bug#69305: outline-minor-mode for tabulated-list-mode

[Juri Linkov <juri@linkov.net>]

>> >> >> But tabulated-list-groups is not a defcustom.
>> >> >> There are no defcustoms here.
>> >> >
>> >> > Buffer-menu-group-by _is_ a defcustom, and it is documented thusly:
>> >> >
>> >> >   +  "If non-nil, buffers are grouped by function.
>> >> >   +This function takes one argument: a list of entries in the same format
>> >> >   +as in `tabulated-list-entries', and should return a list in the format
>> >> >   +suitable for `tabulated-list-groups'.
>> >> >
>> >> > So you are inviting users to write functions that return values of a
>> >> > certain format, but the format's documentation IMO doesn't make it
>> >> > clear how to specify a grouping, and for its important part alludes to
>> >> > another variable (tabulated-list-entries), whose documentation is
>> >> > suitable for different kind of uses, not for grouping of entries.
>> >>
>> >> The format 'tabulated-list-entries' is input.
>> >> The format 'tabulated-list-groups' is output.
>> >
>> > Yes, I'm well aware of that.
>> >
>> >> > Since tabulated-list-groups is such an important part of the
>> >> > documentation of Buffer-menu-group-by, the doc string of
>> >> > tabulated-list-groups is in effect part of the doc string of
>> >> > Buffer-menu-group-by, and should IMO be worded as appropriate for user
>> >> > options.
>> >>
>> >> The docstring of 'tabulated-list-groups' already
>> >> sufficiently documents the format:
>> >
>> > No, it doesn't document it sufficiently.  As I already said several
>> > times.
>> >
>> >>   (defvar-local tabulated-list-groups nil
>> >>     "Groups displayed in the current Tabulated List buffer.
>> >>   This should be either a function, or a list.
>> >>   If a list, each element has the form (GROUP-NAME ENTRIES),
>> >>   where:
>> >>
>> >>    - GROUP-NAME is a group name as a string, which is displayed
>> >>      at the top line of each group.
>> >>
>> >>    - ENTRIES is a list described in `tabulated-list-entries'.
>> >>
>> >> There is no need to duplicate the description of linked
>> >> 'tabulated-list-entries'.
>> >
>> > This is not about duplication.  This is about adding information that
>> > currently isn't there.
>> >
>> >> And here it's clear that the function should add GROUP-NAME to
>> >> groups from 'tabulated-list-entries'.
>> >
>> > Which doesn't clarify the issue at hand even a single bit.
>>
>> Sorry, I don't understand what is missing here.
>> Please show an example of changes that you want.
>
> Sorry, I cannot show an example of something whose use I don't
> understand.  Which is why I asked _you_ to show an example: how to set
> up Buffer-menu-group-by to group buffers by major-mode.

Ah, you wanted to include a buffer example in "Tabulated List Mode".
Ok, here it is:

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 630e42e6878..e79b73eb081 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -1246,6 +1246,41 @@ Tabulated List Mode
 above form when called with no arguments.
 @end defvar
 
+@defvar tabulated-list-groups
+This buffer-local variable specifies the groups of entries displayed in
+the Tabulated List buffer.  Its value should be either a list, or a
+function.
+
+If the value is a list, each list element corresponds to one group, and
+should have the form @w{@code{(@var{group-name} @var{entries})}}, where
+@var{group-name} is a string inserted before all group entries, and
+@var{entries} have the same format as @code{tabulated-list-entries}
+(see above).
+
+Otherwise, the value should be a function which returns a list of the
+above form when called with no arguments.
+@end defvar
+
+You can use @code{seq-group-by} to create @code{tabulated-list-groups}
+from @code{tabulated-list-entries}.  For example:
+
+@smallexample
+@group
+  (setq tabulated-list-groups
+        (seq-group-by 'Buffer-menu-group-by-mode
+                      tabulated-list-entries))
+@end group
+@end smallexample
+
+where you can define @code{Buffer-menu-group-by-mode} like this:
+
+@smallexample
+@group
+(defun Buffer-menu-group-by-mode (entry)
+  (concat "* " (aref (cadr entry) 5)))
+@end group
+@end smallexample
+
 @defvar tabulated-list-revert-hook
 This normal hook is run prior to reverting a Tabulated List buffer.  A
 derived mode can add a function to this hook to recompute