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

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

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  • Assign method in Scala.

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

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

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

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  • Perfect hash in Scala.

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

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  • scala: Adding attributes (odd and even rows) to xml table

    - by Debilski
    In a Lift application, I’d like to add a special tag which takes the <tbody> part of the next table and adds odd and even classes (for example) to each <tr> tag. Alternating, of course. While I have found a way to add another attribute to all <tr> tags, there are still a few problems left (see code below). First, it doesn’t work. cycle.next is called too often, so in the end, everything is an odd row. Other problems are that the code doesn’t exclude inner tables (so a nested <tr> would be transformed as well) and that it also affects the <thead> part of the table. Ideas to make this code work? (Of course, if there already is a lift-based solution – without jQuery – for this, I’ll gratefully take it.) class Loop(val strs: String*) { val stream_iter = Stream.continually(strs.toStream).flatten.iterator def next = stream_iter.next } val cycle = new Loop("even", "odd") val rr = new RewriteRule { override def transform(n: Node): Seq[Node] = n match { case elem : Elem => elem match { case Elem(_, "tr", att @ _, _, _*) => elem % Attribute(None, "class", Text( List(att.get("class").getOrElse("").toString, cycle.next).reduceLeft(_+" "+_).trim ), Null) toSeq case other => other } case other => other } } val rt = new RuleTransformer(rr) val code = <table> <thead><tr><td>Don’t</td><td>transform this</td></tr></thead> <tbody> <tr class="otherclass"> <td>r1c1</td><td>r1c2</td> </tr> <tr> <td>r2c1</td><td>r2c2</td> </tr> <tr> <td>r3c1</td><td>r3c2</td> </tr> </tbody> </table> println(rt(code))

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  • Naming case classes in Scala.

    - by Lukasz Lew
    I tend to have this redundant naming in case classes: abstract class MyTree case class MyTreeNode (...) case class MyTreeLeaf (...) Isn't it possible to define Node and Leaf inside of MyTree? What are best practices here?

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  • best scala idiom for find & return

    - by IttayD
    This is something I encounter frequently, but I don't know the elegant way of doing. I have a collection of Foo objects. Foo has a method bar() that may return null or a Bar object. I want to scan the collection, calling each object's bar() method and stop on the first one returning an actual reference and return that reference from the scan. Obviously: foos.find(_.bar != null).bar does the trick, but calls #bar twice.

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  • Can't update scala on Gentoo

