[R] deferred call

[Whit Armstrong]

Bert and Gabor,

Thanks for clarifying. Much appreciated.

One more question.  Do either of you know the location of the code
that does the lookup of the default arguments?

I would like to be able to capture the implicit dependency of the
function on the default arguments.

Hence, I would write something like:

capture.dag <- function(fun) {
    ans <- list()
    args <- formals(fun)
    for(nm in names(args)) {
        ans[[ nm ]] <- lookup.the.object(args[[nm]])
    }

    ans
}

However, it seems that this logic is already implemented in the core
of R, and it would be great to re-use it.

Thanks,
Whit


On Thu, Apr 12, 2012 at 1:00 AM, Bert Gunter <gunter.berton at gene.com> wrote:
> 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)
>>
>> --
>> Statistics & Software Consulting
>> GKX Group, GKX Associates Inc.
>> tel: 1-877-GKX-GROUP
>> email: ggrothendieck at gmail.com
>>
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>
>
>
> --
>
> Bert Gunter
> Genentech Nonclinical Biostatistics
>
> Internal Contact Info:
> Phone: 467-7374
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