R help - Jan 2006 - creating objects with a slot of class formula, using new

Hi, 
 
This works fine.
>setClass("a",representation(b="list",c="list"))
>new("a",b=list(7))
>An object of class "a"
Slot "b":
[[1]]
[1] 7
 

Slot "c":
list()
 
But, now suppose you want a slot to accept an object of class
formula...
>setClass("a",representation(b="list",c="formula"))
>new("a",b=list(7))
>Error in validObject(.Object) : invalid class "a" object: invalid
object
for slot "c" in class "a": got class "NULL",
should be or extend class
"formula"
 
Why can't new handle this? Why must the slot be defined? 
 
If I call new without any named arguments, it works fine
> new("a")
An object of class "a"
Slot "b":
list()
 
Slot "c":
NULL
 
If I call new with only a formula, it works fine.
> new("a",c=formula(x~y))
An object of class "a"
Slot "b":
list()
 
Slot "c":
x ~ y
 
How can I get R to do this?
>setClass("a",representation(b="list",c="formula"))
>new("a",b=list(7))
An object of class "a"
Slot "b":
[[1]]
[1] 7 
 
Slot "c":
NULL
 
Thanks, 
Steve
 
platform i386-pc-mingw32
arch     i386           
os       mingw32        
system   i386, mingw32  
status                  
major    2              
minor    1.1            
year     2005           
month    06             
day      20             
language R 

	[[alternative HTML version deleted]]

Create a virtual class that can either be a formula or be NULL:

   setClassUnion("formulaOrNULL", c("formula",
"NULL"))
  
setClass("a",representation(b="list",c="formulaOrNULL"))
   new("a", b = list(7))

On 1/18/06, Steven Lacey <slacey at umich.edu>
wrote:
> Hi,
>
> This works fine.
>
>setClass("a",representation(b="list",c="list"))
> >new("a",b=list(7))
> >An object of class "a"
> Slot "b":
> [[1]]
> [1] 7
>
>
> Slot "c":
> list()
>
> But, now suppose you want a slot to accept an object of class formula...
>
>setClass("a",representation(b="list",c="formula"))
> >new("a",b=list(7))
> >Error in validObject(.Object) : invalid class "a" object:
invalid object
> for slot "c" in class "a": got class "NULL",
should be or extend class
> "formula"
>
> Why can't new handle this? Why must the slot be defined?
>
> If I call new without any named arguments, it works fine
> > new("a")
> An object of class "a"
> Slot "b":
> list()
>
> Slot "c":
> NULL
>
> If I call new with only a formula, it works fine.
> > new("a",c=formula(x~y))
> An object of class "a"
> Slot "b":
> list()
>
> Slot "c":
> x ~ y
>
> How can I get R to do this?
>
>setClass("a",representation(b="list",c="formula"))
> >new("a",b=list(7))
> An object of class "a"
> Slot "b":
> [[1]]
> [1] 7
>
> Slot "c":
> NULL
>
> Thanks,
> Steve
>
> platform i386-pc-mingw32
> arch     i386
> os       mingw32
> system   i386, mingw32
> status
> major    2
> minor    1.1
> year     2005
> month    06
> day      20
> language R
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>

On 18 Jan 2006, slacey at umich.edu wrote:
> But, now suppose you want a slot to accept an object of class
> formula...
>>
setClass("a",representation(b="list",c="formula"))
>> new("a",b=list(7)) Error in validObject(.Object) : invalid
class
>> "a" object: invalid object
> for slot "c" in class "a": got class "NULL",
should be or extend
> class "formula"
>
> Why can't new handle this? Why must the slot be defined? 
Because 'formula' is not a formal S4 class.  new("formula")
doesn't
work.

And because isVirtualClass(getClass("formula")) returns TRUE, which I
find surprising since one can have an instance of a formula.

One workaround is to define a prototype:

setClass("a", representation(b="list",
c="formula"),
         prototype=prototype(c=formula(~1)))

+ seth