[Rd] Allowing S3 methods of rounding functions to take `...`

[Davis Vaughan]

I should also say that I would be willing to attempt a patch for this, if
others agree that this would be useful.

- Davis

On Thu, Jan 28, 2021 at 9:14 AM Davis Vaughan <davis using rstudio.com> wrote:

> Hi all,
>
> I would like to propose adding `...` to the signatures of the following
> rounding functions:
>
> - floor(x)
> - ceiling(x)
> - round(x, digits = 0)
> - And possibly signif(x, digits = 6)
>
> The purpose would be to allow S3 methods to add additional arguments as
> required.
>
> A few arguments in favor of this change:
>
> `trunc(x, ...)` already takes dots, which sets a precedent for the others
> to do so as well. It is documented in the same help file as the other
> rounding functions.
>
> Internally at the C level, a check is done to ensure that there is exactly
> 1 arg for floor() and ceiling(), and either 1 or 2 args for round(). The
> actual names of those arguments are not checked, however, and I believe
> this is what allows `round.Date(x, ...)` and `round.POSIXt(x, unit)` to
> exist, solely because they have 2 arguments. It seems like this is a bit of
> a hack, since you couldn't create something similar for floor, like
> `floor.POSIXt(x, unit)` (not saying this should exist, it is just for
> argument's sake), because the 1 argument check would error on this. I think
> adding `...` to the signature of the generics would better support what is
> being done here.
>
> Additionally, I have a custom date-like S3 class of my own that I would
> like to write floor(), ceiling(), and round() methods for, and they would
> require passing additional arguments.
>
> If R core would like to make this change, they could probably tweak
> `do_trunc()` to be a bit more general, and use it for floor() and
> ceiling(), since it already allows `...`.
>
> A few references:
>
> Check for 1 arg in do_math1(), used by floor() and ceiling()
>
> https://github.com/wch/r-source/blob/fe82da3baf849fcd3cc7dbc31c6abc72b57aa083/src/main/arithmetic.c#L1270
>
> Check for 2 args in do_Math2(), used by round()
>
> https://github.com/wch/r-source/blob/fe82da3baf849fcd3cc7dbc31c6abc72b57aa083/src/main/arithmetic.c#L1655
>
> do_trunc() definition that allows `...`
>
> https://github.com/wch/r-source/blob/fe82da3baf849fcd3cc7dbc31c6abc72b57aa083/src/main/arithmetic.c#L1329-L1340
>
> - Davis
>

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