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% File src/library/methods/man/dotsMethods.Rd

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% Copyright 1995-2016 R Core Team

% Distributed under GPL 2 or later

\name{dotsMethods}

\alias{dotsMethods}

\title{The Use of \code{...} in Method Signatures}

\description{

  The \dQuote{\dots} argument in \R functions is treated specially, in that it

  matches zero, one or more actual arguments (and so, objects).  A

  mechanism has been added to \R to allow \dQuote{\dots} as the signature of a

  generic function.  Methods defined for such functions will be

  selected and called when \emph{all}  the arguments matching \dQuote{\dots}

  are from the specified class or from some subclass of that class.

}

\section{Using "..." in a Signature}{

  Beginning with version 2.8.0 of \R, S4 methods can be dispatched

  (selected and called) corresponding to the special argument \dQuote{\dots}.

  Currently, \dQuote{\dots} cannot be mixed with other formal arguments:

  either the signature of the generic function is \dQuote{\dots} only, or it

  does not contain \dQuote{\dots}.  (This restriction may be lifted in a future

  version.)

  Given a suitable generic function, methods are specified in the

  usual way by a call to \code{\link{setMethod}}.  The method

  definition must be written expecting all the arguments corresponding

  to \dQuote{\dots} to be from the class specified in the method's signature,

  or from a class that extends that class (i.e., a subclass of that

  class).

  Typically the methods will pass \dQuote{\dots} down to another function or

  will create a list of the arguments and iterate over that.  See the

  examples below.

  When you have a computation that is suitable for more than one existing

  class, a convenient approach may be to define a union of these

  classes by a call to \code{\link{setClassUnion}}. See the example

  below.

}

\section{Method Selection and Dispatch for "..."}{

  See \link{Methods_Details} for a general discussion.  The following assumes

  you have read the \dQuote{Method Selection and Dispatch} section of

  that documentation.

  A method selecting on \dQuote{\dots} is specified by a single class in the

  call to \code{\link{setMethod}}.  If all the actual arguments

  corresponding to \dQuote{\dots} have this class, the corresponding method is

  selected directly.

  Otherwise, the class of each argument and that class' superclasses are

  computed, beginning with the first \dQuote{\dots} argument.  For the first

  argument, eligible methods are those for any of the classes.   For

  each succeeding argument that introduces a class not considered previously, the eligible methods are further

  restricted to those matching the argument's class or

  superclasses. If no further eligible classes exist, the iteration

  breaks out and the default method, if any, is selected.

  At the end of the iteration, one or more methods may be eligible.

  If more than one, the selection looks for the method with the least

  distance to the actual arguments.  For each argument, any inherited

  method corresponds to a distance, available from the \code{contains}

  slot of the class definition.  Since the same class can arise for

  more than one argument, there may be several distances associated

  with it.  Combining them is inevitably arbitrary:  the current

  computation uses the minimum distance.  Thus, for example, if a

  method matched one argument directly, one as  first generation

  superclass and another as a second generation superclass, the

  distances are 0, 1 and 2.  The current selection computation would

  use distance 0 for this

  method.  In particular, this selection criterion tends to use a method that

  matches exactly one or more of the arguments' class.

  As with ordinary method selection, there may be multiple methods

  with the same distance.  A warning  message is issued and one of the

  methods is chosen (the first encountered, which in this case is

  rather arbitrary).

  Notice that, while the computation examines all arguments, the

  essential cost of dispatch goes up with the number of

  \emph{distinct} classes among the arguments, likely to be much

  smaller than the number of arguments when the latter is large.

}

\section{Implementation Details}{

  Methods dispatching on \dQuote{\dots} were introduced in version 2.8.0 of

  \R.  The initial implementation of the corresponding selection and

  dispatch is in an R function, for flexibility while the new

  mechanism is being studied.  In this implementation, a local version

  of \code{standardGeneric} is inserted in the generic function's

  environment.  The local version selects a method according to the

  criteria above and calls that method, from the environment of the

  generic function.  This is slightly different from the action taken

  by the C implementation when \dQuote{\dots} is not involved.  Aside from the

  extra computing time required, the method is evaluated in a true

  function call, as opposed to the special context constructed by the

  C version (which cannot be exactly replicated in R code.)  However,

  situations in which different computational results would

  be obtained have not been encountered so far, and seem very

  unlikely.

  Methods dispatching on arguments other than \dQuote{\dots} are \emph{cached} by storing

  the inherited method in the table of all methods, where it will be

  found on the next selection with the same combination of classes

  in the actual arguments (but not used for inheritance searches).

  Methods based on \dQuote{\dots} are also cached, but not found quite

  as immediately.  As noted, the selected method depends only on the

  set of classes that occur in the \dQuote{\dots} arguments.  Each of

  these classes can appear one or more times, so many combinations of

  actual argument classes will give rise to the same effective

  signature.  The selection computation first computes and sorts the

  distinct classes encountered.  This gives a label that will be

  cached in the table of all methods, avoiding any further search for

  inherited classes after the first occurrence.  A call to

  \code{\link{showMethods}} will expose such inherited methods.

  The intention is that the \dQuote{\dots} features will be added to the

  standard C code when enough experience with them has been obtained.

  It is possible that at the same time, combinations of \dQuote{\dots} with

  other arguments in signatures may be supported.

}

\examples{

cc <- function(...)c(...)

setGeneric("cc")

setMethod("cc", "character", function(...)paste(...))

setClassUnion("Number", c("numeric", "complex"))

setMethod("cc", "Number", function(...) sum(...))

setClass("cdate", contains = "character", slots = c(date = "Date"))

setClass("vdate", contains = "vector", slots = c(date = "Date"))

cd1 <- new("cdate", "abcdef", date = Sys.Date())

cd2 <- new("vdate", "abcdef", date = Sys.Date())

stopifnot(identical(cc(letters, character(), cd1),

           paste(letters, character(), cd1))) # the "character" method

stopifnot(identical(cc(letters, character(), cd2),

                    c(letters, character(), cd2)))

# the default, because "vdate" doesn't extend "character"

stopifnot(identical(cc(1:10, 1+1i), sum(1:10, 1+1i))) # the "Number" method

stopifnot(identical(cc(1:10, 1+1i, TRUE), c(1:10, 1+1i, TRUE))) # the default

stopifnot(identical(cc(), c())) # no arguments implies the default method

setGeneric("numMax", function(...)standardGeneric("numMax"))

setMethod("numMax", "numeric", function(...)max(...))

# won't work for complex data

setMethod("numMax", "Number", function(...) paste(...))

# should not be selected w/o complex args

stopifnot(identical(numMax(1:10, pi, 1+1i), paste(1:10, pi, 1+1i)))

stopifnot(identical(numMax(1:10, pi, 1), max(1:10, pi, 1)))

try(numMax(1:10, pi, TRUE)) # should be an error:  no default method

## A generic version of paste(), dispatching on the "..." argument:

setGeneric("paste", signature = "...")

setMethod("paste", "Number", function(..., sep, collapse) c(...))

stopifnot(identical(paste(1:10, pi, 1), c(1:10, pi, 1)))

\dontshow{

for(gen in c("numMax", "cc", "paste")) removeGeneric(gen)

for(cl in c("Number", "vdate", "cdate")) removeClass(cl)

}

}

\references{

  \bibshow{R:Chambers:2008}

  (For the R version.)

  \bibshow{R:Chambers:1998}

  (For the original S4 version.)

}

\seealso{

For the general discussion of methods, see  \link{Methods_Details} and links

from there.

}

\keyword{programming}

\keyword{classes}

\keyword{methods}