Great question about how rlang's := operator works—let’s unpack this step by step, since it’s a bit tricky due to how non-standard evaluation (NSE) operates in rlang.

First: The R Function You Found Is a "Safety Net" #

You came across this R code for :=:

#' @rdname dyn-dots
#' @usage NULL
#' @export
`:=` <- function(x, y) { abort("`:=` can only be used within dynamic dots.", call = caller_env()) }

This function is not the actual implementation of the operator’s core logic. Instead, it’s a fallback mechanism: R requires all operators to have a corresponding function definition (otherwise you’d get a confusing "could not find function":=" error if you tried to reference it anywhere). But when you use := correctly within dynamic dots (like with !!!, dplyr verbs, or rlang’s dots_list()), this function is never actually executed.

The Real Work Happens in C Code #

:= is a core component of rlang’s dynamic dots system, which relies on syntax manipulation during parsing—before your code is evaluated. This kind of low-level expression handling is implemented in C for performance, and to directly hook into R’s evaluation engine.

The actual logic for processing := lives in rlang’s C source files, primarily in modules that handle dot processing and quosures. Key places to look include:

• src/dots.c: Contains code that parses and transforms dynamic dots, including recognizing and handling := pairs.
• src/quosure.c and src/expr.c: Hold related logic for working with quosures (rlang’s wrapped expressions) and unquoting behavior.

When you write code like !!var_name := new_value in a function that uses dynamic dots, rlang’s parser intercepts the := at parse time:

1. It evaluates the left-hand side (LHS) of := (using unquoting tools like !!) to resolve it into a valid variable name.
2. It treats the right-hand side (RHS) as the value associated with that name.
3. It converts this pair into a standard named argument that R’s evaluation engine can process—all before any R-level function execution occurs.

Why the R Function Exists #

That R-level := function serves two key purposes:

• It satisfies R’s syntax requirement that all operators have a defined function.
• It provides a clear, helpful error message if someone mistakenly uses := outside of dynamic dots (e.g., in regular R code), so you don’t get a vague "undefined operator" error.

内容的提问来源于stack exchange，提问作者Yuli S