来自?dotsMethods

 Beginning with version 2.8.0 of R, S4 methods can be dispatched
 (selected and called) corresponding to the special argument “...”.
 Currently, “...” cannot be mixed with other formal arguments:
 either the signature of the generic function is “...” only, or it
 does not contain “...”.  (This restriction may be lifted in a
 future version.)


这是EBImage包中的一些代码:

## image IO, display
setGeneric ("image", function (x, ...) standardGeneric("image") )

## statistics
setGeneric ("hist", function (x, ...) standardGeneric("hist") )


这显然违反了“...” cannot be mixed with other formal arguments规则。这是否意味着该限制已被取消但未记录在案?

最佳答案

您引用的部分中的关键字是“分派”。这里

setGeneric("foo", function(x, ...) standardGeneric("foo"))


您可以基于“ x”类编写方法。

.A = setClass("A", "integer")
.B = setClass("B", "integer")
setMethod("foo", "A", function(x, ...) "foo,A-method")


仍然可以使用“ ...”,例如,提供特定于方法的参数,但没有用于“ ...”的分派。

setMethod("foo", "B", function(x, barg, ...) sprintf("barg=%d", barg))




> foo(.B(), barg=123)
[1] "barg=123"


这就是EBImage使用“ ...”的方式,这是一个非常常见的用例。

这里

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


您可以编写在“ ...”上分派的方法,前提是所有类均相同

setMethod("bar", "A", function(...) "bar,A-method")




> bar(.A(), .A())
[1] "bar,A-method"
> bar(.A(), .B())
Error in standardGeneric("bar") :
  no method or default matching the "..." arguments in bar(.A(), .B())
> setMethod("bar", c("A", "B"), function(...) "bar,A,B-method")
Error in matchSignature(signature, fdef) :
  more elements in the method signature (2) than in the generic signature (1) for function 'bar'


上面使用隐式规则来确定签名,并且getGeneric()显示用于分发的参数,其中输出指示“可以为参数定义方法:”,例如,

> getGeneric("foo")
standardGeneric for "foo" defined from package ".GlobalEnv"

function (x, ...)
standardGeneric("foo")
<environment: 0x2ba550a0>
Methods may be defined for arguments: x
Use  showMethods("foo")  for currently available ones.
> getGeneric("bar")
standardGeneric for "bar" defined from package ".GlobalEnv"

function (...)
standardGeneric("bar")
<environment: 0x2c127e58>
Methods may be defined for arguments: ...
Use  showMethods("bar")  for currently available ones.


R似乎可以让您定义在x上混合使用...

> setGeneric("baz", function(x, ...) standardGeneric("baz"),
             signature=c("x", "..."))
[1] "baz"


但实际上并非如此

> getGeneric("baz")
standardGeneric for "baz" defined from package ".GlobalEnv"

function (x, ...)
standardGeneric("baz")
<environment: 0x2c704cc0>
Methods may be defined for arguments: x
Use  showMethods("baz")  for currently available ones.

10-07 19:42
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