Covariant Typing Policy

Covariance in Nit has two causes:

• generic types are covariant on the base type: ie Array[Cat] is a subtype of Array[Object] because Cat is 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.