Add x[subset] to effectively be short for as.list(x)[subset]?

[HenrikBengtsson]

Question

To better emulate list:s, should subsetting via [ also be supported?

Example

Currently, we get:

> x <- listenv()
> x$a <- 1
> x$b <- 2
> x$c <- 3
> as.list(x)[2:3]
$b
[1] 2

$c
[1] 3

> x[2:3]
Error in x[2:3] : object of type 'environment' is not subsettable

After adding support for [, we should get a list:

> x[2:3]
$b
[1] 2

$c
[1] 3

...or a list environment? (see below).

Requirements

• To minimize overhead, and to avoid blocking scenarios (e.g. via futures), the subsetting should implemented such that only the elements requested are queried/touched/retrieved and all other are ignored.
• Should it return a list or a listenv? Note that it is not possible to return a listenv while not copying the elements. In other words, doing y <- x[1:2]; y[1] <- 4 will not change x[1]. Because of this, does it make more sense to return a list? Also, note that environment cannot be subetted; if they would then listenv would also be subsettable automatically.

[HenrikBengtsson]

Decided to have x[1:3] return a list environment.