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  • Can we match Any to a generic type? [Scala 2.8]

    - by Jus12
    Please point me to correct link if this has been answered before. I have this code: def getResult(a:Any):Any = a def getAnswer[T](i:Int) = { val result = getResult(i) result match { case t:T => Some(t) case _ => None } } This gives me a unchecked warning and everything matches to T. For instance, when I do getAnswer[Int](2), I get Some(2) (as expected). However, if I do getAnswer[String](2), I also get Some(2) which is not expected (I need None). Is there any way to work around type erasure and somehow get getAnswer to work correctly (i.e., return Some(result) if and only if the result is of type T)? Thanks in advance.

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  • What's the best Scala build system?

    - by gatoatigrado
    I've seen questions about IDE's here -- Which is the best IDE for Scala development? and What is the current state of tooling for Scala?, but I've had mixed experiences with IDEs. Right now, I'm using the Eclipse IDE with the automatic workspace refresh option, and KDE 4's Kate as my text editor. Here are some of the problems I'd like to solve: use my own editor IDEs are really geared at everyone using their components. I like Kate better, but the refresh system is very annoying (it doesn't use inotify, rather, maybe a 10s polling interval). The reason I don't use the built-in text editor is because broken auto-complete functionalities cause the IDE to hang for maybe 10s. rebuild only modified files The Eclipse build system is broken. It doesn't know when to rebuild classes. I find myself almost half of the time going to project-clean. Worse, it seems even after it has finished building my project, a few minutes later it will pop up with some bizarre error (edit - these errors appear to be things that were previously solved with a project clean, but then come back up...). Finally, setting "Preferences / Continue launch if project contains errors" to "prompt" seems to have no effect for Scala projects (i.e. it always launches even if there are errors). build customization I can use the "nightly" release, but I'll want to modify and use my own Scala builds, not the compiler that's built into the IDE's plugin. It would also be nice to pass [e.g.] -Xprint:jvm to the compiler (to print out lowered code). fast compiling Though Eclipse doesn't always build right, it does seem snappy -- even more so than fsc. I looked at Ant and Maven, though haven't employed either yet (I'll also need to spend time solving #3 and #4). I wanted to see if anyone has other suggestions before I spend time getting a suboptimal build system working. Thanks in advance! UPDATE - I'm now using Maven, passing a project as a compiler plugin to it. It seems fast enough; I'm not sure what kind of jar caching Maven does. A current repository for Scala 2.8.0 is available [link]. The archetypes are very cool, and cross-platform support seems very good. However, about compile issues, I'm not sure if fsc is actually fixed, or my project is stable enough (e.g. class names aren't changing) -- running it manually doesn't bother me as much. If you'd like to see an example, feel free to browse the pom.xml files I'm using [github]. UPDATE 2 - from benchmarks I've seen, Daniel Spiewak is right that buildr's faster than Maven (and, if one is doing incremental changes, Maven's 10 second latency gets annoying), so if one can craft a compatible build file, then it's probably worth it...

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  • Is there an easy way to get the Scala REPL to reload a class or package?

    - by Rex Kerr
    I almost always have a Scala REPL session or two open, which makes it very easy to give Java or Scala classes a quick test. But if I change a class and recompile it, the REPL continues with the old one loaded. Is there a way to get it to reload the class, rather than having to restart the REPL? Just to give a concrete example, suppose we have the file Test.scala: object Test { def hello = "Hello World" } We compile it and start the REPL: ~/pkg/scala-2.8.0.Beta1-prerelease$ bin/scala Welcome to Scala version 2.8.0.Beta1-prerelease (Java HotSpot(TM) Server VM, Java 1.6.0_16). Type in expressions to have them evaluated. Type :help for more information. scala> Test.hello res0: java.lang.String = Hello World Then we change the source file to object Test { def hello = "Hello World" def goodbye = "Goodbye, Cruel World" } but we can't use it: scala> Test.goodbye <console>:5: error: value goodbye is not a member of object Test Test.goodbye ^ scala> import Test; <console>:1: error: '.' expected but ';' found. import Test;

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  • Installation failed for Scala eclipse plugin

