R help - Jun 2008 - Existence of formal arguments.

I just discovered what seems to me to be a slight funny in respect
of formal argument names.  If I define a function

	foo <- function(a,b){ ... whatever ...}

then ``inside'' foo() the exists() function will return TRUE
from ``exists("a") whether an object named ``a'' exists or
not.
But get("a") will yield an error ``object "a" not
found''
in these circumstances.

I presume there is a reason for specifying that an object named
by a formal argument always exists --- but it is mysterious by my
standards.  Can anyone explain the reason for this behaviour?

This is just idle curiosity --- or my hunger for knowledge, whichever
way you want to look at it :-) --- it doesn't really matter.

	cheers,

		Rolf Turner

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On 05/06/2008 8:23 PM, Rolf Turner wrote:
> I just discovered what seems to me to be a slight funny in respect
> of formal argument names.  If I define a function
> 
> 	foo <- function(a,b){ ... whatever ...}
> 
> then ``inside'' foo() the exists() function will return TRUE
> from ``exists("a") whether an object named ``a'' exists
or not.
> But get("a") will yield an error ``object "a" not
found''
> in these circumstances.
I don't see that.  I see this:

 > foo <- function(a, b) { get("a") }
 > get("a")
Error in get("a") : variable "a" was not found
 > foo()

I'm guessing that after you got a, you tried to do something with it, 
but I can't guess what:  I get a different error in this code.

 > foo <- function(a, b) { x <- get("a"); x }
 > foo()
Error in foo() : argument "x" is missing, with no default


The thing is, formals are always defined in the local scope when you 
call a function.  But if you didn't assign a value or a default to them, 
they are defined as a special missing value.  As the second example 
shows, you can move that missing around, but not do anything with it.

Duncan Murdoch



> 
> I presume there is a reason for specifying that an object named
> by a formal argument always exists --- but it is mysterious by my
> standards.  Can anyone explain the reason for this behaviour?
> 
> This is just idle curiosity --- or my hunger for knowledge, whichever
> way you want to look at it :-) --- it doesn't really matter.
> 
> 	cheers,
> 
> 		Rolf Turner
> 
> ######################################################################
> Attention:\ This e-mail message is privileged and confid...{{dropped:9}}
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Note the difference between

test <- function(a) {
   exists("a", mode = "symbol")
}
test()

and

test2 <- function(a) {
   exists("a", mode = "numeric") #say
}
test2()

and then note that the default mode argument to exists is "any".

Rolf Turner wrote:
> 
> I just discovered what seems to me to be a slight funny in respect
> of formal argument names.  If I define a function
> 
>     foo <- function(a,b){ ... whatever ...}
> 
> then ``inside'' foo() the exists() function will return TRUE
> from ``exists("a") whether an object named ``a'' exists
or not.
> But get("a") will yield an error ``object "a" not
found''
> in these circumstances.
> 
> I presume there is a reason for specifying that an object named
> by a formal argument always exists --- but it is mysterious by my
> standards.  Can anyone explain the reason for this behaviour?
> 
> This is just idle curiosity --- or my hunger for knowledge, whichever
> way you want to look at it :-) --- it doesn't really matter.
> 
>     cheers,
> 
>         Rolf Turner
> 
> ######################################################################
> Attention:\ This e-mail message is privileged and confid...{{dropped:9}}
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

On 05/06/2008 8:23 PM, Rolf Turner wrote:
> I just discovered what seems to me to be a slight funny in respect
> of formal argument names.  If I define a function
> 
> 	foo <- function(a,b){ ... whatever ...}
> 
> then ``inside'' foo() the exists() function will return TRUE
> from ``exists("a") whether an object named ``a'' exists
or not.
> But get("a") will yield an error ``object "a" not
found''
> in these circumstances.
> 
> I presume there is a reason for specifying that an object named
> by a formal argument always exists --- but it is mysterious by my
> standards.  Can anyone explain the reason for this behaviour?
Oops, I didn't explain why this is the way it should be.

Say your "whatever" above makes use of a, but you didn't pass an a
in.
Then you'd like an error, or you'd like "missing(a)" to
evaluate to
TRUE, or something along those lines.  But if a was completely undefined 
and nonexistent, R would just go looking for a global, and make use of 
that.  So it has to be marked as missing.

Duncan Murdoch