R devel - Mar 2024 - capture "->"

On 01/03/2024 4:17 a.m., Dmitri Popavenko wrote:
> Hi everyone,
> 
> I am aware this is a parser issue, but is there any possibility to capture
> the use of the inverse assignment operator into a formula?
> 
> Something like:
> 
>> foo <- function(x) substitute(x)
> 
> gives:
> 
>> foo(A -> B)
> B <- A
> 
> I wonder if there is any possibility whatsoever to signal the use of ->
> instead of <-
If you parse it with srcrefs, you could look at the source.  The parser 
doesn't record whether it was A -> B or B <- A anywhere else.

Duncan Murdoch

Dear Duncan,

On Fri, Mar 1, 2024 at 11:30?AM Duncan Murdoch <murdoch.duncan at
gmail.com>
wrote:
> ...
> If you parse it with srcrefs, you could look at the source.  The parser
> doesn't record whether it was A -> B or B <- A anywhere else.
>
Thank you, this gets me closer but it still needs a little push:
> foo <- function(x) {
  x <- substitute(x)
  return(attr(x, "srcref")[[2]])
}
> foo(A -> B)
NULL

This seems to work, however:
> foo({A -> B})
A -> B

Is there a way to treat the formula as if it was enclosed between the curly
brackets?
Dmitri

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