[Rd] Suggestion: default print method for S3 generics could offer some insights on '...' among registered methods

[Mikael Jagan]

I don't really understand the premise.  Any function F with '...' as a formal
argument can pass '...' to another function G.  The actual arguments matching
'...' in the call to F will be matched to the formal arguments of G.  So the
the maintainer of F may want to alert the user of F to the existence of G and
the user of F may want to consult the documentation of G.

Whether F is S3 generic and G is registered as a method for F seems irrelevant.

That is a conceptual issue.  There are practical issues, too:

     * print.default is used "everywhere".  Backwards incompatible changes to
       default behaviour have the potential to break a lot of code out there.

     * Testing that a function F is S3 generic seems nontrivial.  You have to
       deal with internally generic functions and for closures recurse through
       body(F) looking for a call to UseMethod.

     * I would not want the output of print(F) to depend on details external to
       F or the method call, such as the state of the table of registered S3
       methods which changes as packages are loaded.  AFAIK, it is intended that
       options() is the only exception to the rule.

     * More harmonious would be to implement the feature ("give me more
       information about S3 methods") as an option (disabled by default) of
       utils::.S3methods if not as a new function altogether.

Mikael

> Date: Fri, 6 Jun 2025 11:59:08 -0700
> From: Michael Chirico<michaelchirico4 using gmail.com>
> 
> There is a big difference in how to think of '...' for non-generic
> functions like data.frame() vs. S3 generics.
> 
> In the former, it means "any number of inputs" [e.g. columns]; in the
> latter, it means "any number of inputs [think c()], as well as any
> arguments that might be interpreted by class implementations".
> 
> Understanding the difference for a given generic can require carefully
> reading lots of documentation. print(<generic>), which is useful for
> so many other contexts, can be a dead end.
> 
> One idea is to extend the print() method to suggest to the reader
> which other arguments are available (among registered generics). Often
> ?<generic> will include the most common implementation, but not always
> so.
> 
> For rbind (in a --vanilla session), we currently have one method,
> rbind.data.frame, that offers three arguments not present in the
> generic: make.row.names, stringsAsFactors, and factor.exclude. The
> proposal would be to mention this in the print(rbind) output somehow,
> e.g.
> 
>> print(rbind)
> function (..., deparse.level = 1)
> .Internal(rbind(deparse.level, ...))
> <bytecode: 0x73d4fd824e20>
> <environment: namespace:base>
> 
> +Other arguments implemented by methods
> +  factor.exclude: rbind.data.frame
> +  make.row.names: rbind.data.frame
> +  stringsAsFactors: rbind.data.frame
> 
> I suggest grouping by argument, not generic, although something like
> this could be OK too:
> 
> +Signatures of other methods
> +  rbind.data.frame(..., deparse.level = 1, make.row.names = TRUE,
> stringsAsFactors = FALSE,
> +      factor.exclude = TRUE)
> 
> Where it gets more interesting is when there are many methods, e.g.
> for as.data.frame (again, in a --vanilla session):
> 
>> print(as.data.frame)
> function (x, row.names = NULL, optional = FALSE, ...)
> {
>      if (is.null(x))
>          return(as.data.frame(list()))
>      UseMethod("as.data.frame")
> }
> <bytecode: 0x73d4fc1e70d0>
> <environment: namespace:base>
> 
> +Other arguments implemented by methods
> +  base: as.data.frame.table
> +  check.names: as.data.frame.list
> +  col.names: as.data.frame.list
> +  cut.names: as.data.frame.list
> +  fix.empty.names: as.data.frame.list
> +  make.names: as.data.frame.matrix, as.data.frame.model.matrix
> +  new.names: as.data.frame.list
> +  nm: as.data.frame.bibentry, as.data.frame.complex, as.data.frame.Date,
> +    as.data.frame.difftime, as.data.frame.factor, as.data.frame.integer,
