R 中的词汇作用域

Lexical scoping, also known as static scoping, is a fundamental convention for looking up variable names in a 编程语言. In R, when a function needs to access a variable that is not defined locally (a “free variable”), it searches for that variable in the environment where the function was created. This “creation environment” is permanently attached to the function. This contrasts with dynamic scoping, used in some older languages, where the search would proceed up the chain of function calls (the “calling environment”).

For example, consider a function `f()` that uses a variable `x` but doesn’t define it. If `f()` is defined in the global environment where `x` is 10, then no matter where `f()` is called from, it will always use `x = 10`. Even if it’s called from another function that has its own local `x = 20`, `f()` will ignore the calling environment’s `x` and use the one from its definition environment. This behavior makes code easier to debug and reason about, as a function’s behavior is determined solely by its own code and the environment in which it was defined.

This feature is crucial for functional programming paradigms, enabling powerful constructs like closures, where a function can “remember” the environment in which it was created. This is heavily utilized in advanced R programming and package development, such as in the `lapply` family of functions and in frameworks like Shiny for web applications.