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
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