Is Option GenTraversableOnce?
I am confused. In TraversableLike, there is a function flatMap with the signature flatMap[B](f: (A) ⇒ GenTraversableOnce[B]): Iterable[B] However, I can use it this way scala> Iterable(1,2,3,4,5).flatMap{i=>if (i%2==0) {None} else {Some(i)}} res1: Iterable[Int] = List(1, 3, 5) Why is it possible? How is Option converted to GenTraversableOnce? It doesn't seem like a subclass...
Type-level map over list of types
If you absolutely have to do this kind of thing, Shapeless can make it both cleaner and safer: import shapeless._ def findByType[T](xs: Seq[Any])(implicit t: Typeable[T]) = xs.flatMap(t.cast).headOption Note that unlike your implementation, this one won't give wrong answers on many of the standard library's generic types: scala> findByType[List[String]](Seq(List(1), List("a"), 'foo)) res3: Option[
What does the `#` operator mean in Scala?
I see this code in this blog: Type-Level Programming in Scala: // define the abstract types and bounds trait Recurse { type Next <: Recurse // this is the recursive function definition type X[R <: Recurse] <: Int } // implementation trait RecurseA extends Recurse { type Next = RecurseA // this is the implementation type X[R <: Recurse] = R#X[R#Next] } object Recurse { // infinite loop type C = Rec
scala tuple unpacking
I know this question has come up many times in different ways. But it is still not clear to me. Is there a way to achieve the following. def foo(a:Int, b:Int) = {} foo(a,b) //right way to invoke foo foo(getParams) // is there a way to get this working without explicitly unpacking the tuple?? def getParams = { //Some calculations (a,b) //where a & b are Int }
How to set heap size for sbt?
I am using SBT 0.12.0. I have read other answers on stack overflow and followed them, however none of them helps, for example: create ForkRun class - I have not observed any forked process during my usage of sbt set environment variable JAVA_OPTS - it is set but sbt's process command line does not seem to use it at all. sbt -J-Xmx2G appends the parameter to sbt process command line, however the ol
How to stop INFO messages displaying on spark console?
I'd like to stop various messages that are coming on spark shell. I tried to edit the log4j.properties file in order to stop these message. Here are the contents of log4j.properties # Define the root logger with appender file log4j.rootCategory=WARN, console log4j.appender.console=org.apache.log4j.ConsoleAppender log4j.appender.console.target=System.err log4j.appender.console.layout=org.apache.log
How to wrap Actions (in any order) when using Play's ActionBuilder?
I am using Play's ActionBuilder to create various Actions that secure my controllers. For instance, I implemented IsAuthenticated to make sure that certain actions can only be accessed if the user would be logged in: case class AuthRequest[A](user: String, request: Request[A]) extends WrappedRequest[A](request) private[controllers] object IsAuthenticated extends ActionBuilder[AuthRequest] { def in
Scala: write string to file in one statement
For reading files in Scala, there is Source.fromFile("file.txt").mkString Is there an equivalent and concise way to write a string to file? Most languages support something like that. My favorite is Groovy: def f = new File("file.txt") // Read def s = f.text // Write f.text = "file contents" I'd like to use the code for programs ranging from a single line to a short page of code. Having to use you
Parsing a simple string with a scala packrat parser
I'm trying to parse a string using scala's parser combinators like this: import scala.util.parsing.combinator._ import scala.util.parsing.input.CharSequenceReader object TestPackratParser extends RegexParsers with PackratParsers { lazy val program: PackratParser[Any] = "start" ~ water ~ "end" ^^ (_ => println("program")) lazy val water: PackratParser[Any] = (""".""".r).* ^^ (_ => println("water"))
AspectJ pointcuts in Scala using SBT
I'm trying to build a hello world example of using AspectJ pointcuts in Scala using the sbt-aspectj plugin. In my plugins.sbt I've added the plugin: addSbtPlugin("com.typesafe.sbt" % "sbt-aspectj" % "0.10.2") and in my build.sbt, I've set it up as follows: import com.typesafe.sbt.SbtAspectj._ aspectjSettings /* project settings */ fork in run := true javaOptions in run <++= AspectjKeys.weaverOptio
Rewrite the Scala actors in Akka
Update: I've rewritten it in wonky-style and posted it on github - I would appreciate some feedback: https://github.com/drozzy/parallel-discrete-event-akka/tree/master/src/main/scala Can anyone help me re-write "Parallel discrete event simulation" example from the "Chapter 32: Actors and Concurrency" from Programming in Scala, 2nd, by Martin Odersky? It was written originally in Scala actors, but
null java list to scala Buffer
Imagine I have a java list val javaList: java.util.List[String] = null If I wanted to use it as scala collection, let's say Buffer, I would simply add the following import (as described many times before) import scala.collection.JavaConversions._ The problem is that I have to check if the list is different from null. This will not work: javaList foreach println //throws java.lang.NullPointerExcept
How to get around the Scala case class limit of 22 fields?
More recently (Oct 2016, six years after the OP), the blog post "Scala and 22" from Richard Dallaway explores that limit: Back in 2014, when Scala 2.11 was released, an important limitation was removed: Case classes with > 22 parameters are now allowed. That said, there still exists a limit on the number of case class fields, please see https://stackoverflow.com/a/55498135/1586965 This may lead yo
Scalaでstack overflowしないFree Monad TransformerやFree MonadPlus - xuwei-k's blog
以前いくつか関連したもの書きましたが 本物のFree MonadPlus？ Free MonadPlusはFree Monad TransformerにListを当てはめたもの？ たぶんFree MonadPlus出来た気がします。 https://github.com/scalaz/scalaz/pull/1056/ そして、scalazにFreeTという (stack overflowしない) Free Monad Transformer 入ったの解説したことなかったですね。 https://github.com/scalaz/scalaz/pull/1035 purescriptのやつ真似したら出来たらしいです。自分も説明できるほどに理解できてません。scalazのものが、微妙にpurescriptのものと違う気がするのですがなんなんでしょう？ というわけで、解説の代わりにいくつかリ
IntelliJ Scala Plugin's case class indentation is absurd
When a case class has many fields and their names are long, it is often a good idea to write each field in each line like: case class Person ( name: String, age: Int ) This resembles C/C++ struct definition and totally readable even when the case class becomes bigger. But IntelliJ IDEA's default Scala plugin automatically changes its indentation: case class Person ( name: String, age: Int ) which
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