On 02/22/2018 02:31 PM, Iñaki Úcar wrote:
2018-02-22 12:39 GMT+01:00 Tomas Kalibera <tomas.kalib...@gmail.com>:
On 02/22/2018 12:07 PM, Iñaki Úcar wrote:
2018-02-22 10:29 GMT+01:00 Tomas Kalibera <tomas.kalib...@gmail.com>:
The example is invoking NextMethod via an anonymous function, which is
allowed (see documentation for NextMethod).
Thanks for your response. I definitely missed that bit.

Normally one gets a runtime
error "'NextMethod' called from an anonymous function", but not here as
anonymous function is called via do.call. I will fix so that there is a
runtime error in this case as well, thanks for uncovering this problem.
Then I did well chosing this list! Please also note that you could
take that anonymous function out of the method and name it, and the
behaviour would be the same. So maybe this case should issue an error
I am not sure I understand how, but if you find a way to bypass the new
check for an anonymous function (I intend to commit tomorrow), I will be
happy to have a look if you provide a reproducible example.
I meant with a named function inside do.call, instead of an anonymous
one. For example:

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

c.foo.proxy <- function(..., recursive=FALSE)
structure(NextMethod("c"), class="foo")

Right now, the effect of the code above is the same as with the
anonymous function. Shouldn't it issue a similar error then?
Yes, it will also result in runtime error after the change is committed:

calling c.foo...
Error in NextMethod("c") : 'NextMethod' called from an anonymous function
I don't think there is a way to replace (unnamed) arguments in dots for
That's a pity. IMHO, it should be some mechanism for that, but dots
are special in inscrutable ways.

Anyway, for anyone insterested, I found a workaround:

Even though technically this won't be too hard, I don't think NextMethod
should be made any more complex than it is now. There should always be a way
to implement special dispatch scenarios in R and your workaround shows it is
possible specifically in your scenario.
My only concern about this workaround is that it triggers the dispatch
stack again from the beginning of the class hierarchy, which seems not
very elegant nor efficient.
There may be a more elegant way, but that'd be a question for R-help and it might be worth giving a broader context for what you want to achieve. Also please note that S3 dispatch is done on the first argument, and c() gives no special meaning to its first argument, what if e.g. the second argument is of class "foo" but the first is not - is S3/NextMethod really a good fit here?




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