[Rd] R-2.0.0 findVar and findFun question

[Prof Brian Ripley]

On Fri, 8 Oct 2004, Matjaz Kukar wrote:

> Dear all,
> 
> when working on a project with embedded R, I found out that R-2.0.0 causes
> a problem where older versions worked OK.
> 
> Rf_findVar(...) causes the following error when used to find a generic function
> (such as print):
> 
> fun = Rf_findVar(...);
> R_tryEval(fun, ...);

I think without knowing what you are putting in '...' this is impossible
to understand.  R_tryEval() does not seem to be part of the public API and
I had to grep the sources to find it.  However, it seems to be intended to
evaluate an expression not a function, as in (tests/Embedding/tryEval.c)

    PROTECT(e = allocVector(LANGSXP, 2));
    SETCAR(e, Rf_install("sqrt"));
    SETCAR(CDR(e), NEW_CHARACTER(1));
    val = R_tryEval(e, NULL, &errorOccurred);

and it is possible that 2.0.0 is detecting an incorrect usage that 1.9.1
did not detect.

Incidentally, your code need not be littered with Rf_*; that indicates 
you are not including the right header files.

> Error in function (object, ...)  : Invalid generic function in usemethod
> 
> Alternatively, using Rf_findFun(...) seems OK in this context, but crashes if you
> call it with an unexisting function name. Rf_findVar does not crash in this case,
> but returns an unbound value.
> 
> My workaround for now is as follows:
> 
>     fun = Rf_findVar(Rf_install(ident), R_GlobalEnv);
>     if (fun != R_UnboundValue) {
>         fun = Rf_findFun(Rf_install(ident), R_GlobalEnv);
>         R_tryEval(fun, ...);
>     }
> 
> I did some checking on the R object pointers and indeed for generic functions
> Rf_findFun and Rf_findVar return different pointers (and therefore different
> function objects).
> 
> What has changed with these functions? Can you comment on my workaround
> and suggest a better (proper?) way of doing this?
> 
> Platform: Windows XP, R-2.0.0, MS VSC++ 7.0
> 
> Matjaz.
> 
> 

-- 
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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