[R] Generic Functions

[Martin Morgan]

See the 'useAsDefault' argument to setGeneric.

As an aside, if 'setType<-' is meant to be a 'setter' to change the 
value of a slot 'type', then I find the syntax a little redundant -- 
it's use

 > setType(x) <- "foo"

implies that it is already a 'setter' without 'set' at the front. Why 
not just

 > type(x) <- "foo"

(though perhaps 'type' is not such a good name, either)?

As a second aside, if you're writing code that you expect to be used 
with fPortfolio, then having two functions with the same name but 
different signatures or overall goals will confuse your user -- with 
fPortfolio, setType<- works fine, but then for mysterious reasons (i.e., 
when your package is loaded, with a different definition of setType<-) 
code that worked before no longer works! So I'd either use setType in a 
way consistent with it's use in fPortfolio, or define a new generic for 
your own purposes (setType<- is not a generic in my version of fPortfolio,

 > packageDescription('fPortfolio')$Version
[1] "260.72"

).

As a third aside, I think questions about S4 probably belong on R-devel, 
as they seem to fall in the realm of 'questions likely to prompt 
discussion unintelligible to non-programmers' (from the R-devel mailing 
list description).

Martin

Dominik Locher wrote:
> Hi
> 
> I have some problems in defining new generic functions and classes. Just
> have a look at the following example:
> 
> 
> require(fPortfolio)
> 
> setClass("PROBECLASS",           
>   representation(               
>     type="character"            
>   )                             
> )       
> 
> isGeneric("setType<-")
> #Returns 
> TRUE
> 
> #I would like to define a specific function for class PROBECLASS with other
> arguments than for the generic function "setType" of fPortfolio.
> setGeneric("setType<-", function(object, value)
> standardGeneric("setType<-"))
> 
> #Returns
> Fehler in makeGeneric(name, fdef, fdeflt, group = group, valueClass =
> valueClass,  : 
>   the formal arguments of the generic function for "setType<-" (object,
> value) differ from those of the non-generic to be used as the default (spec,
> value)
> 
> setReplaceMethod("setType", "PROBECLASS", function(object, value){
> 
>   object at type <- value                           
>   object
> 
> })           
> 
> #Example
> obj = new("PROBECLASS")
> setType(obj) = "test"
> obj         
> 
> 
> ######
> If I don't require fPortfolio it works fine. However, is it not possible to
> create two generic functions with the same name but different arguments?
> setType for fPortfolio may be differ completely from setType of
> PROBECLASS...
> What's the best way to have functions which belongs to an object of a
> specific class? I had a look at the paper "S4 Classes in 15 pages, more or
> less" (feb12, 2003), however, I could not found what I did wrong...
> 
> Any help is highly appreciated. 
> 
> Thanks
> Dominik
> 
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