[Rd] as.missing

[Paul Gilbert]

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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