[Rd] [Q] Get formal arguments of my implemented S4 method

[William Dunlap]

I wish it didn't have to depend on the name '.local'.

Back when I wrote a lot of S4 methods I avoided the auto-generated .local
and named the local function something that made sense so that is was easier
for a user to track down the source of an error.

E.g., define the generic QQQ with numeric and integer methods:
setGeneric("QQQ",
           function(x, ...)NULL)
setMethod("QQQ",
          signature=signature(x="numeric"),
          function(x, lower, ...) {
              if (x<lower) stop("x<lower")
          })
setMethod("QQQ",
          signature=signature(x="integer"),
          function(x, ...) {
              .QQQ.integer <- function(x, lower, ...) if (x<lower)
stop("x<lower")
              .QQQ.integer(x, ...)
          })
and try using them:
  > QQQ(3.4, 10)
  Error in .local(x, ...) : x<lower
  > traceback()
  4: stop("x<lower") at #4
  3: .local(x, ...)
  2: QQQ(3.4, 10)
  1: QQQ(3.4, 10)
  > QQQ(3L, 10)
  Error in .QQQ.integer(x, ...) : x<lower
  > traceback()
  4: stop("x<lower") at #4
  3: .QQQ.integer(x, ...) at #5
  2: QQQ(3L, 10)
  1: QQQ(3L, 10)
I think the latter gives the user more guidance on how to fix the problem.

Perhaps instead of searching for an assignment to '.local' you could
search for an assignment to the name of the function used in the last
function call of the method.



Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Thu, Jan 29, 2015 at 6:34 AM, Hadley Wickham <h.wickham at gmail.com> wrote:

> On Thu, Jan 29, 2015 at 7:57 AM, John Chambers <jmc at r-project.org> wrote:
> >
> > On Jan 28, 2015, at 6:37 PM, Michael Lawrence <lawrence.michael at gene.com>
> wrote:
> >
> >> At this point I would just due:
> >>
> >> formals(body(method)[[2L]])
> >>
> >> At some point we need to figure out what to do with this .local()
> confusion.
> >
> > Agreed, definitely.  The current hack is to avoid re-matching arguments
> on method dispatch, so a fix would need to be fairly deep in the
> implementation.
> >
> > But I don't think the expression above is quite right.
> body(method)[[2L]] is the assignment.  You need to evaluate the rhs.
> >
> > Here is a function that does the same sort of thing, and returns the
> standard formals for the generic if this method does not have nonstandard
> arguments.  We should probably add a version of this function for 3.3.0, so
> user code doesn't have hacks around the current hack.
> >
> > methodFormals <- function(f, signature = character()) {
> >     fdef <- getGeneric(f)
> >     method <- selectMethod(fdef, signature)
> >     genFormals <- base::formals(fdef)
> >     b <- body(method)
> >     if(is(b, "{") && is(b[[2]], "<-") && identical(b[[2]][[2]], as.name(".local")))
> {
> >         local <- eval(b[[2]][[3]])
> >         if(is.function(local))
> >             return(formals(local))
> >         warning("Expected a .local assignment to be a function.
> Corrupted method?")
> >     }
> >     genFormals
> > }
>
> I have similar code in roxygen2:
>
> # When a generic has ... and a method adds new arguments, the S4 method
> # wraps the definition inside another function which has the same arguments
> # as the generic. This function figures out if that's the case, and
> extracts
> # the original function if so.
> #
> # It's based on expression processing based on the structure of the
> # constructed method which looks like:
> #
> # function (x, ...) {
> #   .local <- function (x, ..., y = 7) {}
> #   .local(x, ...)
> # }
> extract_method_fun <- function(x) {
>   fun <- x at .Data
>
>   method_body <- body(fun)
>   if (!is.call(method_body)) return(fun)
>   if (!identical(method_body[[1]], quote(`{`))) return(fun)
>
>   first_line <- method_body[[2]]
>   if (!is.call(first_line)) return(fun)
>   if (!identical(first_line[[1]], quote(`<-`))) return(fun)
>   if (!identical(first_line[[2]], quote(`.local`))) return(fun)
>
>   first_line[[3]]
> }
>
>
> --
> http://had.co.nz/
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

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