Question

When I use the scala App trait, I can’t get println to work.

This simple example prints as expected,

object HelloWorld {
  def main(args: Array[String]) {
    println("Hello, world!")
  }
}

But once I introduce the trait it does not,

object HelloWorld extends App {
  println("Hello, world!")
}

I get no errors but nothing prints to the console.

Answer 1

Did you compile it first (running scalac HelloWorld.scala)? See this comment: http://www.scala-lang.org/node/9483#comment-40627

Edited to add more explanation:
The first version actually was compiled. Scala files without an explicit main method are run uncompiled as scripts. That means that for your second version, the commands in the file are run sequentially, as though they had been entered into the interpreter–so, the object HelloWorld is created, but no method is called on it. There’s more information about Scala as a scripting language here (scroll to Step 5): http://www.artima.com/scalazine/articles/steps.html