Age of an object?

Martin Maechler wrote:

"Trevor" == Trevor Hastie <hastie at stanford.edu>
   on Tue, 13 Dec 2005 12:51:34 -0800 writes:

    Trevor> It would be nice to have a date stamp on an object.

    Trevor> In S/Splus this was always available, because objects were files.

   [are you sure about "always available"? 
    In any case, objects were not single files anymore for a
    long time, at least for S+ on windows, and AFAIK also on
    unixy versions recently ]

This topic has come up before.
IIRC, the answer was that for many of us it doesn't make sense
most of the time: 

I remember it was discussed several times. I don't remember why it was 
considered too difficult to do.

If you work with *.R files ('scripts') in order to ensure
reproducibility, you will rerun -- often source() -- these files,
and the age of the script file is really more interesting.
Also, I *always* use the equivalent of  q(save = "no") and
almost only use save() to particularly save the results of
expensive  computations {often, simulations}.

OK, now let me give examples where having such an information would ease 
the work greatly: you have a (graphical) view of the content of an 
object (for instance, the one using the "view" button in R commander), 
or you have a graphical object explorer that has a cache to speed up 
display of information about objects in a given workspace (for instance, 
the SciViews-R object explorer). What a wonderful feature it will be to 
tell if an object was changed since last query. In the view, one could 
have a visual clue if it is up-to-date or not. In the object explorer, I 
could update information only for objects that have changed...

    Trevor> I have looked around, but I presume this information is not available.

I assume you will get other answers, more useful to you, which
will be based on a class of objects which carry an
'creation-time' attribute.  

Yes, but that would work only for objects designed that way, and only if 
the methods that manipulate that object do the required housework to 
update the 'last-changed' attribute (the question was about last access 
of an object, not about its creation date, so 'last-changed' is a better 
attribute here). If you access the object directly with, let's say, 
myobject at myslot <- newvalue, that attribute is not updated, isn't it?

Best,

Philippe Grosjean

Martin Maechler, ETH Zurich