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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

class Boss
    super Employee
    redef type OFFICE: BossOffice

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.