    - by Ramesh Vel
    Hi, I am Scala newbie. Just started playing. Today i started to install scala plugin for eclipse (in ubuntu machine), but its failed everytime. Below the error message. An error occurred while collecting items to be installed session context was:(profile=PlatformProfile, phase=org.eclipse.equinox.internal.provisional.p2.engine.phases.Collect, operand=, action=). No repository found containing: osgi.bundle,ch.epfl.lamp.sdt.aspects,2.7.7.final No repository found containing: osgi.bundle,ch.epfl.lamp.sdt.core,2.7.7.final No repository found containing: org.eclipse.update.feature,ch.epfl.lamp.sdt,2.7.7.final No repository found containing: osgi.bundle,org.aspectj.runtime,1.6.4.20090304172355 No repository found containing: osgi.bundle,org.aspectj.weaver,1.6.4.20090304172355 No repository found containing: org.eclipse.update.feature,org.eclipse.contribution.weaving,1.6.4.20090313172428 No repository found containing: osgi.bundle,org.eclipse.contribution.weaving.jdt,1.6.4.20090313172428 No repository found containing: osgi.bundle,org.eclipse.equinox.weaving.aspectj,1.0.0.200902241436 No repository found containing: osgi.bundle,org.eclipse.equinox.weaving.hook,1.0.0.200902121956 No repository found containing: osgi.bundle,scala.library,2.7.7.final No repository found containing: osgi.bundle,scala.tools.nsc,2.7.7.final Can someone clarify me what was the problem. Cheer

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  • Using Scala and StringTemplate, how do I loop through a Map

    - by Marcus Kazmierczak
    I have my environment setup nicely using Scala, StringTempalte within the Google AppEngine. I am having trouble looping through a Map and getting it to display in the template. When I assign a simple List of just Strings to the template it works using: In Scala Servlet: var photos = List[String]() //... get photo url and title ... photos = photo_url :: photos template.setAttribute("photos", photos: _*) In Template: $photos: { photo| <div><img src="$photo$_s.jpg"></div> }$ The above works. However, any attempt of creating a Map using url and title and assigning to the template gives me an error. Here is my attempt, which does not work: In Scala Servlet: var photos = List[Map[String,String]]() //... get photo url and title ... photos = Map("url" -> url, "title" -> title) :: photos template.setAttribute("photos", photos: _*) In Template: $photos: { photo| <div><img src="$photo.url$_s.jpg" title="$photo.title$"></div> }$ This gives me the following error Class scala.collection.immutable.Map$Map2 has no such attribute: title in template context Thoughts / Ideas ?

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  • Scala and Java BigDecimal

    - by geejay
    I want to switch from Java to a scripting language for the Math based modules in my app. This is due to the readability, and functional limitations of mathy Java. For e.g, in Java I have this: BigDecimal x = new BigDecimal("1.1"); BigDecimal y = new BigDecimal("1.1"); BigDecimal z = x.multiply(y.exp(new BigDecimal("2")); As you can see, without BigDecimal operator overloading, simple formulas get complicated real quick. With doubles, this looks fine, but I need the precision. I was hoping in Scala I could do this: var x = 1.1; var y = 0.1; print(x + y); And by default I would get decimal-like behaviour, alas Scala doesn't use decimal calculation by default. Then I do this in Scala: var x = BigDecimal(1.1); var y = BigDecimal(0.1); println(x + y); And I still get an imprecise result. Is there something I am not doing right in Scala? Maybe I should use Groovy to maximise readability (it uses decimals by default)?

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  • Instantiating and starting a Scala Actor in a Map

    - by Bruce Ferguson
    I'm experimenting with a map of actors, and would like to know how to instantiate them and start them in one fell swoop... import scala.actors.Actor import scala.actors.Actor._ import scala.collection.mutable._ abstract class Message case class Update extends Message object Test { val groupings = "group1" :: "group2" :: "group3":: Nil val myActorMap = new HashMap[String,MyActor] def main(args : Array[String]) { groupings.foreach(group => myActorMap += (group -> new MyActor)) myActorMap("group2").start myActorMap("group2") ! Update } } class MyActor extends Actor { def act() { loop { react { case Update => println("Received Update") case _ => println("Ignoring event") } } } } The line: myActorMap("group2").start will grab the second instance, and let me start it, but I would like to be able to do something more like: groupings.foreach(group => myActorMap += (group -> (new MyActor).start)) but no matter how I wrap the new Actor, the compiler complains with something along the lines of: type mismatch; found : scala.actors.Actor required: com.myCompany.test.MyActor or various other complaints. I know it must be something simple to do with anonymous classes, but I can't see it right now. Any suggestions? Thanks in advance!!