> +    as.data.frame.logical, as.data.frame.noquote, as.data.frame.numeric,
> +    as.data.frame.numeric_version, as.data.frame.ordered,
> +    as.data.frame.person, as.data.frame.POSIXct, as.data.frame.raw
> +  responseName: as.data.frame.table
> +  sep: as.data.frame.table
> +  stringsAsFactors: as.data.frame.character, as.data.frame.list,
> +    as.data.frame.matrix, as.data.frame.table
> 
> Or
> 
> +Signatures of other methods
> +  as.data.frame.aovproj(x, ...)
> +  as.data.frame.array(x, row.names = NULL, optional = FALSE, ...)
> +  as.data.frame.AsIs(x, row.names = NULL, optional = FALSE, ...)
> +  as.data.frame.bibentry(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.character(x, ..., stringsAsFactors = FALSE)
> +  as.data.frame.citation(x, row.names = NULL, optional = FALSE, ...)
> +  as.data.frame.complex(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.data.frame(x, row.names = NULL, ...)
> +  as.data.frame.Date(x, row.names = NULL, optional = FALSE, ..., nm =
> deparse1(substitute(x)))
> +  as.data.frame.default(x, ...)
> +  as.data.frame.difftime(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.factor(x, row.names = NULL, optional = FALSE, ..., nm
> = deparse1(substitute(x)))
> +  as.data.frame.ftable(x, row.names = NULL, optional = FALSE, ...)
> +  as.data.frame.integer(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.list(x, row.names = NULL, optional = FALSE, ...,
> cut.names = FALSE,
> +      col.names = names(x), fix.empty.names = TRUE, new.names =
> !missing(col.names),
> +      check.names = !optional, stringsAsFactors = FALSE)
> +  as.data.frame.logical(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.logLik(x, ...)
> +  as.data.frame.matrix(x, row.names = NULL, optional = FALSE,
> make.names = TRUE,
> +      ..., stringsAsFactors = FALSE)
> +  as.data.frame.model.matrix(x, row.names = NULL, optional = FALSE,
> make.names = TRUE,
> +      ...)
> +  as.data.frame.noquote(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.numeric(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.numeric_version(x, row.names = NULL, optional =
> FALSE, ..., nm = deparse1(substitute(x)))
> +  as.data.frame.ordered(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.person(x, row.names = NULL, optional = FALSE, ..., nm
> = deparse1(substitute(x)))
> +  as.data.frame.POSIXct(x, row.names = NULL, optional = FALSE, ...,
> nm = deparse1(substitute(x)))
> +  as.data.frame.POSIXlt(x, row.names = NULL, optional = FALSE, ...)
> +  as.data.frame.raw(x, row.names = NULL, optional = FALSE, ..., nm =
> deparse1(substitute(x)))
> +  as.data.frame.table(x, row.names = NULL, ..., responseName =
> "Freq", stringsAsFactors = TRUE,
> +      sep = "", base = list(LETTERS))
> +  as.data.frame.ts(x, ...)
> 
> Obviously that's a bit more cluttered, but as.data.frame() should be a
> pretty unusual case. It also highlights better the differences in the
> two approaches: the former economizes on space and focuses on what
> sorts of arguments are available; the latter shows the defaults, does
> not hide the arguments shared with the generic, and will always
> produce as many lines as there are methods.
> 
> There are other edge cases to think through (multiple registrations,
> interactions with S4, primitives, ...), but I want to first check with
> the list if this looks workable & valuable enough to pursue.
> 
> Mike C
> 
> ----
> 
> Code that helped with the above:
> 
> f = as.data.frame
> # NB: methods() and getAnywhere() require {utils}
> m = methods(f)
> generic_args = names(formals(f))
> f_methods = lapply(m, \(fn) getAnywhere(fn)$objs[[1L]])
> names(f_methods) = m
> new_args = sapply(f_methods, \(g) setdiff(names(formals(g)), generic_args))
> with( # group by argument name
>    data.frame(method = rep(names(new_args), lengths(new_args)), arg =
> unlist(new_args), row.names=NULL),
>    {tbl = tapply(method, arg, toString); writeLines(paste0(names(tbl),
> ": ", tbl))}
> )
> signatures=sapply(f_methods, \(g) paste(head(format(args(g)), -1),
> collapse="\n"))
> writeLines(paste0(names(signatures), gsub("^\\s*function\\s*", "", signatures)))