[R] What purpose is served by reflexive function assignments?

[andrewH]

Dear Ista--
Peter's post has already persuaded me that my original question was based on
several misunderstandings and so difficult if not impossible to follow --
though he did a remarkable job of figuring out where I was going astray and
what examples might set me right. 

But I will post the results of two of my experiments that I still find
puzzling.

This generates a recursive default error in the cat function. I do not see
why it does not print 5:
X <- 2
gg <-  function(X=X){cat("gg: ", X)}
ss<- function(X){
X <- 5
gg()
}
ss()

And this generates an " 'x' is missing " error in x-y. I expected it to
return the number -1:
x<-1
y<-2
foo<- function(x=x,y=y){x-y}
foo() 

Thanks so much for your time and attention!
andrewH


Ista Zahn wrote
> On Sat, Dec 28, 2013 at 10:27 PM, Andrew Hoerner <

> ahoerner@

> > wrote:
>> Let us suppose that we have a function foo(X) which is called inside
>> another function, bar(). Suppose, moreover, that the name "X" has been
>> assigned a value when foo is called:
>>
>> X <- 2
>> bar(X=X){
>> foo(X)
>> }
>>
>> I have noticed that many functions contain arguments with defaults of the
>> form X=X.
> 
> An example would be really helpful here.
> 
> Call this reflexive assignment of arguments.
> 
> Why call this anything special? All this does is set the default value
> of the X argument. I'm not sure what makes this "reflexive", or why it
> needs a special descriptive term.
> 
> How is foo(X=X)
>> different from foo(X)? Isn't the environment from which X is located the
> 
> foo(X) is hardcoded, foo(X = X) just sets a default.
> 
>> parent environment of foo() in either case? Or if it looks first in the
>> environment inside of foo, will it not immediately pop up to the parent
>> environment if it is not found in foo? Are reflexive assignments just to
>> keep X from being positionally assigned accidentally, or are they doing
>> something deeper? Moreover, this is the only place I have seen people
>> consistently using an equals sign in place of the usual "<-", and I am
>> confident that there is some subtle difference in how the two assignment
>> operators work, perhaps beyond the ken of lesser mortals like myself,
>> that
>> explains why the "=" is preferred in this particular application.
> 
> Again, some examples would really help here.
> 
>>
>> Actually, although I would like to hear the deep answer, which I am sure
>> has something to do with scoping, as everything really confusing in R
>> does,
>> my real question is, is there some rule of thumb by which one could
>> decide
>> whether or not to do a reflexive assignment in a function definition and
>> be
>> right most of the time?
> 
> I'm still not even sure what reflexive assignment means. Can you
> clarify, preferably with some examples.
> 
>>
>> Lately I have gotten several "Error: Promise is already under evaluation"
>> messages, and my current rule of thumb for dealing with this is to add
>> reflexive assignment to the variable if it is missing and take it out if
>> it
>> is present. This seems to work, but it makes me feel unintelligent. Is
>> there a better rule? I would be most grateful for anyone who could shed
>> light on the subject.
> 
> Perhaps someone can, but you will certainly make their job easier if
> you provide a concrete example that produces this error.
> 
> Best,
> Ista
> 
>>
>> Sincerely, andrewH
>>
>> --
>> J. Andrew Hoerner
>> Director, Sustainable Economics Program
>> Redefining Progress
>> (510) 507-4820
>>
>>         [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> 

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> 
> ______________________________________________

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