R help - Nov 2008 - "recursive default argument" error

Dear R Users,

I have a function foo in a script with default values for some of the 
parameters as follows:

testparams=list(a=1,b=2,c=3)
foo<-function(x,y,testparams=testparams)
        x+y+testparams$a+testparams$b+testparams$c


When I try to run foo(1,2), I get the following error:
> testparams=list(a=1,b=2,c=3)
> foo<-function(x,y,testparams=testparams)
+ x+y+testparams$a+testparams$b+testparams$c
> foo(1,2)
Error in foo(1, 2) : 
  promise already under evaluation: recursive default argument reference 
or earlier problems?
> 

I can obviously rename testparams but I would still like to understand why 
this breaks. I would have thought the right hand side of 
"testparams=testparams" would have been evaluated in the namespace
outside
the script and not the function. How can I force it to be evaluated in the 
namespace outside the function ?

Thanks in advance,
Tolga

Generally, this communication is for informational purposes only
and it is not intended as an offer or solicitation for the purchase
or sale of any financial instrument or as an official confirmation
of any transaction. In the event you are receiving the offering
materials attached below related to your interest in hedge funds or
private equity, this communication may be intended as an offer or
solicitation for the purchase or sale of such fund(s).  All market
prices, data and other information are not warranted as to
completeness or accuracy and are subject to change without notice.
Any comments or statements made herein do not necessarily reflect
those of JPMorgan Chase & Co., its subsidiaries and affiliates.

This transmission may contain information that is privileged,
confidential, legally privileged, and/or exempt from disclosure
under applicable law. If you are not the intended recipient, you
are hereby notified that any disclosure, copying, distribution, or
use of the information contained herein (including any reliance
thereon) is STRICTLY PROHIBITED. Although this transmission and any
attachments are believed to be free of any virus or other defect
that might affect any computer system into which it is received and
opened, it is the responsibility of the recipient to ensure that it
is virus free and no responsibility is accepted by JPMorgan Chase &
Co., its subsidiaries and affiliates, as applicable, for any loss
or damage arising in any way from its use. If you received this
transmission in error, please immediately contact the sender and
destroy the material in its entirety, whether in electronic or hard
copy format. Thank you.
Please refer to http://www.jpmorgan.com/pages/disclosures for
disclosures relating to UK legal entities.
	[[alternative HTML version deleted]]

tolga.i.uzuner at jpmorgan.com wrote:
> Dear R Users,
>
> I have a function foo in a script with default values for some of the 
> parameters as follows:
>
> testparams=list(a=1,b=2,c=3)
> foo<-function(x,y,testparams=testparams)
>         x+y+testparams$a+testparams$b+testparams$c
>
>
> When I try to run foo(1,2), I get the following error:
>
>   
>> testparams=list(a=1,b=2,c=3)
>> foo<-function(x,y,testparams=testparams)
>>     
> + x+y+testparams$a+testparams$b+testparams$c
>   
>> foo(1,2)
>>     
> Error in foo(1, 2) : 
>   promise already under evaluation: recursive default argument reference 
> or earlier problems?
>   
>
>   
sure.  from the r language manual, sec. 4.3.3:

"One of the most important things to know about the evaluation of
arguments to a function is
that supplied arguments and default arguments are treated differently.
The supplied arguments
to a function are evaluated in the evaluation frame of the calling
function. The default arguments
to a function are evaluated in the evaluation frame of the function."

the parameter testparams, when no matching argument is passed, is given
the default value which is the value of the variable testparams
looked-up *not* in the environment where foo is defined, and *not* in
the environment where foo is called, but rather in the local environment
created when the function is called and where parameters are mapped to
values -- and in this environment, testparams is a parameter, which is
already being under evaluation, hence the recursive lookup error.

in some programming languages, testparams (the default value) would come
from the definition-time environment, so you'd not have problems; in
others, it's more like in r:

python> x = 1; def foo(x, y=x): return y; foo(2) # 1
ruby> x = 1; def foo(x, y=x); y; end; foo(2) # 2

you just have to get used to it.



> I can obviously rename testparams but I would still like to understand why 
> this breaks. I would have thought the right hand side of 
> "testparams=testparams" would have been evaluated in the
namespace outside
> the script and not the function. How can I force it to be evaluated in the 
> namespace outside the function ?
>   
something like this ugly hack seems to solve the problem:

foo = function(testparams = get("testparams", envir=environment(foo))


vQ