What if there's nothing to dispatch on?

Wed, Sep 1, 2021 4:29 PM

On 01/09/2021 6:29 p.m., Rolf Turner wrote:

On Wed, 1 Sep 2021 05:35:03 -0400
Duncan Murdoch <murdoch.duncan at 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.

I don't know the header of your foo() method, but let's suppose foo() is

   foo <- function(x, data, ...) {
     UseMethod("foo")
   }

with

   foo.formula <- function(x, data, ...) {
     # do something with the formula x
   }

   foo.default <- function(x, data, ...) {
     # do the default thing.
   }

Now you have

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

You want this to dispatch to the default method, because u is not a 
formula, it's a column in xxx.  But how do you know that?  Maybe in some 
other part of your code you have

   u <- someresponse ~ somepredictor

So now u *is* a formula, and this will dispatch to the formula method, 
causing havoc.

I think Bill's suggestion doesn't help here.  To do what you want to do 
doesn't really match what S3 is designed to do.

Duncan

What if there's nothing to dispatch on?

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