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  • Scala capture group using regex

    - by Geo
    Let's say I have this code: val string = "one493two483three" val pattern = """two(\d+)three""".r pattern.findAllIn(string).foreach(println) I expected findAllIn to only return 483, but instead, it returned two483three. I know I could use unapply to extract only that part, but I'd have to have a pattern for the entire string, something like: val pattern = """one.*two(\d+)three""".r val pattern(aMatch) = string println(aMatch) // prints 483 Is there another way of achieving this, without using the classes from java.util directly, and without using unapply?

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  • Scala parser combinators: how to parse "if(x)" if x can contain a ")"

    - by Germán
    I'm trying to get this to work: def emptyCond: Parser[Cond] = ("if" ~ "(") ~> regularStr <~ ")" ^^ { case s => Cond("",Nil,Nil) } where regularStr is defined to accept a number of things, including ")". Of course, I want this to be an acceptable input: if(foo()). But for any if(x) it is taking the ")" as part of the regularStr and so this parser never succeeds. What am I missing?

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  • scala sbt with pk11 or steps

    - by portoalet
    I am quite frustrated with sbt and pk11/steps I am just trying to run "jetty-run", but i got so many dependency errors, it's not fun anymore. I am stuck with unresolved dependencies for sjson 0.3 Does anyone know which mvn repo can I get sjson 0.3 from?

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  • Is there a good date/time API available for Scala?

    - by Erik Engbrecht
    I'm looking for something akin to JodaTime or JSR 310 for Scala that leverages nice Scala features such as operator overloading and doesn't rely on implicit conversions (I have an irrational fear of implicit conversions). I'm aware of http://github.com/jorgeortiz85/scala-time, but it just pimps JodaTime with implicits.

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  • Can someone exaplain 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 contiguous match

    - by drypot
    pathTokens match { case List("post") => ("post", "index") case List("search") => ("search", "index") case List() => ("home", "index") } match { case (controller, action) => loadController(http, controller, action) case _ => null } I wanted contiguous match. but got compile error. :( (pathTokens match { case List("post") => ("post", "index") case List("search") => ("search", "index") case List() => ("home", "index") }) match { case (controller, action) => loadController(http, controller, action) case _ => null } When I wrapped first match with parenparenthesis, it worked ok. Why I need parenthesis here ?

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  • Scala: how to specify varargs as type?

    - by IttayD
    Instead of def foo(configuration: (String, String)*) I'd like to be able to write: type Configuration = (String, String)* def foo(configuration: Configuration) The main use case is to provide an easy method signature when overriding in subclasses

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  • automatic xml conversion in scala

    - by Jeff Bowman
    Let's say I have the following class: class Person(val firstName:String, val lastName:String) Is there an automatic way to generate xml from this class without having to hand create a toXml() method? Ideally the output would be something like: <Person <firstNameJohn</firstName <lastNameSmith</lastName </Person It seems like there should be a way to do this without having to write all that out manually. Perhaps there is a trait I haven't found yet?

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  • Scala: Recursively building all pathes in a graph?

