Re: predicate group-p to see if a group exists?

[martin rudalics]

> There seems to be no predicate to test whether a particular group is
> defined. Should there be one? (group-p 'foo)
>
> For example, consider a library that could be used with different Emacs
> versions. Suppose that a group `foo' exists in one Emacs version but not in
> another (Buffer-menu is an example - doesn't exist in Emacs 20).
>
> Suppose I want to define a new group `my-foo'. I want it to have group `foo'
> as parent, if group `foo' exists'. If not, I want it to have whatever
> parent(s) `foo' has in the Emacs versions where `foo' is defined.
>
> For that, I could do this, if predicate `group-p' existed:
>
> (defgroup my-foo nil "jjjj"
>   :group (if (group-p 'foo) 'foo 'bar))
>
> Yes, I could just do this:
>
> (defgroup my-foo nil "jjjj"
>   :group 'foo :group 'bar)
>
> But in the later Emacs versions, `my-foo' will also have `bar' as parent,
> not just `foo', which is a bit less tidy.

You could write

(defgroup my-foo nil "jjjj"
  :group (if (get 'foo 'custom-group) 'foo 'bar))

because it's unlikely that a custom group doesn't have any members
(except for the case when you're just specifying it - like my-foo).