RE: custom: how do I augment an option?

["Drew Adams" <drew.adams@oracle.com>]

> 3- submit a patch for Customize which lets the user specify
>    not just a new value but a change (like a "diff") to the
>    default value. For lists representing sets, a way for the
>    user to specify elements to add and elements to remove
>    would be great.  For lists where order matters, the user
>    should additionally have some control over where to add
>    elements.
> Point 3 would be *really* welcome.

+1

One reason this would be helpful would be simplicity of expression.  But
another, quite important reason IMO, would be some ability to better handle
updates of the default value, e.g. in a new version of the given defcustom.

For example, an option value that is a list of mappings of some kind might have
additional mappings by default in a newer version of the library.  If you have
already customized the option then you might not learn of the added
possibilities.  I see this fairly often.  You've customized based on the old
default value, not the new one.  Given some awareness of the change, you might
now want to customize the option differently.

But this problem of a changed default value is presented even for non-collection
options.  It would be good to have a simple (optional) way for users of a
library to be alerted that the default value of a given option or face that they
have customized has changed.

E.g., if the default value of `foo' was 3, you have customized it to 42, and the
default value is now nil, be able to be alerted to that default change.