(Not a) Scala compiler bug? (null pointer exception)

This code

v(2,1)  
var m=Array[String]()
def v(f:Int,t:Int)=
{   var move= (10*f+t).toString

    m :+ "21"

}

(run it as a scala script)

gives a null pointer exception when executing the m:+ "21" line.

This

var m=Array[String]()
def v(f:Int,t:Int)=
{   var move= (10*f+t).toString

    m :+ "21"

}
v(2,1)  

works. I think both should work and this is a compiler bug. Or am I mistaken?

Scala 2开发者_JAVA技巧.8.1, Windows XP

You are mistaken: you are attempting to access the variable m before it has been initialized.

Note that just because the line with m on it is before the "problematic" line (i.e. where the exception is thrown) does not necessarily mean that it is run first.

I made this class (which exhibits the same behaviour when you create it):

class OrderTest {
  foo(1)
  val l = List("one", "two")
  def foo(i : Int) = println(l(i))
}

And then compiled with the -print option:

class OrderTest extends java.lang.Object with ScalaObject {
  private[this] val l: List = _;
  <stable> <accessor> def l(): List = OrderTest.this.l;
  def foo(i: Int): Unit = scala.this.Predef.println(OrderTest.this.l().apply(i));
  def this(): test.OrderTest = {
    OrderTest.super.this();
    OrderTest.this.foo(1);
    OrderTest.this.l = immutable.this.List.apply(scala.this.Predef.wrapRefArray(Array[java.lang.String]{"one", "two"}.$asInstanceOf[Array[java.lang.Object]]()));
    ()
  }
}

You can see quite clearly what is going on