R语言函数形参内存存储位置及未调用时存在性技术问询

Let’s break down these questions one by one—they cut to the core of how R manages functions and their parameters behind the scenes:

1. Where are a function's formals stored in memory in R? #

In R, every function is an object, and its formals (parameter definitions) are stored as a built-in attribute of the function object itself. You can inspect this directly with the formals() function. For example:

my_fun <- function(x, y = 1) {x + y}
formals(my_fun)
# Output: $x; $y = 1

These formals live alongside the function’s body (body(my_fun)) and its enclosing environment (environment(my_fun)), forming the three key components of the function object. They aren’t stored in a separate execution or enclosing environment—they’re part of the function itself.

2. When a function is defined but not yet called, do formals without default values exist? If so, where? #

Yes, formals without default values do exist, but only as named placeholders in the function’s formals attribute. They don’t hold any actual values (since the function hasn’t been called to bind values to them), and they aren’t located in an execution environment (which only gets created when the function runs) or the function’s enclosing environment. They’re purely part of the function object’s parameter list.

Take this example:

my_fun <- function(x, y) {x + y}
formals(my_fun)
# Output: $x; $y

Here, x and y are present as formal parameters, but no values are assigned to them until the function is invoked.

3. When a function is defined but not yet called, do the values of formals with default values exist? If so, in which environment? #

This is a common point of confusion: the actual value of a default parameter does NOT exist until the function is called. What’s stored in the function’s formals is the expression for the default value, not its evaluated result.

For example:

a <- 10
my_fun <- function(x = a) {x}
formals(my_fun)$x
# Output: a

The expression a is stored, not the value 10. When you call my_fun() without passing x, R evaluates this default expression in the function’s enclosing environment (the environment where the function was defined) to get the actual value, which is then bound to x in the function’s temporary execution environment.

4. If the default expression evaluates to a constant, has the formal been assigned a value? #

Even for constant default expressions (like x = 5), the formal hasn’t been assigned a usable value until the function is called. While R might store the constant itself in the formals (instead of an unevaluated expression), this is just part of the function’s parameter definition—not a bound value in any environment.

my_fun <- function(x = 5) {x}
formals(my_fun)$x
# Output: 5

That 5 is stored in the function’s formals, but it doesn’t become the value of x until you call my_fun(). At that point, the constant is evaluated (trivially, since it’s already a value) and bound to x in the function’s execution environment.

内容的提问来源于stack exchange，提问作者andrewH