Hi, On Thursday 20 September 2007, Gabor Grothendieck wrote: > 1. Is there some way to copy a promise so that the copy has the same > expression in its promise as the original.
As far as I am aware (which is not very far at all), there is no way to do
this in R code. However, it's fairly simple to achieve this in C. For
example, the following C function is basically the same as
# assign (name, get (name, envir=fromenv), envir=toenv)
except that it does not force any promises:
SEXP doCopyNoEval (SEXP name, SEXP fromenv, SEXP toenv) {
if(!isString (name) || length (name) != 1) error ("name is not a single
string");
if(!isEnvironment (fromenv)) error ("fromenv is not an environment");
if(!isEnvironment (toenv)) error ("toenv is not an environment");
defineVar (Rf_install (CHAR (STRING_ELT (name, 0))), findVar
(Rf_install
(CHAR (STRING_ELT (name, 0))), fromenv), toenv);
return (R_NilValue);
}
> 2. Is there some way to determine if a variable holds a promise
> without evaluating it?
Similarly, in C, determining whether an object is a promise is as easy as
# if (TYPEOF (object) == PROMSXP) {}
We use both in RKWard to peek at objects while making sure not to force any
promises.
Regards
Thomas
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