    - by DarqMoth
    Trying to build all existing paths for an udirected graph defined as a map of edges using the following algorithm: Start: with a given vertice A Find an edge (X.A, X.B) or (X.B, X.A), add this edge to path Find all edges Ys fpr which either (Y.C, Y.B) or (Y.B, Y.C) is true For each Ys: A=B, goto Start Providing edges are defined as the following map, where keys are tuples consisting of two vertices: val edges = Map( ("n1", "n2") -> "n1n2", ("n1", "n3") -> "n1n3", ("n3", "n4") -> "n3n4", ("n5", "n1") -> "n5n1", ("n5", "n4") -> "n5n4") As an output I need to get a list of ALL pathes where each path is a list of adjecent edges like this: val allPaths = List( List(("n1", "n2") -> "n1n2"), List(("n1", "n3") -> "n1n3", ("n3", "n4") -> "n3n4"), List(("n5", "n1") -> "n5n1"), List(("n5", "n4") -> "n5n4"), List(("n2", "n1") -> "n1n2", ("n1", "n3") -> "n1n3", ("n3", "n4") -> "n3n4", ("n5", "n4") -> "n5n4")) //... //... more pathes to go } Note: Edge XY = (x,y) - "xy" and YX = (y,x) - "yx" exist as one instance only, either as XY or YX So far I have managed to implement code that duplicates edges in the path, which is wrong and I can not find the error: object Graph2 { type Vertice = String type Edge = ((String, String), String) type Path = List[((String, String), String)] val edges = Map( //(("v1", "v2") , "v1v2"), (("v1", "v3") , "v1v3"), (("v3", "v4") , "v3v4") //(("v5", "v1") , "v5v1"), //(("v5", "v4") , "v5v4") ) def main(args: Array[String]): Unit = { val processedVerticies: Map[Vertice, Vertice] = Map() val processedEdges: Map[(Vertice, Vertice), (Vertice, Vertice)] = Map() val path: Path = List() println(buildPath(path, "v1", processedVerticies, processedEdges)) } /** * Builds path from connected by edges vertices starting from given vertice * Input: map of edges * Output: list of connected edges like: List(("n1", "n2") -> "n1n2"), List(("n1", "n3") -> "n1n3", ("n3", "n4") -> "n3n4"), List(("n5", "n1") -> "n5n1"), List(("n5", "n4") -> "n5n4"), List(("n2", "n1") -> "n1n2", ("n1", "n3") -> "n1n3", ("n3", "n4") -> "n3n4", ("n5", "n4") -> "n5n4")) */ def buildPath(path: Path, vertice: Vertice, processedVerticies: Map[Vertice, Vertice], processedEdges: Map[(Vertice, Vertice), (Vertice, Vertice)]): List[Path] = { println("V: " + vertice + " VM: " + processedVerticies + " EM: " + processedEdges) if (!processedVerticies.contains(vertice)) { val edges = children(vertice) println("Edges: " + edges) val x = edges.map(edge => { if (!processedEdges.contains(edge._1)) { addToPath(vertice, processedVerticies.++(Map(vertice -> vertice)), processedEdges, path, edge) } else { println("ALready have edge: "+edge+" Return path:"+path) path } }) val y = x.toList y } else { List(path) } } def addToPath( vertice: Vertice, processedVerticies: Map[Vertice, Vertice], processedEdges: Map[(Vertice, Vertice), (Vertice, Vertice)], path: Path, edge: Edge): Path = { val newPath: Path = path ::: List(edge) val key = edge._1 val nextVertice = neighbor(vertice, key) val x = buildPath (newPath, nextVertice, processedVerticies, processedEdges ++ (Map((vertice, nextVertice) -> (vertice, nextVertice))) ).flatten // need define buidPath type x } def children(vertice: Vertice) = { edges.filter(p => (p._1)._1 == vertice || (p._1)._2 == vertice) } def containsPair(x: (Vertice, Vertice), m: Map[(Vertice, Vertice), (Vertice, Vertice)]): Boolean = { m.contains((x._1, x._2)) || m.contains((x._2, x._1)) } def neighbor(vertice: String, key: (String, String)): String = key match { case (`vertice`, x) => x case (x, `vertice`) => x } } Running this results in: List(List(((v1,v3),v1v3), ((v1,v3),v1v3), ((v3,v4),v3v4))) Why is that?

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  • Scala: Correcting type inference of representation type over if statement

