Salut Vicente, :)
From: vicente.botet
There is a thing that i don't like in the design, the fact that you can change the constraint at runtime.
If you use a constraint that works statically then there is no way to change it. It is bound to the constrained type.
I would prefer to have two separated hierarchies, one for constrained values that preserv its constraints, error handling, ... staticaly, and one for those tyhe constraint can be changed at runtime.
I wouldn't prefer to have two separate hierarchies with almost identical functionality and differing only in details.
I expect that the preserving constrained values be implemented in a more space and time efficient way.
Did you find any problems with suboptimal work of the current implementation for the static cases?
For the constrained values that can change its constraint at runtime I see two cases, the constraint is attached to the instance, which is your case, or it attached to a type.
mutating_type :== by_instance | by_type
When attached to a type, the type needs to maintain the set of instances. Instead of changing the type a split operation can be provided resulting in a transfer of the instances satisfying the new constraint to the new type. Of course this meens more space and time consumming, but ...
There was a similar idea on the users list and I'm still not convinced this leads to something good. If you request to change the constraint of a whole type and some instances don't obey your request, then what is the point in doing this? Best regards, Robert