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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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  • 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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  • PlayFramework with Scala and Morphia

    - by AKRamkumar
    I keep getting this exception: Oops: CannotCompileException An unexpected error occured caused by exception CannotCompileException: [source error] ds() not found in models.dc What is wrong with my code? Here is models.ds package models import com.google.code.morphia.annotations._ @Embedded class ds{ @Indexed var xs : Double=0 @Indexed var xc : Double=0 @Indexed var ys : Double=0 @Indexed var yc : Double=0 @Indexed var zs : Double=0 @Indexed var zc : Double=0 } Here is models.dc package models import com.google.code.morphia.annotations.{Embedded, Entity, Indexed} @Entity class dc{ @Indexed var name : String = null @Embedded var summary : ds = new ds() }

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  • Slow Scala assert

    - by Dave
    We've been profiling our code recently and we've come across a few annoying hotspots. They're in the form assert(a == b, a + " is not equal to " + b) Because some of these asserts can be in code called a huge amount of times the string concat starts to add up. assert is defined as: def assert(assumption : Boolean, message : Any) = .... why isn't it defined as: def assert(assumption : Boolean, message : => Any) = .... That way it would evaluate lazily. Given that it's not defined that way is there an inline way of calling assert with a message param that is evaluated lazily? Thanks

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  • Scala: "using" keyword

    - by Albert Cenkier
    i've defined 'using' keyword as following: def using[A, B <: {def close(): Unit}] (closeable: B) (f: B => A): A = try { f(closeable) } finally { closeable.close() } i can use it like that: using(new PrintWriter("sample.txt")){ out => out.println("hellow world!") } now i'm curious how to define 'using' keyword to take any number of parameters, and be able to access them separately: using(new BufferedReader(new FileReader("in.txt")), new PrintWriter("out.txt")){ (in, out) => out.println(in.readLIne) }

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  • A question about Scala Objects

    - by Randin
    In the example for coding with Json using Databinder Dispatch Nathan uses an Object (Http) without a method, shown here: import dispatch._ import Http._ Http("http://www.fox.com/dollhouse/" >>> System.out ) How is he doing this?

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  • How to access "overridden" inner class in Scala?

    - by doom2.wad
    I have two traits, one extending the other, each with an inner class, one extending the other, with the same names: trait A { class X { def x() = doSomething() } } trait B extends A { class X extends super.X { override def x() = doSomethingElse() } } class C extends B { val x = new X() // here B.X is instantiated val y = new A.X() // does not compile val z = new A.this.X() // does not compile } How do I access A.X class in the C class's body? Renaming B.X not to hide A.X is not a preferred way. To make things a bit complicated, in the situation I have encountered this problem the traits have type parameters (not shown in this example).

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  • Can someone explain me implicit parameters in Scala?

    - by Oscar Reyes
    And more specifically how does the BigInt works for convert int to BigInt? In the source code it reads: ... implicit def int2bigInt(i: Int): BigInt = apply(i) ... How is this code invoked? I can understand how this other sample: "Date literals" works. In. val christmas = 24 Dec 2010 Defined by: implicit def dateLiterals(date: Int) = new { import java.util.Date def Dec(year: Int) = new Date(year, 11, date) } When int get's passed the message Dec with an int as parameter, the system looks for another method that can handle the request, in this case Dec(year:Int) Q1. Am I right in my understanding of Date literals? Q2. How does it apply to BigInt? Thanks

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  • Scala Lift - Robust method to protect files from hotlinking

    - by sirjamm
    I'm attempting to implement a way to stop hotlinking and/or un-authorised access to resources within my app. The method I'm trying to add is something I've used before in PHP apps. Basically a session is set when the page is first called. The images are added to the page via the image tag with the session value as a parameter: <img src="/files/image/[handle]?session=12345" /> When the image is requested the script checks to see if the session is set and matches the provided value. If the condition is not met the serving page returns null. Right at the end to the code I unset the session so further requests from outside the scope of the page will return null. What would be the best implementation of this method within the lift framework? Thanks in advance for any help, much appreciated :)

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  • Compiling Scala scripts. How works scalac?

    - by Arturo Herrero
    Groovy Groovy comes with a compiler called groovyc. For each script, groovyc generates a class that extends groovy.lang.Script, which contains a main method so that Java can execute it. The name of the compiled class matches the name of the script being compiled. For example, with this HelloWorld.groovy script: println "Hello World" That becomes something like this code: class HelloWorld extends Script { public static void main(String[] args) { println "Hello World" } } Scala Scala comes with a compiler called scalac. I don't know how it works. For example, with the same HelloWorld.scala script: println("Hello World") The code is not valid for scalac, because the compiler expected class or object definition, but works in Scala REPL interpreter. How is possible? Is it wrapped in a class before execution?

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  • Scala: custom control structures with several code blocks

    - by Vilius Normantas
    Is it possible to create a custom control structure with several code blocks, in the fashion of before { block1 } then { block2 } finally { block3 }? The question is about the sugar part only - I know the functionality can be easily achieved by passing the three blocks to a method, like doInSequence(block1, block2, block3). A real life example. For my testing utilities I'd like to create a structure like this: getTime(1000) { // Stuff I want to repeat 1000 times. } after { (n, t) => println("Average time: " + t / n) }

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  • Can someone explain me implicit conversions in Scala?