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  • Protected Members of Other Instances in Scala

    - by Masterofpsi
    I just ran into a difficulty while learning Scala. I have an inheritance hierarchy that is essentially equivalent to this: class A { protected def myMethod() = println("myMethod() from A") } class B extends A { def invokeMyMethod(a: A) = a.myMethod() } But trying to compile this sample, I get the error "test.scala:7: error: method myMethod cannot be accessed in A". Coming from Java, my understanding is that protected members should be accessible at any point from a derived class, and nowhere have I seen anything that tells me that protected members in Scala are limited by instance. Does anyone have an explanation for this?

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  • Scala n00b: Critique my code

    - by Peter
    G'day everyone, I'm a Scala n00b (but am experienced with other languages) and am learning the language as I find time - very much enjoying it so far! Usually when learning a new language the first thing I do is implement Conway's Game of Life, since it's just complex enough to give a good sense of the language, but small enough in scope to be able to whip up in a couple of hours (most of which is spent wrestling with syntax). Anyhoo, having gone through this exercise with Scala I was hoping the Scala gurus out there might take a look at the code I've ended up with and provide feedback on it. I'm after anything - algorithmic improvements (particularly concurrent solutions!), stylistic improvements, alternative APIs or language constructs, disgust at the length of my function names - whatever feedback you've got, I'm keen to hear it! You should be able to run the following script via "scala GameOfLife.scala" - by default it will run a 20x20 board with a single glider on it - please feel free to experiment. // CONWAY'S GAME OF LIFE (SCALA) abstract class GameOfLifeBoard(val aliveCells : Set[Tuple2[Int, Int]]) { // Executes a "time tick" - returns a new board containing the next generation def tick : GameOfLifeBoard // Is the board empty? def empty : Boolean = aliveCells.size == 0 // Is the given cell alive? protected def alive(cell : Tuple2[Int, Int]) : Boolean = aliveCells contains cell // Is the given cell dead? protected def dead(cell : Tuple2[Int, Int]) : Boolean = !alive(cell) } class InfiniteGameOfLifeBoard(aliveCells : Set[Tuple2[Int, Int]]) extends GameOfLifeBoard(aliveCells) { // Executes a "time tick" - returns a new board containing the next generation override def tick : GameOfLifeBoard = new InfiniteGameOfLifeBoard(nextGeneration) // The next generation of this board protected def nextGeneration : Set[Tuple2[Int, Int]] = aliveCells flatMap neighbours filter shouldCellLiveInNextGeneration // Should the given cell should live in the next generation? protected def shouldCellLiveInNextGeneration(cell : Tuple2[Int, Int]) : Boolean = (alive(cell) && (numberOfAliveNeighbours(cell) == 2 || numberOfAliveNeighbours(cell) == 3)) || (dead(cell) && numberOfAliveNeighbours(cell) == 3) // The number of alive neighbours for the given cell protected def numberOfAliveNeighbours(cell : Tuple2[Int, Int]) : Int = aliveNeighbours(cell) size // Returns the alive neighbours for the given cell protected def aliveNeighbours(cell : Tuple2[Int, Int]) : Set[Tuple2[Int, Int]] = aliveCells intersect neighbours(cell) // Returns all neighbours (whether dead or alive) for the given cell protected def neighbours(cell : Tuple2[Int, Int]) : Set[Tuple2[Int, Int]] = Set((cell._1-1, cell._2-1), (cell._1, cell._2-1), (cell._1+1, cell._2-1), (cell._1-1, cell._2), (cell._1+1, cell._2), (cell._1-1, cell._2+1), (cell._1, cell._2+1), (cell._1+1, cell._2+1)) // Information on where the currently live cells are protected def xVals = aliveCells map { cell => cell._1 } protected def xMin = (xVals reduceLeft (_ min _)) - 1 protected def xMax = (xVals reduceLeft (_ max _)) + 1 protected def xRange = xMin until xMax + 1 protected def yVals = aliveCells map { cell => cell._2 } protected def yMin = (yVals reduceLeft (_ min _)) - 1 protected def yMax = (yVals reduceLeft (_ max _)) + 1 protected def yRange = yMin until yMax + 1 // Returns a simple graphical representation of this board override def toString : String = { var result = "" for (y <- yRange) { for (x <- xRange) { if (alive (x,y)) result += "# " else result += ". " } result += "\n" } result } // Equality stuff override def equals(other : Any) : Boolean = { other match { case that : InfiniteGameOfLifeBoard => (that canEqual this) && that.aliveCells == this.aliveCells case _ => false } } def canEqual(other : Any) : Boolean = other.isInstanceOf[InfiniteGameOfLifeBoard] override def hashCode = aliveCells.hashCode } class FiniteGameOfLifeBoard(val boardWidth : Int, val boardHeight : Int, aliveCells : Set[Tuple2[Int, Int]]) extends InfiniteGameOfLifeBoard(aliveCells) { override def tick : GameOfLifeBoard = new FiniteGameOfLifeBoard(boardWidth, boardHeight, nextGeneration) // Determines the coordinates of all of the neighbours of the given cell override protected def neighbours(cell : Tuple2[Int, Int]) : Set[Tuple2[Int, Int]] = super.neighbours(cell) filter { cell => cell._1 >= 0 && cell._1 < boardWidth && cell._2 >= 0 && cell._2 < boardHeight } // Information on where the currently live cells are override protected def xRange = 0 until boardWidth override protected def yRange = 0 until boardHeight // Equality stuff override def equals(other : Any) : Boolean = { other match { case that : FiniteGameOfLifeBoard => (that canEqual this) && that.boardWidth == this.boardWidth && that.boardHeight == this.boardHeight && that.aliveCells == this.aliveCells case _ => false } } override def canEqual(other : Any) : Boolean = other.isInstanceOf[FiniteGameOfLifeBoard] override def hashCode : Int = { 41 * ( 41 * ( 41 + super.hashCode ) + boardHeight.hashCode ) + boardWidth.hashCode } } class GameOfLife(initialBoard: GameOfLifeBoard) { // Run the game of life until the board is empty or the exact same board is seen twice // Important note: this method does NOT necessarily terminate!! def go : Unit = { var currentBoard = initialBoard var previousBoards = List[GameOfLifeBoard]() while (!currentBoard.empty && !(previousBoards contains currentBoard)) { print(27.toChar + "[2J") // ANSI: clear screen print(27.toChar + "[;H") // ANSI: move cursor to top left corner of screen println(currentBoard.toString) Thread.sleep(75) // Warning: unbounded list concatenation can result in OutOfMemoryExceptions ####TODO: replace with LRU bounded list previousBoards = List(currentBoard) ::: previousBoards currentBoard = currentBoard tick } // Print the final board print(27.toChar + "[2J") // ANSI: clear screen print(27.toChar + "[;H") // ANSI: move cursor to top left corner of screen println(currentBoard.toString) } } // Script starts here val simple = Set((1,1)) val square = Set((4,4), (4,5), (5,4), (5,5)) val glider = Set((2,1), (3,2), (1,3), (2,3), (3,3)) val initialBoard = glider (new GameOfLife(new FiniteGameOfLifeBoard(20, 20, initialBoard))).go //(new GameOfLife(new InfiniteGameOfLifeBoard(initialBoard))).go // COPYRIGHT PETER MONKS 2010 Thanks! Peter

