On 5/10/21 10:47 PM, Juri Linkov wrote:
> Probably you meant to name it `completions-group-sort`
> since the name `completions-sort` is too ambiguous?

Yes, I meant to add a `completions-group-sort(-function)` customizable
variable.

I attached the updated patch set, where the last patch
"0005-minibuffer-completion-help-Add-group-sorting.patch" adds the
`completions-group-sort-function` variable. (The behavior is equivalent
to the previous patch set where the `group-function` has a generalized
action argument.)

> Then such an option could support any function to perform sorting,
> and when necessary the API user could let-bind it around
> the API call, e.g. in the case of the ucs-char-names:> ...
> This is just an example, I'm not sure if read-char-by-name-group-sort is
> really needed, but this example demonstrates that it's up to the user to
> define options more specific to the completion table.

Of course, a completion command can let-bind/override certain
dynamically bound variables which influence the behavior of the
completion UI. However there are two downsides to be noted:

1. By doing such an override the ability of the user to configure the
completion UI is subverted.
2. Since the variables are let-bound, they influence the behavior of
nested recursive completion sessions. It is better to override the
variables with `setq-local` in a `minibuffer-setup-hook`.

Daniel