[Rd] capture "->"

[Duncan Murdoch]

On 01/03/2024 8:51 a.m., Dmitri Popavenko wrote:
> On Fri, Mar 1, 2024 at 1:00 PM Duncan Murdoch <murdoch.duncan using gmail.com 
> <mailto:murdoch.duncan using gmail.com>> wrote:
> 
>     ...
>     I was thinking more of you doing something like
> 
>        parse(text = "A -> B", keep.source = TRUE)
> 
>     I forget what the exact rules are for attaching srcrefs to arguments of
>     functions, but I do remember they are a little strange, because not
>     every possible argument can accept a srcref attribute.  For example,
>     you
>     can't attach one to NULL, or to a name.
> 
>     Srcrefs are also fairly big and building them is slow, so I think we
>     tried to limit them to where they were needed, we didn't try to attach
>     them to every subexpression, just one per statement.  Each expression
>     within {} is a separate statement, so we get srcrefs attached to the {.
>     But in "foo(A -> B)" probably you only get one on the foo call.
> 
>     In some circumstances you could get the srcref on that call by looking
>     at sys.call().  But then things are complicated again, because R
>     doesn't
>     attach srcrefs to things typed at the console, only to things that are
>     sourced from files or text strings (and parsed with keep.source=TRUE).
> 
>     So I think you should probably require input from a string or a
>     file, or
>     not expect foo(A -> B) to work without some decoration.
> 
> 
> Indeed, the more challenging task is to identify "->" at the console
> (from a script or a string, seems trivial now).
> 
> I would be willing to decorate as much as it takes to make this work, I 
> am just empty on more ideas how to persuade the parser.

By "decorate", I meant putting it in quotes and parsing it using 
parse(text=...), or putting it in braces as you found.  I think parsing 
a string is most likely to be reliable because someone might turn off 
`keep.source` and then the braced approach would fail.  But you have 
control over it when you call parse() yourself.

Duncan Murdoch