capture.output(eval(..., envir)) not evaluate in the expected(?) environment

IMHO this has nothing to do with capture.output() per se - it's simply lazy evaluation that gets you. Add force(envir) before capture.output and it works as you expected - the parent.frame() will be different inside capture.output than outside.

Cheers,
Simon


On Nov 23, 2011, at 9:36 PM, Henrik Bengtsson wrote:

I've noticed the following oddity where capture.output() prevents
eval() from evaluating an expression in the specified environment.
I'm not sure if it is an undocumented feature or a bug. ?It caused me
many hours of troubleshooting. ?By posting it here, it might save
someone else from doing the same exercise.

Start by defining foo() which evaluates an expression locally in a
given environment and catches the output via capture.output():

foo <- function(..., envir=parent.frame()) {
?capture.output({
? ?eval(substitute({x <- 1}), envir=envir)
?})
} # foo()

Then call:

suppressWarnings(rm(x)); foo(envir=globalenv()); print(x);

character(0)
[1] 1

This works as expected. ?However, if argument 'envir' is not specified
explicitly, you get:

suppressWarnings(rm(x)); foo(); str(x);

character(0)
Error in str(x) : object 'x' not found

which shows that the internal expression of foo() is *not* evaluated
in the parent.frame(), i.e. the caller of foo(), which here should be
globalenv(). ? It appears that capture.output() prevents this, because
by dropping the latter:

foo <- function(..., envir=parent.frame()) {
?eval(substitute({x <- 1}), envir=envir)
} # foo()

it works:

suppressWarnings(rm(x)); foo(); str(x);

[1] 1

The workaround when still using capture.output() is to force an
explicit evaluation of argument 'envir' inside of foo() before:

foo <- function(..., envir=parent.frame()) {
?stopifnot(is.environment(envir)) ?# Workaround
?capture.output({
? ?eval(substitute({x <- 1}), envir=envir)
?})
} # foo()

which gives:

suppressWarnings(rm(x)); foo(); str(x);

character(0)
num 1

This occurs with R v2.14.0 patched and R devel:

sessionInfo()

R version 2.14.0 Patched (2011-11-20 r57720)
Platform: x86_64-pc-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats ? ? graphics ?grDevices utils ? ? datasets ?methods ? base

sessionInfo()

R Under development (unstable) (2011-11-20 r57720)
Platform: x86_64-pc-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252

attached base packages:
[1] stats ? ? graphics ?grDevices utils ? ? datasets ?methods ? base

/Henrik