alist with type checking

[Urs Liska]

Hi all,

I'm working on (or rather with) functions managing nested association lists
(taken from Jan-Peter's 
https://github.com/openlilylib/openlilylib/blob/master/ly/_internal/utilities/alist-access.ily).

This is intended to manage a hierarchical object with configuration 
options for a project or a library.

One thing I didn't manage to come up with so far is a way to add "type 
checking".

So far a key is created and initialized with a command
\registerOption path.to.key initial-value

Later only registered keys can be modified with a
\setOption new-value
 command.
This is already great, but it would be cool if \registerOption could 
accept an (optional) predicate argument and store that predicate so that 
a subsequent \setOption will only be successful if the new value 
satisfies the predicate.

Any ideas how to achieve that?
I thought about storing a pair of predicate and value instead of only 
the value but so far it failed completely ;-)

Thanks for any suggestions
Urs