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purecopying strings in add-hook?

From: Stefan Monnier
Subject: purecopying strings in add-hook?
Date: Tue, 30 Apr 2019 15:59:27 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Hi Dan, 

Do you remember why we purecopy the "function" argument of `add-hook`
when it's a string(!)?

This was introduced by commit ff917d632134b76be4c764dd6cf3ac1984b371a4,
appended below,


diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 1f0ddc1217..3b58fe1125 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,5 +1,8 @@
 2009-11-11  Dan Nicolaescu  <address@hidden>
+       * subr.el (add-hook): Purecopy strings.
+       (eval-after-load): Purecopy load-history-regexp and the form.
        * custom.el (custom-declare-group): Purecopy load-file-name.
        * subr.el (menu-bar-separator): New defconst.
diff --git a/lisp/subr.el b/lisp/subr.el
index bb3b37ebbf..fb832608cd 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -1218,6 +1218,8 @@ add-hook
       (setq hook-value (list hook-value)))
     ;; Do the actual addition if necessary
     (unless (member function hook-value)
+      (when (stringp function)
+       (setq function (purecopy function)))
       (setq hook-value
            (if append
                (append hook-value (list function))
@@ -1660,14 +1662,14 @@ eval-after-load
   ;; Add this FORM into after-load-alist (regardless of whether we'll be
   ;; evaluating it now).
   (let* ((regexp-or-feature
-         (if (stringp file) (load-history-regexp file) file))
+         (if (stringp file) (setq file (purecopy (load-history-regexp file))) 
         (elt (assoc regexp-or-feature after-load-alist)))
     (unless elt
       (setq elt (list regexp-or-feature))
       (push elt after-load-alist))
     ;; Add FORM to the element unless it's already there.
     (unless (member form (cdr elt))
-      (nconc elt (list form)))
+      (nconc elt (purecopy (list form))))
     ;; Is there an already loaded file whose name (or `provide' name)
     ;; matches FILE?

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