scala-2.10

问题 I have a WeakTypeTag of some type in my macro, and I want to generate code as follows: macroCreate[SomeObject] // => SomeObject(1) The definition of a macro will be something like this: def macroCreate[A] = macro _macroCreate[A] def _macroCreate[A](c: Context)(implicit wtt: c.WeakTypeTag[A]) = { c.Expr(Apply(Select(???, newTermName("apply")), List(c.literal(1).tree))) } The problem is, how do I get Select for the given type? I can use a workaround of converting the type to string, splitting

问题 Given a type declaration, I am able to resolve the type argument. scala> reflect.runtime.universe.typeOf[List[Int]] match {case x:TypeRef => x.args} res10: List[reflect.runtime.universe.Type] = List(Int) For a runtime value, The same method doesn't work. scala> reflect.runtime.currentMirror.reflect(List(42)).symbol.toType match {case x:TypeRef => x.args} res11: List[reflect.runtime.universe.Type] = List(B) Is there a way to overcome the type erasure for reflected values? 回答1: An example based

问题 After switching to Scala 2.10 I get tons of warnings: reflective access of structural type member method ... should be enabled by making the implicit value language.reflectiveCalls visible What does it mean? 回答1: The warning actually tells where to look in the documentation for an explanation: Test.scala:9: warning: reflective access of structural type member method y should be enabled by making the implicit value language.reflectiveCalls visible. This can be achieved by adding the import

问题 This answer to What's the (hidden) cost of Scala's lazy val? shows how they were implemented in Scala 2.7. But as the comments say, this must have changed since then, so I'm curious, what's the current (2.10) implementation of class lazy val variables? 回答1: Compiled this with scala 2.10.2: class Foo { lazy val bar = math.pow(5, 3) } Then decompiled the result with JD-GUI: import scala.math.package.; import scala.reflect.ScalaSignature; @ScalaSignature(bytes="\006\001e1A!\001\002\001\013\t

问题 I'm relatively new to Scala and am trying to define a generic object method. However, when I refer to the parameterized type within the method I am getting "No ClassTag available for T". Here is a contrived example that illustrates the problem. scala> def foo[T](count: Int, value: T): Array[T] = Array.fill[T](count)(value) <console>:7: error: No ClassTag available for T def foo[T](count: Int, value: T): Array[T] = Array.fill[T](count)(value) ^ Thanks in advance for help in understanding what

问题 I have an RDD, and I would like to sum a part of the list. (key, element2 + element3) (1, List(2.0, 3.0, 4.0, 5.0)), (2, List(1.0, -1.0, -2.0, -3.0)) output should look like this, (1, 7.0), (2, -3.0) Thanks 回答1: You can map and indexing on the second part: yourRddOfTuples.map(tuple => {val list = tuple._2; list(1) + list(2)}) Update after your comment, convert it to Vector : yourRddOfTuples.map(tuple => {val vs = tuple._2.toVector; vs(1) + vs(2)}) Or if you do not want to use conversions:

问题 I am new to Spark. Can you give any idea what is the problem with below code: val rawData="""USA | E001 | ABC DE | 19850607 | IT | $100 UK | E005 | CHAN CL | 19870512 | OP | $200 USA | E003 | XYZ AB | 19890101 | IT | $250 USA | E002 | XYZ AB | 19890705 | IT | $200""" val sc = ... val data= rawData.split("\n") val rdd= sc.parallelize(data) val data1=rdd.flatMap(line=> line.split(" | ")) val data2 = data1.map(arr => (arr(2), arr.mkString(""))).sortByKey(false) data2.saveAsTextFile("./sample

问题 Is it possible to show the full type signature (with erased parameterized types) saved in the bytecode with the new Reflection library? For example the type Any => Unit should be displayed as "scala.Function1<java.lang.Object,scala.runtime.BoxedUnit>" because that's the type stored in the bytecode. It is possible to show this type with javap . First one needs to compile some code with scalac : object X { def m(f: Any => Unit) = f } The command javap -c -s -l -verbose X$ shows: ... const #25 =

问题 Is it possible to show the full type signature (with erased parameterized types) saved in the bytecode with the new Reflection library? For example the type Any => Unit should be displayed as "scala.Function1<java.lang.Object,scala.runtime.BoxedUnit>" because that's the type stored in the bytecode. It is possible to show this type with javap . First one needs to compile some code with scalac : object X { def m(f: Any => Unit) = f } The command javap -c -s -l -verbose X$ shows: ... const #25 =

问题 I am getting a taste of Scala through the artima "Programming in Scala" book. While presenting the Map traits, the authors go to some lengths to describe the -> syntax as a method that can be applied to any type to get a tuple. And indeed: scala> (2->"two") res1: (Int, String) = (2,two) scala> (2,"two") res2: (Int, String) = (2,two) scala> (2->"two") == (2, "two") res3: Boolean = true But those are not equivalent: scala> Map(1->"one") + (2->"two") res4: scala.collection.immutable.Map[Int