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Problem statement

This is a demo repository for the issue described in the R-devel mailing list.

Suppose we have R objects of class c("foo", "bar"), and there are two S3 methods, c.foo and c.bar. In c.foo, we want to modify the arguments passed by ... and forward the dispatch using NextMethod. The following code tries to solve this problem, but it results in an infinite recursion (!):

c.foo <- function(..., recursive=FALSE) {
  message("calling c.foo...")
  dots <- list(...)
  # inspect and modify dots; for example:
  if (length(dots > 1))
    dots[[2]] <- 2
  do.call(
    function(..., recursive=FALSE) structure(NextMethod("c"), class="foo"),
    c(dots, recursive=recursive)
  )
}

c.bar <- function(..., recursive=FALSE) {
  message("calling c.bar...")
  NextMethod()
}

foobar <- 1
class(foobar) <- c("foo", "bar")

c(foobar, foobar)
#> calling c.foo...
#> ...
#> Error: C stack usage  7970788 is too close to the limit

The very same methods are deployed into two packages included in this repo. Let's install them and try the same operation:

rm(c.foo, c.bar)
devtools::install("package.foo")
devtools::install("package.bar")
library(package.foo)
library(package.bar)

c(foobar, foobar)
#> calling c.foo...
#> [1] 1 2
#> attr(,"class")
#> [1] "foo"

and there's no recursion this time, but c.bar was not called (!).

Update

As Tomas Kalibera pointed out, the documentation states that calling NextMethod from an anonymous function is not allowed (I missed that bit!), so this code should issue an error. Apparently, there is no way to replace (unnamed) arguments in dots for NextMethod.

Workaround

A possible workaround for this is to reinitialise the dispatch stack by calling the generic again if any argument was modified, and finally call NextMethod cleanly. This works, but it means that c.foo will be called twice every time an argument is modified:

c.foo <- function(..., recursive=FALSE) {
  message("calling c.foo...")
  modified <- FALSE
  dots <- list(...)
  # inspect and modify dots; for example:
  if (length(dots > 1) && dots[[2]] != 2) {
    dots[[2]] <- 2
    modified <- TRUE
  }
  if (modified)
    do.call(c, c(dots, recursive=recursive))
  else structure(NextMethod(), class="foo")
}

c(foobar, foobar)
#> calling c.foo...
#> calling c.foo...
#> calling c.bar...
#> [1] 1 2
#> attr(,"class")
#> [1] "foo"

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