[noidi]

I think generally object validation should remain separate from construction. The documentation for Python's FormEncode library explain this well:

> Validation is contextual; what validation you apply is dependent on the source of the data.

> Often in an API we are more restrictive than we may be in a user interface, demanding that everything be specified explicitly. In a UI we may assist the user by filling in values on their behalf. The specifics of this depend on the UI and the objects in question.

> At the same time, we are often more restrictive in a UI. For instance, we may demand that the user enter something that appears to be a valid phone number. But for historical reasons, we may not make that demand for objects that already exist, or we may put in a tight restriction on the UI keeping in mind that it can more easily be relaxed and refined than a restriction in the domain objects or underlying database. Also, we may trust the programmer to use the API in a reasonable way, but we seldom trust user data in the same way.

-- http://formencode.org/Design.html#validation-as-directional-...

Lately I've even played with the idea that maybe Clojure style strict separation of data and functions is just taking the single responsibility principle of OO design to it's ultimate conclusion as the responsibilities of storing a piece of data in memory and the responsibility of deriving new data from it are separated...