    - by xhochy
    As I wanted to test Scala 2.9.2 on my gentoo system I tried updated the package but ended up with this error. I can't figure out where the problem may be: Calculating dependencies ...... done! >>> Verifying ebuild manifests >>> Jobs: 0 of 1 complete, 1 running Load avg: 0.23, 0.16, 0.20 >>> Emerging (1 of 1) dev-lang/scala-2.9.2 >>> Jobs: 0 of 1 complete, 1 running Load avg: 0.23, 0.16, 0.20 >>> Failed to emerge dev-lang/scala-2.9.2, Log file: >>> Jobs: 0 of 1 complete, 1 running Load avg: 0.23, 0.16, 0.20 >>> '/var/tmp/portage/dev-lang/scala-2.9.2/temp/build.log' >>> Jobs: 0 of 1 complete, 1 running Load avg: 0.23, 0.16, 0.20 >>> Jobs: 0 of 1 complete, 1 running, 1 failed Load avg: 0.23, 0.16, 0.20 >>> Jobs: 0 of 1 complete, 1 failed Load avg: 0.23, 0.16, 0.20 * Package: dev-lang/scala-2.9.2 * Repository: gentoo * Maintainer: [email protected] * USE: amd64 elibc_glibc kernel_linux multilib userland_GNU * FEATURES: sandbox [01m[31;06m!!! ERROR: Couldn't find suitable VM. Possible invalid dependency string. Due to jdk-with-com-sun requiring a target of 1.7 but the virtual machines constrained by virtual/jdk-1.6 and/or this package requiring virtual(s) jdk-with-com-sun[0m * Unable to determine VM for building from dependencies: NV_DEPEND: virtual/jdk:1.6 java-virtuals/jdk-with-com-sun !binary? ( dev-java/ant-contrib:0 ) app-arch/xz-utils >=dev-java/java-config-2.1.9-r1 source? ( app-arch/zip ) >=dev-java/ant-core-1.7.0 dev-java/ant-nodeps >=dev-java/javatoolkit-0.3.0-r2 >=dev-lang/python-2.4 * ERROR: dev-lang/scala-2.9.2 failed (setup phase): * Failed to determine VM for building. * * Call stack: * ebuild.sh, line 93: Called pkg_setup * scala-2.9.2.ebuild, line 43: Called java-pkg-2_pkg_setup * java-pkg-2.eclass, line 53: Called java-pkg_init * java-utils-2.eclass, line 2187: Called java-pkg_switch-vm * java-utils-2.eclass, line 2674: Called die * The specific snippet of code: * die "Failed to determine VM for building." * * If you need support, post the output of `emerge --info '=dev-lang/scala-2.9.2'`, * the complete build log and the output of `emerge -pqv '=dev-lang/scala-2.9.2'`. !!! When you file a bug report, please include the following information: GENTOO_VM= CLASSPATH="" JAVA_HOME="" JAVACFLAGS="" COMPILER="" and of course, the output of emerge --info * The complete build log is located at '/var/tmp/portage/dev-lang/scala-2.9.2/temp/build.log'. * The ebuild environment file is located at '/var/tmp/portage/dev-lang/scala-2.9.2/temp/die.env'. * Working directory: '/var/tmp/portage/dev-lang/scala-2.9.2' * S: '/var/tmp/portage/dev-lang/scala-2.9.2/work/scala-2.9.2-sources' * Messages for package dev-lang/scala-2.9.2: * Unable to determine VM for building from dependencies: * ERROR: dev-lang/scala-2.9.2 failed (setup phase): * Failed to determine VM for building. * * Call stack: * ebuild.sh, line 93: Called pkg_setup * scala-2.9.2.ebuild, line 43: Called java-pkg-2_pkg_setup * java-pkg-2.eclass, line 53: Called java-pkg_init * java-utils-2.eclass, line 2187: Called java-pkg_switch-vm * java-utils-2.eclass, line 2674: Called die * The specific snippet of code: * die "Failed to determine VM for building." * * If you need support, post the output of `emerge --info '=dev-lang/scala-2.9.2'`, * the complete build log and the output of `emerge -pqv '=dev-lang/scala-2.9.2'`. * The complete build log is located at '/var/tmp/portage/dev-lang/scala-2.9.2/temp/build.log'. * The ebuild environment file is located at '/var/tmp/portage/dev-lang/scala-2.9.2/temp/die.env'. * Working directory: '/var/tmp/portage/dev-lang/scala-2.9.2' * S: '/var/tmp/portage/dev-lang/scala-2.9.2/work/scala-2.9.2-sources' The following eix output may help: % eix java-virtuals/jdk-with-com-sun [I] java-virtuals/jdk-with-com-sun Available versions: 20111111 {{ELIBC="FreeBSD"}} Installed versions: 20111111(16:08:51 18/04/12)(ELIBC="-FreeBSD") Homepage: http://www.gentoo.org Description: Virtual ebuilds that require internal com.sun classes from a JDK Both virtual jdks 1.6 and 1.7 are installed: % eix virtual/jdk [I] virtual/jdk Available versions: (1.4) ~1.4.2-r1[1] (1.5) 1.5.0 ~1.5.0-r3[1] (1.6) 1.6.0 1.6.0-r1 (1.7) (~)1.7.0 Installed versions: 1.6.0-r1(1.6)(23:22:48 10/11/12) 1.7.0(1.7)(23:21:09 10/11/12) Description: Virtual for JDK [1] "java-overlay" /var/lib/layman/java-overlay

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  • scala syntax highlighting in bluefish

    - by Synesso
    Scala comes packaged with bluefish syntax config in misc/scala-tool-support/bluefish/ I have attempted to configure this, as per the README, but there is no effect. cp ~/.bluefish/highlighting ~/.bluefish/highlighting_ cat ~/.bluefish/highlighting_ /opt/scala/scala-2.7.7.final/misc/scala-tool-support/bluefish/highlighting > ~/.bluefish/highlighting I have ensured highlighting is turned on in the bluefish config. I have not used bluefish before and am trying it for the first time because there is syntax highlighting support for it. The README says any file I open that has a .scala extension will have the scala highlighting applied. Instead I see no highlighting.

