at java.lang.ClassLoader.loadClass(ClassLoader.java:252) at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320) ... 8 more i don't konw how can cause it. by the way .my scala version is 2.7.5

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• Using a Scala symbol literal results in NoSuchMethod

  - by Benson

  I have recently begun using Scala. I've written a DSL in it which can be used to describe a processing pipeline in medici. In my DSL, I've made use of symbols to signify an anchor, which can be used to put a fork (or a tee, if you prefer) in the pipeline. Here's a small sample program that runs correctly: object Test extends PipelineBuilder { connector("TCP") / Map("tcpProtocol" -> new DirectProtocol()) "tcp://localhost:4858" --> "ByteToStringProcessor" --> Symbol("hello") "stdio://in?promptMessage=enter name:%20" --> Symbol("hello") Symbol("hello") --> "SayHello" / Map("prefix" -> "\n\t") --> "stdio://out" } For some reason, when I use a symbol literal in my program, I get a NoSuchMethod exception at runtime: java.lang.NoSuchMethodError: scala.Symbol.intern()Lscala/Symbol; at gov.pnnl.mif.scaladsl.Test$.<init>(Test.scala:7) at gov.pnnl.mif.scaladsl.Test$.<clinit>(Test.scala) at gov.pnnl.mif.scaladsl.Test.main(Test.scala) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at scala.tools.nsc.ObjectRunner$$anonfun$run$1.apply(ObjectRunner.scala:75) at scala.tools.nsc.ObjectRunner$.withContextClassLoader(ObjectRunner.scala:49) at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:74) at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:154) at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala) This happens regardless of how the symbol is used. Specifically, I've tried using the symbol in the pipeline, and in a simple println('foo) statement. The question: What could possibly cause a symbol literal's mere existence to cause a NoSuchMethodError? In my DSL I am using an implicit function which converts symbols to instances of the Anchor class, like so: implicit def makeAnchor(a: Symbol):Anchor = anchor(a) Sadly, my understanding of Scala is weak enough that I can't think of why that might be causing my NoSuchMethodError.

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• Error with default argument in Source.getLines (Scala 2.8.0 RC1)

  - by Derek

  assuming I running Scala 2.8.0 RC1, the following scala code should print out the content of the file "c:/hello.txt" for ( line<-Source.fromPath( "c:/hello.txt" ).getLines ) println( line ) However, when I run it, I get the following error <console>:10: error: missing arguments for method getLines in class Source; follow this method with `_' if you want to treat it as a partially applied function Error occured in an application involving default arguments. val it = Source.fromPath("c:/hello.scala").getLines From what I understand, Scala should use the default argument "compat.Platform.EOL" for "getLines". I am wondering if I did wrong or is it a bug in scala 2.8 Thanks

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• Java SortedMap to Scala TreeMap

  - by Dave

  I'm having trouble converting a java SortedMap into a scala TreeMap. The SortedMap comes from deserialization and needs to be converted into a scala structure before being used. Some background, for the curious, is that the serialized structure is written through XStream and on desializing I register a converter that says anything that can be assigned to SortedMap[Comparable[_],_] should be given to me. So my convert method gets called and is given an Object that I can safely cast because I know it's of type SortedMap[Comparable[_],_]. That's where it gets interesting. Here's some sample code that might help explain it. // a conversion from comparable to ordering scala> implicit def comparable2ordering[A <: Comparable[A]](x: A): Ordering[A] = new Ordering[A] { | def compare(x: A, y: A) = x.compareTo(y) | } comparable2ordering: [A <: java.lang.Comparable[A]](x: A)Ordering[A] // jm is how I see the map in the converter. Just as an object. I know the key // is of type Comparable[_] scala> val jm : Object = new java.util.TreeMap[Comparable[_], String]() jm: java.lang.Object = {} // It's safe to cast as the converter only gets called for SortedMap[Comparable[_],_] scala> val b = jm.asInstanceOf[java.util.SortedMap[Comparable[_],_]] b: java.util.SortedMap[java.lang.Comparable[_], _] = {} // Now I want to convert this to a tree map scala> collection.immutable.TreeMap() ++ (for(k <- b.keySet) yield { (k, b.get(k)) }) <console>:15: error: diverging implicit expansion for type Ordering[A] starting with method Tuple9 in object Ordering collection.immutable.TreeMap() ++ (for(k <- b.keySet) yield { (k, b.get(k)) })

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• What kind of things are easy in Haskell and hard in Scala, and vice versa?

  - by Daniel C. Sobral

  There has been some intermingling of Scala and Haskell communities, and I have noticed now and then people commenting on stuff that's supposed to be easy in Haskell and hard and Scala. Less often (maybe because I read Scala questions, not Haskell ones), I see someone mentioning that something in Scala is easier than in Haskell. So. I'd like to know from people who are knowledgeable in both what kind of things are easy in Haskell and hard in Scala, and, conversely, what kind of things are easy in Scala and hard in Haskell.

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• Why was Scala not implemented with C or C++

  - by jpartogi

  Does anybody know why was Scala implemented in Java and .NET instead of C or C++? Most languages are implemented with Cor C++ [i.e Erlang, Python, PHP, Ruby, Perl]. What are the advantages for Scala implemented in Java and .NET other than giving access to Java and .NET libraries? UPDATE Woudln't Scala gain more benefit if it is implemented in C because it can be tuned better rather than relying on JVM?

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