[Rd] The default behaviour of a missing entry in an environment

Duncan Murdoch murdoch at stats.uwo.ca
Sat Nov 14 01:21:54 CET 2009

On 13/11/2009 6:39 PM, Gabor Grothendieck wrote:
> Note that one should use inherits = FALSE argument on get and exists
> to avoid returning objects from the parent, the parent of the parent,
> etc.

I disagree.  Normally you would want to receive those objects.  If you 
didn't, why didn't you set the parent of the environment to emptyenv() 
when you created it?

Duncan Murdoch

> On Fri, Nov 13, 2009 at 2:27 PM, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
>> On 11/13/2009 2:03 PM, Trishank Karthik Kuppusamy wrote:
>>> Greetings everyone,
>>> I have a question about the default behaviour of a missing entry in an
>>> environment.
>>> Let us look at the following sequence of R statements:
>>>> e <- new.env()
>>>> e$a <- 1
>>>> e$a
>>> [1] 1
>>>> e$b
>>> NULL
>>> I think I understand the logic for returning NULL to a missing entry in an
>>> environment,
>>> but I do not think that it is fully justified.
>>> I am sure that the R developers must have seen this argument before,
>>> but I wish to call for attention to this problem again,
>>> because I think that it is important to the default safety of the R
>>> programming language.
>> You get the same behaviour when asking for a nonexistent element of a list,
>> or a nonexistent attribute.   If you want stricter checking, don't use $,
>> use get():
>>> get("b", e)
>> Error in get("b", e) : object 'b' not found
>> or check first with exists():
>>> exists("b", e)
>> [1] FALSE
>>> I suppose that one could argue that a good R programmer must be careful
>>> not to use NULL in any of his environment entries,
>>> but I think it is better to remove altogether this burden from the
>>> programmer
>>> and simply raise a good, old-fashioned exception when the "$" operator
>>> encounters a missing entry in an environment.
>> But then it would be inconsistent with what it does in other situations.
>> Duncan Murdoch
>>> The biggest advantage is that it will easily eliminate a whole class of
>>> programming error.
>>> The biggest disadvantage is that it is not backwards-compatible with old R
>>> programs.
>>> I suppose a personal solution would be to simply redefine the "$" operator
>>> in my programs.
>>> However, I really do think that the default safety of an R environment
>>> matters very much.
>>> At the very least, it would be nice to be able to configure the safety of
>>> a new environment,
>>> perhaps through a parameter.
>>> -Trishank
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