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  • Typeclass instances for unnamed types in Scala

    - by ncreep
    How would one encode the following constraint in Scala (pseudocode)? def foo(x: T forSome { type T has a Numeric[T] instance in scope }) = { val n= implicitly[...] // obtain the Numeric instance for x n.negate(x) // and use it with x } In words: I need a type class instance for my input argument, but I don't care about the argument's type, I just need to obtain the instance and use it on my argument. It doesn't have to be an existential type, but I need to avoid type parameters in the def's signature. Thanks.

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  • The Scala way to use one actor per socket connection

    - by Stefan
    I am wondering how it is possible to avoid one socket connection pr. thread in Scala. I have thought a lot about it, but I always end up with some code which is listening for incoming data for each client connection. The problem is that I want to develop an application which should simultanously handle perhaps a couple of thousand connections. However I will of course not want to create a thread for each connection because of the lack of scalability and context switching. What would be the "right" way to do this. In my world it should be possible to have one actor for each connection without the need to block one thread per actor.

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  • Scala importing a file in all files of a package

    - by Core_Dumped
    I need to use an implicit ordering that has been defined in an object in a file abc in the following way: object abc{ implicit def localTimeOrdering: Ordering[LocalDate] = Ordering.fromLessThan(_.isBefore(_)) } So, I make a package object xyz inside a file 'package.scala' that in turn is in the package 'xyz' that has files in which I need the implicit ordering to be applicable. I write something like this: package object xyz{ import abc._ } It does not seem to work. If I manually write the implicit definition statement inside the package object, it works perfectly. What is the correct way to import the object (abc) such that all of its objects/classes/definitions can be used in my entire package 'xyz' ?

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  • How to combine Option values in Scala?

    - by Jeff
    Hi! I want to be able to apply an operation f: (T,T) => T to two Option[T] values in Scala. I want the result to be None if any of the two values is None. More specifically, I want to know if is there a shorter way to do the following: def opt_apply[T](f: (T,T) => T, x: Option[T], y: Option[T]): Option[T] = { (x,y) match { case (Some(u),Some(v)) => Some(f(u,v)) case _ => None } } I have tryied (x zip y) map {case (u,v) => f(u,v)} but the result is an Iterator[T] not an Option[T]. Any help will be appreciated. Thanks.

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  • Pattern matching against Scala Map type

    - by Tom Morris
    Imagine I have a Map[String, String] in Scala. I want to match against the full set of key–value pairings in the map. Something like this ought to be possible val record = Map("amenity" -> "restaurant", "cuisine" -> "chinese", "name" -> "Golden Palace") record match { case Map("amenity" -> "restaurant", "cuisine" -> "chinese") => "a Chinese restaurant" case Map("amenity" -> "restaurant", "cuisine" -> "italian") => "an Italian restaurant" case Map("amenity" -> "restaurant") => "some other restaurant" case _ => "something else entirely" } The compiler complains thulsy: error: value Map is not a case class constructor, nor does it have an unapply/unapplySeq method What currently is the best way to pattern match for key–value combinations in a Map?

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  • [Scala] Applying overloaded, typed methods on a collection

    - by stephanos
    I'm quite new to Scala and struggling with the following: I have database objects (type of BaseDoc) and value objects (type of BaseVO). Now there are multiple convert methods (all called 'convert') that take an instance of an object and convert it to the other type accordingly - like this: def convert(doc: ClickDoc): ClickVO = ... def convert(doc: PointDoc): PointVO = ... def convert(doc: WindowDoc): WindowVO = ... Now I sometimes need to convert a list of objects. How would I do this - I tried: def convert[D <: BaseDoc, V <: BaseVO](docs: List[D]):List[V] = docs match { case List() => List() case xs => xs.map(doc => convert(doc)) } Which results in 'overloaded method value convert with alternatives ...'. I tried to add manifest information to it, but couldn't make it work. I couldn't even create one method for each because it'd say that they have the same parameter type after type erasure (List). Ideas welcome!

