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• How would one do dependency injection in scala?

  - by Fabian

  I'm still at the beginning in learning scala in addition to java and i didn't get it how is one supposed to do DI there? can or should i use an existing DI library, should it be done manually or is there another way?

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• Understanding Scope on Scala's For Loops (For Comprehension)

  - by T. Stone

  In Chapter 3 of Programming Scala, the author gives two examples of for loops / for comprehensions, but switches between using ()'s and {}'s. Why is this the case, as these inherently look like they're doing the same thing? Is there a reason breed <- dogBreeds is on the 2nd line in example #2? // #1 ()'s for (breed <- dogBreeds if breed.contains("Terrier"); if !breed.startsWith("Yorkshire") ) println(breed) // #2 {}'s for { breed <- dogBreeds upcasedBreed = breed.toUpperCase() } println(upcasedBreed)

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• stored procedures in scala.dbc

  - by tuxjay

  I wonder if scala.dbc supports stored procedures? The mysql jdbc backend I'm using supports it, bonus points for a code illustration to show how it works.

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• Scala unsatisfiable cyclic dependency in "table-layout" library (Toolkit class)

  - by Atol

  When I try to compile with sbt some code containing an instance of a Table from this library I get this error: java.lang.AssertionError: assertion failed: unsatisfiable cyclic dependency in 'class Toolkit' It seems to work with Java so I don't understand why it fails in Scala. Here is the toolkit class: http://code.google.com/p/table-layout/source/browse/branches/v1/tablelayout/src/com/esotericsoftware/tablelayout/Toolkit.java As long as I get this error I'm totally stopped in my project :(.

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• Scala: Simple webservice

  - by Chris

  I'd like to create a simple webservice in scala similar to: http://www.artima.com/lejava/articles/threeminutes.html Is there an easy way to do this? EDIT: I found a tutorial for lift http://wiki.liftweb.net/index.php/HowTo_do_Web_Services. However this seems to be way too complicated for what I'm looking for.

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• Would it make sense to have a separate Scala library in Android market?

  - by soc

  As far as I understand it is necessary for people using Scala for Android applications to bundle the Scala classes they used with their application. Considering this adds hundreds of kilobytes to each Scala app redundantly, would it be possible to build a Scala library which can be delivered over the market, so app writers can just depend on that library instead of bundling it themselves?

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• Difference betwean RegexpParsers,StandardTokenParsers and JavaTokenParsers in scala

  - by bveedu

  I am learning Parser Combinators in scala and seeing different ways of parsing.I mainly see three different kind of parsers ie.RegexpParsers,StandardTokenParsers and JavaTokenParsers.I am new to parsing and not getting the idea how we will choose the suitable Parser according to our requirement.Can any one please explain how these different parsers work and when to use them.

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• Enhance Predfined Methods in Scala

  - by fratnk

  Base question: Why can I write in Scala just: println(10) Why don't I need to write: Console println(10) Followup question: How can I introduce a new method "foo" which is everywhere visible and usable like "println"?

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• How to convert a Seq[A] to a Map[Int, A] using a value of A as the key in the map?

  - by Jesper

  I have a Seq containing objects of a class that looks like this: class A (val key: Int, ...) Now I want to convert this Seq to a Map, using the key value of each object as the key, and the object itself as the value. So: val seq: Seq[A] = ... val map: Map[Int, A] = ... // How to convert seq to map? How can I does this efficiently and in an elegant way in Scala 2.8?

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• What is a "context bound"?

  - by Jesper

  One of the new features of Scala 2.8 are context bounds. What is a context bound and where is it useful? Of course I searched first (and found for example this) but I couldn't find any really clear and detailed information.

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• What's the difference between these two calls to a function taking a collection of structural types?

