R/S compatibility in passing a formula

[Prof Brian D Ripley]

On 11 Apr 1999, Peter Dalgaard BSA wrote:

> Douglas Bates <bates@stat.wisc.edu> writes:
> 
> > In S3 if you are passing an evaluated formula to another function that
> > will store it and use it in printing (such as a model-fitting function), you
> > usually want to unclass the formula.  Otherwise, the formula that gets
> > stored looks very ugly when printed.  A trick to do the unclassing is to pass
> >  form = c( myFormula )
> > In R this has the effect of making form a list rather than a call.  It
> > seems that passing
> >  form = as.vector( myFormula )
> > will work in both dialects.  Anyone else have experience with this?
> 
> Apparently
> 
> (a) R does not print the class on a formula (because print.formula
> unclasses it)
> (b) as.vector does not unclass in R but does in S3. Try
> class(as.vector(form)) 
> 
> So the result of as.vector *looks* alike, but isn't. Smells like
> trouble... 
> 
> Whatever is wrong with using unclass() for unclassing anyway??

Because it is oldUnclass in recent S's, and having to use code translators
for S3/R/S-PLUS 5.x is a pain!   Nevertheless, that is what I do: write
oldUnclass() and translate.  (The reverse does not work: in S-PLUS 5 one
sometimes needs unclass()).

(I am pickily using S-PLUS 5 not S4: there is no public version of S4, but
S-PLUS 5 is quite different from S4 as documented in Chambers, 1998.)

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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