Question

http://www.dartlang.org/docs/spec/dartLangSpec.pdf

The language spec for Dart mentions below

Dart supports optional typing based on interface types.
The type system is unsound, due to the covariance of generic types. This is
a deliberate choice (and undoubtedly controversial). Experience has shown that
sound type rules for generics fly in the face of programmer intuition. It is ..

• Can someone elaborate further on the reason type system is unsound?
• What was the Dart lang spec writers thinking when they say sound type rules for generics fly in the face of intuition?

Answer 1

What was the Dart lang spec writers thinking when they say sound type rules for generics fly in the face of intuition?

Take a look at the related questions to the right of this one. I see:

• Why is List<Number> not a sub-type of List<Object>?
• Why generic interfaces are not co/contravariant by default?
• Why can’t I assign a List<Derived> to a List<Base>?
• Why cant I cast from a list<MyClass> to List<object>?
• Why Animals[] animals = new Cat[5] compiles, but List<Animal> animals = new List<Cat>() does not?

While covariance isn’t sound (for many mutable types), it is the behavior that many programmers intuitively expect when they first start working with generic types.