scala

问题 When looking at the source of some Scala libraries, e.g. shapeless, I often find traits named LowPriorityImplicits . Can you please explain this pattern? What is the problem that is solved, and how does the pattern solve it? 回答1: That pattern allows you to have hierarchy of implicits avoiding ambiguity-related errors by the compiler and providing a way to prioritise them. As an example consider the following: trait MyTypeclass[T] { def foo: String } object MyTypeclass { implicit def

问题 I have a small utilities scala build with test classes under a dedicated test folder. Compiling and then publish-local creates the package in my local repository. As expected, the test folder is automatically excluded from the local jar of the utilities package. However, the resulting POM still contains the related dependencies as defined in the sbt. The SBT dependencies: libraryDependencies ++= Seq( "org.scalactic" %% "scalactic" % "3.0.0" % Test, "org.scalatest" %% "scalatest" % "3.0.0" %

问题 I have written some code and exported it as a jar file. In this jar there is a file named automation.properties with defaults that I'm loading using val automationPropertiesFileURL = getClass.getResource("/automation.properties") if (automationPropertiesFileURL != null) { val source = Source.fromURL(automationPropertiesFileURL) config = new Properties() config.load(source.bufferedReader()) } But when this jar file gets added as a gradle dependency in C:\User\abc\.gradle and I want to read

问题 I have written some code and exported it as a jar file. In this jar there is a file named automation.properties with defaults that I'm loading using val automationPropertiesFileURL = getClass.getResource("/automation.properties") if (automationPropertiesFileURL != null) { val source = Source.fromURL(automationPropertiesFileURL) config = new Properties() config.load(source.bufferedReader()) } But when this jar file gets added as a gradle dependency in C:\User\abc\.gradle and I want to read

问题 Here is a simple example: trait Base { type Out def v: Out } object Base { type Aux[T] = Base { type Out = T } class ForH() extends Base { type Out = HNil override def v: Out = HNil } object ForH extends ForH } class TypeClass[B] trait TypeClassLevel1 { def summon[B](b: B)(implicit ev: TypeClass[B]): TypeClass[B] = ev } object TypeClass extends TypeClassLevel1 { implicit def t1: TypeClass[Base.Aux[HNil]] = new TypeClass[Base.Aux[HNil]] implicit def t2: TypeClass[Int] = new TypeClass[Int] } it

问题 To iterate through columns of a Spark Dataframe created from Hive table and update all occurrences of desired column values, I tried the following code. import org.apache.spark.sql.{DataFrame} import org.apache.spark.sql.functions._ import org.apache.spark.sql.functions.udf val a: DataFrame = spark.sql(s"select * from default.table_a") val column_names: Array[String] = a.columns val required_columns: Array[String] = column_names.filter(name => name.endsWith("_date")) val func = udf((value:

问题 To iterate through columns of a Spark Dataframe created from Hive table and update all occurrences of desired column values, I tried the following code. import org.apache.spark.sql.{DataFrame} import org.apache.spark.sql.functions._ import org.apache.spark.sql.functions.udf val a: DataFrame = spark.sql(s"select * from default.table_a") val column_names: Array[String] = a.columns val required_columns: Array[String] = column_names.filter(name => name.endsWith("_date")) val func = udf((value:

问题 Does Scala have Double Side Queue similar to Java Deque or Python deque? I see only Stack and Queue in Scala 2.12 API but just want to double check. 回答1: You can use Vector.drop or Vector.dropRight val v = Vector(1,2,3) v :+ 4 // Vector(1, 2, 3, 4) 0 +: v // Vector(0, 1, 2, 3) v.drop(1) // Vector(2, 3) v.dropRight(1) // Vector(1, 2) 回答2: Scala 2.13 has ArrayDeque that internally uses a resizable circular buffer. Reference: https://downloads.lightbend.com/website/scaladays/2018/ny/slides

问题 Does Scala have Double Side Queue similar to Java Deque or Python deque? I see only Stack and Queue in Scala 2.12 API but just want to double check. 回答1: You can use Vector.drop or Vector.dropRight val v = Vector(1,2,3) v :+ 4 // Vector(1, 2, 3, 4) 0 +: v // Vector(0, 1, 2, 3) v.drop(1) // Vector(2, 3) v.dropRight(1) // Vector(1, 2) 回答2: Scala 2.13 has ArrayDeque that internally uses a resizable circular buffer. Reference: https://downloads.lightbend.com/website/scaladays/2018/ny/slides

问题 Does Scala have Double Side Queue similar to Java Deque or Python deque? I see only Stack and Queue in Scala 2.12 API but just want to double check. 回答1: You can use Vector.drop or Vector.dropRight val v = Vector(1,2,3) v :+ 4 // Vector(1, 2, 3, 4) 0 +: v // Vector(0, 1, 2, 3) v.drop(1) // Vector(2, 3) v.dropRight(1) // Vector(1, 2) 回答2: Scala 2.13 has ArrayDeque that internally uses a resizable circular buffer. Reference: https://downloads.lightbend.com/website/scaladays/2018/ny/slides