`...` 是什么对象？
What kind of object is `...`?
我最近考虑了函数的 ... 参数，并注意到 R 不允许检查对象的 class。
f <- function(...) {
class(...)
}
f(1, 2, 3)
## Error in class(...) : 3 arguments passed to 'class' which requires 1
现在引用
“To understand computations in R, two slogans are helpful:
• Everything that exists is an object. • Everything that happens is a
function call."
— John Chambers
我在想：... 是个什么样的物体？
多么有趣的问题！
点-点-点 ... 是一个对象（John Chambers 是对的！），它是一种配对列表。嗯，我查了下文档，分享给大家：
The ‘...’ object type is stored as a type of pairlist. The components of ‘...’ can be accessed in the usual pairlist manner from C code, but is not easily accessed as an object in interpreted code. The object can be captured as a list.
Another chapter 详细定义配对列表：
Pairlist objects are similar to Lisp’s dotted-pair lists.
Pairlists are handled in the R language in exactly the same way as generic vectors (“lists”).
关于 Generic and Dotted Pairs 的帮助说：
Almost all lists in R internally are Generic Vectors, whereas traditional dotted pair lists (as in LISP) remain available but rarely seen by users (except as formals of functions).
上有一个很好的总结！
我最近考虑了函数的 ... 参数，并注意到 R 不允许检查对象的 class。
f <- function(...) {
class(...)
}
f(1, 2, 3)
## Error in class(...) : 3 arguments passed to 'class' which requires 1
现在引用
“To understand computations in R, two slogans are helpful:
• Everything that exists is an object. • Everything that happens is a function call."
— John Chambers
我在想：... 是个什么样的物体？
多么有趣的问题！
点-点-点 ... 是一个对象（John Chambers 是对的！），它是一种配对列表。嗯，我查了下文档，分享给大家：
The ‘...’ object type is stored as a type of pairlist. The components of ‘...’ can be accessed in the usual pairlist manner from C code, but is not easily accessed as an object in interpreted code. The object can be captured as a list.
Another chapter 详细定义配对列表：
Pairlist objects are similar to Lisp’s dotted-pair lists.
Pairlists are handled in the R language in exactly the same way as generic vectors (“lists”).
关于 Generic and Dotted Pairs 的帮助说：
Almost all lists in R internally are Generic Vectors, whereas traditional dotted pair lists (as in LISP) remain available but rarely seen by users (except as formals of functions).