    - by Oscar Reyes
    And more specifically how does the BigInt works for convert int to BigInt? In the source code it reads: ... implicit def int2bigInt(i: Int): BigInt = apply(i) ... How is this code invoked? I can understand how this other sample: "Date literals" works. In. val christmas = 24 Dec 2010 Defined by: implicit def dateLiterals(date: Int) = new { import java.util.Date def Dec(year: Int) = new Date(year, 11, date) } When int get's passed the message Dec with an int as parameter, the system looks for another method that can handle the request, in this case Dec(year:Int) Q1. Am I right in my understanding of Date literals? Q2. How does it apply to BigInt? Thanks

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  • Scala type system : basic type mismatch

    - by SiM
    I have a basic type system type mismatch problem: I have a class with a method def Create(nodeItem : NodeItem) = {p_nodeStart.addEndNode(nodeItem)} where p_nodeStart is NodeCache class NodeCache[END_T<:BaseNode] private(node: Node) extends BaseNode { def addEndNode(endNode : END_T) = {this.CACHE_HAS_ENDNODES.Create(endNode)} and the error its giving me is: error: type mismatch; found : nodes.NodeItem required: Nothing def Create(nodeItem : NodeItem) = {p_nodeStart.addEndNode(nodeItem)} while the NodeCache is defined as object NodeTrigger { def Create() { val nodeTimeCache = NodeCache.Create[NodeItem](node) and in object NodeCache object NodeCache { def Create[END_T<:BaseNode]() { val nodeCache = new NodeCache[END_T](node); Any ideas, how to fix the error?

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  • assigning to local variables in scala template in play framework

    - by user3548344
    I am trying to define the local variables and assign to them as below : @defining((Json.parse(value), ("GGGGGG"))) {case (json:JsValue, lb)=> @{lb=json\\"myTestField"} } but getting the error reassignment to val. So I tried to declare lb as var like @defining((Json.parse(value), ("GGGGGG"))) {case (json:JsValue, lb:var)=> @{lb=json\\"myTestField"} } but getting the error identifier expected but 'var' found How can I assign to variable lb?

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  • Self-type mismatch in Scala

    - by Alexey Romanov
    Given this: abstract class ViewPresenterPair { type V <: View type P <: Presenter trait View {self: V => val presenter: P } trait Presenter {self: P => var view: V } } I am trying to define an implementation in this way: case class SensorViewPresenter[T] extends ViewPresenterPair { type V = SensorView[T] type P = SensorPresenter[T] trait SensorView[T] extends View { } class SensorViewImpl[T](val presenter: P) extends SensorView[T] { presenter.view = this } class SensorPresenter[T] extends Presenter { var view: V } } Which gives me the following errors: error: illegal inheritance; self-type SensorViewPresenter.this.SensorView[T] does not conform to SensorViewPresenter.this.View's selftype SensorViewPresenter.this.V trait SensorView[T] extends View { ^ <console>:13: error: type mismatch; found : SensorViewPresenter.this.SensorViewImpl[T] required: SensorViewPresenter.this.V presenter.view = this ^ <console>:16: error: illegal inheritance; self-type SensorViewPresenter.this.SensorPresenter[T] does not conform to SensorViewPresenter.this.Presenter's selftype SensorViewPresenter.this.P class SensorPresenter[T] extends Presenter { ^ I don't understand why. After all, V is just an alias for SensorView[T], and the paths are the same, so how can it not conform?

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  • How do I disable auto-compilation of Scala source in jEdit?

    - by Daniel
    I have always liked the auto-compilation feature of jEdit with Scala sources. Now, however, I'm using "mvn scala:cc" and JavaRebel with a Lift project, which provides better compilation than what jEdit does, and I'd like to disable jEdit's auto-compilation. How do I disable auto-compilation in jEdit, of Scala sources, in particular?

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  • Another Java vs. Scala perspective - is this typical?

    - by Alex R
    I have been reading about Scala for a while and even wrote some small programs to better understand some of the more exoteric features. Today I decided to do my first "real project", translating some 60 lines of ugly Java code to Scala to rewrite it using the better pattern-matching features (why? because the Java version was becoming hard to maintain due to excessive combination of regex and conditionals). About halfway through the editing process, Eclipse thew up this error: I get the general impression that the Scala IDE in Eclipse is a lot buggier and less complete than its Java equivalent. Is this correct or do I just have a bad installation? Is there a better IDE for Scala?

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  • Scala - Enumeration vs. Case-Classes

    - by tzofia
    I've created akka actor called LogActor. The LogActors's receive method handling messages from other actors and logging them to the specified log level. I can distinguish between the different levels in 2 ways. The first one: import LogLevel._ object LogLevel extends Enumeration { type LogLevel = Value val Error, Warning, Info, Debug = Value } case class LogMessage(level : LogLevel, msg : String) The second: (EDIT) abstract class LogMessage(msg : String) case class LogMessageError(msg : String) extends LogMessage(msg) case class LogMessageWarning(msg : String) extends LogMessage(msg) case class LogMessageInfo(msg : String) extends LogMessage(msg) case class LogMessageDebug(msg : String) extends LogMessage(msg) Which way is more efficient? does it take less time to match case class or to match enum value? (I read this question but there isn't any answer referring to the runtime issue)

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