Why should someone use {} for initializing an empty object in R?

Question

It seems that some programmers are using:

a = {}
a$foo = 1
a$bar = 2

What is the benefit over a = list(foo = 1, bar = 2)?

Answer 1

Per R's documentation on braces and parentheses (type ?'{' to read them), braces return the last expression evaluated within them.

In this case, a <- {} essentially "returns" a null object, and is therefore equivalent to a <- NULL, which establishes an empty variable that can then be treated as a list.

Incidentally, this is why it is possible to write R functions in which the function's output is returned simply by writing the name of the returned variable as the function's final statement. For example:

function(x) {
    y <- x * 2
    return(y)
}

Is equivalent to:

function(x) {
    y <- x * 2
    y
}

Or even:

function(x) {
    y <- x * 2
}

The final line of the function being an assignment suppresses printing of the result in the console, but the function definitely returns the expected value if it saved into a variable.