how to determine if a function's result is invisible

On 10/26/06, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:

[...]

Actually, there is a way, but it's undocumented (i.e., use at your own
risk).  It's the eval.with.vis function.  This is an internal function

Yes... and there are three problems here:
1) To spot the undocumented function one is looking for,
2) To figure out how to use it,
3) To rewrite your code regularly if you maintain packages that use 
several of such undocumented functions. This is the case of many R GUI 
projects... and one reason why staying up-to-date with the new versions 
of R (every 6 months) is a nightmare for these GUIs!

For instance, I use eval.with.vis() in the latest version of svSockets 
package in the SciViews bundle, but I am afraid to release it on CRAN 
because I know of the nightware I will face if this function (silently) 
changes its behavior in subsequent versions of R.

I guess there is no solution to this problem, since there is certainly a 
good reason for keeping portions of R code undocumented (and thus 
flagged as :" use-it-at-your-own-risk!"), but it does not eases our life!

Best,

Philippe Grosjean

that is used within source() and capture.output(); you'll have to guess
from the usage there what the args are.   But here's an F that does
something close to what you want:

 > fix(F)
 > f <- function() 1
 > g <- function() invisible(1)
 >
 > F <- function (expr)

+ {
+     expr <- substitute(expr)
+     pf <- parent.frame()
+     tmp <- .Internal(eval.with.vis(expr, pf,
+         baseenv()))
+     tmp
+ }

 > F(f())

$value
[1] 1

$visible
[1] TRUE

 > F(g())

$value
[1] 1

$visible
[1] FALSE