R devel - Sep 2007 - copying promise

1. Is there some way to copy a promise so that the copy has the same
expression in its promise as the original.  In the following we
y is a promise that we want to copy to z.  We
want z to be a promise based on the expression x since y is a
promise based on the expression x.  Thus the answer to the code
below is desired to be z=2 but its 1, 1 and y in the next three
examples so they are not the answer. See examples at end.

2. Is there some way to determine if a variable holds a promise
without evaluating it?

This code relates to question 1.

# example 1
x <- 0
delayedAssign("y", x)
x <- 1
# this forces y which is not what we want
z <- y
x <- 2
z # 1

# example 2
# this connects to z to x via y which is not what we want
#  since if y is forced then z takes its value from that, not x
x <- 0
delayedAssign("y", x)
delayedAssign("z", y)
x <- 1
y
x <- 2
z # 1

# example 3
# this attempts to assign the expression underlying promise y to z
# which seems closest in spirit to what we want
# but it does not work as intended
x <- 0
delayedAssign("y", x)
delayedAssign("z", substitute(y))
x <- 1
y
x <- 2
z # y
> R.version.string # Vista
[1] "R version 2.6.0 alpha (2007-09-06 r42791)"

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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