The tricky part about your example is that each of the arguments in "..."
could be originating from different frames.
The S4Vectors:::top_prenv_dots function will give you a list of
environments, one for each argument in "...". An example of its use can be
found in IRanges::transform.DataTable.
I bet top_prenv_dots will be moving to a different package soon, so don't
get too attached to anything.
Michael
On Wed, Apr 23, 2014 at 11:02 AM, Kevin Ushey <kevinus...@gmail.com> wrote:
> Hi everyone,
>
> I'm trying to investigate lazy evaluation with S4, and specifically
> the environment chain available for lookup when an S4 method is
> evaluated.
>
> With 'vanilla' R functions, we can write something like the following:
>
> lazy <- function(...) {
> call <- substitute(list(...))
> for (i in 2:length(call)) {
> eval(call[[i]])
> }
> }
>
> and evaluation will succeed if this function is called by another function:
>
> f <- function() {
> var <- 10
> lazy(x = var)
> }
>
> f()
>
> The evaluation of the symbol 'var' is able to find it in the 'f'
> environment.
>
> setClass("Lazy", list(data="list"))
> setGeneric("lazy", function(x, ...) standardGeneric("lazy"))
> setMethod("lazy", list(x="Lazy"), function(x, ...) {
> call <- substitute(list(...))
> for (i in 2:length(call)) {
> eval( call[[i]] )
> }
> })
>
> f <- function() {
> var <- new("Lazy", data=list(1))
> y <- 1
> cat("\n")
> lazy(var, a=y)
> }
>
> f()
>
> gives ' Error in eval(expr, envir, enclos) : object 'y' not found '.
> It seems like the S4 method is evaluated in an environment detached
> from the scope of the caller (which is probably outlined in ?Methods,
> but I am having trouble parsing everything in there).
>
> My main question is -- is there a nice way to work around this, in a
> way that is transparent to the user of a function?
>
> Further motivation available in the GitHub issue for flowCore here:
> https://github.com/RGLab/ncdfFlow/issues/31
>
> Thanks,
> Kevin
>
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>
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