Dynamic Scope with Special Variables

Older dialects of Lisp did not limit the scopes of variables to function bodies or other restricted regions of the program.

Variable values could be accessed anywhere in the program, provided the bindings were still in effect and were not shadowed by other bindings of the same symbols.

Common Lisp supports this dynamic scoping if you ask for it.

(defun double (x)

(declare (special x))

(announce)

(+ x x) )

(defun announce ()

(format t "The value of X is ~A." x) )

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