R devel - Nov 2019 - calls with comment attribute

In general R doesn't print the "comment" attribute of an object
   > structure(1:3, comment=c("a comment", "another
comment"))
   [1] 1 2 3
but if the object is a call it prints it in an unusual format
   > structure(quote(func(arg)), comment=c("a comment",
"another comment"))
   a comment
   another comment
   func(arg)

What is the rationale for the special treatment of calls?

Bill Dunlap
TIBCO Software
wdunlap tibco.com

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Bill,

Without being involved in that code at all, it seems that this could be
used to (re)create commented source code from R objects. That format seems
to correspond directly to a call in a .R file with two comments above it. A
bit weird there's no comment character there but I guess thats expected to
be in the values of the comment attribute?

Anyway, just a guess.

Best,
~G

On Tue, Nov 12, 2019 at 2:02 PM William Dunlap via R-devel <
r-devel at r-project.org> wrote:
> In general R doesn't print the "comment" attribute of an
object
>    > structure(1:3, comment=c("a comment", "another
comment"))
>    [1] 1 2 3
> but if the object is a call it prints it in an unusual format
>    > structure(quote(func(arg)), comment=c("a comment",
"another comment"))
>    a comment
>    another comment
>    func(arg)
>
> What is the rationale for the special treatment of calls?
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
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On 12/11/2019 5:01 p.m., William Dunlap via R-devel
wrote:
> In general R doesn't print the "comment" attribute of an
object
>     > structure(1:3, comment=c("a comment", "another
comment"))
>     [1] 1 2 3
> but if the object is a call it prints it in an unusual format
>     > structure(quote(func(arg)), comment=c("a comment",
"another comment"))
>     a comment
>     another comment
>     func(arg)
> 
> What is the rationale for the special treatment of calls?
It was there in revision 2 of src/main/deparse.c in 1997.  (For those 
unfamiliar with R history:  the current revision of R is 77405.  That 
particular file has been revised 248 times since rev 2.)

I suspect either nobody has noticed it before, or nobody had the nerve 
to touch it.

Duncan Murdoch

I suspect that the parser used it to store comments, including the initial
"#", before R started using the srcref attribute.  (S also stored
comments
in the parse tree.)

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Tue, Nov 12, 2019 at 4:16 PM Duncan Murdoch <murdoch.duncan at
gmail.com>
wrote:
> On 12/11/2019 5:01 p.m., William Dunlap via R-devel wrote:
> > In general R doesn't print the "comment" attribute of an
object
> >     > structure(1:3, comment=c("a comment", "another
comment"))
> >     [1] 1 2 3
> > but if the object is a call it prints it in an unusual format
> >     > structure(quote(func(arg)), comment=c("a comment",
"another
> comment"))
> >     a comment
> >     another comment
> >     func(arg)
> >
> > What is the rationale for the special treatment of calls?
>
> It was there in revision 2 of src/main/deparse.c in 1997.  (For those
> unfamiliar with R history:  the current revision of R is 77405.  That
> particular file has been revised 248 times since rev 2.)
>
> I suspect either nobody has noticed it before, or nobody had the nerve
> to touch it.
>
> Duncan Murdoch
>
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