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  • Scala updating/creating value in a hashmap

    - by user1672739
    I don't understand this with Scala hasmaps: How do I create a value or update one if it does not exist? I am tryng to count the number of characters in a list of Strings. I've tried this code but it doesn't work : def times(chars: List[Char]): List[(Char, Int)] = { val map = new HashMap[Char, Int]() chars.foreach( (c : Char) => { map.update(c, map.get(c) + 1) }) } I understand the returning type isn't correct. But is my foreach loop wrong? Is there a prettier way to write it?

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  • How to test if Scala combinator parser matches a string

    - by W.P. McNeill
    I have a Scala combinator parser that handles comma-delimited lists of decimal numbers. object NumberListParser extends RegexParsers { def number: Parser[Double] = """\d+(\.\d*)?""".r ^^ (_.toDouble) def numbers: Parser[List[Double]] = rep1sep(number, ",") def itMatches(s: String): Boolean = parseAll(numbers, s) match { case _: Success[_] => true case _ => false } } The itMatches function returns true when given a string that matches the pattern. For example: NumberListParser.itMatches("12.4,3.141") // returns true NumberListParser.itMatches("bogus") // returns false Is there a more terse way to do this? I couldn't find one in the documentation, but my function sees a bit verbose, so I wonder if I'm overlooking something.

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  • Automatic conversion between methods and functions in Scala

    - by fikovnik
    I would like to understand the rules when can Scala automatically convert methods into functions. For example, if I have following two methods: def d1(a: Int, b: Int) {} def r[A, B](delegate: (A, B) ? Unit) {} I can do this: r(d1) But, when overloading r it will no longer work: def r[A, B, C](delegate: (A, B, C) ? Unit) {} r(d1) // no longer compiles and I have to explicitly convert method into partially applied function: r(d1 _) Is there any way to accomplish following with the explicit conversion? def r[A, B](delegate: (A, B) ? Unit) {} def r[A, B, C](delegate: (A, B, C) ? Unit) {} def d1(a: Int, b: Int) {} def d2(a: Int, b: Int, c: Int) {} r(d1) // only compiles with r(d1 _) r(d2) // only compiles with r(d2 _) There is somewhat similar question, but it is not fully explained.

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  • WPF designer gives exception when databinding a label to a checkbox

    - by John
    I'm sure it's something stupid, but I'm playing around with databinding. I have a checkbox and a label on a form. What I'm trying to do is simply bind the Content of the label to the checkbox's IsChecked value. What I've done runs fine (no compilation errors and acts as expected), but if I touch the label in the XAML, the designer trows an exception: System.NullReferenceException Object reference not set to an instance of an object. at MS.Internal.Designer.PropertyEditing.Editors.MarkupExtensionInlineEditorControl.BuildBindingString(Boolean modeSupported, PropertyEntry propertyEntry) at <Window x:Class="UnitTestHelper.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:FileSysCtls="clr-namespace:WPFFileSystemUserControls;assembly=WPFFileSystemUserControls" xmlns:HelperClasses="clr-namespace:UnitTestHelper" Title="MainWindow" Height="406" Width="531"> <Window.Resources> <HelperClasses:ThreestateToBinary x:Key="CheckConverter" /> </Window.Resources> <Grid Height="367" Width="509"> <CheckBox Content="Step into subfolders" Height="16" HorizontalAlignment="Left" Margin="17,254,0,0" Name="chkSubfolders" VerticalAlignment="Top" Width="130" IsThreeState="False" /> <Label Height="28" HorizontalAlignment="Left" Margin="376,254,0,0" Name="lblStepResult" VerticalAlignment="Top" Width="120" IsEnabled="True" Content="{Binding IsChecked, ElementName=chkSubfolders, Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource CheckConverter}}" /> </Grid> The ThreeStateToBinary class is as follows: class ThreestateToBinary : IValueConverter { #region IValueConverter Members public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) { if ((bool)value) return "Checked"; else return "Not checked"; //throw new NotImplementedException(); } public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) { return ((string)value == "Checked"); //throw new NotImplementedException(); } #endregion } Quite honestly, I'm playing around with it at this point. It was originally simpler (not using the ValueConverter) but was displaying similar behavior when I simply had the content set to: Content="{Binding IsChecked, ElementName=chkSubfolders, UpdateSourceTrigger=PropertyChanged}" Any ideas? Thanks, John

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  • NoSuchMethod exception when using scala Regex class... confused...

