Since nobody else has respond, I thought I'd take a stab.  Maybe if I'm
wrong enough somebody will correct me, but my understanding is that that
kind of situation, ie the pain of getting the correct method called when
there is a dependency on the type of more than one argument is part of the
motivation behind the S4 classes.  So maybe it really just is that kludgy
for s3 classes.

Jamie Olson
School of Computer Science
Carnegie Mellon University
5000 Forbes Ave.
Pittsburgh, PA 15213
jfolson@cs.cmu.edu


On Mon, Jun 6, 2011 at 12:19 PM, Janko Thyson <
janko.thyson.rstuff@googlemail.com> wrote:

> Okay, I found something that is working, but it looks and feels pretty
> awkward as the method def and method lookup takes place in one function ;-)
>
> setRefClass("A", fields=list(X="numeric"))
> setRefClass("B", contains="A", fields=list(Y="character"))
>
> mySetAs <- function(
>     from,
>     to
> ){
>     if(!existsMethod(f="coerce", signature=c(from=class(from), to=to))){
>         setAs(from=class(from), to=to,
>              def=function(from, to){
>                 out <- getRefClass(to)$new(X=from)
>                 return(out)
>             }
>         )
>     }
>      mthd <- selectMethod("coerce", signature=c(from=class(from),
> to=to), useInherited= c(from=TRUE, to=TRUE))
>     out <- mthd(from=from, to=to)
>     return(out)
> }
>
> a <- mySetAs(from=1:5, to="A")
> a$X
> b <- mySetAs(from=1:5, to="B")
> b$X
>
> I'm sure there are much better ways. I'd appreciate any comments
> whatsoever.
>
> Best regards,
> Janko
>
> On 06.06.2011 17:46, Janko Thyson wrote:
> > Somehow I don't see my own postings in the list, so sorry for replying
> > to my own message and not the one that went out to the list.
> >
> > I got a little further and I think I found exactly the thing that is
> > bothering me: how to get "extended" method dispatch going in 'setAs()':
> >
> > setRefClass("A", fields=list(X="numeric"))
> > setRefClass("B", contains="A", fields=list(Y="character"))
> >
> > setAs(from="numeric", to="A",
> >     def=function(from,to){
> >         out <- getRefClass(to)$new(X=from)
> >         return(out)
> >     }
> > )
> > a <- as(1:5, "A")
> > a$X
> >
> > b <- as(1:5, "B")
> >
> > My problem is the last statement (b <- as(1:5, "B") which fails. I
> > want to get around having to write new 'setAs' methods for all classes
> > extending class 'A'. If 'B' inherits from 'A', shouldn't it then be
> > possible to tell 'setAs' to look for the next suitable method, i.e.
> > the method defined for 'A'? I tried 'NextMethod()' inside the body of
> > 'setAs' but that didn't work out.
> >
> > Thanks a lot,
> > Janko
> >
> > On 06.06.2011 17:15, Janko Thyson wrote:
> >> Dear list,
> >>
> >> I wonder how to write methods for the function 'as' in the sense that
> >> I can call 'as(object, Class, strict=TRUE, ext)' and let method
> >> dispatch figure out the correct method.
> >>
> >> AFAIU, there is a difference between, e.g. 'as.data.frame' and the
> >> methods of 'as()' as stated above since the former depends on arg 'x'
> >> instead of 'object', 'Class' etc.?
> >>
> >> > methods("as")
> >> > as.data.frame
> >>
> >> I have to admit that I'm not really familiar with the S3 style of
> >> defining methods as I have been coding in S4 a lot, but my first
> >> attempt was to write something like this:
> >>
> >> as.myClass <- function(x, ...){
> >>     if(is(x, "data.frame"){
> >>         x <- as.list(x)
> >>     }
> >>     if(is(x, "character"){
> >>         x <- as.list(x)
> >>     }
> >>     ...
> >>     out <- getRefClass("myClass")$new(X=x)
> >>     return(out)
> >> }
> >>
> >> But that way I'd have to explicitly call 'as.myClass(x)' whereas I'd
> >> simply like to type 'as(x, "myClass")'.
> >> Also, how is it possible to have method dispatch recognize two
> >> signature arguments in S3? I.e., how can I define something like
> >> 'as.data.frame.character' in order to have explicit "sub" methods for
> >> all the data types of 'x' so I wouldn't have to process them all in
> >> the definition of 'as.myClass' as I did above?
> >>
> >> Thanks for your help,
> >> Janko
>
>        [[alternative HTML version deleted]]
>
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