    - by drhagen
    This is a follow-up to two questions on representation types, which are type parameters of a trait designed to represent the type underlying a bounded type member (or something like that). I've had success creating instances of classes, e.g ConcreteGarage, that have instances cars of bounded type members CarType. trait Garage { type CarType <: Car[CarType] def cars: Seq[CarType] def copy(cars: Seq[CarType]): Garage def refuel(car: CarType, fuel: CarType#FuelType): Garage = copy( cars.map { case `car` => car.refuel(fuel) case other => other }) } class ConcreteGarage[C <: Car[C]](val cars: Seq[C]) extends Garage { type CarType = C def copy(cars: Seq[C]) = new ConcreteGarage(cars) } trait Car[C <: Car[C]] { type FuelType <: Fuel def fuel: FuelType def copy(fuel: C#FuelType): C def refuel(fuel: C#FuelType): C = copy(fuel) } class Ferrari(val fuel: Benzin) extends Car[Ferrari] { type FuelType = Benzin def copy(fuel: Benzin) = new Ferrari(fuel) } class Mustang(val fuel: Benzin) extends Car[Mustang] { type FuelType = Benzin def copy(fuel: Benzin) = new Mustang(fuel) } trait Fuel case class Benzin() extends Fuel I can easily create instances of Cars like Ferraris and Mustangs and put them into a ConcreteGarage, as long as it's simple: val newFerrari = new Ferrari(Benzin()) val newMustang = new Mustang(Benzin()) val ferrariGarage = new ConcreteGarage(Seq(newFerrari)) val mustangGarage = new ConcreteGarage(Seq(newMustang)) However, if I merely return one or the other, based on a flag, and try to put the result into a garage, it fails: val likesFord = true val new_car = if (likesFord) newFerrari else newMustang val switchedGarage = new ConcreteGarage(Seq(new_car)) // Fails here The switch alone works fine, it is the call to ConcreteGarage constructor that fails with the rather mystical error: error: inferred type arguments [this.Car[_ >: this.Ferrari with this.Mustang <: this.Car[_ >: this.Ferrari with this.Mustang <: ScalaObject]{def fuel: this.Benzin; type FuelType<: this.Benzin}]{def fuel: this.Benzin; type FuelType<: this.Benzin}] do not conform to class ConcreteGarage's type parameter bounds [C <: this.Car[C]] val switchedGarage = new ConcreteGarage(Seq(new_car)) // Fails here ^ I have tried putting those magic [C <: Car[C]] representation type parameters everywhere, but without success in finding the magic spot.

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  • How do I alias the scala setter method 'myvar_$(myval)' to something more pleasing when in java?

    - by feydr
    I've been converting some code from java to scala lately trying to tech myself the language. Suppose we have this scala class: class Person() { var name:String = "joebob" } Now I want to access it from java so I can't use dot-notation like I would if I was in scala. So I can get my var's contents by issuing: person = Person.new(); System.out.println(person.name()); and set it via: person = Person.new(); person.name_$eq("sallysue"); System.out.println(person.name()); This holds true cause our Person Class looks like this in javap: Compiled from "Person.scala" public class Person extends java.lang.Object implements scala.ScalaObject{ public Person(); public void name_$eq(java.lang.String); public java.lang.String name(); public int $tag() throws java.rmi.RemoteException; } Yes, I could write my own getters/setters but I hate filling classes up with that and it doesn't make a ton of sense considering I already have them -- I just want to alias the _$eq method better. (This actually gets worse when you are dealing with stuff like antlr because then you have to escape it and it ends up looking like person.name_\$eq("newname"); Note: I'd much rather have to put up with this rather than fill my classes with more setter methods. So what would you do in this situation?

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  • Passing dependent objects to a parent constructor in Scala

    - by Nick Johnson
    Suppose I have the following class heirarchy: class A() class B(a:A) class C(b:B) class BaseClass(b:B, c:C) Now I want to implement a subclass of BaseClass, which is given an instance of A, and constructs instances of B and C, which it passes to its superclass constructor. If I could use arbitrary expressions, I'd do something like this: b = new B(a) c = new C(b) super(b, c) Because the second argument to the parent constructor depends on the value of the first argument, though, I can't see any way to do this, without using a factory function, or a gratuitous hack, such as : class IntermediateSubclass(b:B) extends BaseClass(b, new C(b)) class RealSubclass(a:A) extends IntermediateSubclass(new B(a)) Is there clean way to do this?

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  • Step by Step / Deep explain: The Power of (Co)Yoneda (preferably in scala) through Coroutines

