From murdoch at stats.uwo.ca  Mon Nov 16 18:26:05 2009
From: murdoch at stats.uwo.ca (Duncan Murdoch)
Date: Mon, 16 Nov 2009 12:26:05 -0500
Subject: [Rd] The default behaviour of a missing entry in an environment
In-Reply-To: <b796582f0911160907k4d15751nc4cd14e85a005e93@mail.gmail.com>
References: <D737F65B-A317-44C1-8421-E8DE2D0E0740@nyu.edu>	<4AFDB33D.5010607@stats.uwo.ca>	<971536df0911131539o5d702723u63645580bef5f1f@mail.gmail.com>	<4AFDF822.5050602@stats.uwo.ca>	<971536df0911131626k21bd7cefk391ad821b42170b6@mail.gmail.com>	<4AFDFFFA.9070306@stats.uwo.ca>
	<b796582f0911160907k4d15751nc4cd14e85a005e93@mail.gmail.com>
Message-ID: <4B018B2D.6090103@stats.uwo.ca>

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



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