[R] deferred call

[Bert Gunter]

On Wed, Apr 11, 2012 at 8:12 PM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
> On Wed, Apr 11, 2012 at 10:17 PM, Whit Armstrong
> <armstrong.whit at gmail.com> wrote:
>> I must admit I'm a little ashamed to have been using R for so long,
>> and still lack a sound understanding of deferred calls, eval, deparse,
>> substitute, and friends.
>>
>> I'm attempting to make a deferred call to a function which has default
>> arguments in the following way:
>>
>> call.foo <- function(f) {
>>    x <- f()
>> }
>>
>> x <- 1:10
>> f <- function(x=x) { x^2 }
>> call.foo(f)
>>
>> However, I'm getting the error:
>> Error in x^2 : 'x' is missing
>>
>> Is there a common R idiom for calling 'formals' on the function, and
>> then grabbing the named default arguments from the enclosing frame?
>>
>> I naively thought that since function 'f' was defined w/ a default
>> argument of 'x' and x is defined in the same envir as the function
>> itself, that the call would succeed.
>>
>
> f <- function(x=x) x^2 is an endless recursion.  Try

To amplify just a bit on Gabor's comment, section 4.3.3 of the R
language definition explicitly states:

"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."

So foo <- function (x = x) {...}
tries to define foo with the default argument x, which is evaluated in
the ** environment of the function ** not in the caller's environment,
where it is 1:10.  So if x ever needs to be evaluated within foo
(i.e., its promise is forced), then it looks for the value of the rhs
of the x=x  assignment within foo, which is (the promise for) x,
again, within foo, which is x again,...

HTH

-- Bert

>
>  f <- function(x.=x) { x^2 }
>
> (note the dot)
>
> --
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>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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