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

[Gabor Grothendieck]

On 10/25/06, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> 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
>

Perhaps R_Visible be made available at the R level in the future.
It would be helpful in situations where you are transforming a
function but want to keep aspects of it such as whether the
return result is invisible.