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

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Mon, 16 Nov 2009 12:26:05 -0500

On 11/16/2009 12:07 PM, Robert Gentleman wrote:
> Hi,
>
> On Fri, Nov 13, 2009 at 4:55 PM, Duncan Murdoch <murdoch@stats.uwo.ca> wrote:

>> On 13/11/2009 7:26 PM, Gabor Grothendieck wrote:
>>>
>>> On Fri, Nov 13, 2009 at 7:21 PM, Duncan Murdoch <murdoch_at_stats.uwo.ca>
>>> wrote:
>>>>
>>>> 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?
>>>>
>>>
>>> $ does not look into the parent so if you are trying to get those
>>> semantics you must use inherits = FALSE.
>>
>> Whoops, yes.  That's another complaint about $ on environments.

>
> That was an intentional choice. AFAIR neither $ nor [[ on
> environments was not meant to mimic get, but rather to work on the
> current environment as if it were a hash-like object. One can always
> get the inherits semantics by simple programming, but under the model
> you seem to be suggesting, preventing such behavior when you don't own
> the environments in question is problematic.

Sure, I agree with how you did that; I'm not sure you had any choice at the time (didn't all environments have base as a parent then?). Even now, you do want both inherits=TRUE and inherits=FALSE behaviour in different circumstances, and $ has to pick just one. Probably my wording should have been "That's another gotcha about $ on environments."

Duncan Murdoch



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