    - by hbatista
    Hi there, I have a simple Scala project that runs without any problems inside Eclipse, however, when packaged into a .jar I receive this exception when running it: Exception in thread "AWT-EventQueue-0" java.lang.NoSuchMethodError: scala.util.matching.Regex.replaceAllIn(Ljava/lang/CharSequence;Lscala/Function1;)Ljava/lang/String; What is going on here?... The code line in question, and the full stack are below. This is the offending line: "alt=\"[^>]+\">".r.replaceAllIn(inputStr, {_.replace(">", "/>")}) Full stack: Exception in thread "AWT-EventQueue-0" java.lang.NoSuchMethodError: scala.util.matching.Regex.replaceAllIn(Ljava/lang/CharSequence;Lscala/Function1;)Ljava/lang/String; at com.inosat.fuel.FuelStationDgge.fixhtml(FuelStationDgge.scala:40) at com.inosat.fuel.FuelStationDgge.setDetails(FuelStationDgge.scala:82) at com.inosat.fuel.DggeParser$$anon$1.propertyChange(DggeParser.scala:49) at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source) at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source) at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source) at org.jdesktop.beans.AbstractBean.firePropertyChange(AbstractBean.java:302) at org.jdesktop.http.async.AsyncHttpRequest.setReadyState(AsyncHttpRequest.java:705) at org.jdesktop.http.async.AsyncHttpRequest.access$600(AsyncHttpRequest.java:79) at org.jdesktop.http.async.AsyncHttpRequest$AsyncWorker.done(AsyncHttpRequest.java:831) at javax.swing.SwingWorker$5.run(Unknown Source) at javax.swing.SwingWorker$DoSubmitAccumulativeRunnable.run(Unknown Source) at sun.swing.AccumulativeRunnable.run(Unknown Source) at javax.swing.SwingWorker$DoSubmitAccumulativeRunnable.actionPerformed(Unknown Source) at javax.swing.Timer.fireActionPerformed(Unknown Source) at javax.swing.Timer$DoPostEvent.run(Unknown Source) at java.awt.event.InvocationEvent.dispatch(Unknown Source) at java.awt.EventQueue.dispatchEvent(Unknown Source) at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.run(Unknown Source)

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  • Using Scala structural types with abstract types

    - by Joshua Hartman
    I'm trying to define a structural type defining anything that has an "add" method (for instance, a java collection or a java map). Using this, I want to define a few higher order functions that operate on a certain collection object GenericTypes { type GenericCollection[T] = { def add(value: T): java.lang.Boolean} } import GenericTypes._ trait HigherOrderFunctions[T, CollectionType[X] <: GenericCollection[X]] { def map[V](fn: (T) => V): CollectionType[V] .... } class RichJList[T](list: List[T]) extends HigherOrderFunctions[T, java.util.List] This does not compile with the following error error: Parameter type in structural refinement may not refer to abstract type defined outside that same refinement I tried removing the parameter on GenericCollection and putting it on the method: object GenericTypes { type GenericCollection = { def add[T](value: T): java.lang.Boolean} } import GenericTypes._ trait HigherOrderFunctions[T, CollectionType[X] <: GenericCollection] class RichJList[T](list: List[T]) extends HigherOrderFunctions[T, java.util.List] but I get another error: error: type arguments [T,java.util.List] do not conform to trait HigherOrderFunctions's type parameter bounds [T,CollectionType[X] <: org.scala_tools.javautils.j2s.GenericTypes.GenericCollection] Can anyone give me some advice on how to use structural typing with abstract typed parameters in Scala? Or how to achieve what I'm looking to accomplish? Thanks so much!

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  • Need help figuring out scala compiler errors.