    - by Mzk
    some background code /** FunctorStr: ? F[-]. (? A B. (A -> B) -> F[A] -> F[B]) */ trait FunctorStr[F[_]] { self => def map[A, B](f: A => B): F[A] => F[B] } trait Yoneda[F[_], A] { yo => def apply[B](f: A => B): F[B] def run: F[A] = yo(x => x) def map[B](f: A => B): Yoneda[F, B] = new Yoneda[F, B] { def apply[X](g: B => X) = yo(f andThen g) } } object Yoneda { implicit def yonedafunctor[F[_]]: FunctorStr[({ type l[x] = Yoneda[F, x] })#l] = new FunctorStr[({ type l[x] = Yoneda[F, x] })#l] { def map[A, B](f: A => B): Yoneda[F, A] => Yoneda[F, B] = _ map f } def apply[F[_]: FunctorStr, X](x: F[X]): Yoneda[F, X] = new Yoneda[F, X] { def apply[Y](f: X => Y) = Functor[F].map(f) apply x } } trait Coyoneda[F[_], A] { co => type I def fi: F[I] def k: I => A final def map[B](f: A => B): Coyoneda.Aux[F, B, I] = Coyoneda(fi)(f compose k) } object Coyoneda { type Aux[F[_], A, B] = Coyoneda[F, A] { type I = B } def apply[F[_], B, A](x: F[B])(f: B => A): Aux[F, A, B] = new Coyoneda[F, A] { type I = B val fi = x val k = f } implicit def coyonedaFunctor[F[_]]: FunctorStr[({ type l[x] = Coyoneda[F, x] })#l] = new CoyonedaFunctor[F] {} trait CoyonedaFunctor[F[_]] extends FunctorStr[({type l[x] = Coyoneda[F, x]})#l] { override def map[A, B](f: A => B): Coyoneda[F, A] => Coyoneda[F, B] = x => apply(x.fi)(f compose x.k) } def liftCoyoneda[T[_], A](x: T[A]): Coyoneda[T, A] = apply(x)(a => a) } Now I thought I understood yoneda and coyoneda a bit just from the types – i.e. that they quantify / abstract over map fixed in some type constructor F and some type a, to any type B returning F[B] or (Co)Yoneda[F, B]. Thus providing map fusion for free (? is this kind of like a cut rule for map ?). But I see that Coyoneda is a functor for any type constructor F regardless of F being a Functor, and that I don't fully grasp. Now I'm in a situation where I'm trying to define a Coroutine type, (I'm looking at https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/coroutines-for-streaming/part-2-coroutines for the types to get started with) case class Coroutine[S[_], M[_], R](resume: M[CoroutineState[S, M, R]]) sealed trait CoroutineState[S[_], M[_], R] object CoroutineState { case class Run[S[_], M[_], R](x: S[Coroutine[S, M, R]]) extends CoroutineState[S, M, R] case class Done[R](x: R) extends CoroutineState[Nothing, Nothing, R] class CoroutineStateFunctor[S[_], M[_]](F: FunctorStr[S]) extends FunctorStr[({ type l[x] = CoroutineState[S, M, x]})#l] { override def map[A, B](f : A => B) : CoroutineState[S, M, A] => CoroutineState[S, M, B] = { ??? } } } and I think that if I understood Coyoneda better I could leverage it to make S & M type constructors functors way easy, plus I see Coyoneda potentially playing a role in defining recursion schemes as the functor requirement is pervasive. So how could I use coyoneda to make type constructors functors like for example coroutine state? or something like a Pause functor ?

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  • How do I alias the scala setter method 'myvar_$eq(myval)' to something more pleasing when in java?

    - by feydr
    I've been converting some code from java to scala lately trying to teach myself the language. Suppose we have this scala class: class Person() { var name:String = "joebob" } Now I want to access it from java so I can't use dot-notation like I would if I was in scala. So I can get my var's contents by issuing: person = Person.new(); System.out.println(person.name()); and set it via: person = Person.new(); person.name_$eq("sallysue"); System.out.println(person.name()); This holds true cause our Person Class looks like this in javap: Compiled from "Person.scala" public class Person extends java.lang.Object implements scala.ScalaObject{ public Person(); public void name_$eq(java.lang.String); public java.lang.String name(); public int $tag() throws java.rmi.RemoteException; } Yes, I could write my own getters/setters but I hate filling classes up with that and it doesn't make a ton of sense considering I already have them -- I just want to alias the _$eq method better. (This actually gets worse when you are dealing with stuff like antlr because then you have to escape it and it ends up looking like person.name_\$eq("newname"); Note: I'd much rather have to put up with this rather than fill my classes with more setter methods. So what would you do in this situation?

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  • Why do Scala maps have poor performance relative to Java?

