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

[John Fox]

Dear Duncan,

Thanks for this -- it'll enable me to get rid of a list of functions
returning invisible output that the Rcmdr maintains.

Regards,
 John

--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 

> -----Original Message-----
> From: r-devel-bounces at r-project.org 
> [mailto:r-devel-bounces at r-project.org] On Behalf Of Duncan Murdoch
> Sent: Saturday, October 28, 2006 12:13 PM
> To: Gabor Grothendieck
> Cc: Luke Tierney; Martin Maechler; R Development Mailing List
> Subject: Re: [Rd] how to determine if a function's result is invisible
> 
> On 10/26/2006 5:26 AM, Gabor Grothendieck wrote:
> > 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!!!!
> 
> I've just added this function to R-devel (to become 2.5.0 
> next spring):
> 
> withVisible <- function(x) {
>      x <- substitute(x)
>      v <- .Internal(eval.with.vis(x, parent.frame(), baseenv()))
>      v
> }
> 
> Luke Tierney suggested simplifying the interface (no need to 
> duplicate the 3 parameter eval interface, you can just wrap 
> this in evalq() if you need that flexibility); the name 
> "with.vis" was suggested, but it looks like an S3 method for 
> the with() generic, so I renamed it.
> 
> Duncan Murdoch
> 
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