Re: save-excursion doesn't restore point with json-pretty-print

[Tassilo Horn]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Robert Pluim <rpluim@gmail.com>
>> Cc: help-gnu-emacs@gnu.org
>> Date: Fri, 01 Feb 2019 11:33:23 +0100
>> 
>> > Perhaps json-pretty-print could use the new replace-buffer-contents
>> > function?  Although I'm not sure this will cure the problem, at least
>> > not in all cases.
>> 
>> replace-buffer-contents would be a lot easier to use in this context
>> if it could take a string. Otherwise you have to create a buffer,
>> select it, insert a string, then pass that buffer to
>> replace-buffer-contents, delete the buffer.
>
> json-read-from-string uses a temporary buffer anyway, so a couple of
> trivial changes in json-pretty-print should do the trick.

Is this what you have in mind?  Seems to work well.  I'm just not sure
if the save-excursion should be in `json-pretty-print' itself.  I'd
always want to have it but maybe there are use-cases where it would be
wrong.

--8<---------------cut here---------------start------------->8---
diff --git a/lisp/json.el b/lisp/json.el
index 26cd48f41d..966b2e680c 100644
--- a/lisp/json.el
+++ b/lisp/json.el
@@ -740,14 +740,23 @@ json-pretty-print-buffer
 (defun json-pretty-print (begin end)
   "Pretty-print selected region."
   (interactive "r")
-  (atomic-change-group
-    (let ((json-encoding-pretty-print t)
-          ;; Distinguish an empty objects from 'null'
-          (json-null :json-null)
-          ;; Ensure that ordering is maintained
-          (json-object-type 'alist)
-          (txt (delete-and-extract-region begin end)))
-      (insert (json-encode (json-read-from-string txt))))))
+  (save-excursion
+    (save-restriction
+      (narrow-to-region begin end)
+      (goto-char (point-min))
+      (atomic-change-group
+        (let ((json-encoding-pretty-print t)
+              ;; Distinguish an empty objects from 'null'
+              (json-null :json-null)
+              ;; Ensure that ordering is maintained
+              (json-object-type 'alist)
+              (obj (json-read))
+              (orig-buffer (current-buffer)))
+          (with-temp-buffer
+            (insert (json-encode obj))
+            (let ((tmp-buffer (current-buffer)))
+              (set-buffer orig-buffer)
+              (replace-buffer-contents tmp-buffer))))))))
 
 (defun json-pretty-print-buffer-ordered ()
   "Pretty-print current buffer with object keys ordered."
--8<---------------cut here---------------end--------------->8---

Oh, one part where that's not completely right is undoing.  That is,
pretty-printing keeps point on the character it has been before but
undoing the pretty-printing will place point at the end of the json.

Bye,
Tassilo