On 17/11/2014 10:41 AM, Hadley Wickham wrote:
> 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
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