    - by Mike Hanafey
    I am working on a Scala app that consumes large amounts of CPU time, so performance matters. The prototype of the system was written in Python, and performance was unacceptable. The application does a lot with inserting and manipulating data in maps. Rex Kerr's Thyme was used to look at the performance of updating and retrieving data from maps. Basically "n" random Ints were stored in maps, and retrieved from the maps, with the time relative to java.util.HashMap used as a reference. The full results for a range of "n" are here. Sample (n=100,000) performance relative to java, smaller is worse: Update Read Mutable 16.06% 76.51% Immutable 31.30% 20.68% I do not understand why the scala immutable map beats the scala mutable map in update performance. Using the sizeHint on the mutable map does not help (it appears to be ignored in the tested implementation, 2.10.3). Even more surprisingly the immutable read performance is worse than the mutable read performance, more significantly so with larger maps. The update performance of the scala mutable map is surprisingly bad, relative to both scala immutable and plain Java. What is the explanation?

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  • Scala match/compare enumerations

    - by williamstw
    I have an enumeration that I want to use in pattern matches in an actor. I'm not getting what i'd expect and, now, I'm suspecting I'm missing something simple. My enumeration, object Ops extends Enumeration { val Create = Value("create") val Delete = Value("delete") } Then, I create an Ops from a String: val op = Ops.valueOf("create") Inside my match, I have: case (Ops.Create, ...) But Ops.Create doesn't seem to equal ops.valueOf("create") The former is just an atom 'create' and the later is Some(create) Hopefully, this is enough info for someone to tell me what I'm missing... Thanks

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  • initialise a var in scala

    - by user unknown
    I have a class where I like to initialize my var by reading a configfile, which produces intermediate objects/vals, which I would like to group and hide in a method. Here is the bare minimum of the problem - I call the ctor with a param i, in reality a File to parse, and the init-method generates the String s, in reality more complicated than here, with a lot of intermediate objects being created: class Foo (val i: Int) { var s : String; def init () { s = "" + i } init () } This will produce the error: class Foo needs to be abstract, since variable s is not defined. In this example it is easy to solve by setting the String to "": var s = "";, but in reality the object is more complex than String, without an apropriate Null-implementation. I know, I can use an Option, which works for more complicated things than String too: var s : Option [String] = None def init () { s = Some ("" + i) } or I can dispense with my methodcall. Using an Option will force me to write Some over and over again, without much benefit, since there is no need for a None else than to initialize it that way I thought I could. Is there another way to achieve my goal?

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  • Scala: XML Attribute parsing

    - by Chris
    I'm trying to parse a rss feed that looks like this for the attribute "date": <rss version="2.0"> <channel> <item> <y:c date="AA"></y:c> </item> </channel> </rss> I tried several different versions of this: (rssFeed contains the RSS data) println(((rssFeed \\ "channel" \\ "item" \ "y:c" \"date").toString)) But nothing seems to work. What am I missing? Any help would really be appreciated!

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  • Scala: getting the name of the class the trait is mixed in

    - by Alexey Romanov
    Given an instance of a class, we can obviously return its name: trait MixedInClassDiscovery { val className = this.getClass.getName } class AClass extends MixedInClassDiscovery { ... this.className // returns "AClass" ... } But this way uses reflection, once for every instance of AClass. Can the same be done once for every class, instead? One solution which comes to mind is to mix it into companion objects instead of classes themselves.

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  • Scala: can't write setter without getter?

    - by IttayD
    This works: class ButtonCountObserver { private var cnt = 0 // private field def count = cnt // reader method def count_=(newCount: Int) = cnt = newCount // writer method // ... } val b = new ButtonCountObserver b.count = 0 But this doesn't class ButtonCountObserver { private var cnt = 0 // private field def count_=(newCount: Int) = cnt = newCount // writer method // ... } val b = new ButtonCountObserver b.count = 0 I get: error: value count is not a member of ButtonCountObserver Is it possible to create a setter (with the syntactic sugar) without a getter?

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  • What features of Scala cannot be translated to Java?

    - by Paul
    The Scala compiler compiles direct to Java byte code (or .NET CIL). Some of the features of Scala could be re-done in Java straightforwardly (e.g. simple for comprehensions, classes, translating anonymous/inner functionc etc). What are the features that cannot be translated that way? That is presumably mostly of academic interest. More usefully, perhaps, what are the key features or idioms of Scala that YOU use that cannot be easily represented in Java? Are there any the other way about? Things that can be done straightforwardly in Java that have no straightforward equivalent in Scala? Idioms in Java that don't translate?

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