Age of an object?

If R would have timestamps telling when any object was last modified, we 
could extend R with a 'GNU make'-style functionality (or syntax) 
together with some fancy caching to persistent storage (files, data 
bases, ...).  That would really nice!  As B.R. and M.M. writes, 
timestamping is most suited for higher level data structures and not 
simple data types, because the over head would be too large.

/Henrik


Philippe Grosjean wrote:

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