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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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• 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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• How to unimport String "+" operator in Scala?

  - by Alex R

  I'm writing a DSL where the "+" operator is strictly numeric, and the String "+" operator is messing up my implicit conversions. What's the syntax for unimporting an operator of the String class? Thanks

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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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• Awesome WM: Goodbye UI

  - by Håvard Geithus

  Sometimes some of my UI's disappear, but the processes they belong to lives on. So far it has happened to gnome-terminal, emacs, eclipse and evince. I've only experienced this behavior with the Awesome Window Manager. Any ideas how to fix it? Update: It has also happened to a popped out Gmail chat window . When I close the main Gmail window, it warns me that the invisible chat window will also be closed.

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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 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 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: 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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• Scala :: operator, how it works?

  - by Felix

  Hello Guys, in Scala, I can make a caseclass case class Foo(x:Int) and then put it in a list like so: List(Foo(42)) Now, nothing strange here. The following is strange to me. The operator :: is a function on a list, right? With any function with 1 argument in Scala, I can call it with infix notation. An example is 1 + 2 is a function (+) on the object Int. The class Foo I just defined does not have the :: operator, so how is the following possible: Foo(40) :: List(Foo(2)) ? In scala 2.8 rc1, I get the following output from the interactive prompt: scala> case class Foo(x:Int) defined class Foo scala> Foo(40) :: List(Foo(2)) res2: List[Foo] = List(Foo(40), Foo(2)) scala> I can go on and use it, but if someone can explain it I will be glad :)

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• Simple Scala syntax - trying to define "==" operator - what am I missing?

  - by Alex R

  While experimenting with some stuff on the REPL, I got to a point where I needed something like this: scala class A(x:Int) { println(x); def ==(a:A) : Boolean = { this.x == a.x; } } Just a simple class with an "==" operator. Why doesn't it work??? Here's the result: :10: error: type mismatch; found : A required: ?{val x: ?} Note that implicit conversions are not applicable because they are ambiguous: both method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A] and method any2Ensuring in object Predef of type [A](x: A)Ensuring[A] are possible conversion functions from A to ?{val x: ?} class A(x:Int) { println(x); def ==(a:A) : Boolean = { this.x == a.x; } } ^ This is scala 2.8 RC1. Thanks

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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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• Why is this Scala example of implicit paremeter not working?

  - by Alex R

  simple REPL test... def g(a:Int)(implicit b:Int) = {a+b} Why do neither of these attempted usages work? 1. scala class A { var b:Int =8; var c = g(2) } :6: error: could not find implicit value for parameter b: Int class A { var b:Int =8; var c = g(2) } 2. scala class A(var b:Int) { var c = g(2) } :6: error: could not find implicit value for parameter b: Int class A(var b:Int) { var c = g(2) } ^ Thanks

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• Awesome Window Manager - Rule assigning programs to tags

  - by Peter

  I have a set of tags 'main', 'www', and 3: -- {{{ Tags tags = { names = {"main", "www", 3}, for s = 1, screen.count() do tags[s] = awful.tag(tags.names, s, tags.layout) end } -- }}} I want Firefox to open in tag "www". I've tried the following rule: -- {{{ Rules awful.rules.rules = { { rule = {class = "Firefox" }, properties = { tag = tags[1]["www"] }}, } -- }}} However, if I hit Super+R then type 'firefox' or run 'firefox &' from a terminal, Firefox opens in whatever tag I'm viewing. I've tried {class = "firefox"} and properties = { tag = tags[1][2] } but those modified rules did not work either. What is the correct way to set up a rule to make firefox always open in a particular tag?

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• Font-awesome, input type 'submit'

  - by denis.peplin

  It seems no class for input type 'submit' in font-awesome. Is it possible to use some class from font-awesome for button input? I've added icons to all buttons (which actually links with class 'btn' from twitter-bootstrap) in my applications, but can't add icons on 'input type submit'. Or, how to use this code: input#image-button{ background: #ccc url('icon.png') no-repeat top left; padding-left: 16px; height: 16px; } html: <input type="submit" id="image-button">Text</input> (which I took from HTML: How to make a submit button with text + image in it?) with font-awesome?

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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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• Clojure Protocols vs Scala Structural Types

  - by Vasil Remeniuk

  After watching the interview with Rich Hickey on Protocols in Clojure 1.2, and knowing very little about Clojure, I have some questions on Clojure Protocols: Are they intended to do the same thing as Structural Types in Scala? What benefits do Protocols have over Structural Types (performance, flexibility, code clarity, etc.)? Are they implemented through reflections? Questions on interoperability with Scala: Can Protocols be used instead of Structural Types in Scala? Can they be extended (if 'extension' term can be applied to Protocols) in Scala?

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