bug#45147: Org-like cycling in outline-minor-mode

[Juri Linkov <juri@linkov.net>]

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>> +(defcustom describe-bindings-after-hook '(describe-bindings-outline)
>> +  "Hook run at the end of `describe-bindings'."
>> +  :type 'hook
>> +  :options '(describe-bindings-outline)
>> +  :group 'help
>> +  :version "28.1")
>
> What's the rationale for populating the hook by default?  That's not
> what we normally do: hooks are for Lisp programs to use, not for the
> core functionality to invoke itself.

This was an attempt of generalization.

>> +(defun describe-bindings-outline ()
>> +  "Hook to enable outlines in the output buffer of `describe-bindings'."
>
> First, this is not a hook.
>
> And second, if we want a feature whereby the buffer describing
> bindings could be put in Outline mode, why not offer a simple option
> for users to customize, not a hook for users to tweak?  Once again,
> having user options that accept only function values is
> user-unfriendly.

Function values are user-friendly when their defcustom provides
human-readable tags.

But in this case it's fine to have a simple option like in this patch:


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diff --git a/lisp/help.el b/lisp/help.el
index babaf4adc7..2409636b48 100644
--- a/lisp/help.el
+++ b/lisp/help.el
@@ -561,6 +561,12 @@ help--key-description-fontified
               'font-lock-face 'help-key-binding
               'face 'help-key-binding))
 
+(defcustom describe-bindings-outline t
+  "Non-nil enables outlines in the output buffer of `describe-bindings'."
+  :type 'boolean
+  :group 'help
+  :version "28.1")
+
 (defun describe-bindings (&optional prefix buffer)
   "Display a buffer showing a list of all defined keys, and their definitions.
 The keys are displayed in order of precedence.
@@ -578,7 +584,26 @@ describe-bindings
     ;; Be aware that `describe-buffer-bindings' puts its output into
     ;; the current buffer.
     (with-current-buffer (help-buffer)
-      (describe-buffer-bindings buffer prefix))))
+      (describe-buffer-bindings buffer prefix)
+
+      (when describe-bindings-outline
+        (setq-local outline-regexp ".*:$")
+        (setq-local outline-heading-end-regexp ":\n")
+        (setq-local outline-level (lambda () 1))
+        (setq-local outline-minor-mode-cycle t
+                    outline-minor-mode-highlight t)
+        (outline-minor-mode 1)
+        (save-excursion
+          (let ((inhibit-read-only t))
+            (goto-char (point-min))
+            (insert (substitute-command-keys
+                     (concat "\\<outline-mode-cycle-map>Type "
+                             "\\[outline-cycle] or \\[outline-cycle-buffer] "
+                             "on headings to cycle their visibility.\n\n")))
+            ;; Hide the longest body
+            (when (and (re-search-forward "Key translations" nil t)
+                       (fboundp 'outline-cycle))
+              (outline-cycle))))))))
 
 (defun describe-bindings-internal (&optional menus prefix)
   "Show a list of all defined keys, and their definitions.