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

[Deepayan Sarkar]

On 10/25/06, Marc Schwartz <MSchwartz at mn.rr.com> wrote:
> 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

That's not going to work, since invisibility can also be a side effect
of assignment, e.g.

g <- function(x) { x <- x }

-Deepayan