[Rd] how to determine if a function's result is invisible

[Duncan Murdoch]

On 10/25/2006 8:14 PM, Gabor Grothendieck wrote:
> Suppose we have a function such as the following
> 
> F <- function(f, x) f(x)+1
> 
> which runs function f and then transforms it.  I would like the
> corresponding function which works the same except that
> unlike F returns an invisible result if and only if f does.
> 
> Is there some way of determining whether f returns
> an invisible result or not?
> 
> Thus we want this:
> 
> f <- function(x) x
> g <- function(x) invisible(x)
> 
>> F(f, 1)
> 2
> 
>> F(g, 1)

I don't think there's a way to do that.  Internally there's a global 
flag called R_Visible; if it is set to zero, the value won't print.  But 
  it gets reset to 1 very easily (e.g. by adding 1 to the result of an 
invisible function), and it's not available in the API for you to write 
C code to look at it.

I think you'll just have to do require the user of your F to tell you 
that they want the result to be invisible.

Duncan Murdoch