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

[David Winsemius]

On Dec 29, 2013, at 3:57 PM, andrewH wrote:

> Dear David--
>
> Thanks so much for your helpful reply!
>
> David Winsemius wrote:
>>> The LHS X becomes a name, the RHS X will be looked up in the calling
> environment and fails if no value is positionally matched and then  
> no X is
> found (at the time of the function definition.
>
> Does X really have to exist when the function is defined?

No


> I thought it was
> enough if it existed in the environment of the calling function, or
> somewhere up the environment chain of the calling function. If this  
> is not
> true, then that means it matters a lot whether you write a function  
> inside
> another function or just call it in that function.  Suppose a  
> function with
> a reflexive assignment X=X

Arrrgh. The is no "reflexive assignment". You are making up a concept.

> is defined in the global environment but called
> inside another function, and X has a different value in those two  
> places.
> Will it look first in the global environment and only then in the  
> calling
> environment? And is this different from the behavior without the  
> reflexive
> assignment?
>
> I should not bother you with those questions. I should just run it  
> both ways
> and see what happens.calling function and will it look first in the
>
>>> If you use`X <- value` in the argument list, then what is returned  
>>> is only
> the value and the name `X` may be lost. Or in the case of data.frame  
> morphed
> into a strange name:
>
> [example omitted]
> I am not sure that I am understanding you correctly here. Are you  
> saying
> that assignment using the "=" retains the name (and other  
> attributes? which
> ones?) of the RHS, while "<-" does not?

Using "=" assigns a name. Using "<-" retruns a value and whether the  
value gets a name depends on the particular function.

 > foo <- function(X <- V) { print(X)}
Error: unexpected assignment in "foo <- function(X <-"
 > foo <- function(X = V) { print(X)}
 > foo(4)
[1] 4
 > foo <- function(X = V) { print(V)}
 > foo(4)
Error in print(V) : object 'V' not found


-- 

David Winsemius, MD
Alameda, CA, USA