Indexing a list with an empty index

Question

The technique of indexing a data frame with an empty index features several times in Hadley Wickam's Advanced R, but is only explained there in passing. I'm trying to figure out the rules governing indexing a list with an empty index. Consider the following four statements.

> (l <- list(a = 1, b = 2))
$a
[1] 1

$b
[1] 2

> (l[] <- list(c = 3))
$c
[1] 3

> l
$a
[1] 3

$b
[1] 3

> l[]
$a
[1] 3

$b
[1] 3

Questions:

1. Why is the output from second statement different from the output from the third statement? Isn't assignment supposed to return the object being assigned to, in which case the second statement should yield the same output as the third one?
2. How come did the assignment in the second statement result in the output shown after the third statement? What are the rules governing assignment to an emptily indexed list?
3. How come did the fourth statement yield the output shown? What are the rules governing indexing a list with an empty index when it is not on the left hand side of an assignment?

Answer 1

In short l[] will return the whole list.

(l <- list(a = 1, b = 2))
l[]

l[] <- list(c=3) is essentially reassigning what was assigned to each index to now be the result of list(c=3). For this example, it is the same as saying l[[1]] <- 3 and l[[2]] <- 3. From the ?'[' page, which mentions empty indexing a few times:

When an index expression appears on the left side of an assignment (known as subassignment) then that part of x is set to the value of the right hand side of the assignment.

and also

An empty index selects all values: this is most often used to replace all the entries but keep the attributes.

So, I roughly take this to mean each index of l should evaluate to list(c=3).

When you enter (l[] <- list(c = 3)) what is being returned is the replacement value. When you then enter l or l[] you will see that the values at each index have been replaced by list(c=3).