G'day Brian,
>>>>> "BDR" == Prof Brian Ripley <[EMAIL PROTECTED]> writes:
>> I found in the R language definition manual the passage that
>> discourages users of assigning objects within function calls
>> since it is not guaranteed that the assignment is ever made
>> because of R's lazy evaluation model. But this does not seem
>> to explain the above behaviour since the argument to print is
>> evaluated. If I replace sys.on.exit() with, say, ls() in both
>> functions, then they produce the same output (and the output
>> that I expect). Why does f2() not work with sys.on.exit()?
BDR> It does work, but you seems to have misunderstood what it
BDR> does. See e.g. the `bug' report discussed at
BDR> http://tolstoy.newcastle.edu.au/~rking/R/devel/05/02/2112.html
Mmh, mea culpa, I am a bit red-faced here, in my quest to find out
what was going on, I checked about everything (including a private
e-mail to an R-core member) but the bug repository and the help page
of sys.on.exit(). I guess that if I have read the latter, I would
have figured out what was going on. Instead I was satisfied with what
the on.exit() help page stated in its "see also" section:
\seealso{
\code{\link{sys.on.exit}} to see the current expression.
}
May I request that this help page is changed according to the patch
attached below to clarify on the on.exit() help page what it is that
sys.on.exit() is returning?
BDR> You might find
BDR> g <- function(test) {}
BDR> f2 <- function(){
BDR> par.def <- par(no.readonly=TRUE)
BDR> on.exit(par(par.def))
BDR> g(tt <- sys.on.exit())
BDR> str(tt)
BDR> invisible()
BDR> }
BDR> f2()
BDR> illuminating.
Mmh, in this code the function g is not evaluating its argument. So I
would expect an error message in f2() that tt is not defined no matter
which function (sys.on.exit, ls, ...) I am using in the call to g()
within f(). Exactly the situation about which the R language
definition is warning, so this seems to be a different trap than the
one I stepped into....
Cheers,
Berwin
Index: src/library/base/man/on.exit.Rd
===================================================================
--- src/library/base/man/on.exit.Rd (revision 36611)
+++ src/library/base/man/on.exit.Rd (working copy)
@@ -19,7 +19,8 @@
\arguments{
\item{expr}{an expression to be executed.}
\item{add}{if TRUE, add \code{expr} to be executed after any previously
- set expressions.}
+ set expressions; otherwise (the default) \code{expr} will overwrite
+ any previously set expressions.}
}
\references{
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
@@ -27,7 +28,9 @@
Wadsworth \& Brooks/Cole.
}
\seealso{
- \code{\link{sys.on.exit}} to see the current expression.
+ \code{\link{sys.on.exit}} which returns the expression stored for use
+ by \code{on.exit()} in the function in which \code{sys.on.exit()} is
+ evaluated.
}
\examples{
opar <- par(mai = c(1,1,1,1))
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