  - by James Moore

  Why does the call to fn(Iterator("foo") compile, but the call to fn(fooIterator) fail with an error "type mismatch; found : Iterator[java.lang.String] required: scala.Iterator[com.banshee.Qx.HasLength]" object Qx { type HasLength = {def length: Int} def fn(xs: Iterator[HasLength]) = 3 var tn = fn(Iterator("foo")) var fooIterator = Iterator("foo") var tnFails = fn(fooIterator) //doesn't compile } Aren't they the same thing?

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• Don't understand the typing of Scala's delimited continuations (A @cps[B,C])

  - by jkff

  I'm struggling to understand what precisely does it mean when a value has type A @cps[B,C] and what types of this form should I assign to my values when using the delimited continuations facility. I've looked at some sources: http://lamp.epfl.ch/~rompf/continuations-icfp09.pdf http://www.scala-lang.org/node/2096 http://dcsobral.blogspot.com/2009/07/delimited-continuations-explained-in.html http://blog.richdougherty.com/2009/02/delimited-continuations-in-scala_24.html but they didn't give me much intuition into this. In the last link, the author tries to give an explicit explanation, but it is not clear enough anyway. The A here represents the output of the computation, which is also the input to its continuation. The B represents the return type of that continuation, and the C represents its "final" return type—because shift can do further processing to the returned value and change its type. I don't understand the difference between "output of the computation", "return type of the continuation" and "final return type of the continuation". They sound like synonyms.

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• Which Java-specific annoyance fixed in Scala reduces surprises like the ones discussed in Java Puzzl

  - by soc

  Example: In Java this code falls through and prints "Mhhh..." Integer i = new Integer(1); Integer j = new Integer(1); if (i == j) { System.out.println("Equal"); } else if (i < j) { System.out.println("Smaller"); } else if (i > j) { System.out.println("Bigger"); } else {System.out.println("Mhhh...");} In Scala the equivalent code does not even compile: val a = new Integer(1) val b = new Integer(1) println { if(a == b) "Equal" else if(a < b) "Smaller" else if (a > b) "Bigger" else "Mhhh..."}

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• Scala: working around the "illegal cyclic reference"

  - by Paul Milovanov

  Hi all, I'm trying to implement a HashMap-based tree that'd support O(1) subtree lookup for a given root key. To that goal, I'm trying to do the following: scala> type Q = HashMap[Char, Q] <console>:6: error: illegal cyclic reference involving type Q type Q = HashMap[Char, Q] ^ So the question is, is there a way for me to do something of the sort without resorting to the ugly HashMap[Char, Any] with subsequent casting of values to HashMap[Char, Any]? Now, I also see that I can use something like the following to avoid the cyclic-reference error, and it might even be cleaner -- but it'd be nice to find out how to correctly do it the first way, just for the educational value. import collections.mutable.HashMap class LTree { val children = new HashMap[Char, LTree] } Thanks a bunch.

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• ZeroC Ice "checked casts" in Scala

  - by Alexey Romanov

  ZeroC Ice for Java translates every Slice interface Simple into (among other things) a proxy interface SimplePrx and a proxy SimplePrxHelper. If I have an ObjectPrx (the base interface for all proxies), I can check whether it actually has interface Simple by using a static method on SimplePrxHelper: val obj : Ice.ObjectPrx = ...; // Get a proxy from somewhere... val simple : SimplePrx = SimplePrxHelper.checkedCast(obj); if (simple != null) // Object supports the Simple interface... else // Object is not of type Simple... I wanted to write a method castTo so that I could replace the second line with val simple = castTo[SimplePrx](obj) or val simple = castTo[SimplePrxHelper](obj) So far as I can see, Scala's type system is not expressive enough to allow me to define castTo. Is this correct?

