common base functions stripping S3 class

Generally the idea is that the class should be stripped because R has no
way of knowing if the new object, for example unique(obj), still has the
necessary properties to be considered to be of the same class as obj.
Only the author of the class knows that.  S4 would help a bit here, but
only structurally (it could detect when the object couldn't possibly be
of the right class), not semantically.

There are two possible ways that S3 methods could handle subclasses:

* preserve by default (would also have preserve all attributes)
* drop by default

If you could really on either system consistently, I think you could
write correct code. It's very hard when the defaults vary.

(In other words, I agree with everything you said, except I think if
the default was to preserve you could still write correct code)

I don't see how default preserving could work.

For example, I might define a "SortedNumbers" class, which is a vector 
of numbers in non-decreasing order.  I could define min() and max() 
methods for it which would be really fast, because they only need to 
look at the first or last elements.  But a rev() method wouldn't make 
sense, so I wouldn't define one of those.

If the rev() default method left the class as "SortedNumbers", then my 
min() and max() calculations would end up broken.
So maybe I should have defined a rev() method that just stops with an 
error.  But classes don't own methods, so I'd have no way of knowing 
that someone else defined a new generic (e.g. shuffle()) that broke 
things.  I don't see any way around this within the S3 system.

In fact, some default methods do preserve the class, for example the 
replacement method `[<-`.  I could take a SortedNumbers vector of the 
numbers 1:10, and set element 1 to 11, and end up breaking min() and 
max().  This is a problem with the current design.

Probably we should do a better job of documenting which methods preserve 
the class and which ones don't.  (For example, `[` doesn't preserve the 
class, even though it would be fine to do so in this example.)  But 
there are a lot of things to do, and this is one thing that is pretty 
easy to figure out without documentation, so I'd say it's a low priority.

Duncan Murdoch