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  • Scala type inference failure on "? extends" in Java code

    - by oxbow_lakes
    I have the following simple Java code: package testj; import java.util.*; public class Query<T> { private static List<Object> l = Arrays.<Object>asList(1, "Hello", 3.0); private final Class<? extends T> clazz; public static Query<Object> newQuery() { return new Query<Object>(Object.class); } public Query(Class<? extends T> clazz) { this.clazz = clazz; } public <S extends T> Query<S> refine(Class<? extends S> clazz) { return new Query<S>(clazz); } public List<T> run() { List<T> r = new LinkedList<T>(); for (Object o : l) { if (clazz.isInstance(o)) r.add(clazz.cast(o)); } return r; } } I can call this from Java as follows: Query<String> sq = Query.newQuery().refine(String.class); //NOTE NO <String> But if I try and do the same from Scala: val sq = Query.newQuery().refine(classOf[String]) I get the following error: error: type mismatch found :lang.this.class[scala.this.Predef.String] required: lang.this.class[?0] forSome{ type ?0 <: ? } val sq = Query.newQuery().refine(classOf[String]) This is only fixed by the insertion of the correct type parameter! val sq = Query.newQuery().refine[String](classOf[String]) Why can't scala infer this from my argument? Note I am using Scala 2.7

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  • What's the problem with Scala's XML literals?

    - by Oak
    In this post, Martin (the language's head honcho) writes: [XML literals] Seemed a great idea at the time, now it sticks out like a sore thumb. I believe with the new string interpolation scheme we will be able to put all of XML processing in the libraries, which should be a big win. Being interested in language design myself, I'm wondering: Why does he write that it was a mistake to incorporate XML literals into the language? What is the controversy regarding this feature?

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  • Should Scala IDE Worksheets be part of your open git repository?

    - by JacobusR
    Those familiar with Scala IDE will know about the great testing environment offered by the Scala Worksheet. You can scribble and scratch, much like in the REPL, but with all the goodness added by the IDE as a whole (refactoring, saving, error checking, etc). When you create a worksheet, it is created with the .sc extension, and also creates a artifact under a hidden directory called .worksheet. This is all fine and dandy, but should one include these in your public .git repositories? People who does not use Scala IDE (or older versions) may find these files confusing. On the other hand, making some of your experiments public to developers who are using Scala IDE, may give them a quick start into experimenting and learning the project.

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  • Scala libraries and frameworks

    - by folone
    Each technology is powerful with libraries and frameworks, written for it. I understand, that Scala is able to use libraries and frameworks, written for Java. But there are already some frameworks, written for Scala in Scala. Like, for example: Lift Framework ScalaTest Scalaz Do you know any more great libraries and frameworks for Scala, written in Scala?

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  • Scala xhtml attribute breaks because of a question mark

    - by portoalet
    I had a problem with <iframe id="iframe1" src='http://stockcharts.com/h-sc/ui?s=MT&p=D&yr=2&mn=0&dy=0&id=p43321191731' width="300px" height="300px"></iframe> in Lift web framework (Scala) version Message: java.util.NoSuchElementException scala.RandomAccessSeq$$anon$13.next(RandomAccessSeq.scala:165) scala.xml.parsing.MarkupParser$class.normalizeAttributeValue(MarkupParser.scala:1191) It turned out the question mark ? in the iframe src attribute caused this. What can I do to fix this quickly?

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  • Can you return an assignable lvalue in Scala?