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• Error after installing scala plugin of netbeans

  - by ghedas

  I installed the scala plugin on my netbeans and followed the instruction of this page: http://wiki.netbeans.org/Scala68v1#Scala_Plugins_for_NetBeans_6.8_v1.x_.28RC2.29 but after it completed correctly step by step, when I make an empty project (Hello world!), the project has an error! The empty project is here: package scalaapplication1 object Main { /** * @param args the command line arguments */ def main(args: Array[String]): Unit = { println("Hello, world!") } } and the console error report is: ...\NetBeansProjects\ScalaApplication2\nbproject\build-impl.xml:403: The following error occurred while executing this line: ...\NetBeansProjects\ScalaApplication2\nbproject\build-impl.xml:236: scalac doesn't support the "fork" attribute Is there any suggestion about it?!

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• Multiple return points in scala closure/anonymous function

  - by Debilski

  As far as I understand it, there is no way in Scala to have multiple return points in an anonymous function, i.e. someList.map((i) => { if (i%2 == 0) return i // the early return allows me to avoid the else clause doMoreStuffAndReturnSomething(i) }) raises an error: return outside method definition. (And if it weren’t to raise that, the code would not work as I’d like it to work.) One workaround I could thing of would be the following someList.map({ def f(i: Int):Int = { if (i%2 == 0) return i doMoreStuffAndReturnSomething(i) } f }) however, I’d like to know if there is another ‘accepted’ way of doing this. Maybe a possibility to go without a name for the inner function? (A use case would be to emulate some valued continue construct inside the loop.)

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• Advanced control of recursive parser in scala

  - by Jeriho

  val uninterestingthings = ".".r val parser = "(?ui)(regexvalue)".r | (uninterestingthings~>parser) This recursive parser will try to parse "(?ui)(regexvalue)".r until the end of input. Is in scala a way to prohibit parsing when some defined number of characters were consumed by "uninterestingthings" ? UPD: I have one poor solution: object NonRecursiveParser extends RegexParsers with PackratParsers{ var max = -1 val maxInput2Consume = 25 def uninteresting:Regex ={ if(max<maxInput2Consume){ max+=1 ("."+"{0,"+max.toString+"}").r }else{ throw new Exception("I am tired") } } lazy val value = "itt".r def parser:Parser[Any] = (uninteresting~>value)|parser def parseQuery(input:String) = { try{ parse(parser, input) }catch{ case e:Exception => } } } Disadvantages: - not all members are lazy vals so PackratParser will have some time penalty - constructing regexps on every "uninteresting" method call - time penalty - using exception to control program - code style and time penalty

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• Is there a good reflection library available for Scala?

  - by Erik Engbrecht

  I'm working on a library that needs reflection, and needs Scala-specific information as opposed to what is available via the standard Java reflection API. Right now I'm using the undocumented code in scalap (the Scala equivalent to javap) and trying to associate the data it provides with Java reflection objects so that I can call methods and such (scalap just parses class files). I found this but there's nothing there... https://github.com/dubochet/scala-reflection ...and I've searched the Scala SVN repo for traces of it and failed to find it there, either (but it could be I just missed it). Does anyone know of a Scala reflection library that provides information similar to what scalap provides but also allows you to call methods, access fields, etc like the Java reflection library?

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• How to get methods list in scala

  - by skyde

  In language like python and ruby to ask the language what index-related methods its string class supports (which methods’ names contain the word “index”) you can do “”.methods.sort.grep /index/i And in java List results = new ArrayList(); Method[] methods = String.class.getMethods(); for (int i = 0; i < methods.length; i++) { Method m = methods[i]; if (m.getName().toLowerCase().indexOf(“index”) != -1) { results.add(m.getName()); } } String[] names = (String[]) results.toArray(); Arrays.sort(names); return names; How would you do the same thing in Scala?

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• Lexing newlines in scala StdLexical?

