scala

问题 In PlaySlick sample there is file with sample data access object. https://github.com/playframework/play-slick/blob/master/samples/basic/app/dao/CatDAO.scala and table definition: private class CatsTable(tag: Tag) extends Table[Cat](tag, "CAT") { def name = column[String]("NAME", O.PrimaryKey) def color = column[String]("COLOR") def * = (name, color) <> (Cat.tupled, Cat.unapply) } Is it possible to generate a new table using this definition without using play evolutions? If not, why? 回答1:

问题 I am using a spark streaming application(spark 2.1) to consume data from kafka(0.10.1) topics. I want to subscribe to new topic without restarting the streaming context . Is there any way to achieve this? I can see a jira ticket in apache spark project for the same (https://issues.apache.org/jira/browse/SPARK-10320),Even though it is closed in 2.0 version, I couldn't find any documentation or example to do this. If any of you are familiar with this, please provide me documentation link or

问题 I have a spark dataframe like this: word1 word2 co-occur ---- ----- ------- w1 w2 10 w2 w1 15 w2 w3 11 And my expected result is: word1 word2 co-occur ---- ----- ------- w1 w2 25 w2 w3 11 I tried dataframe's groupBy and aggregate functions but I couldn't come up with the solution. 回答1: You need a single column containing both words in sorted order, this column can then be used for the groupBy . You can create a new column with an array containing word1 and word as follows: df.withColumn(

问题 Sometimes, I find myself wishing scala collections to include some missing functionality, and it's rather easy "extending" a collection, and provide a custom method. This is a bit more difficult when it comes to building the collection from scratch. Consider useful methods such as .iterate . I'll demonstrate the usecase with a similar, familiar function: unfold . unfold is a method to construct a collection from an initial state z: S , and a function to generate an optional tuple of the next

问题 This code is from akka documentation. It impelements an actor using the recommended functional style: import akka.actor.typed.Behavior import akka.actor.typed.scaladsl.ActorContext import akka.actor.typed.scaladsl.Behaviors object Counter { sealed trait Command case object Increment extends Command final case class GetValue(replyTo: ActorRef[Value]) extends Command final case class Value(n: Int) def apply(): Behavior[Command] = counter(0) private def counter(n: Int): Behavior[Command] =

问题 Why don't type tags work with type aliases. E.g. given trait Foo object Bar { def apply[A](implicit tpe: reflect.runtime.universe.TypeTag[A]): Bar[A] = ??? } trait Bar[A] I would like to use an alias within the following method, because I need to type A around two dozen times: def test { type A = Foo implicit val fooTpe = reflect.runtime.universe.typeOf[A] // no funciona Bar[A] // no funciona } Next try: def test { type A = Foo implicit val fooTpe = reflect.runtime.universe.typeOf[Foo] // ok

问题 Trying to use the show() method on a dataframe. It is giving Task not serializable Exception. I have tried to extend the Serializable object but still the error persists. object App extends Serializable{ def main(args: Array[String]): Unit = { Logger.getLogger("org.apache").setLevel(Level.WARN); val spark = SparkSession.builder() .appName("LearningSpark") .master("local[*]") .getOrCreate() val sc = spark.sparkContext val inputPath = "./src/resources/2015-03-01-0.json" val ghLog = spark.read

问题 I am reading in dread what will come with Scala 3, paying particular attention to changes to compound types. They were always somewhat of a hack, so clean, true intersection types are certainly an improvement. I couldn't find though anything about what happens to the actual refinement part of the compound type. I rely heavily in my current project on strongly interwoven types in an attempt to have every returned value be as narrow as possible. So, for example, having trait Thing { thisThing =

问题 I'm trying to do a basic CRUD on a scala play mongo prototype. The code works as a standalone main method - but when executed as a play application invoked through a controller, getting runtime exceptions [debug] Running task... Cancel: Null, check cycles: false, forcegc: true [info] play.api.Play - Application started (Dev) [error] application - ! @7b9n058gm - Internal server error, for (GET) [/mongoTestUserCollection] -> play.api.http.HttpErrorHandlerExceptions$$anon$1: Execution exception[

问题 trying to understand some scala syntax, and where to find their spec. Bellow i am confused about statefulMapConcat. The signature is this one: def statefulMapConcat[T](f: () => Out => immutable.Iterable[T]): Repr[T] and we have "be able to restart" in { Source(List(2, 1, 3, 4, 1)) .statefulMapConcat(() => { var prev: Option[Int] = None x => { if (x % 3 == 0) throw ex prev match { case Some(e) => prev = Some(x) (1 to e).map(_ => x) case None => prev = Some(x) List.empty[Int] } } })