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  • Scala: How to combine parser combinators from different objects

    - by eed3si9n
    Given a family of objects that implement parser combinators, how do I combine the parsers? Since Parsers.Parser is an inner class, and in Scala inner classes are bound to the outer object, the story becomes slightly complicated. Here's an example that attempts to combine two parsers from different objects. import scala.util.parsing.combinator._ class BinaryParser extends JavaTokenParsers { def anyrep: Parser[Any] = rep(any) def any: Parser[Any] = zero | one def zero: Parser[Any] = "0" def one: Parser[Any] = "1" } object LongChainParser extends BinaryParser { def parser1: Parser[Any] = zero~zero~one~one } object ShortChainParser extends BinaryParser { def parser2: Parser[Any] = zero~zero } object ExampleParser extends BinaryParser { def parser: Parser[Any] = (LongChainParser.parser1 ||| ShortChainParser.parser2) ~ anyrep def main(args: Array[String]) { println(parseAll(parser, args(0) )) } } This results to the following error: <console>:11: error: type mismatch; found : ShortChainParser.Parser[Any] required: LongChainParser.Parser[?] def parser: Parser[Any] = (LongChainParser.parser1 ||| ShortChainParser.parser2) ~ anyrep I've found the solution to this problem already, but since it was brought up recently on scala-user ML (Problem injecting one parser into another), it's probably worth putting it here too.

