[Rd] Using substitute from inside an S4 method

[Gabor Grothendieck]

Try defining your method like this.  I don't know how generally this
works but it seems to work here.

setMethod("A", signature(x="numeric"),
              function(x, y) as.character(substitute(x, sys.frame(-1))))

On 1/25/06, Seth Falcon <sfalcon at fhcrc.org> wrote:
> Hi all,
>
> I would like to access the name of a variable passed to an S4 method.
> For a function, I would do this:
>
>    f <- function(x) as.character(substitute(x))
>
> This also works for a the examples I have tried for methods that do
> not have extra, non-dispatch args:
>
>    setGeneric("A", function(x, ...) standardGeneric("A"))
>
>    setMethod("A", signature(x="character"),
>              function(x) as.character(substitute(x)))
>
> However, I'm seeing strange behavior if the method uses an extra
> argument:
>
>    setMethod("A", signature(x="numeric"),
>              function(x, y) as.character(substitute(x)))
>
>    num <- 1
>
>    A(num)
>    [1] "x"
>
>    A(num, 2)
>    [1] "x"
>
> Is there a way to make this work?  I came up with one workaround that
> uses a non-standard generic (see below).
>
> It seems that when a method uses extra args matching '...' in the
> generic, an extra frame is used in the evaluation and so substitute()
> isn't reaching the same place as without extra args.
>
> Thanks in advance for pointers to doc or suggestions.
>
>
> + seth
>
> ## here is a non-standard generic that gives me the behavior I want
>
>    setGeneric("B", function(x, ...) {
>        x.name <- as.character(substitute(x))
>        standardGeneric("B")
>        })
>
>    setMethod("B", signature(x="character"),
>              function(x, y) {
>                  penv <- sys.frames()
>                  penv <- penv[[length(penv)-2]]
>                  get("x.name", envir=penv)
>              })
>
> Observation: Without an extra arg in the method, the appropriate
> environment would be penv[[length(penv) - 1]], but the presence of the
> extra arg results in an extra environment in the evaluation, hence we
> need -2.
>
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