[Rd] Bug in new() or validObject() in methods package (PR#7922)

[McGehee, Robert]

The bug might be here:
> is.null(expression())
[1] TRUE

But
> is.null(expression(NULL))
[1] FALSE

So it might look to the methods package like you're passing in a NULL
value for @bar. I might argue that expression() should not be NULL (and
only NULL is NULL) as I have had similar trouble trying to extend
language objects to S4 classes, partially because length 0 name, call
and expression objects are either not available, or in this case,
handled poorly.

Robert

-----Original Message-----
From: Duncan Murdoch [mailto:murdoch at stats.uwo.ca] 
Sent: Monday, June 06, 2005 4:58 PM
To: Peter Dalgaard
Cc: r-devel at stat.math.ethz.ch
Subject: Re: [Rd] Bug in new() or validObject() in methods package
(PR#7922)


On 6/6/2005 4:55 PM, Peter Dalgaard wrote:
> murdoch at stats.uwo.ca writes:
> 
>> Is it doing an extra eval or something?  It looks like it:
>> 
>>  > x <- expression()
>>  > x
>> expression()
>>  > eval(x)
>> NULL
>>  >
>>  > x <- quote(expression())
>>  > x
>> expression()
>>  > eval(x)
>> expression()
>>  > eval(eval(x))
>> NULL
> 
> You do realize that the two expression() results are not identical:
> 
>> x <- quote(expression())
>> class(x)
> [1] "call"
>> x <- expression()
>> class(x)
> [1] "expression"
> 
> Not that I can fathom what bearing that has on the real problem...

I figured they weren't identical, given that they gave different results

  when eval'd, but I didn't know what the difference was exactly.
Thanks!

Duncan

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