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  • How are Scala traits compiled into Java bytecode?

    - by Justin Ardini
    I have played around with Scala for a while now, and I know that traits can act as the Scala equivalent of both interfaces and abstract classes. Recently I've been wondering how exactly traits are compiled into Java bytecode. I found some short explanations that stated traits are compiled exactly like Java interfaces when possible, and interfaces with an additional class otherwise. I still don't understand, however, how Scala achieves class linearization, a feature not available in Java. Could anyone explain or provide a good source explains how traits compile to Java bytecode?

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  • Scala type conversion error, need help!

    - by Mansoor Ashraf
    Hello I am getting a weird error when trying to use a Java map in Scala. This is the snippet of code val value:Double = map.get(name) if (value eq null) map.put(name, time) else map.put(name, value + time) the map is defined as val map=new ConcurrentHashMap[String,Double] and this is the error I am getting error: type mismatch; found : Double required: ?{val eq: ?} Note that implicit conversions are not applicable because they are ambiguous: both method double2Double in object Predef of type (Double)java.lang.Double and method doubleWrapper in object Predef of type (Double)scala.runtime.RichDouble are possible conversion functions from Double to ?{val eq: ?} if (value eq null) map.put(name, time) I am new to Scala so I am having a hard time parsing the stacktrace. Any help would be appreciated

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  • Scala Programming for Android

    - by Lemmy
    I have followed the tutorial at Scala and Android with Scala 2.7.3 final. The resulting Android App works but even the most basic application takes several minutes (!) to compile and needs 900 kb compressed, which is a show stopper for mobile applications. Additionally, the IDE runs out of memory every now and then. I assume dex is not made for big libraries like the scala-library. So my question is: Has anyone actually done this and is there any cure for this?

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  • can't run a scala script on Mac OS X

    - by wataradio
    Running a scala script on Mac OS X results in the following error: Unable to establish connection to compilation daemon I googled and found a page reported the same problem: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=576568 To solve the problem, the page said: Can you please tell what does /etc/sysctl.d/bindv6only.conf contain? If the value is 1, can you try to: 1) sudo sed -i 's/net.ipv6.bindv6only\ =\ 1/net.ipv6.bindv6only\ =\ 0/' \ /etc/sysctl.d/bindv6only.conf && sudo invoke-rc.d procps restart 2) Then, run scala/fsc as usual and see if the bug is still there. But I can't find a bindv6only.conf in my Mac OS X. How can I solve the problem? -- System Information: Scala version 2.7.7.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_20) Mac OS X 10.6.3

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  • Read entire file in Scala?

    - by Brendan OConnor
    What's a simple and canonical way to read an entire file into memory in Scala? (Ideally, with control over character encoding.) The best I can come up with is: scala.io.Source.fromPath("file.txt").getLines.reduceLeft(_+_) or am I supposed to use one of Java's god-awful idioms, the best of which (without using an external library) seems to be: import java.util.Scanner import java.io.File new Scanner(new File("file.txt")).useDelimiter("\\Z").next() From reading mailing list discussions, it's not clear to me that scala.io.Source is even supposed to be the canonical I/O library. I don't understand what its intended purpose is, exactly. ... I'd like something dead-simple and easy to remember. For example, in these languages it's very hard to forget the idiom ... Ruby open("file.txt").read Ruby File.read("file.txt") Python open("file.txt").read()

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  • Referring to the type of an inner class in Scala

    - by saucisson
    The following code tries to mimic Polymorphic Embedding of DSLs: rather than giving the behavior in Inner, it is encoded in the useInner method of its enclosing class. I added the enclosing method so that user has only to keep a reference to Inner instances, but can always get their enclosing instance. By doing this, all Inner instances from a specific Outer instance are bound to only one behavior (but it is wanted here). abstract class Outer { sealed class Inner { def enclosing = Outer.this } def useInner(x:Inner) : Boolean } def toBoolean(x:Outer#Inner) : Boolean = x.enclosing.useInner(x) It does not compile and scala 2.8 complains about: type mismatch; found: sandbox.Outer#Inner required: _81.Inner where val _81:sandbox.Outer From Programming Scala: Nested classes and A Tour of Scala: Inner Classes, it seems to me that the problem is that useInnerexpects as argument an Inner instance from a specific Outer instance. What is the true explanation and how to solve this problem ?

