[Rd] capture "->"

[Dmitri Popavenko]

On Fri, Mar 1, 2024 at 1:00 PM Duncan Murdoch <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.
Dmitri

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