    - by klactose
    Hello all, I have been working on a project in scala, but I am getting some error messages that I don't quite understand. The classes that I am working with are relatively simple. For example: abstract class Shape case class Point(x: Int, y: Int) extends Shape case class Polygon(points: Point*) extends Shape Now suppose that I create a Polygon: val poly = new Polygon(new Point(2,5), new Point(7,0), new Point(3,1)) Then if I attempt to determine the location and size of the smallest possible rectangle that could contain the polygon, I get various errors that I don't quite understand. Below are snippets of different attempts and the corresponding error messages that they produce. val upperLeftX = poly.points.reduceLeft(Math.min(_.x, _.x)) Gives the error: "missing parameter type for expanded function ((x$1) = x$1.x)" val upperLeftX = poly.points.reduceLeft((a: Point, b: Point) => (Math.min(a.x, b.x))) Gives this error: "type mismatch; found : (Point, Point) = Int required: (Any, Point) = Any" I am very confused about both of these error messages. If anyone could explain more clearly what I am doing incorrectly, I would really appreciate it. Yes, I see that the second error says that I need type "Any" but I don't understand exactly how to implement a change that would work as I need it. Obviously simply changing "a: Point" to "a: Any" is not a viable solution, so what am I missing?

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  • Accessing type members outside the class in Scala

    - by Pekka Mattila
    Hi, I am trying to understand type members in Scala. I wrote a simple example that tries to explain my question. First, I created two classes for types: class BaseclassForTypes class OwnType extends BaseclassForTypes Then, I defined an abstract type member in trait and then defined the type member in a concerete class: trait ScalaTypesTest { type T <: BaseclassForTypes def returnType: T } class ScalaTypesTestImpl extends ScalaTypesTest { type T = OwnType override def returnType: T = { new T } } Then, I want to access the type member (yes, the type is not needed here, but this explains my question). Both examples work. Solution 1. Declaring the type, but the problem here is that it does not use the type member and the type information is duplicated (caller and callee). val typeTest = new ScalaTypesTestImpl val typeObject:OwnType = typeTest.returnType // declare the type second time here true must beTrue Solution 2. Initializing the class and using the type through the object. I don't like this, since the class needs to be initialized val typeTest = new ScalaTypesTestImpl val typeObject:typeTest.T = typeTest.returnType // through an instance true must beTrue So, is there a better way of doing this or are type members meant to be used only with the internal implementation of a class?

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  • Dependency between operations in scala actors

    - by paradigmatic
    I am trying to parallelise a code using scala actors. That is my first real code with actors, but I have some experience with Java Mulithreading and MPI in C. However I am completely lost. The workflow I want to realise is a circular pipeline and can be described as the following: Each worker actor has a reference to another one, thus forming a circle There is a coordinator actor which can trigger a computation by sending a StartWork() message When a worker receives a StartWork() message, it process some stuff locally and sends DoWork(...) message to its neighbour in the circle. The neighbours do some other stuff and sends in turn a DoWork(...) message to its own neighbour. This continues until the initial worker receives a DoWork() message. The coordinator can send a GetResult() message to the initial worker and wait for a reply. The point is that the coordinator should only receive a result when data is ready. How can a worker wait that the job returned to it before answering the GetResult() message ? To speed up computation, any worker can receive a StartWork() at any time. Here is my first try pseudo-implementation of the worker: class Worker( neighbor: Worker, numWorkers: Int ) { var ready = Foo() def act() { case StartWork() => { val someData = doStuff() neighbor ! DoWork( someData, numWorkers-1 ) } case DoWork( resultData, remaining ) => if( remaining == 0 ) { ready = resultData } else { val someOtherData = doOtherStuff( resultData ) neighbor ! DoWork( someOtherData, remaining-1 ) } case GetResult() => reply( ready ) } } On the coordinator side: worker ! StartWork() val result = worker !? GetResult() // should wait

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  • initialising a 2-dim Array in Scala