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  • Computationally intensive scala process using actors hangs uncooperatively

    - by Chick Markley
    I have a computationally intensive scala application that hangs. By hangs I means it is sitting in the process stack using 1% CPU but does not respond to kill -QUIT nor can it be attached via jdb attach. Runs 2-12 hours at 800-900% CPU before it gets stuck The application is using ~10 scala.actors. Until now I have had great success with kill -QUIT but I am bit stumped as to how to proceed. The actors write a fair amount to stdout using println which is redirected to a text file but has not been helpful so far diagnostically. I am just hoping there is some obvious technique when kill -QUIT fails that I am ignorant of. Or just confirmation that having multiple actors println asynchronously is a real bad idea (though I've been doing it for a long time only recently with these results) Details scala 2.8.1 & 2.8.0 mac osx 10.6.5 java version "1.6.0_22" Thanks

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  • Static nested class visibility issue with Scala / Java interop

    - by Matt R
    Suppose I have the following Java file in a library: package test; public abstract class AbstractFoo { protected static class FooHelper { public FooHelper() {} } } I would like to extend it from Scala: package test2 import test.AbstractFoo class Foo extends AbstractFoo { new AbstractFoo.FooHelper() } I get an error, "class FooHelper cannot be accessed in object test.AbstractFoo". (I'm using a Scala 2.8 nightly). The following Java compiles correctly: package test2; import test.AbstractFoo; public class Foo2 extends AbstractFoo { { new FooHelper(); } } The Scala version also compiles if it's placed in the test package. Is there another way to get it to compile?

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  • How to write two-dimensional array to xml in Scala 2.8.0

    - by Shadowlands
    The following code (copied from a question from about a year ago) works fine under Scala 2.7.7, but does not behave correctly under Scala 2.8.0 (Beta 1, RC8). import scala.xml class Person(name : String, age : Int) { def toXml(): xml.Elem = <person><name>{ name }</name><age>{ age }</age></person> } def peopleToXml(people: Array[Person]): xml.Elem = { <people>{ for {person <- people} yield person.toXml }</people> } val data = Array(new Person("joe",40), new Person("mary", 35)) println(peopleToXml(data)) The output (according to 2.7.7) should be: <people><person><name>joe</name><age>40</age></person><person><name>mary</name><age>35</age></person></people> but instead comes out as: <people>\[Lscala.xml.Elem;@17821782</people> How do I get this to behave as it did in 2.7.x?

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  • Performing complicated XPath queries in Scala

    - by Ken Bloom
    What's the simplest API to use in scala to perform the following XPath queries on a document? //s:Annotation[@type='attitude']/s:Content/s:Parameter[@role='type' and not(text())] //s:Annotation[s:Content/s:Parameter[@role='id' and not(text())]]/@type The only documentation I can find on Scala's XML libraries has no information on performing complicated real XPath queries. I used to like JDOM for this purpose (in Java), but since JDOM doesn't support generics, it will be painful to work with in Scala. (Other XML libraries for Java have tended to be even more painful in Java, but I admit I don't know the landscape real well.)

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  • Issuing native system commands in Scala

    - by Tony
    I want to issue a native system command from a Scala program, and perhaps trap the output. ("ls" comes to mind. There may be other ways to get directory information without issuing the command, but that's beside the point of my question.) It would correspond to os.system(...) in Python. I've looked in "Programming in Scala". I've looked in O'Reilly's "Programming Scala". I've Googled several combinations of terms. No luck yet. Can someone out there give me an example, or point me at a resource where I can find an example?

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  • When is @uncheckedVariance needed in Scala, and why is it used in GenericTraversableTemplate?

