CL JSON Round Trip Question

ggxx

Assume we're using the following libraries.

Assume the following code exists.

(defun to-camel-case (k)
  "Change the symbol `k' to a camelCase string (for JSON serialization)."
  (change-case:camel-case (string k)))

(defclass post ()
  ((blog-id
    :initarg :blog-id
    :accessor blog-id
    :documentation "")
   (posted-on
    :initarg :posted-on
    :accessor posted-on
    :documentation "datetime of post")
   (title
    :initarg :title
    :accessor title
    :documentation "")
   (content
    :initarg :content
    :accessor content
    :documentation "content of post")))

(defparameter p1 (make-instance 'post :blog-id 1 :posted-on "2026-04-18T14:00:00" :title "Hi" :content "Hello"))

(defparameter json (jzon:stringify p1 :coerce-key #'to-camel-case))

(defparameter data (jzon:parse json :key-fn #'change-case:param-case))

(defparameter p2 (new 'post data))

How would you implement new such that p1 and p2 had the same slot values?

kagevf

Assuming that p1 and p2 are the slot names, something like this:

(defmethod initialize-instance :after ((post post) &key)
  "Initializations: 
- Set p2 to the same value as p1."
  (setf (slot-value post 'p2) (slot-value post 'p1)))```

kagevf

Ah, somehow I missed the bottom 3 lines of your post, so I might have completely missed what you were after. I'm not sure what new is though, even now.

ggxx

kagevf This is what I came up with. It only works if the data is shallow, but it was good enough for me in the moment.

(defun new (class-name hash-table)
  "Instantiate a new instance of `class-name' using the data from `hash-table'."
  (apply #'make-instance
         class-name
         (loop for k being the hash-key using (hash-value v) of hash-table
               append (list (intern (string-upcase k) :keyword) v))))

I just wanted to call make-instance and feed it data from an arbitrary data structure.

kagevf

I see. Code looks nice and clean! Do the keys have to be upcased when interning? I thought that happened automatically. And a quick test shows that, no, upcasing does not happen automatically ... oh, well ...