为什么形式函数在用参数定义的函数上返回NULL？

Question

From base R , formals function should grant access and allow manipulation of the Formal Arguments. So, why does it return NULL on some functions defined with classic arguments ?

> ls
function (name, pos = -1L, envir = as.environment(pos), all.names = FALSE, 
    pattern, sorted = TRUE) 
{ ...

> formals(ls)
$name


$pos
-1L

$envir
as.environment(pos)

$all.names
[1] FALSE

$pattern


$sorted
[1] TRUE


> sum
function (..., na.rm = FALSE)  .Primitive("sum")
> formals(sum)
NULL

I expect formals(sum) to provide a length two list result, not NULL

Answer 1

The problem with your choice of function is apparent from its body. Primitive functions are somewhat different in that their argument list may have some names but their arguments are passed to C code and formals() , body() , and environment() all return NULL . Try it with apply instead.