Hello,
I am curently translating the logic circuit simulator code from SICP into Common Lisp and have run into a snag that I would like to ask about.
The Scheme code is as follows from section 3.3.4 (page 223 of my hardcover edition):
(define (and-gate a1 a2 output) (define (and-action-procedure) (let ((new-value (locical-and (get-signal a1) (get-signal a2)))) (after-delay and-gat-delay (lambda () (set-signal! output new-value))))) (add-action! a1 and-action-procedure) (add-action! a2 and-action-procedure))
The code basically binds the local function and-action-procedure into a list of functions in a1 and a2 which are then called by another routine later to perform the action when a value is set on the wire.
My translated Common Lisp code is:
(defun make-and-gate (a1 a2 output) (flet ((logical-and (a b) (if (= a 1) b a)) (and-action-proc () (let ((new-value (logical-and (get-signal a1) (get-signal a2)))) (set-signal output new-value)))) (add-action a1 #'and-action-proc) (add-action a2 #'and-action-proc)))
The problem is that the bindings of a1 and a2 in and-action-proc are not being bound to the values of the calling function in the Common Lisp version. This causes the calling of and-action-proc to fail because a1 and a2 are not bound and I am assuming being treated as dynamic (special) variables. I have also tried to define and-action-proc using labels with no difference in results.
Is this a total schemantic difference between CL and Scheme? Is every function in Scheme a closure where free variables of sub-functions bound to the calling functions environment? If so, is there a way to emulate this in Common Lisp?
I would also like to note that I am currently trying out writing this code using LispWorks Personal Edition, rather than my usual SBCL environment, if that would make any difference.
-- Burton Samograd