CODEWITHSUNDEEP
scalalazy-evaluationlocal-variablesscala-2.10
Petr
Petr

Reputation: 63359

How are Scala's local lazy variables implemented?

There is a question asking about the implementation of lazy vals if they're class variables. How are implemented local variables, such in

def foo[A](a: => A) = {
  lazy val x: A = a
  // return something that uses x
}

Upvotes: 5

Views: 1007

Answers (1)

Rex Kerr
Rex Kerr

Reputation: 167891

It's a bit less efficient to use lazy vals in a method. The reason is that you can't actually nest functions so the lazy bits which nominally are allocated on the stack actually need to go on the heap. So you have to create at least one extra object, and it turns out that Scala actually creates two.

class Baz{
  def baz(i: => Int, b: Boolean) = {
    lazy val j = i
    if (b) j else 0
  }
}

turns into, among other things

public int baz(scala.Function0, boolean);
  Code:
   0:   new #12; //class scala/runtime/IntRef
   3:   dup
   4:   iconst_0
   5:   invokespecial   #16; //Method scala/runtime/IntRef."<init>":(I)V
   8:   astore_3
   9:   new #18; //class scala/runtime/VolatileByteRef
   12:  dup
   13:  iconst_0
   14:  invokespecial   #21; //Method scala/runtime/VolatileByteRef."<init>":(B)V
   17:  astore  4
   19:  iload_2
   20:  ifeq    34
   23:  aload_0
   24:  aload_1
   25:  aload_3
   26:  aload   4
   28:  invokespecial   #25; //Method j$1:(Lscala/Function0;Lscala/runtime/IntRef;
                                               Lscala/runtime/VolatileByteRef;)I
   31:  goto    35
   34:  iconst_0
   35:  ireturn

See the creation of IntRef and VolatileByteRef? These are in place of what would normally be just private vars to handle the lazy val. And now j$1, the accessor method created to handle retrieval and/or creation of the lazy val, has to take those two newly created classes as parameters (in addition to the by-name function).

So while the underlying mechanics are the same, the implementation details are different and less efficient than if you already had another class around in which to stick the vars.

Upvotes: 3

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