Covariant Typing Policy
Covariance in Nit has two causes:
- generic types are covariant on the base type: ie
Array[Cat]is a subtype of
Catis a subtype of
Object. This is not true in most of object-oriented languages.
- Virtual Types can be redefined
Advantages and drawbacks of the covariance
The main advantage of the covariance is the improved expressiveness to the Nit models.
class Employee type OFFICE: Office var office: OFFICE end class Boss super Employee redef type OFFICE: BossOffice end var b: Boss = ... var o: Office = ... b.office = o # Compilation Error! Expected a BossOffice
Invariant languages (eg. Java, C++) have to use unsafe downcasts to achieve the same effects.