Change variable defined in let function

Question

Given the following let function:

(let ((foo (list a b c d)))
foo)

How can i modify the list foo?

(let ((foo (list a b c d)))
;some code
foo)

so the returned foo looks like eks: '(some new list) or (a b modified d)

i tried (set) but foo will still return as the original list.

Answer 1

You can use setf to modify a specific element in a list.

(let ((foo (list 'a 'b 'c 'd)))
  (setf (third foo) 'modified)
  foo)

This will return (a b modified d).

Or if you want to replace the whole variable, you can assign directly to it with setq:

(let ((foo (list 'a 'b 'c 'd)))
  (setq foo (list 'some 'new 'list))
  foo)

You can't use set with lexical variables, only special variables.