extra arguments to generic functions & bug in model.frame

[Jim Robison-Cox]

R developers,

2 things:  a bug in model.frame and a question about setup of generic 
functions. 
 
   I don't understand the following behavior for generic functions:
Suppose I'm working with the cats data in the MASS library and I want to 
create a formula object to model Hwt on Sex:
  # This works:
> formula(Hwt ~ Sex)
Hwt ~ Sex
  # But the following does not:
> formula(Hwt ~ Sex, data = cats)
Error in formula.formula(Hwt ~ Sex, cats) : unused argument to function
  # because I used an extra argument.

The odd thing to me, is that formula() itself allows the argument:
> formula
function (x, ...) 
UseMethod("formula")

But formula.default does not:

> args(formula.default)
function (x) 

The same thing happens with terms:
> args(terms)
function (x, ...) 
> args(terms.default)
function (x) 

And this caused me a problem when using model.frame() since 
model.frame.default includes the lines:

	    if (!inherits(formula, "terms")) 
	    	    formula <- terms(formula, data = data)

Using 2 arguments in term() is a bug since hitting the line 
will cause an error:

Error in terms.default(formula, data = data) : unused argument to function

  Summary:

1.  There seems to be a bug in model.frame() using terms() with 2 arguments.

2.  What is standard practice for setting up instances of a generic function?
    It seems sensible to me that argument lists should not shrink when 
    going from the generic function to an instance of that function.

Cheers,
Jim

Jim Robison-Cox                 ____________            
Department of Math Sciences    |            |           phone: (406)994-5340
2-214 Wilson Hall               \   BZN, MT |           FAX:   (406)994-1789
Montana State University         |  *_______|
Bozeman, MT 59717                 \_|         e-mail: jimrc@math.montana.edu 


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