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[RFC]: replace-region-contents
From: |
Tassilo Horn |
Subject: |
[RFC]: replace-region-contents |
Date: |
Fri, 01 Feb 2019 22:20:37 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) |
Hi all,
on gnu-emacs-help I've asked why my wrapper command around
`json-pretty-print' doesn't restore point although it uses
`save-excursion'. The reason is that `json-pretty-print' replaces the
region with copy, delete, and insert. Robert and Eli pointed me to the
new `replace-buffer-contents' which allowed me to rewrite
`json-pretty-print' in a way where point stays where it has been before
pretty-printing:
--8<---------------cut here---------------start------------->8---
(defun json-pretty-print (begin end)
"Pretty-print selected region."
(interactive "r")
(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))))))))
--8<---------------cut here---------------end--------------->8---
I think that replacing a region by transforming its contents in some
arbitray way is some generally useful feature, so I'd propose to extract
that functionality into some new function `replace-region-contents' I'd
like to add to subr-x.el. Here it is and how it is applied in the
json-pretty-printing scenario.
--8<---------------cut here---------------start------------->8---
;; in subr-x.el (or wherever you please)
(defun replace-region-contents (beg end extract-fn inject-fn)
"Replace the region between BEG and END using EXTRACT-FN and INJECT-FN.
The current buffer is narrowed to the region between BEG and END,
then EXTRACT-FN is called in order to extract some value.
Thereafter, INJECT-FN is called with that value in a temporary
buffer which it should populate.
Finally, the region in the source buffer is replaced with the
contents of the temporary buffer prepared by INJECT-FN using
`replace-buffer-contents'."
(save-excursion
(save-restriction
(narrow-to-region beg end)
(goto-char (point-min))
(atomic-change-group
(let ((source-buffer (current-buffer))
(extracted (funcall extract-fn)))
(with-temp-buffer
(funcall inject-fn extracted)
(let ((tmp-buffer (current-buffer)))
(set-buffer source-buffer)
(replace-buffer-contents tmp-buffer))))))))
;; in json.el
(defun json-pretty-print (begin end)
"Pretty-print selected region."
(interactive "r")
(replace-region-contents
begin end
(lambda ()
(let ((json-null :json-null)
;; Ensure that ordering is maintained
(json-object-type 'alist))
(json-read)))
(lambda (json-obj)
(let ((json-encoding-pretty-print t)
;; Distinguish an empty objects from 'null'
(json-null :json-null))
(insert (json-encode json-obj))))))
--8<---------------cut here---------------end--------------->8---
Does that seem like a good idea, is there anything to improve, or should
I just fix `json-pretty-print' as in the first snippet?
Bye,
Tassilo