R help - Sep 2021 - What if there's nothing to dispatch on?

I'm trying to build a pair of (S3) methods, a "formula" method and
a
"default" method.  The methods have a "data" argument.  If
the variables
in question cannot be found in "data" then they should be sought in
the global environment.

My problem is that the generic dispatches on its first argument, which
may be a formula (in which case it of course dispatches to the formula
method) or the first of the variables.  If this variable exists in
the global environment then all is well.  But if it doesn't exist there,
then the generic falls over with an error of the form "object 'x'
not
found" --- because there isn't anything to dispatch on.

I'd *like* to be able to tell the generic that if "x" is not found
then
it should dispatch to the default method (which will, if the call is
sensible, find "x" in "data").

Is there any way to tell the generic to do this?

Or is there any other way out of this dilemma? (Other than "Give up and
go to the pub", which I cannot currently do since Auckland is in Level 4
lockdown. :-) )

Thanks for any enlightenment.

cheers,

Rolf Turner

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

On 31/08/2021 11:59 p.m., Rolf Turner wrote:
> 
> I'm trying to build a pair of (S3) methods, a "formula"
method and a
> "default" method.  The methods have a "data" argument. 
If the variables
> in question cannot be found in "data" then they should be sought
in
> the global environment.
> 
> My problem is that the generic dispatches on its first argument, which
> may be a formula (in which case it of course dispatches to the formula
> method) or the first of the variables.  If this variable exists in
> the global environment then all is well.  But if it doesn't exist
there,
> then the generic falls over with an error of the form "object
'x' not
> found" --- because there isn't anything to dispatch on.
> 
> I'd *like* to be able to tell the generic that if "x" is not
found then
> it should dispatch to the default method (which will, if the call is
> sensible, find "x" in "data").
> 
> Is there any way to tell the generic to do this?
> 
> Or is there any other way out of this dilemma? (Other than "Give up
and
> go to the pub", which I cannot currently do since Auckland is in Level
4
> lockdown. :-) )
> 
That design is probably not a good idea:  what if one of the variables 
in data matches the name of some other object in the global environment? 
  Then it would dispatch on that other object, and things won't go well.

But here's a way to shoot yourself in the foot:

function(x) {
   x1 <- try(x, silent = TRUE)
   if (inherits(x1, "try-error"))
     foo.default(x)
   else
     UseMethod("foo", x)
}

Happy shooting!

Duncan