Append lists smartly?

Question

How do I append two lists together, and delete duplicates?

For example:

(setq list1 ("a" "b" "c"))
(setq list2 ("b" "c" "d"))

(setq list3 (a b c))
(setq list4 (c d e))

;; use case
(append-list-to-list list1 list2) ; ("a" "b" "c" "d")
(append-list-to-list list3 list4) ; (a b c d e)

UPDATE after the cl-union answer: I need to preserve the order of the resulting list.

And when there is a nested list, how do I append and make element is unique in list.

For example:

(a (b c) d) or ((a b) c d) etc
add list (x y z) to list ((a b) c d) to become ((a b x y z) c d)

Then how to append a new list to upper nested list?

Answer 1

I think you meant your first case to result in '(a b c d) but correct me if I'm wrong.

It seems like you're looking for the logical union, this is available as cl-union or if you want to use the dash.el library you can use their provided -union.

(cl-union '(a b c) '(b c d)) -> '(a b c d)

Answer 2

append does what the first part of your question asks:

(append &rest SEQUENCES)

Concatenate all the arguments and make the result a list. The result is a list whose elements are the elements of all the arguments. Each argument may be a list, vector or string. The last argument is not copied, just used as the tail of the new list.

delete-dups does what the second part of your question asks:

(delete-dups LIST)

Destructively remove equal duplicates from LIST. Store the result in LIST and return it. LIST must be a proper list. Of several equal occurrences of an element in LIST, the first one is kept.

(setq temp1 '(a b c))
(setq temp2 '(c d e))

(append temp1 temp2)                ; => (a b c c d e)
(delete-dups (append temp1 temp2))  ; => (a b c d e)

There are also Common Lisp options in the cl-lib library, if you're interested.

I did not understand what you meant in your question update about nested lists. Let me suggest you remove the nested lists from this question and post it as a separate question.