formula(model.frame(y~.^2, data=d)) does not return formula from terms attribute of the model.frame

In R 2.12.0 I get

 > d <- data.frame(x=1:10, y=log(1:10), f3=LETTERS[rep(1:3,c(3,3,4))])
 > m <- model.frame(y~.^2, data=d)
 > formula(m)

 y ~ x + f3
In S+ formula(m) gives formula given to model.frame(),
but in R you have to do the following get that formula:

 > formula(attr(m, "terms"))

 y ~ (x + f3)^2

But that has the advantage that you almost certainly have a model 
frame and hence that is what you intend.  With 6-6 (or 20-20 in 
Imperial units) hindsight it would have been better to give model 
frames a class inheriting from "data frame", but it seems that the 
presence of attr(, "terms") is the most common test.

Would it break anything to add to the top of

Unfortunately, that is rather hard to tell!

formula.data.frame
something like
 if (!is.null(tms <- attr(x, "terms"))) {
   return(formula(tms))
 }
so that formula() would retrieve the formula buried
in model.frame's output?

I looked (not hard, but without success) for examples of calling 
formula() on a data frame.  I did see that model.frame.default() calls 
as.formula() on a data frame, but only after checking for the absence 
of a "terms" attribute.

Can you explain where it would help?  I think we need to see examples 
to see if a change in meaning would be clearly beneficial.  I can 
envisage cases in which 'x' was a data frame that just happened to 
have been constructed as a model frame and where the currently 
documented meaning was intended.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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Oxford OX1 3TG, UK                Fax:  +44 1865 272595