[R] What if there's nothing to dispatch on?

[Rolf Turner]

On Wed, 1 Sep 2021 05:35:03 -0400
Duncan Murdoch <murdoch.duncan using gmail.com> wrote:

> 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!

Thanks Duncan. I don't understand your warning, but.

If I call foo(y ~ x,data=xxx) I want the generic to dispatch to the
formula method.  That method will then look for y and x first in xxx,
and if it can't find them there it then will look for them in the global
environment.

If I call foo(x,y,data=xxx) I want the generic to dispatch to the
default method, irrespective of whether x exists in the global
environment.  I can't figure out how to arrange this.  As before
(if I could arrange for the dispatch to happen as desired) I would want
the method to look for y and x first in xxx, and if it can't find them
there it then will look for them in the global environment.

It doesn't matter there is an "x" in both xxx and in the global
environment; the methods will/should use the "x" from xxx.

I don't see a problem with respect to this issue.

Whatever.  I can't get your shoot-in-the-foot solution to work anyway.

If I set

    xxx <- data.frame(u=1:10,v=rnorm(10))

and do

    foo(x=u,y=v,data=xxx)

I get

> Error in foo.default(x, y, data) : Cannot find x.

The argument names need to match up.  Note that calling foo.default()
directly works:

    foo.default(x=u,y=v,data=xxx)

runs just fine.

I think I'm going to have to give up on the classes-and-methods
approach.  I *think* I can see a way through with a using a single
function and if-statements based on your "try" idea.

Thanks!!!

cheers,

Rolf

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