NULL assignment will change the expression's class into list

Hello Everyone!

NULL replacement will change expression object into list:

  

te <- expression(a=23*4, b=33-2)
te
    

expression(a = 23 * 4, b = 33 - 2)

  

te[["a"]] <- quote(blabla) #ok
te
    

expression(a = blabla, b = 33 - 2)

  

te[["a"]] <- NULL #change to list
te
    

$b
33 - 2

I am on w32, version 2.11.1 (2010-05-31)
  

That's certainly an inconsistency, still present in a recent R-devel 
(but I haven't checked the latest beta).  I don't know if it's a bug:  
NULL assignments are handled specially in other situations (e.g. if te 
was a list to start, the NULL assignment would remove the "a" entry).

A simple workaround is to use

te["a"] <- expression(NULL)

or te <- te[-1]

instead, depending on what you expected to happen.

Duncan Murdoch

Regards,
Vitally.

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