    - by Stefan W.
    (Scala 2.7.7:) I don't get used to 2d-Arrays. Arrays are mutable, but how do I specify a 2d-Array which is - let's say of size 3x4. The dimension (2D) is fixed, but the size per dimension shall be initializable. I tried this: class Field (val rows: Int, val cols: Int, sc: java.util.Scanner) { var field = new Array [Char](rows)(cols) for (r <- (1 to rows)) { val line = sc.nextLine () val spl = line.split (" ") field (r) = spl.map (_.charAt (0)) } def put (val rows: Int, val cols: Int, c: Char) = todo () } I get this error: :11: error: value update is not a member of Char field (r) = spl.map (_.charAt (0)) If it would be Java, it would be much more code, but I would know how to do it, so I show what I mean: public class Field { private char[][] field; public Field (int rows, int cols, java.util.Scanner sc) { field = new char [rows][cols]; for (int r = 0; r < rows; ++r) { String line = sc.nextLine (); String[] spl = line.split (" "); for (int c = 0; c < cols; ++c) field [r][c] = spl[c].charAt (0); } } public static void main (String args[]) { new Field (3, 4, new java.util.Scanner ("fraese.fld")); } } and fraese.fld would look, for example, like that: M M M M . M I get some steps wide with val field = new Array Array [Char] but how would I then implement 'put'? Or is there a better way to implement the 2D-Array. Yes, I could use a one-dim-Array, and work with put (y, x, c) = field (y * width + x) = c but I would prefer a notation which looks more 2d-ish.

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  • How to store child objects on GAE using JDO from Scala

    - by Gero
    Hi, I'm have a parent-child relation between 2 classes, but the child objects are never stored. I do get an warning: "org.datanucleus.store.appengine.MetaDataValidator checkForIllegalChildField: Unable to validate relation net.vermaas.kivanotify.model.UserCriteria.internalCriteria" but it is unclear to me why this occurs. Already tried several alternatives without luck. The parent class is "UserCriteria" which has a List of "Criteria" as children. The classes are defined as follows (Scala): class UserCriteria(tu: String, crit: Map[String, String]) extends LogHelper { @PrimaryKey @Persistent{val valueStrategy = IdGeneratorStrategy.IDENTITY} var id = KeyFactory.createKey("UserCriteria", System.nanoTime) @Persistent var twitterUser = tu @Persistent var internalCriteria: java.util.List[Criteria] = flatten(crit) def flatten(crits: Map[String, String]) : java.util.List[Criteria] = { val list = new java.util.ArrayList[Criteria] for (key <- crits.keySet) { list.add(new Criteria(this, key, crits(key))) } list } def criteria: Map[String, String] = { val crits = mutable.Map.empty[String, String] for (i <- 0 to internalCriteria.size-1) { crits(internalCriteria.get(i).name) = internalCriteria.get(i).value } Map.empty ++ crits } // Stripped the equals, canEquals, hashCode, toString code to keep the code snippet short... } @PersistenceCapable @EmbeddedOnly class Criteria(uc: UserCriteria, nm: String, vl: String) { @Persistent var userCriteria = uc @Persistent var name = nm @Persistent var value = vl override def toString = { "Criteria name: " + name + " value: " + value } } Any ideas why the childs are not stored? Or why I get the error message? Thanks, Gero

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  • Trait, FunctionN, or trait-inheriting-FunctionN in Scala?

    - by Willis Blackburn
    I have a trait in Scala that has a single method. Call it Computable and the single method is compute(input: Int): Int. I can't figure out whether I should Leave it as a standalone trait with a single method. Inherit from (Int = Int) and rename "compute" to "apply." Just get rid of Computable and use (Int = Int). A factor in favor of it being a trait is that I could usefully add some additional methods. But of course if they were all implemented in terms of the compute method then I could just break them out into a separate object. A factor in favor of just using the function type is simplicity and the fact that the syntax for an anonymous function is more concise than that for an anonymous Computable instance. But then I've no way to distinguish objects that are actually Computable instances from other functions that map Int to Int but aren't meant to be used in the same context as Computable. How do other people approach this type of problem? No right or wrong answers here; I'm just looking for advice.

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  • Implicit conversion between Scala collection types

    - by ebruchez
    I would like to implicitly convert between the Scala XML Elem object and another representation of an XML element, in my case dom4j Element. I wrote the following implicit conversions: implicit def elemToElement(e: Elem): Element = ... do conversion here ... implicit def elementToElem(e: Element): Elem = ... do conversion here ... So far so good, this works. Now I also need collections of said elements to convert both ways. First, do I absolutely need to write additional conversion methods? Things didn't seem to work if I didn't. I tried to write the following: implicit def elemTToElementT(t: Traversable[Elem]) = t map (elemToElement(_)) implicit def elementTToElemT(t: Traversable[Element]) = t map (elementToElem(_)) This doesn't look too ideal because if the conversion method takes a Traversable, then it also returns a Traversable. If I pass a List, I also get a Traversable out. So I assume the conversion should be parametrized somehow. So what's the standard way of writing these conversions in order to be as generic as possible?