    - by retronym
    @uncheckedVariance can be used to bridge the gap between Scala's declaration site variance annotations and Java's invariant generics. scala import java.util.Comparator import java.util.Comparator scala trait Foo[T] extends Comparator[T] defined trait Foo scala trait Foo[-T] extends Comparator[T] :5: error: contravariant type T occurs in invariant position in type [-T]java.lang.Object with java.util.Comparator[T] of trait Foo trait Foo[-T] extends Comparator[T] ^ scala import annotation.unchecked._ import annotation.unchecked._ scala trait Foo[-T] extends Comparator[T @uncheckedVariance] defined trait Foo This says that java.util.Comparator is naturally contra-variant, that is the type parameter T appears in parameters and never in a return type. Which begs the question, why is it also used in the Scala collections library: trait GenericTraversableTemplate[+A, +CC[X] <: Traversable[X]] extends HasNewBuilder[A, CC[A] @uncheckedVariance] What are the valid uses for this annotation?

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  • Inheritance and type parameters of Traversable

    - by Jesper
    I'm studying the source code of the Scala 2.8 collection classes. I have questions about the hierarchy of scala.collection.Traversable. Look at the following declarations: package scala.collection trait Traversable[+A] extends TraversableLike[A, Traversable[A]] with GenericTraversableTemplate[A, Traversable] trait TraversableLike[+A, +Repr] extends HasNewBuilder[A, Repr] with TraversableOnce[A] package scala.collection.generic trait HasNewBuilder[+A, +Repr] trait GenericTraversableTemplate[+A, +CC[X] <: Traversable[X]] extends HasNewBuilder[A, CC[A] @uncheckedVariance] Question: Why does Traversable extend GenericTraversableTemplate with type parameters [A, Traversable] - why not [A, Traversable[A]]? I tried some experimenting with a small program with the same structure and got a strange error message when I tried to change it to Traversable[A]: error: Traversable[A] takes no type parameters, expected: one I guess that the use of the @uncheckedVariance annotation in GenericTraversableTemplate also has to do with this? (That seems like a kind of potentially unsafe hack to force things to work...). Question: When you look at the hierarchy, you see that Traversable inherits HasNewBuilder twice (once via TraversableLike and once via GenericTraversableTemplate), but with slightly different type parameters. How does this work exactly? Why don't the different type parameters cause an error?

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  • Scala's lazy arguments: How do they work?

    - by python dude
    In the file Parsers.scala (Scala 2.9.1) from the parser combinators library I seem to have come across a lesser known Scala feature called "lazy arguments". Here's an example: def ~ [U](q: => Parser[U]): Parser[~[T, U]] = { lazy val p = q // lazy argument (for(a <- this; b <- p) yield new ~(a,b)).named("~") } Apparently, there's something going on here with the assignment of the call-by-name argument q to the lazy val p. So far I have not been able to work out what this does and why it's useful. Can anyone help?

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  • How do I write recursive anonymous functions?

    - by James T Kirk
    In my continued effort to learn scala, I'm working through 'Scala by example' by Odersky and on the chapter on first class functions, the section on anonymous function avoids a situation of recursive anonymous function. I have a solution that seems to work. I'm curious if there is a better answer out there. From the pdf: Code to showcase higher order functions def sum(f: Int => Int, a: Int, b: Int): Int = if (a > b) 0 else f(a) + sum(f, a + 1, b) def id(x: Int): Int = x def square(x: Int): Int = x * x def powerOfTwo(x: Int): Int = if (x == 0) 1 else 2 * powerOfTwo(x-1) def sumInts(a: Int, b: Int): Int = sum(id, a, b) def sumSquares(a: Int, b: Int): Int = sum(square, a, b) def sumPowersOfTwo(a: Int, b: Int): Int = sum(powerOfTwo, a, b) scala> sumPowersOfTwo(2,3) res0: Int = 12 from the pdf: Code to showcase anonymous functions def sum(f: Int => Int, a: Int, b: Int): Int = if (a > b) 0 else f(a) + sum(f, a + 1, b) def sumInts(a: Int, b: Int): Int = sum((x: Int) => x, a, b) def sumSquares(a: Int, b: Int): Int = sum((x: Int) => x * x, a, b) // no sumPowersOfTwo My code: def sumPowersOfTwo(a: Int, b: Int): Int = sum((x: Int) => { def f(y:Int):Int = if (y==0) 1 else 2 * f(y-1); f(x) }, a, b) scala> sumPowersOfTwo(2,3) res0: Int = 12

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