From hadley at rice.edu  Wed Aug 18 04:31:44 2010
From: hadley at rice.edu (Hadley Wickham)
Date: Tue, 17 Aug 2010 21:31:44 -0500
Subject: [Rd] parent.frame(1) of a S4 method is not a calling
	environment.
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References: <87d3tj4ku4.fsf@gmail.com> <4C698DAC.3090704@fhcrc.org>
	<87wrrq2ns2.fsf@gmail.com> <4C6A630C.9020802@gmail.com>
	<87sk2c3k9j.fsf@gmail.com>
Message-ID: <AANLkTikYjCwuMv2LC_Kx2sYmgm6JYLXkEyP7xR0xKark@mail.gmail.com>

On Tuesday, August 17, 2010, Vitaly S. <spinuvit.list at gmail.com> wrote:
> Duncan Murdoch <murdoch.duncan at gmail.com> writes:
>
>> Vitaly S. wrote:
>>> Martin Morgan <mtmorgan at fhcrc.org> writes:
>>>
>>>>> So, ?can I be sure that for such functions parent.frame(2) will always work?
>>>>> What are the additional rules?
>>>>>
>>>> callNextMethod() will cause additional problems; the idea that you'll
>>>> grab things from somewhere other than function arguments doesn't seem
>>>> like a robust design, even if it's used in some important parts of R.
>>>>
>>>> Martin
>>>>
>>>>
>>>
>>> That make it difficult to handle unevaluated expressions in methods. A solution
>>> would be to explicitly require the users to use quote() or expression(), ?and
>>> then to use the "expression" in the signature. Slightly unpleasant, though.
>>>
>>>
>>
>> You could use formulas for that. ?If you pass in
>>
>> formula = ~ x + y*z
>>
>> then environment(formula) will be the right evaluation environment, and formula[[2]] will be the unevaluated x +
>> y*z.
>>
>> Duncan Murdoch
>
> Thank you Duncan, I didn't know that.
>
> For programmatic use though, formula interface is slightly inconvenient. A
> specialized function and class would be desirable. With the advent of more and
> more complex S4 classes, unevaluated expressions in methods calls will became a
> necessity, that's my feeling.

I probably should move the quoting related out of plyr into it's own
package to facilitate this type of reuse. I think the current
structure is quite general.

Hadley

-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/



[Rd] parent.frame(1) of a S4 method is not a calling environment.
Hadley Wickham