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  • Doesn't (didn't) Scala have automatically generated setters?

    - by Malvolio
    Google and my failing memory are both giving me hints that it does, but every attempt is coming up dry. class Y { var y = 0 } var m = new Y() m.y_(3) error: value y_ is not a member of Y Please tell me I am doing something wrong. (Also: please tell me what it is I am doing wrong.) EDIT The thing I am not doing wrong, or at least not the only thing I am doing wrong, is the way I am invoking the setter. The following things also fail, all with the same error message: m.y_ // should be a function valued expression m.y_ = (3) // suggested by Google and by Mchl f(m.y_) // where f takes Int => Unit as an argument f(m.y) // complains that I am passing in Int not a function I am doing this all through SimplyScala, because I'm too lazy and impatient to set up Scala on my tiny home machine. Hope it isn't that... And the winner is ... Fabian, who pointed out that I can't have a space between the _ and the =. I thought out why this should be and then it occurred to me: The name of the setter for y is not y_, it is y_= ! Observe: class Y { var y = 0 } var m = new Y() m.y_=(3) m.y res1: Int = 3 m.y_= error: missing arguments for method y_= in class Y; follow this method with `_` if you want to treat it as a partially applied function m.y_= ^ m.y_=_ res2: (Int) => Unit = def four(f : Int => Unit) = f(4) four(m.y_=) m.y res3: Int = 4 Another successful day on StackExchange.

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  • Scala always returning true....WHY?

    - by jhamm
    I am trying to learn Scala and am a newbie. I know that this is not optimal functional code and welcome any advice that anyone can give me, but I want to understand why I keep getting true for this function. def balance(chars: List[Char]): Boolean = { val newList = chars.filter(x => x.equals('(') || x.equals(')')); return countParams(newList, 0) } def countParams(xs: List[Char], y: Int): Boolean = { println(y + " right Here") if (y < 0) { println(y + " Here") return false } else { println(y + " Greater than 0") if (xs.size > 0) { println(xs.size + " this is the size") xs match { case xs if (xs.head.equals('(')) => countParams(xs.tail, y + 1) case xs if (xs.head.equals(')')) => countParams(xs.tail, y - 1) case xs => 0 } } } return true; } balance("()())))".toList) I know that I am hitting the false branch of my if statement, but it still returns true at the end of my function. Please help me understand. Thanks.

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  • best web database solution for scala for a high traffic site?

    - by egervari
    I am in charge of a rebuilding a website that gets about 250,000 visitors a day. We'd like to use Scala, but it does not work very well with Spring (in some minor cases) and Hibernate (there is a major and very annoying mismatch here if you want to use scala collections, which we do). The application itself is going to have about 40-50 tables. Other than Hibernate, is there an ORM that works awesome with Scala and is as performant and reliable as Hibernate? Does it also have the same capabilities, or are we going to run into leaky-abstractions if we don't use Hibernate? It would be a big risk for us to go with a framework that is newer and doesn't seem to have a lot of industry backing... and at the same time, Hibernate is a real pain to program against when using Scala. 1) The Java Collection <- Scala Collection is absolutely painful. There is a lot more boilerplate and crap to write. 2) The IDE doesn't import JavaConversions and java interfaces automatically... so we this needs to be done manually. Optimizing Imports in IDEA is going to destroy all the manual work. 3) There is also a performance cost to converting back and forth all the time in your domain objects and your dao classes. 4) Not to mention there needs to be a lot of casting, which produces code ugly as sin. I actually would love to write my own orm that is 100% tailored to scala, but obviously this is really outside of the scope of our project for now. So what is the best approach?

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  • How to implement collection with covariance when delegating to another collection for storage?

