Hi Tim,
I have also asked a similar question a couple of months ago, and someone
else did the same recently, maybe on r-devel.
We received no "official" response, but Deepayan Sarkar (R Core Team
member) claimed that:
"
There is no documented reason for this to work (AFAIK), so again, I
would guess this is a side-effect of the implementation, and not a API
feature you should rely on. This is somewhat borne out by the
following:
> foo <- function(...) substitute({...()})
> foo(abc$de, fg[h], i)
{
pairlist(abc$de, fg[h], i)
}
> foo(abc$de, fg[h], , i) # add a missing argument for extra fun
{
as.pairlist(alist(abc$de, fg[h], , i))
}
which is not something you would expect to see at the user level. So
my recommendation: don't use ...() and pretend that you never
discovered it in the first place. Use match.call() instead, as
suggested by Serguei.
[Disclaimer: I have no idea what is actually going on, so these are
just guesses. There are some hints at
https://cran.r-project.org/doc/manuals/r-devel/R-ints.html#Dot_002ddot_002ddot-arguments
if you want to folllow up.]
"
Cheers,
Denes
On 10/6/20 8:38 AM, Tim Taylor wrote:
I probably need to be more specific. What confuses me is not the use
of substitute, but the parenthesis after the dots. It clearly works
and I can make guesses as to why but it is definitely not obvious.
The following function gives the same final result but I can
understand what is happening.
dots <- function (...) {
exprs <- substitute(list(...))
as.list(exprs[-1])
}
In the original, dots <- function(...) as.list(substitute(...())),
Does ...() get parsed in a special way?
Tim
On Tue, 6 Oct 2020 at 05:30, Bert Gunter <[email protected]> wrote:
You need to understand what substitute() does -- see ?substitute and/or a tutorial on
"R computing on the language" or similar.
Here is a simple example that may clarify:
dots <- function(...) as.list(substitute(...()))
dots(log(foo))
[[1]]
log(foo) ## a call, a language object
dots2 <- function(...) as.list(...)
dots2(log(foo))
Error in as.list(...) : object 'foo' not found
## substitute() does not evaluate its argument; as.list() does
Cheers,
Bert Gunter
"The trouble with having an open mind is that people keep coming along and sticking
things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
On Mon, Oct 5, 2020 at 1:37 PM Tim Taylor <[email protected]>
wrote:
Could someone explain what is happening with the ...() of the
following function:
dots <- function(...) as.list(substitute(...()))
I understand what I'm getting as a result but not why. ?dots and
?substitute leave me none the wiser.
regards
Tim
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______________________________________________
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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.
