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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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• Functional Methods on Collections

  - by GlenPeterson

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

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• 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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• scala 2.8 breakout

  - by oxbow_lakes

  In Scala 2.8, there is an object in scala.collection.package.scala: def breakOut[From, T, To](implicit b : CanBuildFrom[Nothing, T, To]) = new CanBuildFrom[From, T, To] { def apply(from: From) = b.apply() ; def apply() = b.apply() } I have been told that this results in: > import scala.collection.breakOut > val map : Map[Int,String] = List("London", "Paris").map(x => (x.length, x))(breakOut) map: Map[Int,String] = Map(6 -> London, 5 -> Paris) What is going on here? Why is breakOut being called as an argument to my List?

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• scala 2.8 implict java collections conversions

  - by nablik

  I have problem with JavaConversions with 2.8 beta: import scala.collection.JavaConversions._ class Utils(dbFile : File, sep: String) extends IUtils { (...) def getFeatures() : java.util.List[String] = csv.attributes.toList } And then exception: [INFO] Utils.scala:20: error: type mismatch; [INFO] found : List[String] [INFO] required: java.util.List[String] [INFO] def getFeatures() : java.util.List[String] = csv.attributes.toList [INFO]

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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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• Is the Scala 2.8 collections library a case of "the longest suicide note in history" ?

  - by oxbow_lakes

  First note the inflammatory subject title is a quotation made about the manifesto of a UK political party in the early 1980s. This question is subjective but it is a genuine question, I've made it CW and I'd like some opinions on the matter. Despite whatever my wife and coworkers keep telling me, I don't think I'm an idiot: I have a good degree in mathematics from the University of Oxford and I've been programming commercially for almost 12 years and in Scala for about a year (also commercially). I have just started to look at the Scala collections library re-implementation which is coming in the imminent 2.8 release. Those familiar with the library from 2.7 will notice that the library, from a usage perspective, has changed little. For example... > List("Paris", "London").map(_.length) res0: List[Int] List(5, 6) ...would work in either versions. The library is eminently useable: in fact it's fantastic. However, those previously unfamiliar with Scala and poking around to get a feel for the language now have to make sense of method signatures like: def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That For such simple functionality, this is a daunting signature and one which I find myself struggling to understand. Not that I think Scala was ever likely to be the next Java (or /C/C++/C#) - I don't believe its creators were aiming it at that market - but I think it is/was certainly feasible for Scala to become the next Ruby or Python (i.e. to gain a significant commercial user-base) Is this going to put people off coming to Scala? Is this going to give Scala a bad name in the commercial world as an academic plaything that only dedicated PhD students can understand? Are CTOs and heads of software going to get scared off? Was the library re-design a sensible idea? If you're using Scala commercially, are you worried about this? Are you planning to adopt 2.8 immediately or wait to see what happens? Steve Yegge once attacked Scala (mistakenly in my opinion) for what he saw as its overcomplicated type-system. I worry that someone is going to have a field day spreading fud with this API (similarly to how Josh Bloch scared the JCP out of adding closures to Java). Note - I should be clear that, whilst I believe that Josh Bloch was influential in the rejection of the BGGA closures proposal, I don't ascribe this to anything other than his honestly-held beliefs that the proposal represented a mistake.

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• Use example of Scala ObservableSet Trait

  - by isola009

  Could anyone help me telling me how to use scala's ObservableSet trait? Thank you very much in advance

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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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• Use example of Scala OservableSet Trait

  - by isola009

  Could anyone help me telling me how to use scala's ObservableSet trait? Thank you very much in advance

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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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• 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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• 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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• How do I implement a collection in Scala 2.8?

  - by Simon Reinhardt

  In trying to write an API I'm struggling with Scala's collections in 2.8(.0-beta1). Basically what I need is to write something that: adds functionality to immutable sets of a certain type where all methods like filter and map return a collection of the same type without having to override everything (which is why I went for 2.8 in the first place) where all collections you gain through those methods are constructed with the same parameters the original collection had (similar to how SortedSet hands through an ordering via implicits) which is still a trait in itself, independent of any set implementations. Additionally I want to define a default implementation, for example based on a HashSet. The companion object of the trait might use this default implementation. I'm not sure yet if I need the full power of builder factories to map my collection type to other collection types. I read the paper on the redesign of the collections API but it seems like things have changed a bit since then and I'm missing some details in there. I've also digged through the collections source code but I'm not sure it's very consistent yet. Ideally what I'd like to see is either a hands-on tutorial that tells me step-by-step just the bits that I need or an extensive description of all the details so I can judge myself which bits I need. I liked the chapter on object equality in "Programming in Scala". :-) But I appreciate any pointers to documentation or examples that help me understand the new collections design better.

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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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• In Scala 2.8 collections, why was the Traversable type added above Iterable?

  - by Seth Tisue

  I know that to be Traversable, you need only have a foreach method. Iterable requires an iterator method. Both the Scala 2.8 collections SID and the "Fighting Bitrot with Types" paper are basically silent on the subject of why Traversable was added. The SID only says "David McIver... proposed Traversable as a generalization of Iterable." I have vaguely gathered from discussions on IRC that it has to do with reclaiming resources when traversal of a collection terminates? The following is probably related to my question. There are some odd-looking function definitions in TraversableLike.scala, for example: def isEmpty: Boolean = { var result = true breakable { for (x <- this) { result = false break } } result } I assume there's a good reason that wasn't just written as: def isEmpty: Boolean = { for (x <- this) return false true }

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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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