[R] turning R expressions into functions?

[William Dunlap]

I tend not to use match.call for this because it feels like I'm repeating
work (argument matching) that has already been done.  Also, match.call's
output needs to be processed a bit to get the expressions.

The following 2 functions give the same results, a "pairlist" of the unevaluated
arguments matching the "..." in the function definition.
   f1 <- function(x, ..., atEnd) substitute(...())
   f2 <- function(x, ..., atEnd) match.call(expand.dots=FALSE)$...
E.g.,
  > str(f1(1, stop("don't evaluate me!"), Log=log(-10:1), atEnd=Inf))
  Dotted pair list of 2
   $    : language stop("don't evaluate me!")
   $ Log: language log(-10:1)
  > str(f2(1, stop("don't evaluate me!"), Log=log(-10:1), atEnd=Inf))
  Dotted pair list of 2
   $    : language stop("don't evaluate me!")
   $ Log: language log(-10:1)
The former appears to be about 3 times as fast as the latter, but you need
to run it a lot of times (>10^4) to see the difference.  There may be a bigger
speedup if there are lots of non-"..." arguments, but I haven't tested that.
  
I also use the following, which works in both S+ and R:
   > f3 <- function(x, ..., atEnd) as.list(substitute(junk(...)))[-1] 
   > str(f3(1, stop("don't evaluate me!"), Log=log(-10:1), atEnd=Inf))
   List of 2
    $    : language stop("don't evaluate me!")
    $ Log: language log(-10:1)

It is probably best to bury this in a utility function with an intuitive name instead
of trying to remember the idioms.  Perhaps there already is one.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: Bert Gunter [mailto:gunter.berton at gene.com]
> Sent: Monday, July 23, 2012 2:45 PM
> To: William Dunlap
> Cc: S Ellison; Jochen Voß; r-help at r-project.org
> Subject: Re: [R] turning R expressions into functions?
> 
> Bill:
> 
> Is there some reason to prefer your "odd idiom" to match.call, perhaps
> as as.list(match.call()), as proposed by Hadley?
> 
> -- Bert
> 
> On Mon, Jul 23, 2012 at 2:25 PM, William Dunlap <wdunlap at tibco.com> wrote:
> > list(...) evaluates the things in ...
> > E.g.,
> >    > f0 <- function(x, ...) list(...)
> >    > f0(1, warning("Hmm"), stop("Oops"), cat("some output\n"))[[2]]
> >    Error in f0(1, warning("Hmm"), stop("Oops"), cat("some output\n")) : Oops
> >    In addition: Warning message:
> >    In f0(1, warning("Hmm"), stop("Oops"), cat("some output\n")) : Hmm
> >
> > You can use the odd idiom substitute(...()) to get the unevaluated ... arguments:
> >    > f1 <- function(x, ...) substitute(...())
> >    > f1(1, warning("Hmm"), stop("Oops"), cat("some output\n"))
> >    [[1]]
> >    warning("Hmm")
> >
> >    [[2]]
> >    stop("Oops")
> >
> >    [[3]]
> >    cat("some output\n")
> >
> >
> > Bill Dunlap
> > Spotfire, TIBCO Software
> > wdunlap tibco.com
> >
> >
> >> -----Original Message-----
> >> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
> >> Behalf Of S Ellison
> >> Sent: Monday, July 23, 2012 2:12 PM
> >> To: Jochen Voß
> >> Cc: r-help at r-project.org
> >> Subject: Re: [R] turning R expressions into functions?
> >>
> >> >> One of the things I would love to add to my package would be the
> >> >> ability to compare more than two expressions in one call.  But
> >> >> unfortunately, I haven't found out so far whether (and if so, how) it
> >> >> is possible to extract the elements of a "..." object without
> >> >> evaluating them.
> >> >
> >> >Have a look at match.call.
> >>
> >> ... or use
> >> dotlist <- list(...)
> >>
> >> to get a list of everything included in ...
> >>
> >> S Ellison
> >>
> >>
> *****************************************************************
> >> **
> >> This email and any attachments are confidential. Any use...{{dropped:8}}
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list
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> >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> 
> 
> 
> --
> 
> Bert Gunter
> Genentech Nonclinical Biostatistics
> 
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