What if there's nothing to dispatch on?
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