    - by memelet
    I'm trying to implement a type of SortedMap with extended semantics. I'm trying to delegate to SortedMap as the storage but can't get around the variance constraints: class IntervalMap[A, +B](implicit val ordering: Ordering[A]) //extends ... { var underlying = SortedMap.empty[A, List[B]] } Here is the error I get. I understand why I get the error (I understand variance). What I don't get is how to implement this type of delegation. And yes, the covariance on B is required. error: covariant type B occurs in contravariant position in type scala.collection.immutable.SortedMap[A,List[B]] of parameter of setter underlying_=

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  • [Scala] Using overloaded, typed methods on a collection

    - by stephanos
    I'm quite new to Scala and struggling with the following: I have database objects (type of BaseDoc) and value objects (type of BaseVO). Now there are multiple convert methods (all called 'convert') that take an instance of an object and convert it to the other type accordingly. For example: def convert(doc: ClickDoc): ClickVO = doc match { case null => null case _ => val result = new ClickVO result.x = doc.x result.y = doc.y result } Now I sometimes need to convert a list of objects. How would I do this - I tried: def convert[D <: MyBaseDoc, V <: BaseVO](docs: List[D]):List[V] = docs match { case List() => List() case xs => xs.map(doc => convert(doc)) } Which results in 'overloaded method value convert with alternatives ...'. I tried to add manifest information to it, but couldn't make it work. I couldn't even create one method for each because it'd say that they have the same parameter type after type erasure (List). Ideas welcome!

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  • Odd MVC 4 Beta Razor Designer Issue

    - by Rick Strahl
    This post is a small cry for help along with an explanation of a problem that is hard to describe on twitter or even a connect bug and written in hopes somebody has seen this before and any ideas on what might cause this. Lots of helpful people had comments on Twitter for me, but they all assumed that the code doesn't run, which is not the case - it's a designer issue. A few days ago I started getting some odd problems in my MVC 4 designer for an app I've been working on for the past 2 weeks. Basically the MVC 4 Razor designer keeps popping me errors, about the call signature to various Html Helper methods being incorrect. It also complains about the ViewBag object and not supporting dynamic requesting to load assemblies into the project. Here's what the designer errors look like: You can see the red error underlines under the ViewBag and an Html Helper I plopped in at the top to demonstrate the behavior. Basically any HtmlHelper I'm accessing is showing the same errors. Note that the code *runs just fine* - it's just the designer that is complaining with Errors. What's odd about this is that *I think* this started only a few days ago and nothing consequential that I can think of has happened to the project or overall installations. These errors aren't critical since the code runs but pretty annoying especially if you're building and have .csHtml files open in Visual Studio mixing these fake errors with real compiler errors. What I've checked Looking at the errors it indeed looks like certain references are missing. I can't make sense of the Html Helpers error, but certainly the ViewBag dynamic error looks like System.Core or Microsoft.CSharp assemblies are missing. Rest assured they are there and the code DOES run just fine at runtime. This is a designer issue only. I went ahead and checked the namespaces that MVC has access to in Razor which lives in the Views folder's web.config file: /Views/web.config For good measure I added <system.web.webPages.razor> <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, <split for layout> Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" /> <pages pageBaseType="System.Web.Mvc.WebViewPage"> <namespaces> <add namespace="System.Web.Mvc" /> <add namespace="System.Web.Mvc.Ajax" /> <add namespace="System.Web.Mvc.Html" /> <add namespace="System.Web.Routing" /> <add namespace="System.Linq" /> <add namespace="System.Linq.Expressions" /> <add namespace="ClassifiedsBusiness" /> <add namespace="ClassifiedsWeb"/> <add namespace="Westwind.Utilities" /> <add namespace="Westwind.Web" /> <add namespace="Westwind.Web.Mvc" /> </namespaces> </pages> </system.web.webPages.razor> For good measure I added System.Linq and System.Linq.Expression on which some of the Html.xxxxFor() methods rely, but no luck. So, has anybody seen this before? Any ideas on what might be causing these issues only at design time rather, when the final compiled code runs just fine?© Rick Strahl, West Wind Technologies, 2005-2012Posted in Razor  MVC   Tweet !function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0];if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src="//platform.twitter.com/widgets.js";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs"); (function() { var po = document.createElement('script'); po.type = 'text/javascript'; po.async = true; po.src = 'https://apis.google.com/js/plusone.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(po, s); })();

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