[Rd] substitute() on arguments in ellipsis ("dot dot dot")?

Hadley Wickham h@wickh@m @ending from gm@il@com
Tue Aug 14 01:58:41 CEST 2018

```Since you're already using bang-bang ;)

library(rlang)

dots1 <- function(...) as.list(substitute(list(...)))[-1L]
dots2 <- function(...) as.list(substitute(...()))
dots3 <- function(...) match.call(expand.dots = FALSE)[["..."]]
dots4 <- function(...) exprs(...)

bench::mark(
dots1(1+2, "a", rnorm(3), stop("bang!")),
dots2(1+2, "a", rnorm(3), stop("bang!")),
dots3(1+2, "a", rnorm(3), stop("bang!")),
dots4(1+2, "a", rnorm(3), stop("bang!")),
check = FALSE
)[1:4]
#> # A tibble: 4 x 4
#>   expression                                          min     mean  median
#>   <chr>                                          <bch:tm> <bch:tm> <bch:t>
#> 1 "dots1(1 + 2, \"a\", rnorm(3), stop(\"bang!\"…   3.23µs   4.15µs  3.81µs
#> 2 "dots2(1 + 2, \"a\", rnorm(3), stop(\"bang!\"…   2.72µs   4.48µs  3.37µs
#> 3 "dots3(1 + 2, \"a\", rnorm(3), stop(\"bang!\"…   4.06µs   4.94µs  4.69µs
#> 4 "dots4(1 + 2, \"a\", rnorm(3), stop(\"bang!\"…   3.92µs    4.9µs  4.46µs

On Mon, Aug 13, 2018 at 4:19 AM Henrik Bengtsson
<henrik.bengtsson using gmail.com> wrote:
>
> Thanks all, this was very helpful.  Peter's finding - dots2() below -
> is indeed interesting - I'd be curious to learn what goes on there.
>
> The different alternatives perform approximately the same;
>
> dots1 <- function(...) as.list(substitute(list(...)))[-1L]
> dots2 <- function(...) as.list(substitute(...()))
> dots3 <- function(...) match.call(expand.dots = FALSE)[["..."]]
>
> stats <- microbenchmark::microbenchmark(
>   dots1(1+2, "a", rnorm(3), stop("bang!")),
>   dots2(1+2, "a", rnorm(3), stop("bang!")),
>   dots3(1+2, "a", rnorm(3), stop("bang!")),
>   times = 10e3
> )
> print(stats)
> # Unit: microseconds
> #                                        expr  min   lq mean median
> uq  max neval
> #  dots1(1 + 2, "a", rnorm(3), stop("bang!")) 2.14 2.45 3.04   2.58
> 2.73 1110 10000
> #  dots2(1 + 2, "a", rnorm(3), stop("bang!")) 1.81 2.10 2.47   2.21
> 2.34 1626 10000
> #  dots3(1 + 2, "a", rnorm(3), stop("bang!")) 2.59 2.98 3.36   3.15
> 3.31 1037 10000
>
> /Henrik
>
> On Mon, Aug 13, 2018 at 7:10 AM Peter Meilstrup
> <peter.meilstrup using gmail.com> wrote:
> >
> > Interestingly,
> >
> >    as.list(substitute(...()))
> >
> > also works.
> >
> > On Sun, Aug 12, 2018 at 1:16 PM, Duncan Murdoch
> > <murdoch.duncan using gmail.com> wrote:
> > > On 12/08/2018 4:00 PM, Henrik Bengtsson wrote:
> > >>
> > >> Hi. For any number of *known* arguments, we can do:
> > >>
> > >> one <- function(a) list(a = substitute(a))
> > >> two <- function(a, b) list(a = substitute(a), b = substitute(b))
> > >>
> > >> and so on. But how do I achieve the same when I have:
> > >>
> > >> dots <- function(...) list(???)
> > >>
> > >> I want to implement this such that I can do:
> > >>
> > >>> exprs <- dots(1+2)
> > >>> str(exprs)
> > >>
> > >> List of 1
> > >>   \$ : language 1 + 2
> > >>
> > >> as well as:
> > >>
> > >>> exprs <- dots(1+2, "a", rnorm(3))
> > >>> str(exprs)
> > >>
> > >> List of 3
> > >>   \$ : language 1 + 2
> > >>   \$ : chr "a"
> > >>   \$ : language rnorm(3)
> > >>
> > >> Is this possible to achieve using plain R code?
> > >
> > >
> > > I think so.  substitute(list(...)) gives you a single expression containing
> > > a call to list() with the unevaluated arguments; you can convert that to
> > > what you want using something like
> > >
> > > dots <- function (...) {
> > >   exprs <- substitute(list(...))
> > >   as.list(exprs[-1])
> > > }
> > >
> > > Duncan Murdoch
> > >
> > >
> > > ______________________________________________
> > > R-devel using r-project.org mailing list
> > > https://stat.ethz.ch/mailman/listinfo/r-devel
>
> ______________________________________________
> R-devel using r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

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