[Rd] Odd behavior of symbol objects with classes/attributes

[Winston Chang]

Thanks for the feedback, that clears things up.

-Winston

On Mon, Jun 17, 2013 at 1:29 PM,  <luke-tierney at uiowa.edu> wrote:
> On Mon, 17 Jun 2013, Duncan Murdoch wrote:
>
>> On 17/06/2013 1:01 PM, Winston Chang wrote:
>>>
>>> I've been trying to add classes and attributes to symbol objects
>>> (created with quote()), and the behavior is very strange, as
>>> illustrated in the examples below.
>>>
>>> If symbols aren't meant to have classes and attributes attached to
>>> them, then perhaps R should throw errors when you attempt to do it?
>>
>>
>> I think this is a consequence of another strange property of symbol
>> objects, namely that they are not copied on assignment.  This is also true
>> of environments, NULL, builtin function references, and some rarely
>> encountered types like external pointers and weak references.  You're
>> allowed to assign attributes to all of these other than NULL, but you will
>> find strange things happen if you do it (as demonstrated in your code
>> below).
>
>
> You are only because we haven't yet gotten around to preventing it --
> we will at some point. For now, just don't try to assign attributes to
> symbols.
>
> Best,
>
> luke
>
>
>>
>> I thought I remembered reading a list of these in the documentation
>> somewhere, but when I went to look for it just now, I couldn't find it.
>> (Maybe I'm remembering a comment in the source.)  It would be a useful
>> addition.
>>
>> Duncan Murdoch
>>
>>>
>>>
>>> # Using str() strips class from object
>>> x <- quote(foo)
>>> class(x) <- "bar"
>>> str(x)
>>> # Class 'bar'  symbol foo
>>> str(x)
>>> # symbol foo
>>>
>>>
>>> # Attempting to overwrite doesn't affect class.
>>> # str() still strips class from the object.
>>> x <- quote(foo)
>>> class(x) <- "bar"
>>> x <- quote(foo)
>>> str(x)
>>> # Class 'bar'  symbol foo
>>> str(x)
>>> # symbol foo
>>>
>>>
>>> # Changing class of one object affects other
>>> x <- quote(foo)
>>> y <- quote(foo)
>>> class(x) <- "bar"
>>> class(y)
>>> # [1] "bar"
>>> str(y)
>>> # Class 'bar'  symbol foo
>>> str(y)
>>> # symbol foo
>>> str(x)
>>> # symbol foo
>>>
>>>
>>> # Changing attribute of one object affects other
>>> # Unlike with class, str() doesn't cause other attributes to disappear
>>> x <- quote(foo)
>>> y <- quote(foo)
>>> attr(x, "a") <- "bar"
>>> str(y)
>>> # length 1 foo
>>> # - attr(*, "a")= chr "bar"
>>> str(y)
>>> # length 1 foo
>>> # - attr(*, "a")= chr "bar"
>>> str(quote(foo))
>>> # length 1 foo
>>> # - attr(*, "a")= chr "bar"
>>>
>>>
>>> -Winston
>>>
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>
> --
> Luke Tierney
> Chair, Statistics and Actuarial Science
> Ralph E. Wareham Professor of Mathematical Sciences
> University of Iowa                  Phone:             319-335-3386
> Department of Statistics and        Fax:               319-335-3017
>    Actuarial Science
> 241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu