Problem Statement

Define two associative arrays, where one represents the following "base" data: And the other represents "update" data: Merge these into a new associative array that contains every key found in either of the source ones. Each key should map to the value in the second (update) table if that exists, or else to the value in the first (base) table. If possible, do this in a way that does not mutate the original two associative arrays. Obviously this should be done in a way that would work for any data, not just the specific data given here, but in this example the result should be:

Let's start with the solution:

(append list2 list1)


(defun merge-alists (alist1 alist2)
  (nconc
   (loop :for pair1 :in alist1
         :for pair2 := (assoc (car pair1) alist2)
         :do (setf alist2 (remove pair2 alist2))
         :collect (or pair2 pair1))
   alist2))

(defun merge-plists (plist1 plist2)
  (let ((res '()))
    (loop :for (key val) :on plist1 :by #'cddr
          :do (setf (getf res key) val))
    (loop :for (key val) :on plist2 :by #'cddr
          :do (setf (getf res key) val))
    res))