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

[Gabor Grothendieck]

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.

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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>
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