R devel - Dec 1997 - R-alpha: inheritance ([.ts)

While commenting out the previously mentioned warning seems to work, I have been
reflecting on this problem and there seems to be an issue wrt inheritance that I
do not understand properly. Perhaps some gurus could comment.

As mentioned previously, I have a class "tframe" with more specific
classes
indicating how time is being represented, such as
> class(tframe(data))
[1] "ts"     "tframe"
and in R0.60 "[.ts" produces warning messages when I use
tframe(data)[2].

Now tframe(data) is NOT a "ts", it is a specific kind of
"tframe". When I coded
this I was expecting it would use  "[.default", so to date I guess it
has just
been working accidentally.

Does the fact that there is a class "ts" mean that one should not have
another
class with "ts" as its more specific class? This seems like a pretty
severe
limitation.

Paul Gilbert




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On Thu, 11 Dec 1997, Paul Gilbert wrote:
> While commenting out the previously mentioned warning seems to work, I have
been
> reflecting on this problem and there seems to be an issue wrt inheritance
that I
> do not understand properly. Perhaps some gurus could comment.
There is no real "inheritance" mechanism in R.  Objects have a list of
classes, and UseMethod/NextMethod search this list from left to right,
pasting the class name on to the generic method name, until a function is
found. The ability to mimic inheritance relies on class names being
unique, so that, for example gee inherits from glm which inherits from lm.

> As mentioned previously, I have a class "tframe" with more
specific classes
> indicating how time is being represented, such as
> > class(tframe(data))
> [1] "ts"     "tframe"
> and in R0.60 "[.ts" produces warning messages when I use
tframe(data)[2].
 
> Does the fact that there is a class "ts" mean that one should not
have another
> class with "ts" as its more specific class? This seems like a
pretty severe
> limitation.
Yes it does. It is a limitation especially as there is only one namespace.
It is also unfortunate that a lot of people have used method-like names
the other way around, so that eg cox.zph is a proportional hazards test (z
ph) for a cox model, not a "cox" for a "zph" object.

The fix to this is to use something like 
	c("ts.tsframe", "tsframe")
for a more specific ts subclass of tsframe.  It's a bit ugly, but it
works.

	-thomas

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