support of `substitute(...())`

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support of `substitute(...())`

Dénes Tóth-2
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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Re: support of `substitute(...())`

R devel mailing list
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))

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

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Thu, Mar 12, 2020 at 2:09 AM Dénes Tóth <[hidden email]> 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
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

        [[alternative HTML version deleted]]

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Re: support of `substitute(...())`

Dénes Tóth-2

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 <[hidden email]
> <mailto:[hidden email]>> 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
>
>     ______________________________________________
>     [hidden email] <mailto:[hidden email]> mailing list
>     https://stat.ethz.ch/mailman/listinfo/r-devel
>

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