  - by Nick Fortescue

  I'm trying to lex (then parse) a C like language. In C there are preprocessor directives where line breaks are significant, then the actual code where they are just whitespace. One way of doing this would be do a two pass process like early C compilers - have a separate preprocessor for the # directives, then lex the output of that. However, I wondered if it was possible to do it in a single lexer. I'm pretty happy with writing the scala parser-combinator code, but I'm not so sure of how StdLexical handles whitespace. Could someone write some simple sample code which say could lex a #include line (using the newline) and some trivial code (ignoring the newline)? Or is this not possible, and it is better to go with the 2-pass appproach?

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• what is wrong: "value Parsers is not a member of package scala.util.parsing.combinator"?

  - by Nick Fortescue

  I've got the above odd error message that I don't understand "value Parsers is not a member of package scala.util.parsing.combinator". I'm trying to learn Parser combinators by writing a C parser step by step. I started at token, so I have the classes: import util.parsing.combinator.JavaTokenParsers object CeeParser extends JavaTokenParsers { def token: Parser[CeeExpr] = ident } abstract class CeeExpr case class Token(name: String) extends CeeExpr This is as simple as I could make it. The code below works fine, but if I uncomment the commented line I get the error message given above: object Play { def main(args: Array[String]) { //val parser: _root_.scala.util.parsing.combinator.Parsers.Parser[CeeExpr] CeeParser.token val x = CeeParser.token print(x) } } In case it is a problem with my setup, I'm using scala 2.7.6 via the scala-plugin for intellij. Can anyone shed any light on this? The message is wrong, Parsers is a member of scala.util.parsing.combinator.

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• Repeating a List in Scala

  - by Ralph

  I am a Scala noob. I have decided to write a spider solitaire solver as a first exercise to learn the language and functional programming in general. I would like to generate a randomly shuffled deck of cards containing 1, 2, or 4 suits. Here is what I came up with: val numberOfSuits = 1 (List["clubs", "diamonds", "hearts", "spades"].take(numberOfSuits) * 4).take(4) which should return List["clubs", "clubs", "clubs", "clubs"] List["clubs", "diamonds", "clubs", "diamonds"] List["clubs", "diamonds", "hearts", "spades"] depending on the value of numberOfSuits, except there is no List "multiply" operation that I can find. Did I miss it? Is there a better way to generate the complete deck before shuffling? BTW, I plan on using an Enumeration for the suits, but it was easier to type my question with strings. I will take the List generated above and using a for comprehension, iterate over the suits and a similar List of card "ranks" to generate a complete deck.

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• Scala Tuple Deconstruction

  - by dbyrne

  I am new to Scala, and ran across a small hiccup that has been annoying me. Initializing two vars in parallel works great: var (x,y) = (1,2) However I can't find a way to assign new values in parallel: (x,y) = (x+y,y-x) //invalid syntax I end up writing something like this: val xtmp = x+y; y = x-y; x = xtmp I realize writing functional code is one way of avoiding this, but there are certain situations where vars just make more sense. I have two questions: 1) Is there a better way of doing this? Am I missing something? 2) What is the reason for not allowing true parallel assignment?

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• How do I set up Scala plugin for NetBeans to copy the Scala runtime library?

  - by Alexey Romanov

  Versions: NetBeans 6.8, Scala Kit 0.16.1 When I compile my project, I get the following output: init: deps-jar: Compiling 2 source files to F:\MyProgramming\NorvigSpellChecker\build\classes compile: Created dir: F:\MyProgramming\NorvigSpellChecker\dist Building jar: F:\MyProgramming\NorvigSpellChecker\dist\NorvigSpellChecker.jar Not copying the libraries. To run this application from the command line without Ant, try: java -jar "F:\MyProgramming\NorvigSpellChecker\dist\NorvigSpellChecker.jar" jar: BUILD SUCCESSFUL (total time: 3 seconds) Of course, the libraries should be copied, so I can't actually run it by using this command line. I don't see any options to copy the library in the project configuration. The plugin uses Ant for building, but I don't have any experience with it; presumably it should be easy enough to tell Ant to copy the libraries. Here is build-impl.xml, what should I do in build.xml?

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