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(elisp)Scope

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Scope
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   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:

   * If we call `user' directly without calling `binder' at all, then
     whatever binding of `x' is found, it cannot come from `binder'.

   * If we define `foo' as follows and then call `binder', then the
     binding made in `binder' will be seen in `user':

          (defun foo (lose)
            (user))

   * However, if we define `foo' as follows and then call `binder',
     then the binding made in `binder' _will not_ be seen in `user':

          (defun foo (x)
            (user))

     Here, when `foo' is called by `binder', it binds `x'.  (The
     binding in `foo' is said to "shadow" the one made in `binder'.)
     Therefore, `user' will access the `x' bound by `foo' instead of
     the one bound by `binder'.

   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.