Allow saving ispell-local-words in .dir-locals.el

TODO: Automatically move local words to .dir-locals on save

init.el

@@ -400,6 +400,10 @@
     (:bind "c" #'+Info-copy-current-node-name
            "w" #'+Info-copy-current-node-name)))
 
+(setup ispell
+  (:also-load +ispell)
+  (put 'ispell-buffer-session-localwords 'safe-local-variable #'+ispell-safe-local-p))
+
 (setup kmacro
   (:also-load +kmacro)
   (with-eval-after-load '+kmacro

lisp/+ispell.el

@@ -0,0 +1,82 @@
+;;; +ispell.el --- Customizations for `ispell' -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl)
+
+;; Utility function TODO: move elsewhere
+(defun +ispell-append-removing-duplicates (&rest lists)
+  "Append LISTS, removing duplicates from the result.
+Any keyword arguments to `cl-remove-duplicates' should come
+before the LISTS."
+  (let (cl-remove-duplicates-args)
+    (while (keywordp (car lists))
+      (push (pop lists) cl-remove-duplicates-args)
+      (push (pop lists) cl-remove-duplicates-args))
+    (apply #'cl-remove-duplicates (apply #'append lists)
+           (nreverse cl-remove-duplicates-args))))
+
+;;; Ispell in .dir-locals
+
+;; Let Emacs know a list of strings is safe
+(defun +ispell-safe-local-p (list)
+  (and (listp list)
+       (seq-every-p #'stringp list)))
+
+;; Can I instruct ispell to insert LocalWords in a different file?
+;; https://emacs.stackexchange.com/q/31396/2264
+
+;; How can I move all my file-local LocalWords to .dir-locals.el?
+;; https://emacs.stackexchange.com/q/31419
+
+;; Adapted from ispell.el:ispell-buffer-local-words
+(defun +ispell-buffer-local-words-list ()
+  (let (words)
+    (or ispell-buffer-local-name
+        (setq ispell-buffer-local-name (buffer-name)))
+    (save-excursion
+      (goto-char (point-min))
+      (while (search-forward ispell-words-keyword nil t)
+        (let ((end (point-at-eol))
+              (ispell-casechars (ispell-get-casechars))
+              string)
+          (while (re-search-forward " *\\([^ ]+\\)" end t)
+            (setq string (match-string-no-properties 1))
+            (if (and (< 1 (length string))
+                     (equal 0 (string-match ispell-casechars string)))
+                (push string words))))))
+    words))
+
+(defun +ispell-move-buffer-words-to-dir-locals ()
+  (interactive)
+  (unless (buffer-file-name)
+    (user-error "Buffer not attached to file"))
+  (hack-dir-local-variables)
+  (let ((words (+ispell-buffer-local-words-list))
+        (dir-local-words (+ispell-append-removing-duplicates
+                          (alist-get 'ispell-buffer-session-localwords
+                                     dir-local-variables-alist)
+                          (alist-get 'ispell-buffer-session-localwords
+                                     file-local-variables-alist))))
+    (save-excursion
+      (add-dir-local-variable
+       major-mode
+       'ispell-buffer-session-localwords
+       (setq ispell-buffer-session-localwords
+             (+ispell-append-removing-duplicates
+              :test #'string=
+              dir-local-words ispell-buffer-session-localwords words)))
+      (when (y-or-n-p "Save .dir-locals.el?")
+        (save-buffer))
+      (bury-buffer))
+    (or ispell-buffer-local-name
+        (setq ispell-buffer-local-name (buffer-name)))
+    (save-excursion
+      (goto-char (point-min))
+      (while (search-forward ispell-words-keyword nil t)
+        (delete-region (point-at-bol) (1+ (point-at-eol)))))))
+
+(provide '+ispell)
+;;; +ispell.el ends here