[Rd] support of `substitute(...())`

[Dénes Tóth]

On 3/12/20 4:06 PM, William Dunlap wrote:
> Note that substitute(...()) and substitute(someFunc(...))[-1] give 
> slightly different results, the former a pairlist and the latter a call.
>    > str((function(...)substitute(...()))(stop(1),stop(2),stop(3)))
>    Dotted pair list of 3
>     $ : language stop(1)
>     $ : language stop(2)
>     $ : language stop(3)
>    > 
> str((function(...)substitute(someFunc(...))[-1])(stop(1),stop(2),stop(3)))
>     language stop(1)(stop(2), stop(3))

Yes, I am aware of this difference. In my use cases, the ...() form 
gives the result that I prefer (a pairlist).

> 
> The ...() idiom has been around for a long time, but more recently 
> (slightly after R-3.4.0?) the ...elt(n) and ...length() functions were 
> introduced so you don't have to use it much.  

Yes, I know both.

> I don't see a ...names() 
> function that would give the names of the ... arguments - 
> names(substitute(...())).

Exactly, this is a frequent use case. Occasionally I use it in other 
cases as well where I deliberately do not want to evaluate the arguments 
passed as dots.

What I am most interested in is whether this is a 'trick' or a legal use 
of a (rather unadvertised) feature of the language.

> 
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com <http://tibco.com>
> 
> 
> On Thu, Mar 12, 2020 at 2:09 AM Dénes Tóth <toth.denes using kogentum.hu 
> <mailto:toth.denes using kogentum.hu>> wrote:
> 
>     Dear R Core Team,
> 
>     I learnt approx. two years ago in this mailing list that one can use
>     the
>     following "trick" to get a (dotted pair)list of the ellipsis arguments
>     inside a function:
> 
>     `substitute(...())`
> 
>     Now my problem is that I can not find any occurrence of this call
>     within
>     the R source - the most frequent solution there is
>     `substitute(list(...))[-1L] `
> 
>     I would like to know if:
>     1) substitute(...()) is a trick or a feature in the language;
>     2) it will be supported in the future;
>     3) when (in which R version) it was introduced.
> 
>     A hint on where to look for the machinery in the R source would be also
>     appreciated.
> 
>     Regards,
>     Denes
> 
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