From pgilbert at bank-banque-canada.ca  Fri Oct 27 17:30:21 2006
From: pgilbert at bank-banque-canada.ca (Paul Gilbert)
Date: Fri, 27 Oct 2006 11:30:21 -0400
Subject: [Rd] as.missing
In-Reply-To: <x2pscdltv9.fsf@turmalin.kubism.ku.dk>
References: <453E462C.6050103@bank-banque-canada.ca>	<971536df0610250928q615c2e4dl950e8f576c492789@mail.gmail.com>	<4540CC71.1010108@bank-banque-canada.ca>	<971536df0610261830o4d36376bk4b90d42e218eb2d0@mail.gmail.com>	<m0hcxq3z2a.fsf@bar.nemo-project.org>	<45420FE2.5090000@bank-banque-canada.ca>
	<x2pscdltv9.fsf@turmalin.kubism.ku.dk>
Message-ID: <4542260D.2000606@bank-banque-canada.ca>



Peter Dalgaard wrote:

>Paul Gilbert <pgilbert at bank-banque-canada.ca> writes:
>
>  
>
>>>I.e., when x is missing in g, and g calls f(3,x), f will use its
>>>default value for x.
>>> 
>>>
>>>      
>>>
>>Yes, that is the behaviour I am looking for. That is, f  should do what 
>>it normal would do if it were called with x missing.
>>    
>>
>
>But if x has a default in g then that default should presumably be
>used? 
>
Yes.  The value of x in g would get passed to f, default or otherwise. 
If that value is something that indicates x is missing, then it should 
be treated as if it is missing in f. This means f should use its default 
value, rather than throw an error saying x is missing.

>And what if x is given a value in the evaluation frame of g
>before it is used by f (which can happen, you know, even after the
>evaluation of f has begun)? Now imagine a longer chain of calls.
>
>I think what you're asking for is essentially dynamic scoping for
>missing arguments: you'd have to backtrack along the call chain to
>find the first instance where x is either given a value or has a
>default. This sounds messy.
>  
>
You understand this better than I do, but I don't think I am asking to 
do this. Currently I think f  looks back too far and finds x is missing 
and g does not have a default value for x, so it throws an error.  Why 
can't f find its own default value for x?
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[Rd] as.missing
Paul Gilbert