    - by Alex R
    (note, lvalue is actually a term from the C grammar, I don't know what it's called in Scala!) Trying to learn Scala... this evening I'm working on an internal DSL for a dynamically scoped language that might resemble PHP syntax. My REPL is: Welcome to Scala version 2.7.6.final (Java HotSpot(TM) Client VM, Java 1.6.0). I have some made-up example code: class $(any: Any) { def update(sym: Symbol, any: Any) { println("line 2 executed");} def -(sym: Symbol) : $ = { println("line 1 executed"); return this } def update(any: Any) { println("line 3 executed");} } The following works as expected: scala var a = new $(0) a: $ = $@19238ad scala a('x) = "blah" line 2 executed On the other hand, why does the following not invoke the 1-parameter update method? scala a = 1 :6: error: type mismatch; found : Int(1) required: $ a = 1 ^ Ultimately, I would like this to work: a-'x = "blah" Thanks

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  • How to use Scala interpreter options with SBT?

    - by John Threepwood
    When using the Scala interpreter, one could start it with an option like: C:\Users\John>scala -unchecked Welcome to Scala version 2.9.2 (Java HotSpot(TM) Client VM, Java 1.6.0_32). Type in expressions to have them evaluated. Type :help for more information. scala> When using sbt, how can one start the Scala interpreter with options ? The following try will not work: C:\Users\John\Test Scala Project 1>sbt [...] [info] Loading global plugins from C:\Users\John\.sbt\plugins [info] Set current project to default-8d4ecc (in build file:/C:/Users/John/Tes t%20Scala%20Project%201/) > console -unchecked [error] Expected end of input. [error] console -unchecked [error] ^ With Google & Co I could not figure out how to do this from within the sbt shell. Does anyone know ?

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  • Scala class to implement two Java Interfaces - how?

    - by puudeli
    Hi, I have just started learning Scala and I'm now wondering how I could implement two different Java interfaces with one Scala class? Let's say I have the following interfaces written in Java public interface EventRecorder { public void abstract record(Event event); } public interface TransactionCapable { public void abstract commit(); } But a Scala class can extend only one class at a time. How can I have a Scala class that could fulfill both contracts? Do I have to map those interfaces into traits? Note, my Scala classes would be used from Java as I am trying to inject new functionality written in Scala into an existing Java application. And the existing framework expects that both interface contracts are fulfilled.

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  • scala REPL is slow on vista

    - by Jacques René Mesrine
    I installed scala-2.8.0.RC3 by extracting the tgz file into my cygwin (vista) home directory. I made sure to set $PATH to scala-2.8.0.RC3/bin. I start the REPL by typing: $ scala Welcome to Scala version 2.8.0.RC3 (Java HotSpot(TM) Client VM, Java 1.6.0_20). Type in expressions to have them evaluated. Type :help for more information. scala> Now when I tried to enter an expression scala> 1 + 'a' the cursor hangs there without any response. Granted that I have chrome open with a million tabs and VLC playing in the background, but CPU utilization was 12% and virtual memory was about 75% utilized. What's going on ? Do I have to set the CLASSPATH or perform other steps.

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  • Scala: Mixing traits with private fields

    - by Vilius Normantas
    It's not much of a question, it's rather my excitement that it's possible at all! I wrote this little example just to prove the opposite - I expected either a compiler error or one of the values (111 or 222, I wasn't sure). scala> trait T1 { private val v = 111; def getValueT1 = v } scala> trait T2 { private val v = 222; def getValueT2 = v } scala> class T12 extends T1 with T2 scala> val t = new T12 scala> t.getValueT1 res9: Int = 111 scala> t.getValueT2 res10: Int = 222 Why doesn't the v get overridden? Off course this works only as long as vs are private, but still.

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  • Scala Eclipse IDE suddenly ignoring breakpoints

    - by malsmith
    I've been using Scala 2.8RC1 and Scala Eclipse plugin for 2.8 RC1 happily for a few days. However, last night after adding a couple jar files to my environment (apache http client jars) the debugger just stopped stopping at breakpoints in scala code. Java code stops fine at breakpoints. I tried creating a new mimimal scala app breakpoints don't stop. I've tried switching to sun-jre-1.6.0.20 from the openjdk-1.6.18 I had been using. I've switched to the scala 2.8 nightly and also eclipse plugin for scala nightly builds. No luck. I would greatly appreciate ideas for fixes. Rather frustrating as the initial experience with 2.8 was really great.

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