[R] creating objects with a slot of class formula, using new

[Seth Falcon]

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