[R] why-s of method dispatching

[Vadim Ogranovich]

I see. Thank you very much!

This brings another question. Does R-Core have any plan to promote
data.frame to an S4 class? In general, is there any "road-map" (formal
or informal) to phasing out S3 classes?

Thanks,
Vadim

> -----Original Message-----
> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk]
> Sent: Thursday, March 18, 2004 12:24 AM
> To: Vadim Ogranovich
> Cc: R Help List
> Subject: Re: [R] why-s of method dispatching
> 
> 
> On Wed, 17 Mar 2004, Vadim Ogranovich wrote:
> 
> > I am having a problem to understand why as.data.frame
> method doesn't
> > dispatch properly on my class:
> > 
> > > setClass("Foo", "character")
> > [1] "Foo"
> > > as.data.frame(list(foo=new("Foo", .Data="a")))
> > Error in as.data.frame.default(x[[i]], optional = TRUE) :  can't 
> > coerce Foo into a data.frame
> > 
> > I was expecting that this would call as.data.frame.character.
> 
> You have set an S4 class and as.data.frame is an S3 generic.
> 
> > list(foo=new("Foo", .Data="a"))
> $foo
> An object of class "Foo"
> [1] "a"
> 
> and what as.data.frame sees is
> 
> > attributes(list(foo=new("Foo", .Data="a"))$foo)
> $class
> [1] "Foo"
> attr(,"package")
> [1] ".GlobalEnv"
> 
> so thinks this is an S3 class it knows nothing about.
> 
> > Another puzzle. If I explicitly call as.data.frame.character() it
> > would fail but for a different reason:
> > 
> > > as.data.frame.character(list(foo=new("Foo", .Data="a")))
> > Error in unique.default(x) : unique() applies only to vectors
> > 
> > I was under an impression that an instance of "Foo" would
> be welcome
> > anywhere a "character" was, but it seems to be more subtle.
> What am I
> > missing?
> 
> The difference between S3 and S4 classes.
> 
> -- 
> 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
> 
>