Lexical vs special variables

Global special variables are special everywhere

Thus these variables will use dynamic binding.

(defparameter count 0)
;; All uses of count will refer to this one 

(defun handle-number (number)
  (incf count)
  (format t "~&~d~%" number))
  
(dotimes (count 4)
  ;; count is shadowed, but still special
  (handle-number count))
  
(format t "~&Calls: ~d~%" count)
==>
0
2
Calls: 0

Give special variables distinct names to avoid this problem:

(defparameter *count* 0)

(defun handle-number (number)
  (incf *count*)
  (format t "~&~d~%" number))
  
(dotimes (count 4)
  (handle-number count))
  
(format t "~&Calls: ~d~%" *count*)
==>
0
1
2
3
Calls: 4

Note 1: it is not possible to make a global variable non-special in a certain scope. There is no declaration to make a variable lexical.

Note 2: it is possible to declare a variable special in a local context using the special declaration. If there is no global special declaration for that variable, the declaration is only locally and can be shadowed.

(defun bar ()
  (declare (special a))
  a)                       ; value of A is looked up from the dynamic binding

(defun foo ()
  (let ((a 42))            ; <- this variable A is special and
                           ;    dynamically bound
    (declare (special a))
    (list (bar)
          (let ((a 0))     ; <- this variable A is lexical
            (bar)))))


> (foo)
(42 42)


2016-07-24
2016-07-25
common-lisp Pedia
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