merge

util.lisp

(defun merge-structs (type &rest objs)
  "values appearing earlier have higher precedence. nil interpreted as undefined"
  (loop with ret = (make-instance type)
     with slots = (loop for slot in (sb-mop:class-direct-slots (find-class type))
                     collect (slot-value slot 'SB-PCL::NAME))
     for obj in (reverse objs) if obj do
       (loop for slot in slots
          as val = (slot-value obj slot)
          if val do
            (setf (slot-value ret slot) val))
     finally (return ret)))