newLISP: modifying association list

Question

I have a problem with modifying entries of an association list. When I run this code

Example A

(set 'Dict '(("foo" "bar")))

(letn (key "foo"
       entry (assoc key Dict))
  (setf (assoc key Dict) (list key "new value")))

(println Dict)

the result is:

(("foo" "new value")) ; OK

which is expected. With this code

Example B

(set 'Dict '(("foo" "bar")))

(letn (key "foo"
       entry (assoc key Dict))
  (setf entry (list key "new value"))) ; the only change is here

(println Dict)

the result is:

(("foo" "bar")) ; huh?

Why the Dict is not being updated in the second case?

Edit

What I want is to check if an entry is in the Dict and if it is - update it, otherwise leave it alone. With letn I want to avoid a duplicated code

(letn (key "foo"
       entry (assoc key Dict))
  (if entry ; update only if the entry is there
    (setf entry (list key "new value")))

Answer 1

In the letn expression the variable entry contains a copy of the association not a reference. Set the association directly as shown in Cormullion's example:

(setf (assoc key Dict) (list key "new value"))

In the newLISP programming model everything can be referenced only once. Assignment always makes a copy.

Answer 2

My understanding of association lists is that they work like this:

> (set 'data '((apples 123) (bananas 123 45) (pears 7)))
((apples 123) (bananas 123 45) (pears 7))
> (assoc 'pears data)
(pears 7)
> (setf (assoc 'pears data) '(pears 8))
(pears 8)
> data
((apples 123) (bananas 123 45) (pears 8))
> (assoc 'pears data)
(pears 8)
>

If you want to check for the existence of a key and update its value, do something like this:

(letn (key "foo")
   (if (lookup key Dict)
       (setf (assoc key Dict) (list key "new value"))))