[Rd] Packages, generics, S3 and S4

[Heywood, Giles]

My question has two parts.

The first is with regard to the frame or environment in which generic 
functions are defined in packages.  It seems as though they are defined 
(i.e. exist as objects) in frame 1, even when defined in a package.

The following is a short example:

setClass("track",representation(x="numeric",y="numeric"))
plotTrack <- function(x,y,...) plot(x at x,x at y,xlab="",ylab="Value",...)
setGeneric("plotT",def=function(x,y,...) standardGeneric("plotT"))
setMethod("plotT",signature(x="track",y="missing"),plotTrack)

This which works with my 'examples', as follows:

x <- new("track",x=1:10,y=rnorm(10))
plotT(x)

The above also passes Rcmd CHECK with the following warnings, which I 
think are not related to my main question:

Undocumented code objects: "plotTrack"
Objects with usage in documentation object 'track' but missing from code:
"plotT"

Slightly more worrying, I get the following:

* checking examples ... ERROR
Running examples failed.

Ignoring this (only for the time being), I now do 'make pkg-track', 
and encounter no errors.  Loading the package, my 'examples' work fine.  
Returning to the ERROR, it is clear how this has come about. 

ls(.GlobalEnv)
[1] "plotT"

So the genericFunction object plotT resides in frame 1  i.e. .GlobalEnv.  
It gets deleted when cleanEx(env=.GlobalEnv) does its job in Rcmd CHECK.

The other object defined in this package, plotTrack, is in frame 2:

ls(2)
[1] "plotTrack"

I have tried unsuccessfully to put the genericFunction object plotT into 
frame 2, using the 'where' argument in setGeneric.  To cut a long story 
short, I can put it anywhere _except_ frame 2.

Now it seems to me that I am barking up the wrong tree, but I don't know 
how to proceed.  Any guidance will be gratefully received.

The second part of my question is this.  I clearly want to define a plot 
method (not naming it plotT)for the 'track' class, which indeed is what 
is described in the 'Green Book' p43.  However the 'Green Book' is largely 
unencumbered by S3 methods AFAIK.  Venables & Ripley (2000) addresses the 
issue of defining an S4 generic on the same name as an S3 generic on p108.  

setMethod("plot",signature(x="fungi",y="missing"),plot.fungi)

This clearly works, but I am not clear whether the definition of an S4
generic 
'on top of' i.e. with the same name 'plot' as an S3 generic (as in the
example 
above) is generally regarded as bad practice, or is quite OK.  If it is
done,
is the above a suitable template, or should the 'def' argument be used?  I
know that the above _works_, I am more looking for guidance on what is
regarded as 'good practice' and will not lead to too many problems as the 
S3->S4 migration proceeds.  Any indication of the planned S3->S4 migration
path 
and timetable would be a welcome bonus.

I realise that the second question has been aired before in different
guises, 
but I regret to say I still do not know the answer.  

I am using NT, and R1.7.1, for the above examples.

Regards

Giles Heywood
Portfolio & Risk Advisory Group
Commerzbank Securities

tel	 + 44 20 7653 7009
fax	 + 44 20 7562 5347
mobile + 44 7973 266343
mailto:giles.heywood at commerzbankib.com
60 Gracechurch Street, London EC3V 0HR, U.K.
http://www.cbksec.com/rsh/portfolio&risk.html


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