[R] as.character methods

[McGehee, Robert]

Professor Ripley, 
You are quite correct, I did have some S3/S4 confusion. Thank you. By
using an S3 method with the documented usage, I had no trouble writing
an appropriate as.character method.

I had also not absorbed the fact that S3 and S4 generics might (and do!)
have different argument lists, as I see that the S3 as.character generic
takes (x, ...), and the S4 generic takes just (x).

I'd also like to ask a follow-up question to clarify this difference.
The documentation for ?.BasicFunsList (first page of methods package
documentation) reads:

"Functions in R that are defined as '.Primitive(<name>)' are not
suitable for formal methods, because they lack the basic reflectance
property."

As "as.character"() calls .Primitive("as.character"), I read the above
sentence to say that S4 methods are not appropriate for this function.
But certainly S4 methods _can_ be defined for .Primitives, and I can
seemingly get the argument list for such a primitive with the
getGeneric() function. Is the point then that S4 cannot pass on extended
(...) argument list for some primitives (such as the message below), and
thus informal S3 methods are required in some cases? Or perhaps I'm
missing the point entirely. Any clarification is greatly appreciated.

Thanks,
Robert

-----Original Message-----
From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk] 
Sent: Thursday, January 13, 2005 5:58 PM
To: McGehee, Robert
Cc: r-help at stat.math.ethz.ch
Subject: Re: [R] as.character methods


You seem to be confusing `generic' with `S4 generic', and `method' with 
`S4 method'.

Note that all references to `generic' and `method' outside the `methods'

package are not to S4 concepts unless explicitly stated.

On Thu, 13 Jan 2005, McGehee, Robert wrote:

> Hello,
> ?as.character says that the as.character function is a generic with
> usage: as.character(x, ...). So, I want to create an S4 object with an
> as.character method following the above usage, but I get the below
error
> telling me that ... isn't in the generic for as.character.
>
>> setClass("tmp", "numeric")
>> setMethod("as.character", "tmp", function(x, ...) paste(x, c(...)))
> Error in rematchDefinition(definition, fdef, mnames, fnames,
signature)
> :
> 	Methods can add arguments to the generic only if "..." is an
> argument to the generic
>
> Am I reading the documentation incorrectly? How do I correctly pass
the
> ... object to the method for this "tmp" object?
>
> However I note that looking at the generic function, I see no mention
of
> ... (despite the documentation).
>> getGeneric("as.character")
> standardGeneric for "as.character" defined from package "base"
>
> function (x)
> standardGeneric("as.character", .Primitive("as.character"))
> <environment: 0145EDC4>
> Methods may be defined for arguments: x
>
>
> So, briefly, is the documentation wrong? Am I doing something wrong?
Can
> I create an as.character method and pass additional arguments to it as
I
> think I should be able to?

Briefly, No, yes, yes.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595