R devel - Oct 2006 - how to determine if a function's result is invisible

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)
>

On Wed, 2006-10-25 at 20:14 -0400, 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)
> >
Gabor,

There may be a better way of doing this and/or this will spark some
thoughts.

Let's create two simple functions:

 f.inv <- function(x) {invisible(x)}

 f <- function(x) {x}

So we now have:
> f.inv(1)
> f(1)
[1] 1

> any(grep("invisible", (deparse(f))))
[1] FALSE
> any(grep("invisible", (deparse(f.inv))))
[1] TRUE


This is not extensively tested of course, but another function that
comes to mind that does return a result 'invisibly' is:
> any(grep("invisible", (deparse(barplot.default))))
[1] TRUE

So there seems to be some foundation for working, as long as the target
function can be deparsed, which may limit things with respect to
C/FORTRAN based functions.

HTH,

Marc Schwartz

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