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Afternoon All, I created two functions that use let. ...

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Question by juanfie
Submitted on 3/25/2004
Related FAQ: FAQ: Lisp Frequently Asked Questions 1/7 [Monthly posting]
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Afternoon All,

I created two functions that use let.  Both are pretty much the same, except that in one, I initialize a list using quote and later on modify it using setf; in the second one I modify the variable using setq.

(defun test (x)
  (let ((lista '(a)))
    (print lista)
    (setf (cdr lista) x)
    (print lista)
    lista))

(defun test2 (x)
  (let ((lista '(a)))
    (print lista)
    (setq lista (cons (car lista) x))
    (print lista)
    lista))

When I run test repeatedly, it seems to keep the variable's value as if it was global.  The second one does not maintain the value from the previous one.  Here's the run under Franz CL (Solaris) - same results using other implementations.

; Loading /users/juan/Homeworks/Lisp/let.lisp
CL-USER(12): (test 3)

(A)
(A . 3)
(A . 3)
CL-USER(13): (test 5)

(A . 3)
(A . 5)
(A . 5)
CL-USER(14): (test2 3)

(A)
(A . 3)
(A . 3)
CL-USER(15): (test2 5)

(A)
(A . 5)
(A . 5)
CL-USER(16): :Ex
; Exiting Lisp

Can anyone explain this behavior?

ThX a million.

Juan Flores


Answer by Tagore Smith
Submitted on 6/24/2004
Rating: Not yet rated Rate this answer: Vote
This is not an issue of scope- it is not the binding of your variable that is being "kept", it is the actual memory used to store the literal '(a) that is being modified. Basically, you should never destructively modify a literal, as your implementation may choose to keep only one copy of a literal and reuse that every time that literal is seen.

So the real answer is- don't do that :). If you are going to destructively modify a list in this fashion you should cons up a fresh one, using the list function (or some other function, like cons, that allocates a new list). So your let should look like: (let ((lista (list 'a)))...)

This is a pretty big gotcha, but it becomes second nature after a while- the basic rule is, if you are going to destructively modify a list, make sure that you created that list at runtime.
  

 

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