What is happening with 0.asInstanceOf[B] in Scala reduceLeft implementation

Question

Below is the source for the reduceLeft method of Scala's TraversableOnce trait. What is happening with the line that reads var acc: B = 0.asInstanceOf[B]?

To me, it seems that if I call this on a list of Strings, such as List("a", "b", "c"), this would result in something like 0.asInstanceOf[String]. However, 0.asInstanceOf[String] throws a ClassCastException at run time if I try it directly.

What is happening with that line, and why is it different than calling 0.asInstanceOf[String] directly when called on a list of Strings?

def reduceLeft[B >: A](op: (B, A) => B): B = {
  if (isEmpty)
    throw new UnsupportedOperationException("empty.reduceLeft")

  var first = true
  var acc: B = 0.asInstanceOf[B]

  for (x <- self) {
    if (first) {
      acc = x
      first = false
    }
    else acc = op(acc, x)
  }
  acc
}

Bonus question: why is acc even initialized to that value? It looks like the code in the first iteration of the for loop will just overwrite that value with the first element in the TraversableOnce object.

Answer 1

Well, the reason this is not failing with a ClassCastException at runtime is either because:

1. The compiler removes the initialization reasoning that it is never used (I doubt this)
2. B is erased to Object (or AnyRef), hence the cast is really 0.asInstanceOf[Object]

You could test the first possibility by checking the bytecode but it seems a pointless piece of code. If it is not elided, then this has the unnecessary overhead of boxing an Int on every call to the method (although not an object creation - see below)!

Figuring out what the compiler produces: 1

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_29).
Type in expressions to have them evaluated.
Type :help for more information.

scala> trait X[A] {
 | def foreach[U](f: A => U): U
 | def isEmpty = {
 | var b = false
 | foreach(_ => b = true)
 | !b
 | }
 | def reduceLeft[B >: A](op: (B, A) => B): B = {
 | if (isEmpty) sys.error("Bad")
 | var first = true
 | var acc: B = 0.asInstanceOf[B]
 | for (x <- this) {
 | if (first) { acc = x; first = false } else acc = op(acc, x)
 | }
 | acc
 | }
 | }
defined trait X

So now:

scala> :javap -v X
Compiled from "<console>"
public interface X extends scala.ScalaObject
  SourceFile: "<console>"
  Scala: length = 0x

  Signature: length = 0x2
   00 0D 
  InnerClass: 
   public abstract #19= #16 of #18; //X=class X of class 
   public final #21; //class X$$anonfun$isEmpty$1
   public final #23; //class X$$anonfun$reduceLeft$1
  minor version: 0
  major version: 49
  Constant pool:
const #1 = Asciz    SourceFile;
const #2 = Asciz    <console>;
const #3 = Asciz    foreach;
const #4 = Asciz    (Lscala/Function1;)Ljava/lang/Object;;
const #5 = Asciz    <U:Ljava/lang/Object;>(Lscala/Function1<TA;TU;>;)TU;;
const #6 = Asciz    Signature;
const #7 = Asciz    isEmpty;
const #8 = Asciz    ()Z;
const #9 = Asciz    reduceLeft;
const #10 = Asciz   (Lscala/Function2;)Ljava/lang/Object;;
const #11 = Asciz   <B:Ljava/lang/Object;>(Lscala/Function2<TB;TA;TB;>;)TB;;
const #12 = Asciz   Scala;
const #13 = Asciz   <A:Ljava/lang/Object;>Ljava/lang/Object;Lscala/ScalaObject;;
const #14 = Asciz   InnerClasses;
const #15 = Asciz   X;
const #16 = class   #15;    //  X
const #17 = Asciz   ;
const #18 = class   #17;    //  
const #19 = Asciz   X;
const #20 = Asciz   X$$anonfun$isEmpty$1;
const #21 = class   #20;    //  X$$anonfun$isEmpty$1
const #22 = Asciz   X$$anonfun$reduceLeft$1;
const #23 = class   #22;    //  X$$anonfun$reduceLeft$1
const #24 = Asciz   java/lang/Object;
const #25 = class   #24;    //  java/lang/Object
const #26 = Asciz   scala/ScalaObject;
const #27 = class   #26;    //  scala/ScalaObject

{
public abstract java.lang.Object foreach(scala.Function1);
  Signature: length = 0x2
   00 05 

public abstract boolean isEmpty();

public abstract java.lang.Object reduceLeft(scala.Function2);
  Signature: length = 0x2
   00 0B 

}

Make of that what you will!

Figuring out what the compiler produces: 2

One other possibility is to put this in a source file and compile it, printing out the intermediate code phase:

./scalac -Xprint:icode X.scala

then you get...

def reduceLeft($this: X, op$1: Function2): java.lang.Object = {
  if ($this.isEmpty())
    scala.sys.`package`.error("Bad")
  else
    ();
  var first$1: scala.runtime.BooleanRef = new scala.runtime.BooleanRef(true);
  var acc$1: scala.runtime.ObjectRef = new scala.runtime.ObjectRef(scala.Int.box(0));
  $this.foreach({
    (new anonymous class X$$anonfun$reduceLeft$1($this, op$1, first$1, acc$1): Function1)
  });
  acc.elem
};

It looks distinctly like the unnecessary boxing is in there! One point is that this won't involve an object creation, merely a lookup, because the boxed values of -127 to 127 are cached.

You can check the erased line by changing the compiler phase in the above print command to "erasure". Hey presto:

var acc: java.lang.Object = scala.Int.box(0).$asInstanceOf[java.lang.Object]();

Answer 2

The answer to the bonus question can be deduced by attempting to propose an alternative. The accumulator is of type B. What B will you assign to it? (There is a better answer, namely null.asInstanceOf[B] which is what is normally done, but I assume that would leave your question in place.)