r - 在 s4 泛型方法中混合 ... 与正式参数

Question

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“...”不能与其他正式参数混合规则。这是否意味着限制已经取消但没有记录？

Answer 1

您引用的部分中的关键字是“已 dispatch ”。这里

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

您可以根据“x”的类编写方法。

.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，但是没有可用于“...”的分派(dispatch)

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

与

> 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(dispatch)的方法，前提是所有类都相同

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

与

> 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，并且派发的参数由 getGeneric() 显示，其中输出指示“Methods may be defined for arguments:”，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 似乎允许您定义在 x 上分派(dispatch)的泛型...

> 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.