Update struct field

Question

Is there a more ergonomic way to apply a function to a field in a struct in Elisp?

Say I have the following:

(cl-defstruct stack xs)

(defvar stack (make-stack :xs '(1 2 3 4 5)))

Is there an easy way to apply functions to the :xs field. I'd like an API like this:

(update-field :xs stack (lambda (xs)
                          (cl-map 'list #'1+ '(1 2 3 4 5))))

Does anyone know if this exists?

Update: I'm looking for a way to DRY up the calls to (stack-xs stack) (see below). What I'm looking for is more similar to something like Map.update from Elixir.

(setf (stack-xs stack) (cl-map 'list #'1+ (stack-xs stack)))

Answer 1

I ended up solving this by writing a macro, struct/update:

(defmacro struct/update (type field f xs)
  "Apply F to FIELD in XS, which is a struct of TYPE.
This is immutable."
  (let ((copier (->> type
                     symbol-name
                     (string/prepend "copy-")
                     intern))
        (accessor (->> field
                       symbol-name
                       (string/prepend (string/concat (symbol-name type) "-"))
                       intern)))
    `(let ((copy (,copier ,xs)))
       (setf (,accessor copy) (funcall ,f (,accessor copy)))
       copy)))

I use this macro as such:

(defun set/add (x xs)
  "Add X to set XS."
  (struct/update set
                 xs
                 (lambda (table)
                   (let ((table-copy (ht-copy table)))
                     (ht-set table-copy x 10)
                     table-copy))
                 xs))

Which will update the following struct:

(cl-defstruct set xs)