[R] on.exit() inside local()

[Gabor Grothendieck]

Gabor Grothendieck <ggrothendieck <at> myway.com> writes:

: 
: Vadim Ogranovich <vograno <at> evafunds.com> writes:
: 
: : 
: : Hi,
: : 
: : Since I routinely open files in a loop I've developed a habit of using
: : on.exit() to close them. Since on.exit() needs to be called within a
: : function I use eval() as a surrogate. For example:
: : 
: : for (fileName in c("a", "b")) eval({
: : 	con <- file(fileName);
: : 	on.exit(close(con))
: : 	}) 
: : 
: : and con will be closed no matter what.
: : 
: : However it stopped working once I wrapped the loop in local():
: : > local(
: : +       for (foo in seq(2)) eval({
: : +         on.exit(cat(foo, "\n"))
: : +       })
: : + )
: : Error in cat(foo, "\n") : Object "foo" not found
: : 
: : 
: : W/o local()it works just fine
: : >       for (foo in seq(2)) eval({
: : +         on.exit(cat(foo, "\n"))
: : +       })
: : 1 
: : 2 
: : 
: : The reason I wanted the local() is to keep 'foo' from interfering with
: : the existing environments, but somehow this breaks the thing.
: : At this point I am stuck. Could someone please tell what's going on?
: 
: The on.exit code is executing in an environment whose parent is
: namespace:base and so cannot access the environment created by
: local.  Use evalq, instead of eval, which has the effect of
: running the on.exit code in the environment created by
: local:
: 
: local({
:    for (i in c("a", "b")) evalq(
:          on.exit(cat(i, "\n"))
:       )
: })
: 
: or use an inner local, which has the effect of creating
: a new environment for each iteration of the loop in which
: the on.exit code runs:
: 
: local({
:    for (i in c("a", "b")) local(
:       on.exit(cat(i, "\n"))
:    )
: })


Just one additional thought.  You could use sapply in which case the
i gets hidden in the function anyways:

   junk <- sapply( c("a", "b"), function(i) 
                        on.exit(cat(i, "\n")) 
           )

or maybe:

   invisible(
      sapply( c("a", "b"), function(i) 
                   on.exit(cat(i, "\n")) 
      )
   )