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GNU Emacs Lisp Reference Manual

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11.9.1 Scope

Emacs Lisp uses indefinite scope for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions:

(defun binder (x)   ; x is bound in binder.
   (foo 5))         ; foo is some other function.

(defun user ()      ; x is used ``free'' in user.
  (list x))

In a lexically scoped language, the binding of x in binder would never be accessible in user, because user is not textually contained within the function binder. However, in dynamically-scoped Emacs Lisp, user may or may not refer to the binding of x established in binder, depending on the circumstances:

Emacs Lisp uses dynamic scoping because simple implementations of lexical scoping are slow. In addition, every Lisp system needs to offer dynamic scoping at least as an option; if lexical scoping is the norm, there must be a way to specify dynamic scoping instead for a particular variable. It might not be a bad thing for Emacs to offer both, but implementing it with dynamic scoping only was much easier.

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