R devel - Aug 2018 - substitute() on arguments in ellipsis ("dot dot dot")?

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?

Thanks,

Henrik

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

On Sun, Aug 12, 2018 at 10:00 PM, Henrik Bengtsson
<henrik.bengtsson at gmail.com> 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?
You could use match.call, for example:

  dots <- function(...) match.call(expand.dots = FALSE)[['...']]

Note that this returns a pairlist, so if you want an ordinary list you
should wrap it in as.list()

Interestingly,

   as.list(substitute(...()))

also works.

On Sun, Aug 12, 2018 at 1:16 PM, Duncan Murdoch
<murdoch.duncan at 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
>
>
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