capture "->"

I am wondering what the specific need for this is or is it just an
exercise?

Where does it matter if a chunk of code assigns using "<-" beforehand or
"->" after hand, or for that matter assigns indirectly without a symbol?

And whatever you come up with, will it also support the global assignment
of "->>" as compared to ""<<-" too?

I do wonder if you can re-declare the assignment operators or would that
mess up the parser.

-----Original Message-----
From: R-devel <r-devel-bounces at r-project.org> On Behalf Of Duncan Murdoch
Sent: Friday, March 1, 2024 9:23 AM
To: Dmitri Popavenko <dmitri.popavenko at gmail.com>
Cc: r-devel <r-devel at r-project.org>
Subject: Re: [Rd] capture "->"

On 01/03/2024 8:51 a.m., Dmitri Popavenko wrote:

On Fri, Mar 1, 2024 at 1:00?PM Duncan Murdoch <murdoch.duncan at gmail.com
<mailto:murdoch.duncan at 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