[Rd] attributes of environments

[Gabor Grothendieck]

On 7/5/06, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> On 7/5/2006 4:33 PM, Gabor Grothendieck wrote:
> > On 7/5/06, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> >> On 7/5/2006 3:47 PM, Gabor Grothendieck wrote:
> >> > On 7/5/06, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> >> >> On 7/5/2006 2:23 PM, Gabor Grothendieck wrote:
> >>
> >> >>  I think by this time I have shown that subclassing of
> >> >> > environments does not work yet it could if it were designed differently
> >> >> > and furthermore there are significant problems with the workarounds.
> >> >>
> >> >> I don't think you've shown that subclassing of environments doesn't
> >> >> work.  You have an example that shows that shows that R implements
> >> >> Henrik's "Case 2" rather than his "Case 1", but as Thomas and I said,
> >> >> that really has nothing to do with subclassing.
> >> >>
> >> >> Subclassing is about defining a new class, not about copying objects.
> >> >> You can (and did!) define a new class which inherits from the
> >> >> environment class.
> >> >
> >> > But by subclassing in the way allowed one comes up with something that
> >> > is not useful.
> >>
> >> You haven't shown that.  Show an example where you define a new class
> >> that should inherit from environment but doesn't.
> >>
> >> All you've shown so far is that when you try to change the class of an
> >> object to a new class, it appears that the class of another object also
> >> changes.  (The explanation being that they are really just different
> >> names for the same object.)
> >
> > But that is not how oo works.  When one defines a child its a
> > delta to the parent.  It does not change the parent.
>
> There are dozens of different definitions of "object oriented", but
> generally in the ones I know about, subclassing is something you do to a
> class, not to an object.  (In some languages classes are objects, but
> not all objects are classes.)
>
> It is possible to have an object with class c('myenv', 'environment').
> As far as I know, methods applied to that object will dispatch to the
> myenv method if one is defined, or to the environment method or default
> method if not.  That's exactly how things should work, and that's how
> they worked in the example you showed.
>
> Because environments have unusual semantics, it wouldn't surprise me
> very much if there were some errors in the implementation of UseMethod
> or NextMethod.  If there are, then you'd have a valid complaint.  But so
> far you've just made an unsupported claim.
>
> > Your parenthesized statement discussed why it works that way
> > under the current design but that is not inevitable.  The current
> > design is not the only possibility.
> >
> >>
> >> > That is why tcltk and Henrik's package wrap environments in lists and define
> >> > a completely different class but by doing that they are not able to take
> >> > advantage of inheritance.
> >>
> >> I think they did that because they wanted explicit references to
> >> objects, rather than the built-in implicit ones.  I've wanted explicit
> >> references to things on a number of occasions too, but that's really
> >> unrelated to inheritance as far as I can see.
> >
> > They are defining environments with special features yet they can't make
> > use of inheritance as they injected the environment object into their object
> > rather than subclassing it -- understandable given the current limitations.
>
> I think their worry was that attaching the special features to the
> environment would leave those features at risk of being thrown away by
> some other code that attached its own features to that environment.  But
> this has nothing to do with subclassing, it has everything to do with
> the semantics of references.
>
> If you want to complain about the semantics of references in R, do that,
> but don't bring up the red herring of subclassing (unless you really
> have code that demonstrates that CallMethod or NextMethod don't work as
> expected.)

Perhaps the example I gave previously does not adequately
convey the problem.  Lets try another example.

#1 uses R as it currently exists.

1. Define an environment e1 with component A.  Now
suppose we wish to define a "subobject" for which
$ is overridden so that it will upper case the
right arg.  We can do this:

e1 <- new.env()
e1$A <- 1

# now define a "subobject" and override $
e2 <- structure(list(env = e1), class = c("myenvironment", "environment"))
"$.myenvironment" <- function(obj, x) obj[["env"]][[toupper(x)]]

e2$a # 1
e2[["A"]]  # NULL since e2 cannot usefully inherit [[

To really get this to work we would have to
redefine all of environment's methods.  I won't do that
here to keep things small.


2. However, if it were possible to have the class
attached to e2 rather than being attached to the environment itself then
e2 could still inherit all the other methods of "environment" yet
be distinct from e1 even though the two would share the same
environment:

# this assumes a attributes are associated with variables
# does not work as intended in R currently since the class(e2)<-
statement also changes e1:

e1 <- new.env()
e1$A <- 1
e2 <- e1
class(e2) <- c("myenvironment", "environment")
"$.myenvironment" <- function(obj, x) obj[[toupper(x)]]
e2$a # 1
e2[["A"]]  # 1

Now all of environment's methods can be used and e1
can still be used in the usual way.

If we typed the above into R now then e1 would be changed
too.  Thus e2 is no longer a "subobject" of e1.  It is e1.
e1$a # 1  -- oops!

Although this example may seem simple I think it represents the essence
of  the situation that exists in a number of packages.  Any package
that currently uses the list(env=...an.environment...) idiom could
likely usefully
make use of this.  Note that in #1 we had to redefine all the methods of
environment to get access to them but if the functionality assumed in
#2 existed then it would inherit them and no further work need be done.