R in a Nutshell, 2nd Edition

Symbols

When you define a variable in R, you are actually assigning a symbol to a value in an environment. For example, when you enter the statement:

> x <- 1

on the R console, it assigns the symbol x to a vector object of length 1 with the constant (double) value 1 in the global environment. When the R interpreter evaluates an expression, it evaluates all symbols. If you compose an object from a set of symbols, R will resolve the symbols at the time the object is constructed:

> x <- 1
> y <- 2
> z <- 3
> v <- c(x, y, z)
> v
[1] 1 2 3
> # v has already been defined, so changing x does not change v
> x <- 10
> v
[1] 1 2 3

It is possible to delay evaluation of an expression so that symbols are not evaluated immediately:

> x <- 1
> y <- 2
> z <- 3
> v <- quote(c(x, y, z))
> eval(v)
[1] 1 2 3
> x <- 5
> eval(v)
[1] 5 2 3

It is also possible to create a promise object in R to delay evaluation of a variable until it is (first) needed. You can create a promise object through the delayedAssign function:

> x <- 1
> y <- 2
> z <- 3
> delayedAssign("v", c(x, y, z))
> x <- 5
> v
[1] 5 2 3

Promise objects are used within packages to make objects available to users without loading them into memory. Unfortunately, it is not possible to determine if an object is a promise object, nor is it possible to figure out the environment in which it was created.