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• Scala, make my loop more functional

  - by Pengin

  I'm trying to reduce the extent to which I write Scala (2.8) like Java. Here's a simplification of a problem I came across. Can you suggest improvements on my solutions that are "more functional"? Transform the map val inputMap = mutable.LinkedHashMap(1->'a',2->'a',3->'b',4->'z',5->'c') by discarding any entries with value 'z' and indexing the characters as they are encountered First try var outputMap = new mutable.HashMap[Char,Int]() var counter = 0 for(kvp <- inputMap){ val character = kvp._2 if(character !='z' && !outputMap.contains(character)){ outputMap += (character -> counter) counter += 1 } } Second try (not much better, but uses an immutable map and a 'foreach') var outputMap = new immutable.HashMap[Char,Int]() var counter = 0 inputMap.foreach{ case(number,character) => { if(character !='z' && !outputMap.contains(character)){ outputMap2 += (character -> counter) counter += 1 } } }

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• Can Scala be considered a functional superset of Java?

  - by Giorgio

  Apart from the differences in syntax, can Scala be considered a superset of Java that adds the functional paradigm to the object-oriented paradigm? Or are there any major features in Java for which there is no direct Scala equivalent? With major features I mean program constructs that would force me to heavily rewrite / restructure my code, e.g., if I had to port a Java program to Scala. Or can I expect that, given a Java program, I can port it to Scala almost line-by-line?

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• Writing functions of tuples conveniently in Scala

  - by Alexey Romanov

  Quite a few functions on Map take a function on a key-value tuple as the argument. E.g. def foreach(f: ((A, B)) ? Unit): Unit. So I looked for a short way to write an argument to foreach: > val map = Map(1 -> 2, 3 -> 4) map: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 3 -> 4) > map.foreach((k, v) => println(k)) error: wrong number of parameters; expected = 1 map.foreach((k, v) => println(k)) ^ > map.foreach({(k, v) => println(k)}) error: wrong number of parameters; expected = 1 map.foreach({(k, v) => println(k)}) ^ > map.foreach(case (k, v) => println(k)) error: illegal start of simple expression map.foreach(case (k, v) => println(k)) ^ I can do > map.foreach(_ match {case (k, v) => println(k)}) 1 3 Any better alternatives?

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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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• running scala apps with java -jar

  - by paintcan

  Yo dawgs, I got some problems with the java. Check it out. sebastian@sebastian-desktop:~/scaaaaaaaaala$ java -cp /home/sebastian/.m2/repository/org/scala-lang/scala-library/2.8.0.RC3/scala-library-2.8.0.RC3.jar:target/scaaaaaaaaala-1.0.jar scaaalaaa.App Hello World! That's cool, right, but how bout this: sebastian@sebastian-desktop:~/scaaaaaaaaala$ java -cp /home/sebastian/.m2/repository/org/scala-lang/scala-library/2.8.0.RC3/scala-library-2.8.0.RC3.jar -jar target/scaaaaaaaaala-1.0.jar Exception in thread "main" java.lang.NoClassDefFoundError: scala/Application at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632) at java.lang.ClassLoader.defineClass(ClassLoader.java:616) at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141) at java.net.URLClassLoader.defineClass(URLClassLoader.java:283) at java.net.URLClassLoader.access$000(URLClassLoader.java:58) at java.net.URLClassLoader$1.run(URLClassLoader.java:197) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:248) at scaaalaaa.App.main(App.scala) Caused by: java.lang.ClassNotFoundException: scala.Application at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:248) ... 13 more Wat the heck? Any idea why the first works and not the 2nd? How do I -jar my scala?? Thanks in advance bro.

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• How can I use the Scala program schema2src?

  - by pr1001

  This perhaps more a Server Fault question... I installed schema2src via sbaz and now I would like to convert a DTD (Apple's plist schema) to Scala source. $ schema2src usage: schema2src [flags] --module mname arg* or schema2src dtd arg* or (experimental) schema2src xsd arg* (this doesn't work at all yet) where supported [flags] may be: --verbose prints some debugging information However, if I try give any argument, it appears it can't find Scala: $ schema2src --verbose Exception in thread "main" java.lang.NoClassDefFoundError: scala/runtime/BoxesUtility at schema2src.Main$.processArgs(Main.scala:56) at schema2src.Main$.main(Main.scala:25) at schema2src.Main.main(Main.scala) Caused by: java.lang.ClassNotFoundException: scala.runtime.BoxesUtility at java.net.URLClassLoader$1.run(URLClassLoader.java:200) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:188) at java.lang.ClassLoader.loadClass(ClassLoader.java:315) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:330) at java.lang.ClassLoader.loadClass(ClassLoader.java:250) at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:398) ... 3 more I have scala in my PATH... Any suggestions? Assuming that problem can be fixed, is this the correct syntax? $ schema2src PropertyList-1.0.dtd

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• Bad class file error when using Scala 2.8.0-rc1 in Javafx 1.2

  - by aoprisan

  When trying to import scala.Option in a javafx script, I get the following javafxc error: bad class file: scala/Option$$anonfun$orNull$1.class(scala:Option$$anonfun$orNull$1.class) undeclared type variable: A1 Please remove or make sure it appears in the correct subdirectory of the classpath. import scala.Option; ^ I am using Scala 2.8.0-RC1, Javafxc 1.2.3_b36, JVM 1.6.0_18-b07, OS Ubuntu 9.10. The same code was working in Scala 2.7.7 .

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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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• Expand a Set[Set[String]] into Set[String] in Scala

  - by huynhjl

  I have the following set of sets. I don't know ahead of time how long it will be. val sets = Set(Set("a","b","c"), Set("1","2"), Set("S","T")) I would like to expand it into a cartesian product: Set("a&1&S", "a&1&T", "a&2&S", ..., "c&2&T") How would you do that?

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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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• Scala 2.8 Actor design document? Akka design document?

  - by Stefan K.

  Is there a design document for Scala 2.8 Actors, like it is for 2.7? Scala Actors: Unifying Thread-based and Event-based Programming Is there one for Akka? The "Scala Improvement Documents Library" doesn't mention Actors.

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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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• How do I exclude/rename some classes from import in Scala?

  - by Alexey Romanov

  Language FAQ says import scala.collection.mutable.{_, Map => _, Set => _} should import all classes from package scala.collection.mutable, except Map and Set. But it gives me this error: error: '}' expected but ',' found. import scala.collection.mutable.{_, Map => _, Set => _} Is there still a way to do this?

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• Expand a Set[Set[String]] into Cartesian Product in Scala

  - by huynhjl

  I have the following set of sets. I don't know ahead of time how long it will be. val sets = Set(Set("a","b","c"), Set("1","2"), Set("S","T")) I would like to expand it into a cartesian product: Set("a&1&S", "a&1&T", "a&2&S", ..., "c&2&T") How would you do that?

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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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• 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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• 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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