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

[Gabor Grothendieck]

On 10/26/06, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> On 10/25/2006 11:02 PM, Gabor Grothendieck wrote:
> > 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.
>
> 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
> 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
>
>


Perfect.  Thanks!!!!