Question

I have written a package for Mathematica called MathOO. In short, it allows you to use object orientation in Mathematica just like you do in Python. Please read the following article in Voofie/MathOO for details:

MathOO: Adding Python style Object Orientation to Mathematica with MathOO (1.0 beta launch) [Alternative to Objectica]

The problem I encountered is that, I would like to have garbage collector, so that user don’t have to explicitly deleting the object after using it. For instance:

NewClass[Object1]
Object1.$init$[self_]:= Return[];

In the above two lines, I just defined Object1 to be a new class, and the constructor to be an empty function. If you are familiar with Python, you should see the similarity with __init__().

To instantiate an Object1, I do:

object1 = new[Object1][]

The output is:

Out: object$13

Here, object$13 is an temporary variable. What I want is, when there are no references to this temporary variable, it should be deleted automatically. But it doesn’t work as expected. I have identified the problem to be the following:

In:  y = Module[{x}, x[1] = 2; x]
Out: x$117

In:  FullDefinition[y]
Out: y = x$117
     Attributes[x$117] = {Temporary}
     x$117[1] = 2

Since y holds a reference of x$117, so x$117 is not removed yet. Now let’s delete the reference by setting the value of y to 1:

In:  y = 1;

However, x$117 is still here:

In:  Definition[x$117]
Out: Attributes[x$117] = {Temporary}
     x$117[1] = 2

But I expected the variable to be removed since it is no longer referenced. From the manual of Mathematica, it said:

Temporary symbols are removed if they are no longer referenced:

So, is it a bug of Mathematica? Or is there any workaround methods? I am using Mathematica 7.0. Thank you very much.

Answer 1

Mathematica really does garbage collects Temporary variables when they have no more references. That said, there’s two reasons that your x$117 is not garbage collected.

1. Remember that Module uses lexical scoping, so the module variables are only “local” in the sense that they are give a unique name “var$modnum” and the Temporary Attribute.
   Since you gave your x a DownValue, it must be cleared before x can be garbage collected.

2. Your y was set to be the temporary variable x$... and the output was assigned to Out[]. So you also need to clear the history: Unprotect[In, Out]; Clear[In, Out]; Protect[In, Out];.

Then your Module example seems to be properly garbage collected.

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When using your MathOO package (that I downloaded yesterday, but haven’t played with yet) maybe you can just set the $HistoryLength to some finite number. And recommend that users suppress the output of instantiations object1 = new[Object1][];