[Rd] S4 methods and polymorophism

Eric Lecoutre lecoutre at stat.ucl.ac.be
Wed Sep 22 10:21:48 CEST 2004

Hello R Power Users,

There is a rather large introduction you may skip to go directly to 
my  question about S4 classes.

I am working on some toy code to help me get in through S4 classes.
Doing this homework, I have some questions about S4 classes. I have 
carefully read available help and some additional material such as Robert 
Gentleman's slides and special attention to "Programming with Data".

I am working with R 1.9 on Windows.

Final goal is the complete rewriting of my R2HTML package from scratch to 
provide to users a convenient way to describe their output and to allow 
different output format (provided by special functions - drivers, "à la" 
Sweave - HTML, LaTeX, XML...).

As BillGates guys often have good ideas (or find or buy them), I take the 
structure of Word documents as a starting point.

A document will be a collection of Objects, which will be either a standard 
R object or a graph, in both cases with some Formatting Options (FO) added. 
Formatting Options (a class) is a list of named components like (font-size: 
16, forecolor: darkred).

Here is what I have for the moment:

 > setClassUnion("ROMA_ID", c("NULL","character"))
 > setClass("Objects",representation("list",ID="ROMA_ID"))

ie: Objects is a list (there will be objects of class "Object" without "s") 
and ID provided to all objects.

Then, as I will manipulate several such collections, such as Documents, 
Graphs, and so on, I define the virtual class Collection. At the moment:

 > setClassUnion("Collection",c("Objects","Documents","Graphs"))

Then, I follow Chambers's samples to add some functionality such as show() 
functions and manipulations. A "+" method applied on two "Object" creates a 
new Objects collection.

As you can see from this output, both the definition of the mehod for "+" 
and for "show" are correct:

---------- output ---------------
 > Object(1)+Object("Oui")
<ROMA> Collection of Objects: 2 objects.

ID:  numeric
         <ROMA> Object
         Classe: numeric
         FO:  alignment

ID:  character
         <ROMA> Object
         Classe: character
         FO:  font;fontsize;fontcolor;alignment
---------- output ---------------

I have succedded adding a method to generic "+" operator, which is 
initially defined as:

 > getMethod("+")

As I understand it, the fact that it is a .Primitive call ensures the 
default method for any extendion (that is: we can add methods).

Now, I try to add a method for length to handle "Collection" classes.

 > getMethod("length")


I dont undertand the following error message when trying to define this method:

 > setMethod("length","Collection",function(object){ length(object at ID)})
Error in conformMethod(signature, mnames, fnames, f) :
         In method for function "length": Formal arguments omitted in the 
method definition cannot be in the signature (x = "Collection")

How is it possible to extend length to my own class?

Be HappyR and HappyR again,


Eric Lecoutre
UCL /  Institut de Statistique
Voie du Roman Pays, 20
1348 Louvain-la-Neuve

tel: (+32)(0)10473050
lecoutre at stat.ucl.ac.be

If the statistics are boring, then you've got the wrong numbers. -Edward 

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