This was discussed very very early. Basically type system would not allow to declare `var` inside class. It would apply to object graph, so immutable collection

This was discussed very very early. Basically type system would not allow to declare `var` inside class. It would apply to object graph, so immutable collection could not store mutable objects etc.. Problem is that type system in Java is dynamic at runtime. It is not possible while maintaining interoperability with Java code.