[Rd] WISHLIST: on.exit(..., add=TRUE, where="first") to address common use cases

[William Dunlap]

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-bounces at r-project.org [mailto:r-devel-bounces at 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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