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• Scala method where type of second parameter equals part of generic type from first parameter

  - by ifischer

  I want to create a specific generic method in Scala. It takes two parameters. The first is of the type of a generic Java Interface (it's from the JPA criteria query). It currently looks like this: def genericFind(attribute:SingularAttribute[Person, _], value:Object) { ... } // The Java Interface which is the type of the first parameter in my find-method: public interface SingularAttribute<X, T> extends Attribute<X, T>, Bindable<T> Now i want to achieve the following: value is currently of type java.lang.Object. But I want to make it more specific. Value has to be the of the same type as the placeholder "_" from the first parameter (and so represents the "T" in the Java interface). Is that somehow possible, and how? BTW Sorry for the stupid question title (any suggestions?)

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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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• Visual C# GUI Designer - Recommended way of removing generated event handler-code & basic tutorial

  - by cusack

  Hi, I'm new to the Visual C# designer so these are general and pretty basic question on how to work with the designer. When we for instance add a label to a form and then double-click on it in the Visual C# designer (I'm using Microsoft Visual C# 2008 Express Edition), the following things happen: The designer generates code within Form1.Designer.cs (assume default names for simplicity) to add the label, then with the double-click it will add the event handler label1_Click to the label within Form1.Designer.cs, using the following code this.label1.Click += new System.EventHandler(this.label1_Click); and it adds the event handler method to Form1.cs private void label1_Click(object sender, EventArgs e) { } If I now remove the label only the code within Form1.Designer.cs will be removed but the label1_Click method will stay within Form1.cs even if it isn't used by anything else. But if I'm using reset within Properties-Events for the Click-event from within the designer even the label1_Click method in Form1.cs will be removed. 1.) Isn't that a little inconsistent behavior? 2.) What is the recommended way of removing such generated event handler-code? 3.) What is the best "mental approach"/best practice for using the designer? I would approach it by mental separation in the way that Form1.cs is 100% my responsibility and that on the other hand I'm not touching the code in Form1.Designer.cs at all. Does that make sense or not? Since sometimes the designer removes sth. from Form1.cs I'm not sure about this. 4.) Can you recommend a simple designer tutorial that assumes no Visual C# designer knowledge but expects/doesn't explain C#. The following one is an example of what I would not want it explains what a c#-comment is and I'd prefer text over video as well: http://msdn.microsoft.com/en-us/beginner/bb964631.aspx

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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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• SQL Server 2008 Designer Behavior Change: Saving Changes Not Permitted

  - by SAMIR BHOGAYTA

  Warning Message: Saving changes is not permitted. The changes you have made require the following tables to be dropped and re-created. You have either made changes to a table that can't be re-created or enabled the option Prevent saving changes that require the table to be re-created. This is by design and can be quickly fixed in Management Studio by unchecking a property. To fix this in Management Studio, go to Tools - Options then go to the Designer Page and uncheck "Prevent saving changes that require table re-creation"

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• When SharePoint Designer has its own designs

  - by PeterBrunone

  Recently, a colleague came to me with a simple task and an inscrutable error.  He just wanted to populate a text field with a querystring value.  If you've ever done this in SPD, you know it's fairly simple:  create a parameter, map it to a querystring value, and then use the resulting parameter name in your form field. Having done so, however, he was told the following by the ASP.NET "yellow barf page": The 'Text' property of 'asp:TextBox' does not allow child objects. As it turns out, he had done everything correctly.  The problem was that SharePoint Designer had decided the best place for his FieldDescription control was INSIDE the TextBox control.  Obviously the compiler doesn't know what to do with that.  When the FieldDescription was moved to a less obtrusive location, everything worked as expected.The moral of the story is, as always, don't trust what any WYSIWYG tool gives you.  If it looks great, then fine.  However, if there's a problem, remember that Design mode was written by human beings who make mistakes... just like the rest of us.Take THAT, Skynet.

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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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