[Rd] capture "->"

[@vi@e@gross m@iii@g oii gm@ii@com]

Adrian,
 
That is indeed a specialized need albeit not necessarily one that cannot be done by requiring an alternate way of typing a formula that avoids being something the parser sees as needed to do at that level.
 
In this case, my other questions become moot as I assume the global assignment operator and somethings like assign(“xyz”, 5) will not be in the way.
 
What I was wondering about is what happens if you temporarily disable the meaning of the assignment operator <- and turn it back on after.
 
In the following code, for no reason, I redefine + to mean – and then undo it:
 
 
> temp <- `+`
> `+` <- `-`
> 5 + 3
[1] 2
> `+` <- temp
> 5 + 3
[1] 8
 
I have no idea if a similar technique could save and later restore the meaning of <- and replace it with something appropriate so that an expression using it is not evaluated the same way and leaves it alone long enough …
 
Of course, even if this works, it could cause side effects if anything else is done between changes that invokes it (maybe not likely) or the change back is not done perhaps due to an error interruption.
 
 
Avi
 
From: Adrian Dușa <dusa.adrian using gmail.com> 
Sent: Friday, March 1, 2024 11:38 AM
To: avi.e.gross using gmail.com
Cc: r-devel <r-devel using r-project.org>; Dmitri Popavenko <dmitri.popavenko using gmail.com>
Subject: Re: [Rd] capture "->"
 
I would also be interested in that.
For me, this is interesting for my QCA package, over which Dmitri and I have exchanged a couple of messages.
The "<-" operator is used to denote necessity, and the "->" is used for sufficiency.
 
Users often make use of Boolean expressions such as A*B + C -> Y
(to calculate if the expression A*B + C is sufficient for the outcome Y)
 
The parser inverses it into Y <- A*B + C, as if the outcome Y is necessary for the expression A*B + C, which changes the nature of the expression.
 
Quoting such expressions is already possible and it works as expected. We were trying to avoid the quotes, if at all possible, to simplify the command use in the manuals.
 
Best wishes,
Adrian
 
On Fri, Mar 1, 2024 at 4:33 PM <avi.e.gross using gmail.com <mailto:avi.e.gross using gmail.com> > wrote:
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 using r-project.org <mailto:r-devel-bounces using r-project.org> > On Behalf Of Duncan Murdoch
Sent: Friday, March 1, 2024 9:23 AM
To: Dmitri Popavenko <dmitri.popavenko using gmail.com <mailto:dmitri.popavenko using gmail.com> >
Cc: r-devel <r-devel using r-project.org <mailto:r-devel using 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 using gmail.com <mailto:murdoch.duncan using gmail.com>  
> <mailto: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

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