I used to worry about the order of evaluation of on.exit expressions and
thought that maybe we needed named on.exit expression so you could
remove particular on.exit expressions. However, now I think I can almost
always
avoid such considerations and make code clearer by making a new function call,
often involving a lazily evaluated expression, whenever I need a new on.exit
expression. E.g., instead of
f0 <- function(x, y) {
oldWarn <- options(warn=0)
on.exit(oldWarn)
oldMar <- par(rep(2,4))
on.exit(par(oldMar))
plot(log(x), y)
}
use
f1 <- function(x, y) {
suppressWarnings(
withPar(list(mar=rep(2,4)),
plot(log(x), y)
)
)
}
suppressWarnings already exists and withPar could be
withPar <- function(parList, expr) {
oldPars <- par(parList)
on.exit(par(oldPars))
expr
}
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
> -----Original Message-----
> From: r-devel-boun...@r-project.org [mailto:r-devel-boun...@r-project.org] On
> Behalf
> Of Henrik Bengtsson
> Sent: Sunday, November 03, 2013 1:42 PM
> To: R-devel
> Subject: [Rd] WISHLIST: on.exit(..., add=TRUE, where="first") to address
> common use
> cases
>
> Before trying to submit a patch(*) to on.exit(), I'd like to check
> whether there is an interest in enhancing on.exit(..., add=TRUE) such
> that it is possible to specify whether the added expression should be
> added before or after already recorded expression. The default is now
> to add it after, but it would often be useful to add it before
> previously recorded expressions.
>
> EXAMPLE:
>
> foo <- function(path="work") {
> # Change working directory. Make sure to reset on exit.
> opwd <- setwd(path)
> on.exit(setwd(opwd))
>
> # Write to a local temporary file. Make sure to remove it on exit.
> cat("Hello", file="local.txt")
> on.exit(file.remove("local.txt"), add=TRUE, where="first")
> }
>
> Without where="first" (i.e. using where="last" as on.exit() does now),
> it all becomes unnecessarily complicated.
>
> Comments?
>
> (*) It seems to come down to adjusting a few lines of code to
> do_onexit() [http://svn.r-project.org/R/trunk/src/main/builtin.c] to
> control whether the expression should be prepended or appended to the
> existing set of recorded expressions.
>
> /Henrik
>
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