[R] Documentation of S3 and S4 classes, inheritance

[Gabor Grothendieck]

On 5/23/05, Ross Boylan <ross at biostat.ucsf.edu> wrote:
> I'd like to have a class A that computes a likelihood, and a subclass B
> that computes the same likelihood by sometimes throws in an additional
> term (B includes measurement error).
> 
> So B's likelihood needs to call A's, and then (sometimes) multiply by an
> additional term.
> 
> It sounds as if, in the S3 scheme, NextMethod is supposed to do this:
> 
> like.A <- function(stuff) compute value
> like.B <- function(stuff) extraFactor*NextMethod()
> ?

Here is a working example to try out.  In the call to lik,
the first class in the class vector is "B" so lik.B gets invoked.  
The next method of x is "A" so NextMethod invokes lik.A from 
within lik.B and then returns its result to lik.A which finishes the 
calculation.

lik <- function(x) UseMethod("lik")
lik.A <- function(x) mean(x)
lik.B <- function(x) NextMethod("lik") + sd(x)

x <- structure(1:3, class = c("B", "A"))
lik(x) # 3

> but, after some study of both the Language Manual (2.1) and the online
> help of NextMethod I can't tell exactly what it does.  In particular,
> I'm not sure if it returns to the calling function, or how it decides
> which flavor to call.  The language manual says the method choice
> depends on the values of .Generic and .Class, but doesn't say how those
> get filled in.  I would expect .Class to be the current class, in which
> case the call is recursive.  The online help says "'NextMethod' invokes
> the next method (determined by the class)" but it doesn't say how it is
> determined.  One ambiguity is whether "class" refers to the class (B) or
> the class attribute ("B", "A").

See above discussion.

> 
> I think the documentation could be clearer.
> 
> Now, probably none of this matters to me, since several sources
> (including the online help for S3)indicate that S4 classes are
> preferred.
> 
> I found the documentation for S4 initially elusive.  As far as I can
> tell, it isn't even mentioned in the "R Language Definition."  While the
> fact that S4 is defined in a package makes clear it is not formally part
> of the base language, this is not a very good way to get people to use
> S4.
> 
> I think by now I've tracked down docs on S4, including the
> intro/overview at http://www.omegahat.org/RSMethods/.
> 
> Finally, I'm a bit concerned that one article mentioned that S4
> inheritance, in practice, is used mostly for data, not methods (Thomas
> Lumley, R News 4(1), June 2004: p. 36).  Am I going down a road I
> shouldn't travel?
> 

This area seems somewhat controversial with different people stating
different opinions.   IMHO you are probably best off to start with S3 since its
simpler and if you do learn S4 later they are not unrelated so it will make 
it easier than jumping straight into it.  Also you may find you never have to 
go beyond S3 in which case you have saved yourself some time.  I 
personally use S3.

By the way if choosing from S3 and S4 is not enough, there are also 
two CRAN packages that provide additional OO models as well: 
R.oo and proto.